PARENT SESSION
Posters P1D Photosynthesis and global change. Abstracts (699-710)

Will photosynthetic gain of boreal evergreen conifers increase in response to a potentially longer growing season? Ingo Ensminger^*,1, Susanne Tittmann¹, Lilian Schmid¹, Olga Shibistova², Jon Lloyd¹, ¹ Max-Planck-Institut für Biogeochemie, Jena, Germany²

ABSTRACT- We studied tree and forest photosynthesis in a boreal climate (Zotino, central Siberia), ranging from the cellular to the ecosystem level. Our aim was to characterise the physiological mechanisms underlying the onset of photosynthesis in Scots pine (Pinus sylvestris) in spring and a downregulation of the photosynthetic capacity in autumn. We measured whole canopy and needle gas exchange and characterised biochemical and molecular properties of the chloroplast. Whole canopy gas exchange revealed a rapid onset of photosynthesis mainly triggered by above-zero air temperatures in spring. A slow but protracted increase followed. The subsequent decline in canopy photosynthesis in autumn was only partially attributable to less favourable environmental conditions. A detailed analysis on the needle level using controlled environmental facilities showed air temperature and light to be the main drivers of spring recovery processes, but that intermittent low temperature events have strong (but reversible) retarding effects on the reconstituting photosynthetic apparatus. Soil temperatures around zero degrees during spring do not inhibit the recovery process but do decrease the potential rate of recovery. For Scots pine exposed to different temperature and daylength treatments during autumn, photosynthetic capacity, was largely retained by warm temperatures and seemed to be independent of daylength. However, endogenous factors were responsible for the further downregulation of photosynthetic capacity in late autumn in all experimental treatments, irrespective of daylength or temperature conditions. There was, however a decreased frost resistance of needles as assessed by chlorophyll fluorescence of Scots pine grown under simulated increased autumn temperature conditions. Our results suggest that boreal evergreen conifers might not be able to exploit the suggested increased length of the growing season due to global warming, because they might fail to properly time the processes of hardening in autumn and dehardening in spring.

KEY WORDS: energy balance, physiological , Scots pine, excitation pressure