Seasonal and interannual ecophysiological responses of beech (Fagus sylvatica) at its south-eastern distribution limit in Europe

Due to its wide European distribution and its drought-susceptibility, beech (Fagus sylvatica L.) received intensive attention recently in the light of global warming. Contrary to central European beech ecosystems, little is known about the ecophysiology of beech at its south-eastern European distribution limit. Here we tested whether climatic fluctuations during a three-year period affected the ecophysiology of a beech site in Greece. Attention was paid at comparing our findings to the intense effects the 2003 extreme drought had on beech forests in central Europe. We assessed the interannual and seasonal variation of certain physiological parameters in a beech stand of north-western Greece during three consecutive growing seasons of the period 2003-2005. Leaf water potential and effective quantum yield of PSII were measured as well-known indicators of plant's responses to environmental stresses. Furthermore, plant carbon isotopic composition (δ¹³C) of tissues and extracts with different turnover times was determined, since it can reveal short- or long-term environmental effects on the water and carbon balance of the plant. Moreover, a number of micrometeorological parameters were measured and their effect on ecophysiological responses was tested. Precipitation of 2003 at the study site was comparable to that in central Europe, but it did not differ from the local range of precipitation in NW-Greece. Still, 2003 was more xeric, compared to 2004 and 2005. Despite this, leaf water potential, effective quantum yield and δ¹⁸O showed no significant variation between years and their values were not indicative of plants suffering from drought stress. Foliar δ¹³C, on the other hand, appeared to be more sensitive to the climatic differences between the years and it was higher during the more xeric 2003 compared to later on. Regression analysis revealed that its response was largely controlled by current soil water content and vapour pressure deficit of the preceding month. Regarding δ¹³C of phloem from both twigs and trunk, their patterns were influenced only by short-term changes in air vapour pressure deficit. Within the climatic range recorded in this study, which is typical for beech ecosystems in Greece, no substantial drought-driven limitations were observed on beech ecophysiology. Our observations contradict those from central European beech sites, rarely subjected to drought, where similarly low water availability had a great impact on the ecophysiology of beech.