Plant-Growth-Stress Model (PGSM) is a physiologically based process mo
del that integrates the effects of ozone, acid deposition, elevated ca
rbon dioxide, temperature, drought, and nutrient deficiency. The model
uses an hourly time step for photosynthesis and a daily time step for
all other plant and soil processes. It can be set up to run for as ma
ny years as needed. The model was applied to simulate the growth patte
rn of ponderosa pine seedlings under the experimental conditions of oz
one and drought stresses. The simulated diameter and biomass of canopy
, roots, and stem were comparable to the measured. Major effect of dro
ught stress was root mortality; however, its sublethal effect on stoma
ta opening caused significant reductions of photosynthesis, canopy, ro
ots, and stem. Ozone stress increased litterfall and decreased canopy.
Trees compensated by growing more new needles. The canopy photosynthe
sis was not reduced to the extent of an across-the-board decrease of r
oots and stem. Ozone and drought stresses had an antagonistic effect,
in which the biomass reductions due to the combined stresses were less
than the sum of reductions from individual stresses. However, the obs
erved data indicate a protective effect, not simulated by the model.