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

Abstract

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.

Introduction

Owing to climate change (Schär et al., 2004, IPCC, 2007) the future survival and sustainability of European beech (Fagus sylvatica) ecosystems in Europe has become of great concern (e.g. Cescatti and Piutti, 1998, Peñuelas and Boada, 2003, Geßler et al., 2004a, Geßler et al., 2007, Bréda et al., 2006), due to the species’ high sensitivity to drought (Fotelli et al., 2001, Leuschner et al., 2001, Granier et al., 2007). Since European beech is one of the widely spread forest species in Europe (Ellenberg, 1996), any possible adverse effects on its sustainability and regeneration may have great ecological and economical impacts.

In the Mediterranean region, beech is limited to mountainous areas where it could not be reached by ice during the Quaternary period allowing, thus, its survival. Although beech populations found close to the southern limit of the species’ distribution are characterized by high genetic diversity compared to northern populations (Demesure et al., 1996, Scarascia-Mugnozza et al., 2000), they might be quite sensitive to extreme environmental conditions since they grow at the limit of their ecological requirements (e.g. Jump et al., 2006a). Therefore, beech ecosystems of the Mediterranean might be especially prone to climate change due to the already xeric conditions of this area (e.g. Scarascia-Mugnozza et al., 2000). A recent study of Jump et al. (2006a) reports a rapid growth decline of beech forests in north-eastern Spain, as a direct consequence of intense drought and warming during the last decades. As a result, beech may be forced to shift to higher altitudes (Peñuelas and Boada, 2003) and restrict its distribution in southern Europe. On the other hand, beech of southern origins might respond better to intensified drought and heat events due to a potential acclimation to such environmental conditions (Tognetti et al., 1995, García-Plazaola et al., 2008). Recently, evidence is provided that current climatic changes are resulting in genetic alterations and, thus, adaptive responses to climate change in certain beech populations of the Mediterranean (Jump et al., 2006b). The observed increase in water use efficiency of lower elevation beech sites in Spain, subjected to intense warming during the last decades (Peñuelas et al., 2008) further emphasizes that some beech population have the potential to respond to climate change.

Studies on the effects of drought on water relations and photosynthetic performance of beech (e.g. Tognetti et al., 1995, Backes and Leuscher, 2000, Fotelli et al., 2001, Leuschner et al., 2001, Bréda et al., 2006) generally showed that although beech possesses mechanisms for responding to water deficits, it is not a drought-tolerant species. To date there is limited information on the ecophysiology of beech in the typically xeric Mediterranean ecosystems (e.g. Tognetti et al., 1995, Aranda et al., 2000, Aranda et al., 2005, Sabate et al., 2002, Skomarkova et al., 2006). Particularly in Greece, the south-eastern distribution limit of beech in Europe, only few studies focused on this species till lately (Raftoyannis and Radoglou, 2002, Nahm et al., 2006b, Zerva et al., 2008), since beech forests in Greece were of low economical importance, but may reveal to be of great ecological importance. Studying the ecophysiology of these beech ecosystems, generally characterized by xeric conditions, may offer valuable information on the species’ potential to acclimate and adapt to climatic changes. For example, the summer of 2003 was extremely dry and the hottest of the last 180 years in central and western Europe (Schär et al., 2004, Ciais et al., 2005, Löw et al., 2006, Rebetez et al., 2006, Granier et al., 2007) and led to intensified research on the physiological responses of beech to drought (e.g. Löw et al., 2006, Geßler et al., 2007, Granier et al., 2007, Nahm et al., 2007). However, little is known about the ecophysiological performance of beech in Greece during this well-studied dry year.

In this study we have measured certain physiological parameters, indicative of the plant's water and carbon balance, in order to assess the performance of beech in NW-Greece under the influence of the typical Mediterranean climate of this region. Carbon isotopic compositions of plant tissues and extracts offer useful insights on plants’ responses to environmental stresses, such as limited water availability (e.g. Damesin et al., 1998, Adams and Grierson, 2001, Fotelli et al., 2003). In C₃ plants, the slower diffusion of the heavier ¹³C isotope, compared to ¹²C, from the atmosphere to the site of carboxylation, and the strong discrimination of Rubisco against ¹³C, are largely responsible for the depletion of plant material in ¹³C relative to the atmosphere. The δ¹³C composition of a plant tissue is described by Farquhar et al. (1989) as:

$$\delta {}^{13}\text{C}_{\text{plant}}=\delta {}^{13}\text{C}_{\text{atm}}-\alpha -(b-\alpha )\frac{C_{\text{i}}}{C_{\alpha }}$$

where δ¹³C is expressed in units of parts per thousand (‰), α is the discrimination during diffusion (∼4.4‰), b is the discrimination during carboxylation by Rubisco (∼29‰), C_i is the CO₂ concentration inside the stomatal cavities, and C_α is the atmospheric CO₂ concentration.

Water deficits lead, thus, in reductions in the C_i/C_α factor and in increases in δ¹³C of organic matter (¹³C-enriched tissues). However, structural carbon of bulk material like leaves, may carry an isotopic signature affected by storage and remobilisation processes (Helle and Schleser, 2004, Skomarkova et al., 2006). On the other hand, δ¹³C of recently fixed carbon, allocated among others in phloem sap, is indicative of short-term environmental fluctuations (Keitel et al., 2003, Scartazza et al., 2004). Moreover, variations in water availability may affect the downstream processes of carbon metabolism and ¹³C-isotopic signature during allocation from leaf to stem (Damesin and Lelarge, 2003) or to root (Keitel et al., 2003). Therefore, δ¹³C analysis of leaves, as well as of phloem sap, which are characterized by different turnover times may reveal differences in the effect of environmental factors, such as water availability (e.g. Peuke et al., 2006).

Fluorescence parameters are also well-established measures of plants’ responses to environmental stresses. Effective quantum yield of PSII in light-adapted leaves is a reliable indicator of photoinhibition in plants in response to stresses (Colom and Vazzana, 2003). Furthermore, leaf water potential is traditionally measured for characterizing plant's responses to drought and other stresses, and is among the very few ones already studied in beech native to Greece (Raftoyannis and Radoglou, 2002, Nahm et al., 2006b), providing thus comparability to existing data.

To assess the seasonal and interannual variation of critical physiological traits of adult beech trees in a beech forest in Greece, carbon isotopic analysis of various plant compounds were combined to chlorophyll fluorescence and water relations measurements. We aimed at comparing the seasonal ecophysiological performance of beech during three consecutive growing periods from 2003 to 2005 and at studying how this performance is affected by climatic conditions. In the light of the great impact the 2003 drought had on beech forests of central Europe, we also focused on comparing our findings with those of studies on beech ecosystems in central Europe, generally characterised by less xeric summers than in Greece.