Stomatal, Mesophyll Conductance and Biochemical Limitations to Photosynthesis as Affected by Drought and Leaf Ontogeny in Ash and Oak Trees

Accurate predictions of net primary production in forest ecosystems require a deteiled assessment of the different limitations imposed on leaf photosynthesis during the whole growing season, resulting from leaf plantation during three whole growing seasons characterised by a different water availability (2001, 2002 and 2003). A novel quantitative limitation analysis was applied to estimate the effects of drought and leaf ontogeny on stomatal (SL) and non-stomatal limitations (NSL) to light-saturated net photosynthesis (Armax). Furthermore, based on combined gas exchange and chlorophyll fluorescence measurements, NSL was partitioned into a diffusive and a biochemical component. On the basis of our results, we conclude that diffusional limitations largely affect net assimilation during most of the year, whereas biochemical limitations are quantitatively important only during leaf development and senescence or with severe droughts.JRC.H.2-Climate chang