Long term N and S deposition and ambient ozone are the three major pollutants impacting forests in California, USA. Little is known about the dose-response relationships under field conditions for tree growth and mortality to the combined exposure to these three pollutants. Data from US Forest Service Forest Inventory and Analysis (FIA) permanent (remeasured) plots were used in combination with modelled atmospheric N and S deposition and an ozone exposure index to evaluate tree growth and mortality responses across California. After controlling for tree size, site productivity, weather, and competition experienced by each remeasured tree, we tested for effects, and possible interactions, of N and S deposition on tree bole growth and tree mortality for 18 common forest tree species in California. The combined effects of the three pollutants were assessed on a portion of the state where ozone data was available. Inventory data used for this analysis were from 4217 FIA plots across California containing forested conditions at both the initial visit, between 2001 and 2005, and the remeasurement visit ten years later.
Results/Conclusions
Seven of 18 tree species showed a significant increase in C increment (CI) with increasing N deposition. In most ecosections, the net effect of N deposition on C sequestration in tree boles was negligible. The clear exception was the significant increase in C increment in response to N deposition in the coastal ecosections of central and northern California where precipitation levels and fog exposure are much greater than in the rest of the state. Sequoia sempervirens (redwood) trees had the highest rate of CI increase in response to N deposition. As a statewide average, CI increased by 12 kg C/ha per kg/ha N deposition. In the Central and Northern California Coast ecosections, where redwood trees are common, CI increased by 51 kg C per unit N deposition. N deposition significantly increased the odds of top damage and tree mortality. Higher ozone values were associated with significantly larger rates of tree growth but declining tree mortality. S deposition in forested California was low, ranging from 0.5 – 3 kg/ha/yr, but surprisingly, was associated with positive growth response in 7 coniferous species. None of the 6 hardwood species responded positively to S deposition. The combined effects of N and S deposition and ozone exposure in California forests appear to cause a net increase in CI, although the effect varies widely among tree species.