MODELING RESPONSES OF PONDEROSA PINE TO INTERACTING STRESSES OF OZONEAND DROUGHT

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.