EPA910-R-99-002
&EFA
United States
Environmental Protection
Agency
Region 10
1200 Sixth Avenue
Seattle WA 98101
Alaska
Idaho
Oregon
Washington
Office of Ecosystems and Communities
May 1999
Biological Aspects of Hybrid
Poplar Cultivation on
Floodplains in Western North
America:
A Review
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on
In
A
Prepared by: Prepared for:
Jeffery H. Braatne, PhD Office of Ecosystems and Communities
Seattle. Washington U. S. Environmental Protection Agency Region 10
braatneiSiLwashington.edu 1200 6th Ave.
Seattle, WA 98101
EPA Project Manager:
Thomas E. Wilson
Purchase Order No. 7Y-0304-NATX
EPA Document No. 910-R-99-002
March, 1999
Acknowledgments
The author and project manager express sincere appreciation to the many parties who provided data and other materials used
in this report. We especially thank those whose intensive peer review* substantially enhanced the final product. We also thank
Doug Norton of EPA 's Office of Wetlands, Oceans, and Watersheds (OWOW), whose policy and fiscal support made this report
possible.
Notice
This document has been subjected to U.S. Environmental Protection Agency review and has been approved for
publication. Publication does not signify thai the contents necessarily reflect the views and policies of the
Environmental Protection Agency or of any other organization represented in this document. Mention of trade names
or commercial products does not constitute endorsement or recommendation for use.
This report should be cited as:
U.S. Environmental Protection Agency. 1999. Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North
America -A Review. (EPA Document No. 910-R-99-002)
To obtain additional copies of this document, contact:
EPA Region 10 Public Environmental Resource Center, Seattle, WA. Phone (800) 424-4372 (within Region 10) or
(206) 553-1200.
This report (with color graphics) is available on the Internet for browsing or download at:
http://www.epa.gov/rlOearth/offices/ecoconini/poplars.pdf
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America — A Review
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Biological Aspects of Hybrid Poplar Cultivation on Flaodplains in Western North America — A Review
Preface
Cottonwoods—both native and hybrid—are receiving a great deal of ecological and economic attention these
days. Historically, native cottonwoods were a vital component of most lowland floodplains and associated riparian
ecosystems, particularly in the western United States. Unfortunately, those trees have been largely eliminated,
leading to widespread stream degradation. Now, however, increasing pressures for stream restoration are inevitably
leading to demands to restore cottonwoods to these critical riparian and floodplain environments.
Today, hybrids derived from those native cottonwoods—referred to as "hybrid poplars"—are being widely
introduced" into those same floodplain habitats. Timber and agricultural interests are planting large acreages of hybrid
poplars for pulp and wood products. Environmental agencies are using these hybrids to cost-effectively treat a variety
of contaminated soils and wastewaters. And natural resource agencies, struggling to restore critical salmon and
riparian habitats, are replanting stream banks with both native cottonwoods and hybrid poplars.
To some, these hybrids represent an opportunity to cost-effectively improve many degraded floodplain and
riparian habitats. For example, they argue that farmers could plant these fast growing, flood tolerant tree crops just
outside of critical riparian areas, thus protecting those sensitive areas from damaging tillage and grazing impacts,
while also intercepting and neutralizing runoff of nutrients and farm chemicals.
Others, however, fear that such uses of hybrids near riparian areas could genetically contaminate native
cottonwoods. or could pose other ecological threats. And a few even oppose the re-introduction of native
cottonwoods into riparian ecosystems, citing fears of water depletion.
Unfortunately, the knowledge needed to help the various parties resolve such conflicts is fragmented among
many scientific disciplines. But clearly, one of the greatest needs is for all parties to have a better understanding of
the basic biology and ecological role of cottonwoods, with particular emphasis on the relative biological distinctions
between native cottonwoods and hybrid poplars.
To meet this need, the U.S. Environmental Protection Agency asked Dr. Jeffrey Braatne to compile the existing
scientific knowledge on selected issues that have arisen most frequently. This is his report, admirably done on a very
limited budget. And to assist those interested in investigating specific issues in greater depth, Dr. Braatne has also
included an extensive bibliography.
In reviewing this report, please keep in mind that few studies have been specifically designed to address many of
these environmental issues. Thus findings in this report necessarily reflect the synthesis of relevant studies conducted
by diverse disciplines. However, several long-term studies are now underway that will provide additional
information on several key issues. In addition, Dr. Braatne has also included his recommendations for future research
needs.
Thank you
Thomas E. Wilson
Senior Policy Advisor
EPA Region 10
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America — A Review
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Biological Aspects of Hybrid Poplar Cultivation on Flaodplains in Western North America — A Review
Abstract
Fast-growing hybrid poplars—the product of selective breeding of native cottonwoods- are being widely planted
to supplement the diminishing supply of natural hardwoods. As effective biofilters, these trees are also being
increasingly used to treat agricultural runoff and municipal wastewaters. Given such uses, the cultivation of hybrid
poplars in floodplain habitats is expected to increase in the coming years. This report discusses the major biological
distinctions between hybrid poplars and native cottonwoods. and explores some of the potential management issues
associated with their cultivation near riparian corridors.
The biological differences between hybrid poplars and native cottonwoods are subtle, with varying levels of
ecological significance. Such differences arise in the following areas: a) genetic and reproductive properties, b) growth
and water-use characteristics, and c) wildlife habitat values. Much of our current knowledge of these factors is derived
from comparative studies of parental (native cottonwoods) and hybrid genotypes (F|5 F,, and backcrosscs). Only a
limited number of studies have compared the ecological properties of commercial poplar plantations with native
habitats, hence the potential outcome of ecological interactions must often be inferred.
Over the years, widespread planting of non-native poplars in the West has had a limited effect upon the genetics
and ecology of native riparian cottonwoods. Restricted levels of gene exchange are related to phenological
incompatibilities and the reduced pollen and seed viability of hybrid crosses. Although hybrid poplars are noted for
rapid growth, their potential impact on groundwater and streamflows is comparable to slightly lower than other
agricultural crops. Hybrids are more drought tolerant than native cottonwoods, yet potentially more vulnerable to
flooding. These physiological differences may favor hybrids in some riparian settings. Since commercial plantations
are not designed to serve as wildlife habitat, lower habitat values relative to native riparian zones are expected.
However, their habitat values are greater than traditional row and pasture crops.
Given our current knowledge, numerous research and management opportunities exist for reducing gene
exchange and improving the habitat properties of hybrid poplar plantations. Future research needs to focus upon these
opportunities, while also promoting the conservation and study of native riparian cottonwood ecosystems.
Keywords
biofilter, cottonwood, ecosystem, floodplain, genetic contamination, habitat, hybridization, hybrid poplar,
populus, riparian corridor, transpiration, tree crop, water use.
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America — A Review
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Biological Aspects of Hybrid Poplar Cultivation on Flaodplains in Western North America — A Review
Contents
I. Introduction Page 1
II. Native poplars and natural zones of hybridization Page 2
III. Domesticated non-native and hybrid poplars Page 5
IV. Reproductive properties of native cottonwoods and hybrid poplars Page 7
V. Natural patterns of seedling recruitment in riparian corridors Page 8
VI. Water-use by hybrid poplars and other agricultural crops Page 9
VII. Drought and flood-tolerance of hybrid poplars and native cottonwoods Page 11
VIII. Clearing of floodplain habitats as a means of conserving water Page 11
IX. Wildlife habitat studies Page 12
X. Conclusions. Recommendations and Rcascarch Needs Page 13
XI. Literature cited Page 15
Appendix A: About the Author Page 23
Appendix B: Bibliography on the Biological Aspects of Populus Spp. and Ecology of the
Riparian Landscapes of Western North America Page 25
Figures and Tables
Table 1, Native Populus Species in North America Page 2
Table 2. Reproductive characteristics of diploid andtriploid F: hybrid poplars Page 8
Table 3. Estimated water use by agricultural crops and hybrid poplars in Eastern Washington Page 10
Figure 1, Distributional range of native riparian cottonwoods in North America Page 3
Figure 2. Black Cottonwood Page2
Figure 3, Eastern Cottonwood Page 4
Figure 4, Natural hybrids of P. fremontii x P. ctngustifolict Page 4
Figure 5, Fj hybrids and advanced generations Page 4
Figure 6, Lombardy Poplar Page 5
Figure 7, White Poplar Page 5
Figure 8, Common leaf forms Page 6
Figure 9, Harvest of seven-year old hybrid poplars Page 6
Figures 10. Poplars are dioecious Page 7
Figure 11, Cottonwood seedlings Page 8
Figure 12, Physiological studies ofttomatal conductance and photosynthesis Page 9
Figure 13, The upper canopy of a four-year old stand of hybrid poplar in Eastern Washington Page 10
Figure 14, Irrigated cornfields in Eastern Washington Page 10
Figure 15, Seasonal flooding of a natural hybrid zone Page 11
Figure 16, The understory of a five-year old stand of hybrid poplar Page 12
Figure 17, Deer and other cosmopolitan wildlife species commonly use hybrid poplar stands Page 13
Figure 18, Riparian hybrid poplar buffers at Carnation Farms Page 14
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Biological Aspects of Hybrid Poplar Cultivation on Flaodplains in Western North America — A Review
I. Introduction
Hybrid poplars are the product of the selective
breeding of native cottonwoods (Populus spp.). Their
rapid growth and capacity to supply a broad range of
wood products has led to their widespread cultivation in
North America (Dickmann and Stuart 1983, Stettler et
al. 1996a). In the West, hybrid poplars are a relatively
recent agricultural commodity, and only a small
proportion of arable lands has been converted from
traditional crops, such as com, hay or pasture, to
commercial poplar plantations (Heilman et al. 1995,
Stettler et al. 1996a). These conversions are largely
driven by dwindling supplies of natural hardwood fiber
(Heilman et al. 1995. Sedjo 1997) and by the impressive
yields of hybrid poplar when grown under short-rotation
intensive culture (Stettler et al. 1988, Heilman and Xie
1993, 1994). Hybrid poplars are thus becoming anew
source of wood and fiber for the pulp and paper industry,
and thereby an alternative cash crop for many fanners
and ranchers.
In addition to their commercial value, hybrid
poplars arc effective in the control of nutrients and toxic
compounds found in agricultural runoff and landfill
effluents (Haycock and Pinay 1993, Liclit 1991, 1993,
1994, Licht and Madison 1995, O'Neill and Gordon
1994, Schultzetal. 1994, 1995, Schnooretal. 1995,
Burken and Schnoor 1997. Chappell 1997, Gordon et al.
1997, Schnoor 1997). For example, a simple four-row
buffer of hybrid poplar can reduce nitrates by over 90%
and significantly lower pesticide concentrations in
agricultural runoff (Licht 1994, Burken and Schnoor
1997). Accordingly, hybrid poplar is being promoted
within the agricultural community as a means of
improving water quality. New field trials are currently
underway in several areas to further assess their efficacy
in the treatment of agricultural runoff. As a result of
these and other research activities, hybrid poplars may
prove to be an economically-attractive approach to the
treatment of "non-point" agricultural pollutants (Licht
1994, Sedjo 1997).
Given their emergence as an agricultural
commodity and biofilter, the cultivation of hybrid
poplars in floodplain habitats is expected to increase
significantly. As a result, resource managers and
landowners need to have a better understanding of the
biology of these trees. On the basis of current research
and published findings, this report discusses the major
biological distinctions between hybrid poplars and native
cottonwoods, and explores some of the potential
management issues associated with their cultivation in
riparian corridors. The topics addressed include: a)
genetic and reproductive properties, b) growth and
water-use characteristics, and c) wildlife habitat values.
Much of this research is derived from comparative
studies of parents (native cottonwoods) and their
offspring (Fp F2 hybrids), as only a limited number of
studies are available from which the ecological
properties of native habitats could be compared with
commercial plantations. This report highlights research
needs and initiates a framework for evaluating the
potential role of hybrid poplars in sustaining the natural
functions of riparian corridors impacted by agricultural
activity.
Those interested in a broader review of the
scientific literature should consult a recently published
book entitled, "The Biology of Populus: implications for
management and conservation" (Stettler et al. 1996a).
An extensive body of literature is cited directly within
this report and an updated bibliography is also provided
in Appendix B. A general familiarity with these sources
of peer-reviewed literature is critical to understanding
the biology of hybrid poplars and the ecology of native
riparian forests.
1.
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America —A Review
II. Native poplars and natural zones of
hybridization
Populus is a large and widespread genus of
dioecious woody plants (30 species worldwide),
commonly found in temperate regions of the world. In
North America, there are eight native species of Populus
(Table 1, Figure 1), with numerous zones of natural
hybridization (Eckenwalder 1977a,b, 1984a,b, c, 1996,
Pregitzer and Barnes 1980). Most of these native poplars
are riparian cottonwoods (Figures 2 and 3), though two
species of aspen are also common in some regions of the
continent (Table 1).
The life cycle of native cottonwoods closely
follows the seasonal dynamics of streams and rivers, and
they are common members of floodplain forest
communities (Johnson 1994, Braatne et al. 1996, Scott
et al. 1997). Many studies have documented the critical
role of cottonwoods and willows in maintaining the
integrity of river channels (Beeson and Doyle 1995,
Scott et al. 1996) and riparian ecosystems (Stromberg et
al. 1991, 1996, Johnson 1994, Rood and Mahoney 1990,
Rood et al. 1995, Scott et al. 1996). Cottonwoods and
willows growing along river channels and backwaters act
to trap sediment and debris and serve as a barrier to the
scour and erosion of riverbanks. Native cottonwoods
provide critical habitat for a diverse assemblage of
amphibians, birds and mammals (Knopf et al. 1988,
Finch and Ruggiero 1993, Martinsen and Whitham
OO
1993, Whitham et al. 1996). They are also an important
source of carbon for riverine invertebrates, as shredders
and decomposers readily breakdown instream woody-
debris derived from cottonwoods (R. Naiman, pers.
comm.). As such, cottonwoods are integral components
of riverine food webs and critical to the ecology of rivers
and streams in the West (Braatne et al. 1996).
Natural hybrids between native cottonwoods
commonly arise within riparian corridors wherever the
distributional ranges of species overlap. As a general
rule, cottonwoods are noted for their lack of genetic
isolation mechanisms. As species segregate along
latitudinal and elevational gradients, hybridization
occurs in discrete zones where different species of
cottonwood come into contact with one another (Figures
1 and 4). Widespread zones of natural hybridization
occur in a) Washington, Oregon, Idaho, Montana ,
Alberta and British Columbia (Populus angustifolia, P.
Figure 2.
Black Cottonwood (Populus trichocarpa) is
commonly found along riparian corridors
throughout the Pacific Northwest. P. trichocarpa
contributes to the height growth and acute branch
angles of interspecific hybrid crosses (F hybrids).
(Photo by R.F. Stettler)
Table 1.
Native Populus Species in North America
Section
Species"
Common name
General distributional range
Aigeiros
Leucoides
Populus
Tacamahaca
P. deltoides
P. fremontii
P. heterophylla
P. grandidentata
P. tremuloides
P. angustifolia
P. balsamifera**
P. trichocarpa**
Eastern Cottonwood
Fremont Cottonwood
Swamp Cottonwood
Bigtooth Aspen
Trembling Aspen
Narrowleaf Cottonwood
Balsam Cottonwood
Black Cottonwood
Eastern N. America to the Arid West
Arid Southwest of N. America
Southeastern N. America
Great Lakes region of N. America
Higher elevations and latitudes
Mid-elevations along Rocky Mts.
Upper latitudes (Canada and Alaska)
Pacific Northwest to Alaska
* Nomenclature follows Eckenwalder 1977a,b, 1984a,b,c, 1996, Burns and Honkala 1990.
Synonymy/taxonomic conflicts: Despite monographs by Eckenwalder (1977a,b, 1984a,b,c), and Brayshaw (1965), the taxonomic status of P. trichocarpa remains controversia
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America —A Review
Map Key
| Populus balsamifera
| Populus trichocarpa
Populus angustifolia
I—I Populus deltoides
I—I var. occldentalis
I I Populus deltoides
I—I var. dettoides
I | Populus fremontll
\ \ Populus heterophylla
Pacific Ocean
1500km
1000 mi
1992 MAGELLAN GeographixSMSanta Barbara, CA (800) 929-4627
Figure 1.
Distributional range of native riparian cottonwoods in North America. Natural hybrids commonly arise
where the distributional range of species overlaps.
3.
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America —A Review
balsamifera, P. deltoides and P. trichocarpa), and b)
throughout the intermountain region of Arizona,
Colorado, Utah, and Nevada (P. angustifolia, P.
deltoides, and P. fremontii).
Natural hybrid zones are usually not very broad
(10-15 km), and contain not only of F: crosses, but also
many advanced-generation hybrids (i.e., F2's and
backcrosses, See Figure 5). As such these hybrid zones
serve as bridges for the introgression of genes (i.e.
genetic exchange between species) and thus are of
evolutionary significance not only for cottonwoods, but
also for pathogens and invertebrates that rely upon
cottonwoods as their host and food source (Paige and
Capman 1993, Whitham et al. 1996, 1999, Floate et al.
1998). Natural hybrid zones are further noted for their
diverse assemblage of other plants, insects and wildlife
species (Martinsen and Whitham 1994, Whitham et al.
1996, 1999, Floate et al. 1998). These natural zones of
hybridization are unique and worthy of special efforts to
promote their conservation and protection (Whitham et
al. 1991, 1996, 1999, Floate et al. 1998).
Figure 3.
Eastern Cottonwood (Populus deltoides) is a
common riparian species throughout eastern and
western regions of temperate North America. Given
its widespread distribution, there are several
recognized varieties/subspecies, including: a) Plains
Cottonwood (P. deltoides var. occidentalis) found
throughout the Great Plains, and b) Rio Grande
Cottonwood (P. deltoides var. wislizenii) extends
northward from the Rio Grande River along the
western slopes of the Rocky Mountains to the
southern border of Wyoming. P. deltoides
contributes to the radial stem growth of interspecific
hybrid crosses (F1 hybrids). (Photo by J.H. Braatne)
Figure 4.
Natural hybrids of P. fremontii x P. angustifolia in
Northern Utah. These areas of natural hybridization
are noted for their high biotic diversity. In particular,
avian nesting success is promoted by the diverse
range of canopy architecture found among parental
species and hybrid crosses. (Photo by R.F. Stettler)
Figure 5.
F1 hybrids and advanced generations (F2 &
backcrosses) are characteristic of natural hybrid
zones. This photo shows the range of leaf
morphology that is characteristic of natural hybrid
crosses. (Photo by R.F. Stettler)
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America —A Review
III. Domesticated non-native and hybrid
poplars
Non-native and hybrid poplars have been planted
throughout temperate regions of North America. The
most common non-native poplars include: Lombardy
poplar (P. nigra var. italica, Figure 6) and white poplar
(P. alba, Figure 7). Both of these species are indigenous
to Eurasia, yet widely cultivated in urban, suburban and
agricultural landscapes. In the West, both species have
long been planted as shade trees and windbreaks on
agricultural landscapes (Burns and Honkala 1990).
Occasionally, non-native poplars colonize adjacent
riparian corridors via asexual propagation (J.H. Braatne,
P.E. Heilman and R.F Stettler, pers. observations), yet
Figure 6.
Lombardy Poplar (Populus nigra var. italica), a
desirable columnar cultivar arose from selective
breeding and propagation in southern Europe. This
male clone is planted as either an ornamental or
windbreak throughout temperate regions of the
world. Pollen release from Lombary Poplar is noted
to give rise to interspecific crosses with Aigeiros
and Tacamahaca poplars (See Table 1).
(Photo by J.H. Braatne)
there are no reports of these species displacing native
cottonwood populations.
The domestication of poplar has largely been a
process of interspecific hybridization and the selection
and propagation of desirable genotypes. Domestication
has spanned several centuries, beginning in Eurasia and
extending more recently to North America (Stettler et al.
1996b, Zsuffaet al. 1996). The most common
interspecific hybrids arise from crosses between
members of the Aigeiros and Tacamahaca sections of
Populus (Table 1, Figure 8). Selective crosses between
Asian and North American species are known as Asian-
american hybrids, between Asian and European species
as Eurasian hybrids, between European and North
America species as Euramerican hybrids, and between
North American species as Intra-american hybrids. The
most common interspecific crosses include:
Asian-american: P. trichocarpa x P. maximowiczii
(TM Clones)
Eurasian: P. maximowiczii x/! nigra
(MN Clones)
Euramerican: P. deltoides x P. nigra
(DN Clones)
P. trichocarpa x P. nigra
(TN Clones)
Intra-american: P. balsamifera x/! deltoides
(BD Clones)
P. trichocarpa x P. deltoides
(TD Clones)
Plant breeders are now developing an array of
hybrid poplars on the basis of these types of interspecific
crosses (Bisoffi and Gullberg 1996, Bradshaw 1996,
Stanton and Villar 1996, Stettler et al. 1996b, Zsuffa et
al. 1996).
Figure 7.
White Poplar (Populus alba) is a drought hardy
species that is native to southern Europe. The
drought resistance of this species has led to its
widespread cultivation in arid agricultural
landscapes. Interspecific crossability is limited
solely to those species found in the Populus section
(See Table 1). (Photo by J.H. Braatne)
5.
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America —A Review
Figure 8.
Common leaf forms associated with interspecific
crosses between P. trichocarpa (female) and P.
deltoides (male). Large F1 hybrid leaves (B-F) are
related in part to the large cell size inherited from P.
trichocarpa (A) and the high cell density from P.
deltoides (G). The undersurface of aneuploid and
triploid hybrid leaves (B-D) is much lighter than
diploid hybrid leaves (E and F). This lighter color is
correlated with higher stomatal density and rates of
water loss. (Photo by R.F. Stettler)
The agronomic interest in hybrid poplars arises
from their rapid juvenile growth and production of
woody biomass (Figure 9, Stettler et al. 1988,
Ceulemans et al. 1992, Heilman and Xie 1993, 1994).
Emphasis on the early part of their life cycle and on
favorable cultural conditions (i.e. high nutrients,
irrigation and control of weedy and herbivorous pests)
has promoted the selection of rapid-growing genotypes.
The ease with which poplars can be propagated
vegetatively (i.e. cloned), allows the selection of the best
performing genotypes and/or cultivars, such as columnar
forms of white poplar and Lombardy poplar. Selective
breeding combined with clonal propagation thus enables
the widespread cultivation and perpetuation of desirable
hybrid cultivars (Stettler et. al 1996b).
The biological consequences arising from the
artificial breeding and selection of hybrid poplars are
many. Of particular interest to their cultivation in
floodplain habitats, are those related to declines in
reproductive fitness (Bisoffi and Gullberg 1996, Stanton
and Villar 1996, Stettler et al. 1996b, Zsuffa et al. 1996)
and reduced resource allocation to defense mechanisms
(Newcombe 1996, Floate and Whitham 1993, Floate et
al. 1993, Whitham et al. 1996). Defense against insects
and pathogens is primarily a function of quantitative and
qualitative variation in biochemical traits. Interspecific
crosses can result in F: genotypes that are more
susceptible to insect and pathogen attack than parental
species. This reduction in defense mechanisms is known
as "hybrid breakdown" and has been observed in hybrids
growing in commercial plantations and natural zones of
hybridization (Floate and Whitham 1993, 1994, Floate et
al. 1996, 1998, Newcombe 1996, Whitham etal. 1996).
Figure 9.
Harvest of seven year old hybrid poplars along the
lower Columbia River. The harvest rotation of
hybrid poplars is commonly seven to eight years for
the purposes of fiber production. The production of
veneer logs for solid wood products would require
approximately 12 to 18 years in most regions.
(Photo by J.H. Braatne)
Thus, whereas hybrid poplars have been bred and
selected to grow extremely well under managed
conditions (i.e. with irrigation, fertilization and pest
control), they may face significant challenges from
competitive interactions and pathogen attack in natural
riparian corridors (Stettler et al. 1996b). Factors related
to lower reproductive fitness are discussed below.
IV. Reproductive properties of native
cottonwoods and hybrid poplars
Populus species are predominantly dioecious; thus
individual trees are either male (Figure lOa) or female
(Figure lOb). The age of reproductive maturity varies
among native species from five to ten years, yet in some
natural populations may not occur until the trees are 15
to 20 years old (Horton et al. 1960, Fenner et al. 1984,
6.
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America —A Review
Cooper 1990, Debell 1990, Dewit and Reid 1992, Van
Haverbeke 1990, B. Stanton, pers. communication). In
contrast, hybrid poplars grown in well-maintained
plantations commonly attain reproductive maturity in
four years (H.D. Bradshaw, B. Stanton and R.F. Stettler,
pers. communication). Pollen is widely dispersed by
wind, which in part helps explain the common
occurrence of natural hybrids. The dispersal of seeds
primarily follows wind vectors, though some seeds are
also water dispersed. These dispersal vectors contribute
to long-distance gene flow (up to several km) and high
levels of genetic diversity within natural populations
(Weber and Stettler 1981, Dunlap etal. 1994, 1995,
Farmer 1996).
Many researchers report that pollen and seed
viability is significantly lower in hybrid poplars (F
hybrids) relative to parental species (Stanton and Villar
1996, Stettler et al. 1996b). This pattern of reduced
fertility among F: hybrids has been widely reported for
agronomic crops and other interspecific Populus crosses
(Henry and Barnes 1977, Pregitzer and Barnes 1980,
Spies and Barnes 1982, Gladysz and Ochlewska 1983).
Recent studies by the Poplar Molecular Genetic
Cooperative (PMGC) and Tree Genetic Engineering
Research Cooperative (TGERC) have documented lower
reproductive activity among diploid and triploid hybrids
(Fj) relative to their parents (Bradshaw and Stettler
1993, Bradshaw 1996). These researchers have studied
variation in the reproductive biology of hybrid poplars
over several breeding seasons. Some recent findings
from TGERC are shown in Table 2.
Although additional large-scale genetic studies are
needed, the general trend of lower reproductive effort by
Fj hybrids is evident (Table 2). The causes, inferred or
known, for declines in the fertility of hybrids are related
to pre- and post-zygotic incompatibility (Bisoffi and
Gullberg 1996, Stanton and Villar 1996, Stettler et al.
1996b, Strauss and DiFazio 1997). Incompatibility in
the temporal patterns of capsule and embryo maturation
is a critical factor regulating fertility. If the capsules and
embryos of hybrid females lack synchronous patterns of
development, capsules will mature prior to embryos
resulting in the production of non-viable seed. In the
case of triploidy, it is the imbalance in chromosome
number among gametes that contributes to their low
fertility. Given the complexity of the reproductive
process, the nature and extent of genetic incompatibility
among Populus spp. warrants further study.
The lower fertility of F: hybrids limits their
potential gene exchange with native species.
Nevertheless, a few cases of gene exchange between
hybrid poplars and native cottonwoods have been
reported in the lower Columbia River (Strauss and
DiFazio 1997). TGERC studies have also shown that
crosses between hybrid males and native females yield
slightly more viable seed than crosses between native
males and hybrid females. This lower seed yield by
Figure 10a.
Figure 10b.
Figure 10.
Poplars are dioecious, thus individual trees are
either male or female. Male catkins of Populus
deltoides (10a) are reddish to purple and rapidly
senescence following pollen release. Female
catkins of Populus trichocarpa (10b) are green and
release seed throughout the early months of
summer. (Photos by J.H. Braatne and R.F. Stetter)
7.
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America —A Review
Table 2.
Reproductive characteristics of diploid and triploid F1 hybrid poplars (TD clones) relative to their maternal
species, Populus trichocarpa*
Reproductive Trait Diploid Hybrids Triploid Hybrid** Populus trichocarpa
Pollen viability (%) 48 22
Seeds per capsule 5 to 25 14 to 17
Capsules per catkin 33 to 38 40 to 46
Catkins per tree 210 to 256 260 to 320
Seed viability (%) 83 to 86 41 to 51
Viable seeds per tree 43,000 to 200,000 -98,000
98
34 to 38
34 to 36
ND
92
ND
Data are based on seven-year old border trees from four different clonal blocks, P. trichocarpa was growing adjacent to the plantation.
* * Only one triploid clone (1 84-402) was included in this study.
ND — No data taken, however several researchers have reported that native cottonwoods release millions of seed annually, up to 25 million or more per tree (Braatne et al.
1996).
hybrid females is related to their lower number of viable
seeds per capsule (Table 2). Given the close spacing of
commercial plantations, hybrid females growing as
interior trees (i.e. non-border row positions) would be
expected to have a much lower number of catkins per
tree than those reported in Table 2. In assessing
potential avenues for gene exchange, it is also important
to remember that there is a high level of variation in the
fecundity of native females.
Many researchers have reported cases of
spontaneous hybridization when non-native poplars are
planted in the vicinity of native populations (Stettler et
al. 1996b). These instances are relatively easy to
identify, as F: crosses display intermediacy with their
parents in a number of morphological traits (i.e. leaves.
branches, buds and bark). In more advanced-generation
hybrids, the diagnosis can be more difficult and require
the use of molecular genetic tools to confirm
hybridization. Some documented examples of gene
exchange between non-native and native poplars include:
a) 18th Century introduction of eastern cottonwood (P.
deltoides) to Europe gave rise to euramerican (DN)
hybrids (Houtzagers 1937), b) widespread cultivation of
euramerican hybrids in Eurasia has compromised the
genetic integrity of native stands of black poplar (P.
nigra) (Prison et al. 1995), c) introduction of white
poplar (P. alba) to the Midwest gave rise to P. alba x P.
grandidentata clones (D. Dickmann and J. Isebrands.
pers. comm.), and d) semi-columnar hybrid poplars may
often be found in the vicinity of Lombardy poplar
(Stettler et al. 1996b, Eckenwalderpers. comm.).
Despite these observations and the widespread planting
of non-native poplars throughout the West, it appears
that their influence on the genetics and ecology of native
cottonwoods has been limited.
Seeds developing from crosses between hybrid
poplars and parental species have a significantly lower
viability (Table 2), and thusfar their establishment on
agricultural floodplains has been extremely limited
(Strauss and DiFazio 1997). Low levels of seedling
establishment by hybrids appear related to a number of
factors including a longer period of seed maturation, and
lower seed output and viability. The interaction among
Figure 11.
Cottonwood seedlings growing on moist, exposed
substrates along the Elk River in Southeastern
British Columbia. Cottonwood seedlings require
peak-flows to create nursery sites and a gradual
stream stage decline during their first growing
season. (Photo by J.H. Braatne)
these factors and their role in limiting the recruitment of
hybrid seedlings warrants further study. Another
important factor is the natural constraints on seedling
recruitment within riparian corridors.
V. Natural patterns of seedling recruitment in
riparian corridors
The reproductive cycle and natural recruitment
patterns of native cottonwoods closely follow the
seasonal dynamics of rivers (Johnson 1994, Braatne et
al. 1996, Scott etal. 1996, 1997). The progression from
pollination to seed dispersal is closely attuned to the
seasonal rise and fall of river levels. In general,
pollination and fertilization occur before leaf bud-break.
either before or during peak springflows. Seeds are
dispersed as river levels decline, such that seedlings
colonize moist, recently exposed soil along gravel bars
and riverbanks within the riparian corridor (Figure 11).
The establishment of cottonwood seedlings is
highly dependent upon flooding events (when overbank
flows inundate the floodplain). Cottonwood seeds are
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America —A Review
small, with little or no endosperm (stored energy) and
will not establish in the shade of competitors (Braatne et
al. 1996). Under favorable conditions (i.e. moist, barren
soil), seeds germinate and their radicle enters the soil
within 24 hr of soil contact (Reed 1995). After
germination, the roots of young seedlings must keep
pace with declining river levels. Several studies have
documented that their rate of root growth averages 0.5 to
1.0 cm per day, with rooting depths commonly
exceeding one meter by the end of the first growing
season (Mahoney and Rood 1991, 1992, 1998,
Segelquist et al. 1993, Johnson 1994, Rood et al. 1995).
If river levels decline too rapidly, seedlings succumb to
drought stress. Seedlings that establish on moist soils at
lower river levels are subject to later removal or damage
by the scouring of ice and floodwaters. Collectively,
these environmental constraints contribute to the
infrequent establishment of cottonwood seedlings, on
the order of every 10 to 20 years depending on climatic
conditions and channel morphology (Bradley and Smith
1986, Baker 1990, Johnson 1994, Rood et al. 1995,
Braatne et al. 1996, Mahoney and Rood 1998, Rood and
Kalischuk 1998).
Given these environmental constraints to seedling
establishment, the delay in seed maturation by hybrid
poplars (2-3 weeks longer than native species) may place
them at a disadvantage relative to native cottonwoods
and willows. Seed dispersal at a later point in the
growing season would coincide with lower river levels,
hence only lower streambanks would be available for
germination. As previously noted, several studies have
documented that seedlings establishing along lower
streambanks are subject to scour by spring- and storm-
related flows (Johnson 1994, Scott et al. 1996, 1997).
After establishment, the continued health and
vigor of riparian cottonwoods is closely tied to the
seasonal dynamics of fluvial systems and the continuity
of water supply throughout their life cycle. The
following sections explore water-use patterns, and the
relative tolerance of hybrids and native cottonwoods to
seasonal drought and flooding.
VI. Water-use by hybrid poplars and other
agricultural crops
It is commonly assumed that riparian woody
plants, such as cottonwood, transpire large volumes of
water relative to agricultural crops and other forest
communities. However, recent studies reveal that water-
use by hybrid poplars and native cottonwoods is
comparable to agricultural crops and some conifer
species (Braatne et al. 1992, Hinckely and Braatne 1994,
Hinckley et al. 1994).
Leaf-level studies of stomatal conductance (Figure
12) suggest that hybrid poplars are capable of transpiring
large volumes of water. Maximum rates of stomatal
Figure 12.
Physiological studies of stomatal conductance and
photosynthesis. Numerous studies have
documented seasonal and diurnal patterns of water-
use and growth characteristics. These studies
reveal that interspecific F1 hybrids are often more
drought-resistant than their parents.
(Photo by J.H. Braatne)
conductance (gmax) for hybrid poplar approach 600 mmol
m~2 s"1, whereas gmax for white oak (Quercus alba),
Pacific silver fir (Abies amabilis), and Douglas fir
(Psedotsuga menziesii) are 300, 240 and 100 mmol m~2
s"1, respectively (Fritchen et al. 1973, Price and Black
1990, Braatne et al. 1992, Hinckley et al. 1992, 1994,
Martin et al. 1997). Among agricultural plants, gmax
typically range from 300 to 500 mmol m~2 s"1 (Nobel
1991, Korner 1994, Larcher 1995). Based on stomatal
conductance values, one might expect large rates of
transpiration by hybrid poplar stands relative to other
plant canopies. However, transpiration is governed by a
number of biophysical and environmental factors,
including: a) size and density of stomata, b) degree of
stomatal opening, c) hydraulics of water-conducting
tissues, d) size, density and distribution of leaves within
the canopy, e) environmental conditions, such as solar
radiation, temperature, humidity, and soil moisture
content, and f) canopy boundary-layer conditions (wind
speed, patterns of turbulent airflow, etc.).
Transpirational water loss by a hybrid poplar
canopy was recently documented on alluvial soils in
Western Washington (Puyallup River Floodplain,
Hinckley et al. 1994). In this study, the maximum rate
of transpiration for a four-year old hybrid poplar stand
was calculated to be 4.92 mm d"1 (0.194 inches d"1) over
a wide range of atmospheric conditions. This value is
slightly more than transpiration rates of Douglas-fir
stands (4.2 to 4.8 mm d'1, up to 0.193 inches d'1;
Fritschen et al. 1973, 1977, McNaughton and Black
1973, Tan et al. 1978, Price and Black 1990). On the
basis of these findings, it appears that hybrid poplars and
some conifers have comparable rates of water use.
Similar rates of stand-level transpiration for
Douglas-fir and hybrid poplar arise from offsetting
differences in the ways these species control
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America —A Review
Figure 13.
The upper canopy of a four-year old stand of hybrid
poplar in eastern Washington. The upper canopy of
hybrid poplars is a complex and highly variable
surface from which water fluxes are governed by an
interaction of numerous environmental factors,
including radiation inputs, turbulent air flows, and
temperature gradients. Seasonal rates of water-use
by mature poplar stands are comparable to slightly
lower than many agricultural crops.
(Photo by J.H. Braatne)
transpiration. In particular, these species differ in their
partitioning of canopy water fluxes between the stomatal
and boundary-layer components of water loss. For
example, thin conifer needles are directly exposed to
wind and solar radiation, and thus readily lose moisture
through a high boundary-layer conductance, even though
their stomatal conductance is low (i.e. the conifer canopy
is tightly coupled with atmospheric conditions). In a
poplar stand, stomatal conductance is high, but their
Table 3.
Estimated water use by agricultural crops and
hybrid poplars in eastern Washington*.
Crop type
Estimated Water Use
(inches acre'yr1)
Alfalfa
Apples w/ cover crop
Onions (dry)
Potatoes
Sweet corn
Winter wheat
Hybrid poplar (1st yr)
Hybrid poplar (2nd to 3rd yr)
Hybrid poplar (4th yr to harvest)
28-451
34-50'
30-36'
28-34'
24-28'
25-311
10-142
22-262
32-36 M
* Significantly lower water-use values would be expected in cooler, maritime
climates.
1) Values are derived from twenty-year water-use records (James et al. 1989).
2) Calculated values based on maximum stand water losses of 0.188-0.194
inches d"1 (Hinckley et al. 1994). Note: Evapotranspiration rates can be
expected to range from 0.3 to 0.4 inches d"1 when mid-day air temps > 35°C
(R. Cuenca, unpubl. data), thus water-use may approach 34 inches
acre^yr1 for 3 yr stands and 40-42 inches acre"1 yr"1 for 4 yr+ stands during
growing seasons that are hotter and longer than normal (J. Eaton, pers.
comm.).
3) The leaf area of a hybrid poplar canopy growing in eastern Washington
remains relatively constant from the end of the fourth yr until harvested at
7-8 yr (Kim Brown and Tom Hinckley, unpubl. data), hence annual rates of
water use would be comparable during the latter stages of the commercial
harvest cycle.
Figure 14.
Irrigated cornfields in eastern Washington. Annual
rates of water use by agricultural crops, such as
sweet corn, are comparable to slightly lower than
hybrid poplars. Differences in the length of growing
season and harvest rotation cycles are critical
variables in comparing long-term rates of water use
by hybrid poplars with agricultural crops.
(Photo by J.H. Braatne)
large, dense canopy (Figure 13) results in a low
boundary-layer conductance (i.e. the poplar canopy is
poorly coupled with the atmosphere).
Related physiological studies suggest that a
homeostasis between stomatal and boundary-layer
conductance results in a relatively constant rate of
maximum transpirational water loss across a range of
canopy and vegetation types. Meinzer and Grantz
(1989, 1991) observed that as leaf area and boundary-
layer conductance of a stand change over time, total
canopy conductance also changes in a manner that
maintains relatively steady rates of water loss (boundary-
layer plus stomatal components). Kelliher et al. (1993)
also reported similar observations across a range of
vegetation types. There are two plausible explanations
for these observations. First, the range of maximum
stomatal conductance appears to be quite conservative
within vegetation types (Nobel 1991, Korner 1994,
Larcher 1995). Second, stands with low leaf areas will
have a lower canopy conductance, yet the corresponding
rate of soil evaporation will increase as higher levels of
solar radiation warm the soil surface. The increased rate
of soil evaporation compensates for a lower canopy
conductance, resulting in relatively steady rates of water
loss across a range of canopy and vegetation types.
Similar trends in water use by agricultural crops
can be seen in Table 3. Contrary to common
misconceptions, water use by agricultural crops is not
solely related to the amount of water applied through
irrigation (Figure 14). Rather water-use is regulated by
solar radiation (McNaughton and Jarvis 1983), whereby
inputs of solar energy drive evapotranspirational fluxes
from the canopies of agricultural plants.
10.
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America —A Review
In viewing the data presented in Table 3, it is
important to note that the growing season of hybrid
poplars is significantly longer (April to October) than
most annual crops. Since hybrid poplars take up to 4
years to reach their maximum transpiration potential.
their long-term water-use will also be significantly lower
than agricultural crops whose annual water demand is
relatively constant from year to year. Detailed models of
evapotranspiration by hybrid poplars are currently being
developed at Washington State University and Oregon
State University. Until these findings are published, the
range of values presented in Table 3 can be used to
approximate water fluxes from hybrid poplar
plantations.
VII. Drought and flood-tolerance of hybrid
poplars and native cottonwoods
There have been numerous water relation studies
of hybrid poplars (F: hybrids) and their parents (native
cottonwoods). Although canopy and stand-level
comparisons are noticeably absent, whole-plant and leaf-
level studies reveal that hybrid poplar rapidly close their
stomata in response to atmospheric and soil-water
deficits (Schulte et al. 1987, Tschaplinski and Blake
1989a,b, Braatne et al. 1992, Hinckley et al. 1992,
Hinckley and Braatne 1994). This type of stomatal
behavior typically results in lower transpiration rates and
a greater drought resistance among hybrids relative to
native species (Tschaplinski and Tuskan 1994,
Tschaplinski et al. 1994, Blake et al. 1996).
Furthermore, these water relation properties enable
hybrids to maintain higher leaf areas for longer periods
during a drought-cycle than native species (Braatne et al.
1992).
Given current trends in climatic warming, these
research findings may be significant as streamside
plantings of hybrid poplar could potentially compensate
for the adaptive limitations of some native species
(notably black cottonwood, P. trichocarpa). The fact
that P. trichocarpa x P. deltoides hybrids could provide
more shade to moderate stream temperatures than native
species is not a minor point, especially in a warming
climate with periodic drought. Hybrids are thus not only
suitable for short-rotation fiber plantations, but may also
hold some promise as supplemental plantings with
native species in the restoration of riparian corridors.
New research is needed to explore the potential
ecological role of hybrid poplar in light of changing
riparian landscapes.
Seasonal flooding is a common feature of riparian
landscapes (Figure 15), and the flood tolerance of native
cottonwoods and hybrid poplars has been subject of
several recent studies (Lui and Dickmann 1992a,b, 1993,
Neuman et al. 1996). The general symptoms associated
with flooding include: a) yellowed leaves, b) leaf curl
(i.e. epinasty), c) formation of adventitious roots, and d)
Figure 15.
Seasonal flooding of a natural hybrid zone (P.
angustifolia x P. trichocarpa) along the upper
Yellowstone River, Montana. The life cycle of
riparian cottonwoods is highly dependent upon
seasonal flooding to create nursery sites for the
seedling recruitment. Periodic flooding is thus
critical to sustaining native riparian cottonwood
populations. Native cottonwoods and hybrid
poplars are very flood-tolerant relative to other
natives and invasive eurasian species as well as
most of the agricultural crops typically planted on
floodplain habitats.
(Photo by J.H. Braatne)
wilting (reviewed by Kozlowski 1984, 1997).
Cottonwoods are considered to be fairly tolerant of
excess soil moisture (Harrington 1987), yet flooded soils
reduce growth and survival in many species (Smit et al.
1989, Lui and Dickmann 1992a). Some cottonwoods
appear to be more flood tolerant than others, yet there
are some disparities among research findings. For
example, Harrington (1987) has shown that 20 d of
flooding did not affect the growth and survival of black
cottonwood (P. trichocarpa). These results contradict
the findings of Smit et al. (1989), where flooding was
observed to significantly reduce leaf growth in this
species. These observations suggest that there is a high
level of genetic variability in flooding responses within
and between species. In fact, Smit (1988) found that
flooding tolerance was more variable within populations
of black cottonwood than between populations. Overall,
native cottonwoods appear to be more flood tolerant than
hybrid poplars, yet both are significantly more flood
tolerant than the agricultural crops commonly planted in
floodplain habitats (Kozlowski 1984, 1997, Neuman et
al. 1996, D.I. Dickmann, pers. communication).
Additional research is needed to assess the physiological
and morphological relationships between flooding and
drought-tolerance in hybrids and native species.
VIM. Clearing of floodplain habitats as a means
of conserving water
On the basis of the misconception that riparian
trees transpire excessive amounts of water, agronomists
11.
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America —A Review
and hydrologic engineers widely promoted their removal
from floodplain habitats during the late 1940's to the
early 1980's (Robinson 1952, 1958, US. Congress 1960,
Horton 1966, Culler 1970, Ritzi et al. 1985, Welder
1973, 1988). The removal of riparian vegetation was
justified in part on poorly conceived physiological
studies of water-use and overly simplistic hydrologic
models of fluvial systems (Gatewood et al. 1950,
Robinson 1958, Bowie and Kam 1968, Van Hylckama
1974, Horton et al. 1976, Culler et al. 1982, Weeks et al.
1987, Welder 1973, 1988). While many of these
programs were undertaken in the arid southwest to
conserve water for crops and municipalities, it is useful
to review what we know about how these fluvial systems
and how they responded to the removal of riparian
woody vegetation.
Government-sponsored programs for the removal
of phreatophytes (i.e. deep-rooted plants that absorb
water from the water table or the soil above it) were
primarily directed at rapidly expanding populations of
salt cedar (Tamarix spp.), though cottonwoods were also
cleared from many riparian corridors (Horton 1966,
1976, Bowie and Kam 1968). Unfortunately, there is
only a limited database on the hydrologic responses of
streams to these vegetation removal programs. Most of
the studies were limited to short-term measurements of
scattered river reaches and/or isolated watersheds
(Robinson 1952, 1958, Bowie and Kam 1968, Welder
1973, 1988). In these studies, summer baseflows
appeared to increase on the order 10 to 15% in the year
following vegetation clearing (Robinson 1958, Bowie
and Kam 1968, Culler 1982, Ritzi et al. 1985, Welder
1973, 1988). These gains in baseflow were attributed to
phreatophyte removal, however, some alternative
explanations include: a) large-scale spatial and temporal
variation in precipitation patterns across arid landscapes,
and b) increased drainage and dewatering of shallow
aquifers lacking vegetative cover. As only moderate
gains were reported, these studies suggest that there
were significant increases in evaporative water loss from
exposed floodplain surfaces.
Today, the general condition of many of these
riparian corridors reveals that the increases in baseflow
following phreatophyte removal were not sustained.
Rather, removal of riparian vegetation contributed to
increased rates of evaporative water loss and localized
streambank erosion. In some cases (primarily low to
mid-order riverine environments), stream channels were
downcut or abraided through a series of erosive events.
Downcutting and streambank erosion further diminished
ground-water recharge and aquifer storage capacity. The
expansion of salt cedar populations has also not
diminished over time (Everitt 1980, Shafroth et al.
1995).
The long-term response of riparian corridors to
phreatophyte control and stream channelization from the
1940's to 1980's still requires systematic documentation,
particularly in light of our current understanding of
evapotranspirational fluxes from plant canopies.
Nevertheless, our knowledge of fluvial responses to
vegetation removal reveals a critical role for riparian
woody plants in maintaining stream channel morphology
and sustaining seasonal baseflows (Scott et al. 1996,
Braatne et al. 1996, Beeson and Doyle 1997, Naiman
and Decamps 1997, Poff et al. 1997, Rood and
Kalischuk 1998).
IX. Wildlife habitat studies
A common observation upon entering a poplar
plantation following canopy closure is the lack of
understory vegetation and wildlife activity (Figure 16).
Given the deciduous nature of these trees, low species
Figure 16.
The understory of a five-year old stand of hybrid
poplar. After canopy closure, a sparse understory is
a common characteristic of hybrid poplar stands.
The general lack of structural diversity in
commercial stands contributes to lower wildlife
usage relative to native riparian plant communities.
(Photo by J.H. Braatne)
12.
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America —A Review
Figure 17.
Deer and other cosmopolitan wildlife species
commonly use hybrid poplar stands for seasonal
cover and resting habitat. (Photo by J.H. Bmatne)
diversity and lack of avian activity is striking in
comparison to natural riparian cottonwood forests.
To date, only a few studies have compared patterns
of wildlife activity in poplar plantations with adjacent
agricultural row-crop and small-grain fields. In these
studies, only minor differences were reported in the
abundance and diversity of wildlife species (Hoffman et
al. 1993, Christian et al. 1997, Hanowski et al. 1997).
In grasslands and areas with limited forest cover,
plantations are commonly used for resting and cover by
deer, rodents, upland game birds and raptors (Figure 17).
Wildlife abundance in many plantations appears to be
significantly lower than native forests (Christian et al.
1997). On the other hand, wildlife data from the
Columbia River Basin show higher summer bird
densities along the edges of young plantations than
surrounding native shrub-steppe habitat. Use of
plantations by raptors during winter months is also
greater than adjacent cover types (Pat Heglund and Brian
Moser, unpubl. data). Additional research is needed to
further clarify localized and regional patterns of wildlife
activity within plantations relative to natural riparian
corridors and other native habitats.
In some studies, avian and mammalian diversity
was higher in isolated patches within plantations where
small groups of trees had died or weed control was less
effective (Christian et al. 1997, Hanowski et al. 1997).
This type of finding suggests that plantations could be
designed to incorporate spatial heterogeneity as a means
of increasing habitat quality. Some possible options
include: a) multi-age clonal rotations, such that large
plantations contain a mosaic of differing stand ages,
patch sizes and canopy structure, and b) restoring native
willows and cottonwoods along streambanks adjacent to
commercial plantations. Additional research is needed
to explore the efficacy of these options.
X. Conclusions, Recommendations and
Research Needs
Native cottonwoods are critical elements in the
structure and function of riparian corridors. As such,
remnant stands of native cottonwood need to be
inventoried, ecological studies initiated and long-term
efforts directed towards their conservation and
restoration. Native germ plasm should be collected from
remnant stands and used to establish ex-situ collections
in designated arboreta throughout the West. Research
should also be initiated that develop options for
promoting the natural recruitment of cottonwoods and
other riparian plants through controlled flow-
modifications of dams and agricultural diversions
(Mahoney and Rood 1998, Rood and Kalischuk 1998),
and other cost-effective practices for large-scale
restoration of riparian forests (Friedman et al. 1995,
Braatne and Rood 1998).
A. Gene exchange between commercial hybrid
plantations and native populations:
If hybrid poplar plantations are established
near native populations, preference toward the
use oftriploids or genetically-engineered
sterile clones would strongly limit gene
exchange. Unfortunately, triploids have lower
growth rates than other hybrids, which limits
their commercial value and biofiltration
capacity. Other options to reduce gene flow
include: a) limited planting oftriploids in
border rows to deter pollen and seed dispersal,
and b) preferential planting of hybrid females
to limit long-distance pollen release to native
females.
Future research needs include exploring: a) the
nature and extent of genetic incompatibility
among Populus spp., b) the development of
sterile clones with comparable growth rates to
existing hybrids, c) patterns of long-distance
gene flow within and between populations
using molecular genetic tools, and d)
phenological barriers to the establishment of
hybrid seedlings within natural riparian
corridors.
B. Growth and water-use characteristics:
hybrid poplars vs. agricultural crops
The growth and vigor of hybrid poplars is
dependent on adequate weed control,
fertilization, and irrigation. Since poplar
stands take several years to reach their
maximum transpiration potential, water-use by
hybrid poplars will be lower than that of many
annual crops. Canopy-level studies of
evapotranspiration are needed to clarify
13.
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America —A Review
seasonal and long-term trends in water-use
and stand-water balance. Research is also
needed to compare water-use by poplar
plantations with native stands of riparian
cottonwood.
C. Drought and flood-tolerance of hybrid
poplars and native cottonwoods:
Native cottonwoods appear to be less drought-
tolerant, yet more flood-tolerant than hybrid
poplars. Agricultural crops traditionally
planted infloodplain habitats are significantly
less flood-tolerant than either hybrid poplar or
native cottonwoods and willows. Additional
research is needed to explore the physiological
and morphological relationships between
flooding and drought-tolerance in hybrids and
native species.
D. Clearing of floodplain habitats to conserve
water:
Removal of riparian trees and shrubs from
streambanks increases erosion, channel
downcutting, sediment loading and localized
dewatering of shallow aquifers. Clearing of
vegetation may give an initial impression of
water conservation, but stream/low increases
are spatially and temporally limited.
Additional research is needed to assess the role
of vegetation in maintaining the functional
integrity of river channels and riparian
corridors.
B. Wildlife habitat studies:
Minor differences in the abundance and
diversity of wildlife have been observed
between hybrid plantations and adjacent row-
crop or small-grain fie Ids. In arid grasslands
or areas with limited forest cover, plantations
provide habitat for deer, upland game birds,
raptors and rodents. In some plantations,
mammalian and avian diversity were higher in
localized patches where hybrid clones had
died, suggesting that plantation designs could
be developed that incorporate spatial and
temporal heterogeneity as an approach to
increase habitat quality.
Additional research is required to understand
the relationships between stand structure,
plantation design and wildlife utilization.
Comparative ecological studies of poplar
plantations and native stands of riparian
cottonwood are needed to assess spatial and
temporal variation in habitat value and
utilization. Studies of silvicultural practices
and harvesting regimes are also needed to
determine how hybrid poplars can be
integrated into agricultural floodplains in a
manner that promotes the natural functions of
riparian corridors (Figure 18).
Figure 18.
Riparian hybrid poplar buffers at Carnation Farms
near Carnation, WA. Floodplain plantings of hybrid
poplar can be used to intercept excess nutrients
associated with the surface runoff from hayfields,
pastures and agricultural crops.
(Photo by J.H. Braatne)
14.
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Biological Aspects of Hybrid Poplar Cultivation on Flaodplains in Western North America — A Review
XI. Literature
Baker. W.L. 1990. Climatic and hydrologic effects
on the regeneration ofPopulus angiistifolia
along the Animas River. Colorado. J. Biogeo.
17:59-73.
Beeson, C.E. and P.P. Doyle. 1995. Comparison of
bank erosion at vegetated and non-vegetated
channel bends. Water Resources Bulletin 31:
983-990.
Bisoffi, S. and U. Gullberg. 1996. Poplar breeding
and selection strategies. In: R.F. Stettler. H.D.
Bradshaw, P.E. Heilman and T.M. Hinckley
(eds.), Biology ofPopulus: Implications for
management and conservation. National
Research Council of Canada, Ottawa: 139-158.
Blake. T.J., J.S. Sperry, T.J. Tschaplinski and S.S.
Wang. 1996. Water relations. In: R.F. Stettler,
H.D. Bradshaw, P.E. Heilman and T.M.
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replacement vegetation in the Pecos River
floodplain between Acem and Artesia, New
Mexico. U.S. Geological Survev Professional
Paper 491-G, 33 p.
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Welder. G.E. 1973. Bascflow in the Accm-Artcsia
reach of the Pccos River. New Mexico. 1957-
1971: U.S. Geological Survey Open-file report.
50 p.
Welder, G.E, 1988. Hydrologic effects of
phreatophyte control, Acem-Artesia Reach of
Pecos River, New Mexico: 1967-82. U.S.
Geological Survey Water-Resources
Investigations Report 87-4148.
Whitliam, T.B., P.A. Morrow, and B.M. Potts.
1991. Conservation of hybrid plants. Science
254: 779-780.
Whitham, T.G., K.D. Floate, G.D. Martinsen, E.M.
Driebe and P. Keim. 1996. Ecological and
evolutionary implications of hybridization:
Populus-herbivore interactions. In: R.F.
Stettler, H.D. Bradshaw, PE. Heilman and
T.M. Hinckley (eds.), Biology of Populus:
Implications for management and conservation.
National Research Council of Canada, Ottawa:
247-275.
Whitman, T.G., G.D. Martinsen, K.D. Floate, H.S.
Durgey, B.M. Potts and P. Klein. 1999. Pland
hybrid zones affect biodiversity: tools for a
genetic-based understanding of community
structure. Ecology 80 (2): 416-428.
Zsuffa, L., E. Giordano, L.D. Pryor, and R.F.
Stettler. 1996. Trends in poplar culture: some
global and regional perspectives. In: R.F.
Stettler, H.D. Bradshaw, P.E. Heilman and
T.M. Hinckley (eds.), Biology of Populus:
Implications for management and conservation.
National Research Council of Canada, Ottawa:
515-539.
21.
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America — A Review
22.
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Biological Aspects of Hybrid Poplar Cultivation on Flaodplains in Western North America — A Review
Appendix A: About the Author
Dr. Jeffrey H.
Education:
1989
1978
Ph.D. Department of Botany,
University of Washington, Seattle
B.A. Departments of Biology &
Botany, University of Montana.
Missoula
Additional training (94-98): Applied Fluvial
Geomorphology I & II, River Restoration & Natural
Channel Design (Wildland Hydrology w/ D. Rosgen,
Pagosa Springs, CO); Stable Isotope Ecology (BIO581,
University of Utah. Salt Lake City); Federal. State and
Local Clean Water and Wetland Regulations I & II
(Seattle Law Review Board).
Background: Dr. Braatne is a physiological plant
ecologist with expertise in the ecology of riparian
landscapes and the physiological ecology of riparian
plants. Over the last ten years, he has been an active
participant in the University of Washington/Washington
State University Black Cottonwood Research Program.
His research and teaching interests focus on the
physiology and ecology of riparian cottonwoods and
willows in western North America. Some recent
research topics include: a) physiological and
morphological responses of willows and cottonwoods to
drought and flooding, b) fluvial/ecological modeling of
the riparian plant communities, and c) impacts of
stream-flow modifications on riparian plant communities
and landscapes. Recent studies have focused upon the
ecology of riparian plant communities along the lower
Snake (Hells Canyon) and Salmon River Corridors. Dr.
Braatne teaches graduate courses on riparian landscape
ecology at the Universities of Washington and Montana.
Employment History (90-99):
Assistant Research Professor: Biology Dept, University
of Lethbridge, Alberta' " 1997-Present
Affiliate Assistant Professor: Forestry Dept., University
of Washington, Seattle, WA 1994-Present
Independent Environmental Consultant: Seattle, WA
1995-Prescnt
Senior Ecologist: National Wetland Science Training
Cooperative;
L.C. Lee & Associates, Inc.; Springwood Associates.
Inc., Seattle, WA 1993-96
Postdoctoral Fellow: Forestry Dept.. University of
Washington, Seattle, WA
1990-93
Selected Publications (96-99):
Braatne, J.H., W.C. Johnson, and S.B. Rood. Riparian
ecosystems: biophysical processes influencing plant and
animal diversity. In: R.C. Wissmar and P.A. Bisson
(eds.), Strategies for Renewing River Ecosystems:
variability and uncertainty of biophysical processes and
their ecological consequences. American Fisheries
Society (In prep)
Braatne, J.H. and S.B. Rood. Life history and ecology of
riparian willows in North America. Rivers (In prep).
Braatne, J.H. Naturalized populations of the Plains
Cottonwood (Populus deltoides var. occidentalis) along
the lower Snake and Columbia Rivers. (For submission
to Madrono).
Braatne, J.H. 1999. The ecological consequences of
hybrid poplar cultivation in riparian settings.
Proceedings of the Society of American Foresters,
Pasco, WA. 25p.
Braatne, JH., S.B. Rood and R. Simons. 1998. Life
history, ecology and distribution of riparian vegetation in
the Hells Canyon National Recreation Area. A detailed
study plan prepared for Idaho Power Company. 218 p.
Braatne, J.H., and S.B. Rood. 1998. Strategies for
promoting natural recruitment and restoration of riparian
cottonwoods and willows. Proceedings of the Society for
Ecological Restoration, Tacoma, WA.. 5p.
Braatne, J.H. 1998. Annual Review of the BLM/USFS
Riparian Cottonwood Restoration Program along the
John Day River, Oregon. Prepared for the US Forest
Service and Bureau of Land Management Prineville.
OR. 33p.
Braatne, J.H., S.B. Rood and P.E. Hcilman. 1996. Life
history, ecology and conservation of riparian
cottonwoods in North America. In: R.F. Stettler. H.D.
Bradshaw, P.E. Heilman and T.M. Hinckley (eds.),
Biology of Populus: Implications for management and
conservation. National Research Council Ottawa: 57-85.
23.
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America — A Review
24.
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Biological Aspects of Hybrid Poplar Cultivation on Flaodplains in Western North America — A Review
Appendix B: Bibliography on the Biological of Populus Spp. Ecology of the Riparian
of Western North America
Ecology & Fluvial
Geomorphology:
Aubie, G.T., J.M. Friedman and M.L. Scott. 1994.
Relating riparian vegetation to present and
future strcamflows. Ecol. Appl. 4:544-554.
Beeson, D.E. and P.P. Doyle. 1995. Comparison
of bank erosion at vegetated and non-vegetated
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990.
Beschta. R.L. 1991. Stream habitat management
for fish in the NW United States: The role of
riparian vegetation. Amer. Fish. Soc. Symp
10:53-58.
Costa, I.E., A.J. Miller, K.W. Potter, P.R. Wilcock
(eds). 1995. Natural and Anthropogenic
Influences in Fluvial Geomorphology: The
Wolman Volume. Geophysical Monograph 89.
America Geophysical Union.
R.T.T. Forman. 1996. Landform Mosaics. Chapter
7: Stream and river corridors.
Gregory, S.V. et al. 1991. An ecosystem
perspective of riparian zones. Bioscience 41:
540-551.
Karr, J.R. 1991. Biological Integrity: a long
neglected aspect of water resource
management. Ecol. Appl. 1: 66-84.
Leopold, LB. 1995. A View of the River.
Academic Press, NY.
Malanson. G.P. 1992. Riparian Landscapes.
Cambridge University Press.
Naiman, R.J. et al. 1992. Fundamental elements of
ecologically healthy watersheds in PNW
coastal ecoregion. In: Watershed Management:
Balancing sustainability and environmental
change: pp. 127-188.
Naiman, R.J. & H. Decamps. 1990. The Ecology
and Management of Aquatic-Terrestrial
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Paris.
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template of lotic systems: recovery in the
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Scott, M.L,, J.M. Friedman, G..T Auble. 1996.
Fluvial processes and the establishment of
bottomland trees. Geomorphology 14:327-339.
II. Of
Braatne, J.H., S.B. Rood and P.E. Heilman. 1996.
Life history, ecology and conservation of
riparian cottonwoods in North America. In:
R.F. Stettler, H.D. Bradshaw, P.E. Heilman and
T.M. Hinckley (eds.). Biology of Populus:
Implications for management and conservation.
National Research Council of Canada, Ottawa:
57-85.
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Cottonwoods on a Grazed and an Ungrazed
Plains Bottomland in Northeastern Colorado.
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25.
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America — A Review
Farmer, R.E.Jr. 1966. Variation in time of
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Floate, K.D., and Whitham, T.G. 1994. Aphid-ant
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Floate, K.D., Martinsen, G., and Whitham, T.G.
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abundance for gall aphids in western North
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Gom, L.A. 1996. The discrimination of
cottonwood clones in a mature population
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Thesis, University of Lethbridge. Alberta.
Howe, W.H., and F.L. Knopf 1991. On the
imminent decline of Rio Grande Cottonwoods
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Ecol. Monographs 64:45-84.
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vegetation and environment on the Missouri
River floodplain in North Dakota. Ecol
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McKay, S.J. The impact of river regulation on the
establishment processes of riparian black
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Washington. Seattle.
Mahoney, J.M. and S.B. Rood. 1993. A model for
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Mahoney, J.M. and S.B. Rood. 1998. Streamflow
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Biological Aspects of Hybrid Poplar Cultivation on Flaodplains in Western North America — A Review
Rood. S.B. ct al. 1995. Instrcam flows and the
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Biological Aspects of Hybrid Poplar Cultivation on Floodplains in Western North America — A Review
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