Forecasting the effect of ozone on mixed deciduous forests

Background/Question/Methods

Ozone pollution can cause extensive damage to trees, but species vary significantly in their tolerance to ozone exposure.  This variation could create a restructuring role for ozone in forests that are experiencing high exposures.  To assess the successional dynamics of a forest community exposed to different ozone regimes, an individual physiological based model (TREGRO) was linked to a spatially explicit forest stand gap model (ZELIG).  Individual mature trees were modeled for three years in TREGRO for 13 broadleaf deciduous tree species including native and invasive species commonly found in the Mid-Atlantic region of the United States.  The results from the TREGRO model were used to modify the growth rate for the response of each species to ozone within the ZELIG model.  The ZELIG trials modeled the succession of a forest on a recently abandoned field in a low, medium, and high ozone environment for 200 years.  One hundred independent simulations were run for each ozone regime.  

Results/Conclusions

High ozone stress facilitated high species diversity (1.95, Shannon diversity index) by weakening the competitive ability of ozone sensitive species, which typically exhibited dominance.  Ozone significantly affected the abundance (F_39,844.69=52.31, p<0.0001), DBH (F_39,844.69=51.59, p<0.0001), and height (F_39,844.69=45.68, p<0.0001).  Abundance was significantly correlated with DBH (r=0.40, p<0.0001) and tree height (r=0.35, p<0.0001).  Height was also significantly correlated with tree height (r=0.94, p<0.0001).  Ozone sensitivity reduced the abundance of some species (both dominant and non-dominant species) while ozone and shade tolerance enabled other species to increase in abundance.  Though new species failed to enter the canopy with increasing ozone stress, tree size decreased and switches in canopy dominance occurred due to varying ozone tolerance among species.  High ozone stress caused a thinning of trees during early forest growth, which enhanced the growth of the remaining trees.  In contrast, moderate ozone levels did not lead to tree thinning, but impaired tree growth due to the positive interaction between increased competition and ozone stress.  Ozone is one of many stressors that currently affects Mid-Atlantic forest communities and is likely to play an important role in the reorganization of forest structure and species composition.