Species-specific responses to ozone and drought in six deciduous trees

Saplings of alder (Alnus glutinosa), birch (Betula pendula), hazel (Corylus avellana), beech (Fagus sylvatica), ash (Fraxinus excelsior) and oak (Quercus robur) were exposed to five episodic ozone regimes in solardomes, with treatment means between 16 and 72 ppb. All trees were kept fully watered for the first five weeks of exposure, after which half the trees continued to be well-watered, whereas the other half were subjected to a moderate drought and received approximately 45% of the amount of water. Species-specific reductions in growth in response to both ozone and drought were found, which could result in reduced potential carbon sequestration in future ozone climates. In well watered conditions the ozone treatments resulted in total biomass reductions for oak (18%), alder (16%), beech (15%), ash (14%), birch (14%) and hazel (7%) in the 72 ppb compared to the 32 ppb treatment. For beech there was a reduction in growth in response to ozone in the well watered treatment, but an increase in growth in response to ozone in the drought treatment, assumed to result from changes in hormonal signalling. For alder, in addition to a decrease in root biomass there was reduced biomass of root nodules with high compared to low ozone for both drought treated and well-watered trees. There was also a large reduction in the biomass of nodules from drought trees compared to well-watered. It is therefore possible that changes in the nitrogen dynamics of alder could occur due to reduced nodulation in both drought and elevated ozone conditions.