Responses of Northern U.S. Forests
to Environmental Change
Chapter 12: Regional Impacts of Ozone on Forest
John A. Laurence, Scott V. Ollinger , and Peter
Mechanistic models simulate changes in forest structure or function
by quantifying fundamental mechanisms or processes, whereas statistical
models rely on analysis of empirical data. Mechanistic models can
therefore be used to extrapolate forest responses beyond conditions
that have already occurred, if the correct response mechanisms are
understood and modeled correctly.
PnET-II, an ecosystem-scale model used to estimate regional forest
production, total ecosystem carbon balances and water yield, and
responses to climate change, was modified to include ozone effects
on productivity including interactions with other stressors. The
approach was to modify the models photosynthesis algorithms
to include leaf-level uptake-response relationships and allow interaction
with factors such as light attenuation, canopy ozone gradients,
and water stress. Results for New England and New York showed decreases
in predicted annual net primary production (NPP) from 2 to 17 percent
as a result of mean ozone levels from 1987-1992, with greatest reductions
occurring where both ozone levels and stomatal conductance were
greatest. Growth declines were greatest on sites with wetter soils.
Another approach to estimating ozone effects was to couple a model
of forest stand development and composition (ZELIG) with a model
of tree-level physiological response to stress (TREGRO). TREGRO
models the acquisition of carbon, water, and nutrients, and allocates
carbon among competing plant parts depending upon resource availability
and phenology. ZELIG is a gap-succession model used to simulate
succession in mixed stands typical of eastern and northern forests,
and has both mechanistic and empirical characteristics. The two
models were coupled by passing calculations from TREGRO to ZELIG
in order to modify the growth relationships in the stand model.
Simulated response to ambient ozone from 1991 using the TREGRO/ZELIG
combination for red oak and sugar maple in the Northeast showed
a small reduction in red oak basal area growth (2-4 percent over
a 100-year simulation) and a compensating small growth increase
in sugar maple.
Model predictions include uncertainty due to (1) incomplete regional
data, (2) incomplete knowledge of how forests respond to particular
stresses, and (3) uncertainty about which processes and parameters
should be included in the model. Some of this uncertainty is related
to spatial interpolation, aggregation, and scaling errors.
net photosynthesis in relation to cumulative ozone uptake (from
Reich, 1987) using data from independent ozone fumigation studies
conducted on a variety of hardwood seedlings.
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