Physiological responses of ponderosa pine to gradients of environmental stress:
![[Photo]:Harvesting tree roots for determination of ozone exposure and drought stress response.](https://webarchive.library.unt.edu/eot2008/20090514084943im_/http://www.fs.fed.us/psw/topics/air_quality/air_pollution/images/08_3.jpg)
To better understand changes in forest stand structure in response to
atmospheric pollutants, we need a basic understanding of why susceptibility
to pollution stress changes from seedlings to pole-sized trees (20-60
yr old) to mature trees. Some physiological attributes remain constant
as a tree ages, such as maximum rates of carbon acquisition under optimum
conditions, but others change radically. Some examples include frequency
and duration of drought stress, biomass allocation to roots, support structures,
or photosynthetic gain. Resource acquisition (e.g., photosynthetic uptake
of C, or fine root uptake of N), allocation (e.g., storage location of
carbohydrates), and partitioning (e.g., anti-oxidative potential) also
changes with environmental stress, whatever the source. The following
studies will address the most pressing gaps in knowledge for understanding
tree response to atmospheric pollutants and drought stress.
The following is a collection of studies conducted to determine physiological
response of ponderosa pine to a strong gradient in ozone exposure across
the San Bernardino Mountains of southern California. Across this gradient,
nitrogen deposition, soil moisture availability, temperature, and season
length also varied, some factors with and some without significant physiological
responses. In addition to these sites, an atmospherically clean site near
Lassen Volcanic National Park in northern California was also studied.
Abstracts of results to date are presented.
Tree-age changes in physiological characteristics of ponderosa pine:
Understanding which physiological characteristics change, and in what
way, as a tree grows is essential for extrapolation of experimental work
with seedlings. At Lassen Volcanic National Park, five tree age classes
were studied seasonally (mid April to mid October): 3-8 yr old seedlings,
10-20 yr old saplings, 21-40 yr old trees, 41-60 yr old trees, and mature
trees averaging 200 yr old. Individuals were chosen based on an ordination
of morphological characteristics to chose representative (average) trees
in each tree age class. Gas exchange, needle chemistry, needle retention,
and needle and branchlet growth were measured monthly for six individuals
in each tree age class. Early summer gas exchange rates were high and
similar across all tree age classes for 1 yr old needles. Late summer
gas exchange rates were low and similar across all tree age classes. However,
younger tree age classes were affected sooner by the onset of low soil
moisture availability in the upper soil horizons (0-40 cm). For gas exchange
rates, foliage on 21-40 yr old trees appear to be analogous to that of
200 yr old trees. Trees less than the 21-40 yr old tree-age class have
significant differences in foliar retention, and needle and branch growth
rates. There was an increase in leaf to fine root biomass from seedlings
through the 21-40 yr old tree age class. Trees with greater coarse root
biomass were larger overall within a tree age class. The 21-40 yr tree
age class is recommended for experimental work, for the best extrapolation
to mature tree response.
Role of early, late, and whole season ozone exposure in
photosynthetic decline of ponderosa pine:
Photosynthetic response of ponderosa pine was studied along a well-described
pollution gradient in the San Bernardino Mountains, California, with documented
deterioration in forest canopy health. Response of ponderosa pine in an
atmospherically clean site near Lassen Volcanic National Park were also
used in the analysis. The role of cumulative ozone exposure in predicting
photosynthetic decline using monthly intervals, early and late season
intervals, or whole growing season was evaluated using the correlation
coefficient, and the statistical significance of the correlation coefficient
of the regression. Monthly intervals had the least predictive capability
of photosynthetic decline, and the two phenophase (early vs. late summer)
intervals had the most statistical power for the single variable, cumulative
ozone. These time frames were mid June to mid July (early summer) and
late July to early October (late summer), corresponding to the period
of the most active gas exchange and growth, and the onset of drought stress,
respectively, in ponderosa pine. In mesic years, the rate of photosynthetic
decline was constant across both phenological periods. In xeric years,
there was no further photosynthetic decline with additional exposure to
ozone. Mature trees averaging 200 yr old followed a seasonal pattern and
rate similar to that of the 21 to 60 yr old trees in a xeric year, whether
the year was xeric or mesic. Ozone indices or models which incorporate
a variable of summer precipitation ([average and above] or [below average])
may be more effective in predicting canopy health responses of ponderosa
pine.
Changes in stomatal functioning of ponderosa pine across a pollution gradient:
The relationship between photosynthesis and stomatal conductance changes
seasonally from early summer (lower water use efficiency, WUE) to late
summer (greater WUE) in ponderosa pine. The seasonal changes in moisture
availability probably contribute to this change in WUE, and low to moderate
ozone exposure (<250 ppm h seasonal total, 24 h basis) does not appear
to alter this response. However, moderate to high ozone exposure (300-320
ppm h) significantly affects stomatal response to light (photosynthetically
active radiation) and vapor pressure deficit (VPD). Greater light levels
are required to achieve the light compensation point, and stomatal conductance
under saturating light is progressively lower with increasing ozone exposure.
Although substomatal CO2 concentration regulates stomatal aperture
under near-ideal conditions, stomatal conductance at the highest ozone
exposure sites appears to be less coupled with both endogenous and exogenous
drivers. Under high ozone exposure but otherwise favorable environmental
conditions (low VPD, saturating light, non-limiting soil moisture), stomatal
conductance is lower than for ponderosa pine under lower ozone exposure.
Under high ozone exposure and one or more environmental stressors (high
VPD, limiting soil moisture), conductance is low, but ponderosa pine is
unable to achieve complete stomatal closure. Based on these results, we
developed an estimator of ozone uptake that allows for differences in
stomatal behavior due to changes in WUE and high ambient ozone exposure.
Ozone and nitrogen deposition lowers root biomass of ponderosa pine:
Lower root biomass in forest trees in response to anthropogenic pollutants
is believed to be one of the first steps in forest health degradation.
Although decreased root biomass has been observed in controlled experiments,
ozone effects on mature tree roots in natural stands has not previously
been documented. Standing root biomass was measured during early and late
summer at three sites in the San Bernardino Mountains distributed along
a known, long-term pollution gradient of ozone and nitrogen deposition.
Trees at each site were assessed for foliar ozone injury and below ground
attributes, in addition to other environmental factors known to influence
root growth. During the period of peak root growth in the spring, root
biomass at the least polluted site was 6-14 times greater than that observed
at the most polluted site. Coarse root starch also was greatest at the
least polluted site, however variability was high. Known differences in
climatic and edaphic factors among the sites potentially contributing
to the observed response were discounted as primary contributors to the
response since in most cases the site differences would have driven the
patterns of root growth in the opposite direction to that observed. Differences
in biotic competitive interactions, also known to affect root growth,
did not explain the observed pattern for the same reason. The data suggests
that elevated ozone, high nitrogen deposition, and possibly other contributing
factors such as soil acidification are primarily responsible for lowering
root biomass in ponderosa pine stands in the San Bernardino Mountains.
Assessing visible ozone-induced foliar injury in ponderosa pine:
Chlorotic mottle of foliage is one of the primary indicators of ozone
injury to conifers. Univariate and multivariate analyses to test for significant
correlations between chlorotic mottle of needles and 38 characteristics
known to be associated with ozone injury in ponderosa pine. Measures of
morphological, physiological, and nearest neighbor characteristics were
compared on the same trees for their relative contribution in determining
the minimum set of characteristics needed to assess ozone injury as indicated
by chlorotic mottle. Nine characteristics were significantly correlated
with the different levels of chlorotic mottle: the number of green whorls
retained, the proportion of foliated versus total branchlet length, branchlet
and bole diameter, radial growth, foliar chlorophyll and nitrogen content,
coarse root sucrose content, and proximity of the nearest Pinus neighbor.
Discriminant analysis applied to the standardized principal components
showed that the first six characteristics were nearly as good as all nine
in classifying the visible foliar injury scores. The series of analyses
performed supported some of the characteristics in use and added others
that should be included in assessing ozone injury in yellow pine.
Does N deposition mitigate ozone injury to yellow
pine and California black oak?
Ozone concentrations frequently reach or exceed 100 ppb in mid-elevation
forests in Sequoia National Park, California. Little is known of the levels
of total N deposition in the Park, although ozone and nitrogenous pollutants
co-occur as the major pollutant types in forested areas of California.
Few studies have addressed the combined effects of these co-occurring
pollutants on field response of mature trees in situ. The primary objective
of the study is to test the hypothesis that increased N fertility mitigates
ozone injury to two important tree species of the Sierran mixed conifer
zone, yellow pine (highly sensitive) and California black oak (moderately
sensitive). Mitigation of foliar ozone injury will be determined by a
multivariate assessment of visible foliar injury and associated characteristics
(foliar retention, foliar N, total chlorophyll, total rubisco activity,
branch and bole growth, coarse root carbohydrate content, fine and medium
root biomass). Differing ozone and N deposition inputs along the elevation
gradient, in combination with N amendment treatments at each elevation,
will provide the ozone x N fertilization combinations needed to test the
above hypotheses. A second objective of the study is to assess the role
of timing of N availability on ozone injury. This will be tested for each
species using sites at different elevations. A physiologically based,
single tree model (TREGRO) will also be used to explore the predicted
responses of pine to ozone exposure at differing foliar N contents. The
proposed study would make a major contribution to evaluating whether N
deposition is an environmental enhancer at current and potentially higher,
future levels, and whether it mitigates a known environmental stressor
(oxidant pollution) in Sequoia National Park.
Funded by DISPRO, Environmental Research Laboratory, Environmental Protection
Agency, and the National Biological Survey. Cooperators include: David
Weinstein (Boyce Thompson Institute), and Robert Heath (Department of
Botany, University of California, Riverside).
Research is being conducted by:
Air Pollution and Global Change Impacts on Western Forest
Ecosystems
(RWU-4451)
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