SEPA
United States
Environmental Protection
Agency
Environmental Research
Laboratory
Corvallis OR 97333
Research and Development EPA-600/D-82-276 August 1982
ENVIRONMENTAL
RESEARCH BRIEF
Oxidant Air Pollution Effects on a
Western Coniferous Forect Ecosystem
P. R. Miller
Pacific Southwest Forest and Range Experimental Station, Riverside, CA
0. C. Taylor
University of California, Riverside, CA
R. G. Wilhour
Environmental Research Laboratory, Corvallis, OR 97330
Abstract
From 1973 to 1978, an interdisciplinary study of the pine
and mixed conifer forests of the San Bernardino Mountains
of southern California measured the effects of 30 years'
exposure to photochemical oxidant air pollution on selected
ecological systems. Average 24-hour ozone concentrations
in the San Bernardino Mountains during the May through
September period ranged from a background of 3-4 pphm
up to a maxima of 10-12 pphm. Ponderosa pine was very
ozone sensitive; foliar injury occurred at 24-hour
concentrations of 5-6 pphm followed by, in decreasing
order of sensitivity, Jeffrey pine, white fir, black oak,
incense cedar, and sugar pine. Foliar injury and premature
leaf fall caused decreased photosynthetlc capacity,
suppressed radial growth of stems (a negative exponential
relationship), and reduced nutrient retention in the green
biomass, all leading to weakened trees. Pines became more
susceptible to root rot (Fames annosus) and pine beetles
(Dendroctonus brevicomis); mortality rates reached 2-3
percent in some years. Litter depth was greatest in stands
receiving the most injury and associated defoliation,
hindering pine seed establishment but encouraging
oxidant-tolerant species in the understory. The living
foliage of smaller trees, which can be easily ignited by
ground fires in the understory combined with increased
litter accumulation, fuels a more destructive type of fire.
Fire and ozone destruction of the pineforest overstory leads
to a dominance of self-perpetuating, fire-adapted,
ozone-tolerant, shrub and oak species mixtures that provide
fewer commodity and amenity values than the former pine
forest. Change was evident in four ecosystem processes:
the flows of water, carbon and nutrient and changes in
patterns of diversity in time and space.
Introduction
The mixed conifer forests in the San Gabriel and San
Bernardino mountain ranges east of Los Angeles have
been exposed to oxidant air pollution since the early
1950's.1 The symptoms of chronic ozone injury to sensitive
tree species are visible up to 120 km east of urban centers
in southern California and from 50 to 70 km east of central
valley cities in parts of the Sierra Nevadas.2'3 The extensive
visible injury, and a concern for possible adverse effects on
forest ecosystem stability under continuing exposure, led
to an interdisciplinary study on the San Bernardino
National Forest (SBNF) with participants from the
University of California at Riverside and Berkeley; the
USDA, Forest Service and the Pacific Southwest Forest and
Range Experiment Station, Riverside, California. This study
was funded principally by the U.S. Environmental
Protection Agency (EPA) through the Environmental
Research Laboratory in Corvallis, Oregon. The research
team investigated two questions: 1) How do the organisms
and biological processes of the conifer forest respond to
different levels of chronic oxidant exposure? and 2) How
can these responses be interpreted within an ecosystem
context?
Due to preexisting damage by air pollution, the SBNF
project was constrained to observe up to 30 years of
accumulated effects on populations and processes at
selected research plots along a gradient of decreasing
ozone stress (Figure 1). The ozone stress gradient was
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Study-Site Locations
Q Major Vegetation Plots
A Air Monitoring Stations
Contour Lines (Meters)
^ Lakes — Nat'l Forest Boundary
1.51012345
Miles
Kilometers
Los Angeles* • San Bernadino Mtn. Area
Figure 1. Location of major study sites in the San Bernardio National Forest.
paralleled by a gradient of decreasing precipitation and air
temperature (with increasing altitude). Consequently, as
precipitation and air temperature decreased, the most
common conifer species forming the matrix of the forest
mixtures shifted from ponderosa pine (Pinus ponderosa
Laws) to Jeffrey pine (P. Jeffrey!Grev. and Balf.). It was not
possible to locate a suitable ponderosa pine stand free from
ozone stress in southern California that could be used as a
control in the study. The topography, geology, soils, climate,
and vegetation of the San Bernardino Mountains have been
described in earlier reports.4"7
One independent research group used some of the study
sites for a survey of the diversity and health of the lichen
flora.8 A second group completed an intensive study of
photosynthesis and transpiration of ponderosa pine
experiencing severe chronic ozone exposure.9
Data Collection and Management
The data collection methods used by individual
investigators have been described in progress reports. 7>1°
Most of this research focused on 18 permanent plots
during the summers of 1972 through 1978 (Figure 1).
Some investigators found it necessary to limit their studies
to particular plots, or to establish additional temporary plots
in order to obtain a forest-wide perspective of vegetation
recovery after fire1' and the activity of insects and diseases
responsible for tree mortality.12
The data base of the entire project was centralized.10 All
data sets, including up to 200 data types, were stored on
the operating system disk of an IBM 370/145, where they
were available for transfer to a mini-disk in the interactive
environment of the computer. Studies by the separate
teams provided data on: 1) air temperature, relative
humidity, precipitation and transpiration regimes; 2) water
availability as a function of soil and site attributes; 3)
concentration of ozone in the study area; 4) foliage injury
along the gradient of ozone dose; 5) leaf litter fall; 6) decay
of litter and partitioning of selected nutrients; 7) tree seed
production; 8) seedling establishment; 9) tree growth; 10)
insect and disease complexes responsible for tree
mortality; and 11) fire history in relation to stand species
and age composition.
For purposes of analysis in an ecological systems analysis
context, the physical and biological components of the
ecosystem, and four essential ecosystem processes were
defined as follows. The major physical components are
water (precipitation), temperature, light, mineral nutrients
(soil substrate), and ozone air pollution. Biological
components include the producers represented by an
assortment of tree species and lichens; the consumers or
wildlife that consume tree seeds and young seedlings, and
insect and disease organisms causing tree mortality; and
the decomposers, principally the populations of
saprophytic fungi particularly responsible for leaf and
woody litter decay.
•'ti ft.
*..*.'..
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The major ecosystem processes represented are: 1) the
flow of carbon from the atmosphere to be incorporated
initially into green plant biomass, and then partitioned
among consumers, litter and decomposer mass, the soil
and back to the atmosphere; 2) the flow of water in the soil-
plant-atmosphere continuum; 3) the flow of mineral
nutrients through the green plant, litter, and soil-water
compartments; and 4) the shift of diversity patterns in time
and space as represented mainly by changes in tree stand
species composition, age, structure, and tree density.
Results
Temporal Variation of Ozone Dose,
Temperature, and Precipitation
Ozone dose, temperature, and precipitation trends during
the study period were critical because these variables drive
the ecosystem processes (Figure 2). During the 1973
through 1978 term of the project, the May through
September ozone dose at Sky Forest indicated a definite
downtrend until 1978, when dose increased again.13 This
trend is more closely correlated with meteorological
variation from year-to-year than with changes in amounts
of ozone precursors, according to another study of South
Coast Air Basin trends.14 The May through September
average hourly air temperature ranged between 14.7 and
17.0°C during the five-year data collection period. The
mean temperature for the five years was 15.6 ± 1.0°C. The
highest average air temperature occurred in 1975, the year
of lowest annual precipitation. The precipitation at nearby
Lake Arrowhead (3 km north of Sky Forest) was below the
30-year average of 1055 mm during the first four
precipitation years and considerably above average during
the last year. However, the monthly distribution of
precipitation is more important than the annual total
because it determines the water available to plants during
the growing season.15 Higher-than-average precipitation
during the late spring, late summer, and early fall is much
more favorable for vegetation growth and other ecosystem
processes that are moisture limited. During 1975-76,
1976-77, and 1977-78, there were unusually large
amounts of precipitation in August and September. This is
a marked departure from the long-term average for late
summer months. This deviation has important implications
for interpretation of the entire study results because the
expected late summer drought did not occur during three
consecutive years. These annual trends of ozone dose,
temperature, and precipitation at Sky Forest-Lake
Arrowhead are considered representative of the entire
study area, after adjustments for distance and elevations.13
Precipitation, oxidant dose and hourly average temperature in the Lake Arrowhead-Sky Forest Area,
1973-1978
PPT = mm x 103
OX = pphm-hr x 104
1973-74
1974-75
1975-76
1976-77
1977-78
Figure 2. Annual trends of ozone dose precipitation and air temperature near the Lake Arrowhead-Sky Forest region of the San
Bernardino National Forest.
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Spatial Variation of Ozone Dose
Oxidant monitoring stations (Figure 3) along the major
west-to-east axis of pollution transport extending for 50 km
at elevations between 1500 and 2300 m showed that
between 30 to 45 km, where the May-September hourly
average ozone concentration ranged from 5-6 pphm,
chronic damage to ponderosa and Jeffrey pine was barely
detectable. The injury threshold between 30 and 45 km was-
highly influenced by terrain features. Beyond 45 km, the
natural background ozone concentration was 3-4 pphm,
comparable to other mountain areas in the United States.
The daylight ozone dose at a mountain station located at 12
km along the 50-km transport axis was 40% higher than the
nearest urban station which was located 1349 m lower and
15 km to the southwest. This targe difference is due to the
processes supporting the nocturnal preservation of ozone
at higher elevations.
Effects of Chronic Ozone Stress on Ecosystem
Processes
Carbon Flow
During a highly instrumented study at a single plot, a
research team from Lawrence Livermore National
Laboratory (LLNL) found that the photosynthetic rate of the
needles of ponderosa pines classed as having slight,
moderate, or severe chronic injury was reduced to about
10% of the maximum observed rate after experiencing 800,
700, and 450 pphm ozone, respectively.9 Stressed trees
also retained a smaller amount of assimilated carbon after
respiration losses.
Three years of parallel observations of ponderosa and
Jeffrey pines at five sites ranging from low to high ozone
environments provided dose-injury relationships based on
the increase of visible needle symptoms and needle
abscission rates.16 The largest increments of needle
symptom increase occurred in the early summer; needle
abscission started during the second year of exposure and
the largest numbers of needles were lost during the winter.
The most important variable governing dose response was
the inherent level of tree sensitivity to ozone which was
defined by the average number of annual needle whorls
retained by each tree.
The changes in the number of annual needle whorls
retained by 951 ponderosa and 769 Jeffrey pines
distributed throughout the 18 study plots indicate a
decrease from 2.5 to 2.0 whorls from 1973-1978 at 12
plots experiencing hourly average ozone concentrations
ranging from 6-12 pphm.'7 Pines at the six plots with lower
doses maintained the same number of whorls or showed a
slight increase.
The proportion of needle whorls retained which had slight
to severe ozone injury symptoms, varied from year to
year.17 In 1975, there was a sudden decline of injured
foliage retained by trees experiencing 6-12 pphm ozone.
This peak of needle abscission followed the highest
seasonal ozone dose of the period, which occurred in 1974,
12
14-18
0.4
02
95% Confidence
Limits of Ratio
Estimate
10
E
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i
o
o
?
<§
I
£
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V
10 15 20 25 30 35 40
West to East Dimension of Study Area, Km
45
50
55
Figure 3. The estimated gradients of ozone concentration along two west to east axes representing the major terrain-influenced
transport patterns.
-------
and was coincident with highest summer temperature and
one of the lowest precipitation amounts for the 1973-78
period. The average number of annual needle whorls
retained by 502 white fir remained about the same during
the 1973-78 period but at all dose levels the proportion of
injured needle whorls decreased and uninjured whorls
increased.17
The 378 California black oaks in the study plots showed a
sensitive leaf injury response to ozone each year,18 but the
97 incense cedars and 68 sugar pines were generally slow
to show ozone injury.18 Measurements of the difference of
diameter-at-breast-height (dbh) between 1975 and 1978
for 951 ponderosaand 769 Jeffrey pines showed moderate
correlation with number of annual needle whorls retained
(a range of 1 -7 whorls). The correlation was closer for the
10-29.9 cm dbh trees than for the trees 30 cm dbh and
larger.'9
Tree ring analysis to investigate the variations in annual
ring growth was approached cautiously because, as a
result of ozone or drought stress, many trees failed to
produce a ring.20 For those trees which fit the master ring
chronology for the part of the study area in question, the
model which best characterized ring growth was a negative
exponential curve.21 The degree of deviation from this
model could be explained by the variability in ring growth
during the 1950-75 period. The logarithm of winter
precipitation and autocorrelation with the previous years'
growth were the strongest variables predicting growth of
trees at plots experiencing no pollutant effect; at the high
ozone dose plots the correlation was much lower for these
variables. The positive relationship between needle whorl
retention and ring width was also significant. The effects of
temperature were ambiguous.21
Measurements of ponderosa pine stem growth by change
in dbh and ring analysis both showed that surviving trees at
high-dose plots demonstrated sudden growth increases 1)
due to thinning resulting from death of sensitive trees or
salvage logging to remove weakened trees19'21 and 2) as a
result of reduced competition from surrounding trees. Both
annual ring and stem diameter difference measurements22
showed radial growth reductions for sensitive blackout22
and white fir.1"1
Cone production in both injured and uninjured ponderosa
and Jeffrey pine stands was mostly influenced by crown
class or the position of the tree's crown relative to its
neighbors. Dominant ponderosa pine comprised 32
percent of the trees and produced 80 percent of the cones.
Cone production also increased with age. However, injured
ponderosa and Jeffrey pines older than 130 years produced
significantly fewer cones per tree than uninjured trees of
the same age.23 Severe injury to both dominant and
codominant ponderosa and Jeffrey pines resulted in fewer
cone crops during the six years of study. The drop in
proportion of trees in severe injury classes that produced
cones was much more dramatic for Jeffrey pine than for
ponderosa. Tree ring analysis showed a positive and
significant relationship between cone production and
radial growth.
Nutrient Flow
The largest amount of needle litter, an average of 357 g/m2,
was found under ponderosa pines moderately damaged by
ozone. There was 90 g/m2 of litter under severely damaged
trees and 131 g/m2 under healthy trees.24 The negative
effects of heavy accumulations include an increased fuel
load and the hinderance of successful pine seedling
establishment. In the event of fires, the large nutrient store
in a thick needle litter layer would be lost by volatilization
and subsequent surface runoff. The benefits of heavy litter
include significant increases in surface soil carbon,
nitrogen, the carbon/nitrogen ratio, and exchangeable
calcium. Lower absolute amounts of N, P, K, Ca, and Mg
were found in the living foliage of injured ponderosa pines.
Back translocation of all five elements and also dry matter
from leaves is curtailed, indicating an interference in the
tree's internal conservation of nutrients.26 Nitrogen,
calcium, and magnesium concentrations in throughfall are
higher under injured trees.26
The decomposition of litter comprised of ozone-injured
needles was more rapid; it was inversely correlated with
solar radiation and positively correlated with litter depth.
Moisture was the single most important variable limiting
decomposition;27 nitrogen and phosphorus did not affect
the process.
The taxonomic richness and population density of fungi
which colonize living needles and later participate in
decomposition were both reduced by ozone injury because
the normal increase with age was prevented by premature
needle senescence and abscission. This change could
weaken the functional stability of the decomposer
community.27
Moisture Flow
Ponderosa pine crowns intercepted 19.0, 22.4, and 21.6
mm of rain at 1, 2, and 3 m, respectively, from the stem,
compared to 19.4 mm for a nearby clearing. As leaf surface
area is decreased by oxidant injury, rain throughfall
increases until it is nearly the same as precipitation
amounts in clearings.26 This implies that fog condensation
would also increase under injured trees. Consequently,
litter moisture could be expected to evaporate more rapidly
since thinner crowns would also allow more radiation to
reach the litter surface.
The summer season flux of moisture in the soil-plant-
atmosphere continuum was investigated by weekly
measurements of available soil moisture at several depths
down to 274 cm15 and biweekly measurements of pre-dawn
twig xylem water potential.16 Trees continued to obtain
moisture from lower depths when moisture in the top 274
cm was depleted. A transpiration simulation model
provided corroborative evidence for the summer water use
pattern and suggested that higher transpiration rate during
early summer months may be partially responsible for the
large incremental increases of ozone injury to needles
observed in June and early July.16
An ozone-induced decline of stomatal conductance was
observed by the LLNL research group after the early
summer ozone exposure. This observation and the fact that
leaf surface area is much lower for the ozone-sensitive
genotypes implies lower transpiration losses for stands
containing many sensitive or moderately sensitive trees.9'28
Diminished competition for water in such stands is
undoubtedly related to the sudden growth improvement of
the more ozone-tolerant genotypes that was detected with
tree ring analysis.21
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Shift of Diversity in Time and Space
The preexisting species mixture in the SBNF conifer zone
includes five types: 1) ponderosa pine; 2) ponderosa pine-
white fir; 3) ponderosa pine-Jeffrey pine; 4) Jeffrey pine-
white fir; and 4) Jeffrey pine. The stands most subject to
ozone damage are those containing ponderosa pine. The
changes in species and age composition after long-term
exposure to ozone were considered the most important
measures of oxidant impact because these qualities have
greatest implications for human welfare. The'effects of air
pollution are superimposed on natural variables which
interact to control species establishment and survival, e.g.,
moisture availability and suitable space, fire frequency and
intensity, and mortality caused by various diseases and
insects.'21
Two distinct patterns of successional development were
recognized in the study area,11 namely autogenic and
allogenic succession. With autogenic succesion, changes
in the forest environment caused by existing trees promote
continuous establishment of species more tolerant to lower
light intensity in the understory. This process is taking place
on sites not recently disturbed by fire, diseases, or insects.
In stands affected by significant ozone damage, pine needle
litter accumulation and a heavy layer of combustible litter
accumulation following pine mortality combined with the
development of a living fuel laddar created by the shade and
oxidant-tolerant species in the understory will lead to
crown fires that could eliminate the entire tree layer. There
would be few surviving pines to provide the seed required to
reestablish the pine overstory.29 Even without a catastrophic
fire event, pine establishment is limited by the lower seed
production of injured trees and the thick litter layer that
contributes to fungal infection and death of germinating
seeds.
Air pollution-injured overstory pines are more readily killed
by Fomes annosus root disease30'31 because the fungus
colonizes freshly cut stump surfaces of weakened trees
more rapidly, thus speeding up the spread rate from stumps
to nearby living roots. The fungus spreads more rapidly in
the roots of weakened trees than in healthy trees. Fewer
western pine beetles (Dendroctonus brevicomis) are
required to kill weakened trees; therefore, in stands with
many weakened trees, a given population of western pine
beetles could kill more trees and increase at a faster rate.32
Fomes annosus and the western pine beetle are often
present in the same tree. The continuous effects of ozone
during the years of low moisture stress results in the
accumulation of weakened trees, therefore, mortality rates
from diseases and insects peaked during years when soil
moisture was limited.
Three ring analysis showed that the period of decreasing
vigor in the 5-10 year period preceding tree death
correlated with tree age, i.e., more younger trees died
during the period of observation than older trees.
The probable elimination of autogenic succession at many
sites following inevitable crown fires is expected to cause a
shift to allogenic succession where changes in plant
communities result from environmental modifications
(repeated fires) not caused by the plants themselves.29 Tree
and shrub species adapted to survive fire by sprouting
from the stem or root crown dominate this vegetation cover.
In the San Bernardino National Forest, there are already vast
acreages converted to various mixtures of oaks and shrubs.
These areas range from impenetrable brush to oak
woodlands, which in most cases are self-perpetuating
because rapid crown closure creates an unfavorable
environment for the return of conifer species, and after
each fire these species resprout.'1
Black oak is an important species in this vegetation mixture.
The crowns of this deciduous oak provides favorable
understory conditions for the best conditions for the
reestablishment of conifers, particularly ponderosa pine.
Black oak is slightly to moderately sensitive to ozone, but
this degree of sensitivity should not seriously disrupt its
"nurse tree" role with respect to ponderosa pine.
Discussion
The essential interactions of physical ecosystem components
and four ecosystem processes, namely the flows of carbon,
r
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Precipitation *
and
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V Attributes J £~ **T \
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Decomposer 52 i
Activity 7-V_y
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Diversity T |
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Mass and
Species Diversity
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Mass end
Species Diversity
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Injured Folia
*— Mass anc
Nutrient Conti
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^~* Litter Mass
Nutrient Con
— -.0
— * Available Nutrii
Figure 4. Diagrammatic model of the effects of chronic oxone ai
-------
nutrients, and water, and the shift of diversity patterns in
time and space, are summarized in Figure 4. This scheme
also suggests the submodels that will be required for future
development of simulation modeling.
Solid lines connecting boxes indicate material flow from
compartment to compartment. Dashed lines indicate the
controlling influence of one variable on the flow between
compartments. The controlling "valve" resembles a bow
tie.
Compartments (2 through 6) represent the flow of water in
the soil-plant-atmosphere continuum which is modified by
several site, soil, and geologic substratum attributes. Water
availability (2 through 5) influences the flux of ozone (15) to
both healthy and injured foliage and therefore exercises
control on foliage injury, abscission, and litter accumulation
(7 through 9). The flow of nutrients to the litter layer (also 7
through 9) is influenced by leaching due to precipitation
contracting foliage during throughfall (2) as well as the rate
of needle fall to the litter surface (9). The relative a mounts of
healthy (7) and injured foliage (8) remaining in the crowns
have a direct effect on carbon assimulation and carbon loss
(13) and finally the growth of trees of all size classes (16,18,
19). Photosynthesis is further controlled by available
nutrients (10), air temperature (11), carbon dioxide
concentration in the atmosphere (12), and light availability
as influenced mainly by tree density. Decomposer activity
and diversity (32) is only indirectly affected by ozone (15);
however, air temperature (11)and particularly surface soil-
available water (3) control the rate of litter decomposition
and availability of nutrients (10). The processes of seed
production, seedling establishment, and reactions to
competition as different species grow through several size
classes are indicated in compartments 17 through 20. These
processes for different species will be decreasingly
influenced by ozone effects in the following order:
Relative Numbers of Ozone
Sensitive Individuals in Each
Species Population
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Tree Stem Growth
(Tree Vigor)
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Residual Mixture
of Living Tree
Species at T=O
Future Mixture
of Tree Species
atT=O + N
western coniferous forest ecosystem.
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ponderosa and Jeffrey pine, white fir, black oak, incense
cedar, and sugar pine. The establishment of the more ozone
sensitive species (particularly ponderosa pine) will be
influenced by diminished seed production (17)and losses to
wildlife, mainly tree squirrels (21). Surface soil-available
water (3) exercises the most important control over the
survival of seedlings (18). Tree mortality rates are closely
related to the number of ozone-sensitive individuals in
each species population (20). The low vigor of ozone-
injured trees (16) encourages the interactions of the insect
and disease complex (29, 30, 31) resulting in tree mortality
(22) and alteration of the stand environment by making new
space available (23). The reduction of competition among
the remaining trees leads to a growth release for ozone-
tolerant genotypes. The continuing effects of ozone (15) and
the pest complex (29, 30/and 31) result in accumulation of
stumps which encourages the Fames annosus root disease
cycle (24, 29). Additional amounts of leaf litter (9) and a
heavy layer of combustible litter (25) have the effect of
increasing the fire hazard (27) tothe residual mixture of tree
species (26), particularly since ozone-tolerant, fire-
sensitive understory trees can form a living fuel ladder that
carries the fire to the crowns of the fire-tolerant overstory
trees. Where crown fires are prevented, the future mixture
of tree species (28) may be dominated by allogenic
succession which will reestablish the desirable ponderosa-
or Jeffrey pine-dominated forest. If crown fires are not
prevented, the autogenic successional process will
dominate and the oak species and less desirable shrubs
that sprout after fire will be perpetuated as the vegetation
cover. Only black oak is ozone-sensitive. Foliose lichen
mass and diversity change (33, 34) provide another
example of the direct impact of ozone.
The effects of chronic ozone stress on this web of
interactions will vary considerably with ozone concentrations
and other physical and biological factors. Predictive
capability will improve with additional experimentation
using the existing data base and computer simulation
models.
Conclusions
1. For the five year period of this study, average 24-hour
ozone concentrations in the San Bernardino Mountains
during the May through September period ranged from
a background of 3-4 pp'hm up to maxima in the range of
10-12 pphm.
2. The first evidences of ozone injury to sensitive
ponderosa and Jeffrey pines were observed at 24-hour
averages in the range of 5-6 pphm.
3. Occurrence of high nocturnal ozone concentrations
at mountain sites, compared to basin sites as reported
elsewhere, was confirmed for the San Bernardino
Mountain area. Here, a mountain station received a
daylight-hour dose 40% higher than a nearby basin
station that was 1378 m lower in elevation, mainly
because higher nocturnal concentrations were still
being seen in the early morning and late evening (3
hours following sunrise and preceding sunset).
4. Ecosystem components most directly affected by ozone
were tree species, the fungal microflora of needle
leaves, and foliose lichens occupying tree bark.
5. The most important ecosystem processes affected
either directly or indirectly were flows of carbon,
mineral nutrients and water, and changes in patterns
of vegetation cover diversity over time and in space.
6. The diminished flow of carbon in the tree layer of the
ecosystem was associated with diminished foliage
surface of affected trees and decreased photosynthetic
capacity of the remaining foliage.
7. Diminished photosynthetic capacity resulted in
decreased stem diameter and height growth and
reduced seed production in the injured ponderosa and
Jeffrey pines.
8. The store of carbon and mineral nutrients accumulated
in the thick needle litter layer under stands of ozone-
injured trees influenced nutrient availability due to
losses by volatilization during fires and in subsequent
surface runoff; the mere increase of litter thickness
inhibits pine seedling establishment. Without fire, the
surface soil may be more enriched by carbon and
mineral nutrients from excessive litter.
9. The last three consecutive summers of data collection
(1976 to 1978) were atypical because unseasonal
rainfall prevented the usual late summer drought
stress. Injured foliage did not drop as readily, there
were significant stem diameter growth differences
between 1975 and 1978, and tree mortality rates
declined. These conditions might not have occurred
during the typical late summer drought.
10. The large proportion of missing rings in ozone-stressed
areas confounded attempts to use tree ring analysis to
fit study trees to a master ring chronology for each of
several distinct regions. The model which best
characterized tree ring growth of the past 30 years was
a negative exponential curve.
11. Interception of rain and fog by the forest canopy
increased precipitation under trees compared to
clearings. The thinning of needles associated with
injured pines allowed increased amounts of moisture
to fall to the forest floor. These high moisture
conditions and the increased radiation that reaches the
litter surface may influence litter decomposition and
tree seedling establishment.
12. The biweekly incremental increases of ozone injury to
needles of ponderosa and Jeffrey pines were larger
during the early season period of high moisture
availability and high transpiration, but a causal
relationship was not established.
13. Fomes annosus root disease can be expected to
increase more rapidly in ozone-injured pine stands
because freshly cut stumps and roots of weakened
trees are more vulnerable to fungus attacks.
14. Fewer western pine beetles (Dendroctonus brevicomisj
are required to kill weak pines. In stands with a high
proportion of ozone-injured trees, a given population of
western pine beetles could kill more trees and increase
at a greater rate.
15. Forest stand age and species structure a re variables that
have the most relevance and direct effect on human
welfare in both recreational and commercial forests.
The interplay of insects and diseases, drought, ozone
8
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injury, and forest fires shapes stand age and species
structure. Forest management practices can alter
these factors significantly.
16. In the absence of fire, the gradual destruction of
ponderosa or Jeffrey pine overstory by ozone and other
agents has lead to an accumulation of heavy woody
fuels and an understory of ozone-tolerant, fire-
sensitive species that form a living fuel ladder, likely
leading to crown fires which will consume the
remaining pines. In devastating fires, there would be
total loss of forest cover, requiring expensive
reforestation. Without the pine-dominated forest, a
less desirable cover of shrub and oak species emerges
as a self-perpetuating community of species that
sprout after fire, quickly obtain crown closure, and
inhibit the natural reestablishment of pines and other
conifers.
Recommendations
Air quality control measures that manage to maintain
summer season, 24-hour ozone concentration averages
below 5-6 pphm in the forested receptor areas of California
are required to prevent injury to sensitive species and to
prevent initiating undesirable changes in carbon, water and
mineral nutrient flows, and patterns of diversity in time and
space.
In circumstances where air quality cannot be maintained
below the 5-6 pphm average, there are still significant
opportunities to ameliorate adverse effects with known,
operational forest management practices. Additional work
is needed to refine these management prescriptions.
References
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10
Acknowledgements
This research brief is the product of a interdisciplinary
study with participants from the University of California at
Riverside and Berkeley; the USDA, Forest Service and the
Pacific Southwest Forest and Range Experiment Station,"
Riverside, California. The editorial assistance of David
O'Guinn, Northrop Services, Inc., is appreciated.
*USGPO:1M2-559-<
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