DYNAMICS OF NON STRUCTURAL CARBOHYDRATES (NSC) IN 10 YEARS-OLD FAGUS SYLVATICA L. YOUNGEST TWIGS FOLLOWING EXPERIMENTAL DEFOLIATION OR DROUGHT WHICH INDUCED MORTALITY

Climate change might result in more intense and more frequent drought events particularly in temperate regions (IPCC, 2013). Even if forest ecosystems are known to be resilient and have faced to disturbance for century, this climate-mediated increases in disturbance could be exceed the capacity of forest to cope with such rapid changes (Reyer et al., 2015; Seidl et al., 2016). Indeed, recent evidence of drought-induced mortality has been reported in many regions of the world resulting of a renewed interest in studying forest mortality in the scientific community (McDowell et al., 2008; Sala et al., 2010; Allen et al., 2010; Anderegg et al., 2015; Johnson et al., 2016). Relevant key physiological mechanisms are needed for models to reliably predict forest responses to environmental changes especially when mortality occurs (McDowell et al., 2013) and carbon metabolism is one of these (Martinez-Vilalta et al., 2002; McDowell et al., Trees fix CO2 from atmosphere via photosynthesis that allows providing critical source of C for various sinks such as growth, respiration, turgor maintenance, supply solutes for water movement or into mechanisms involved in defense (Chapin et al., 1990; Korner, 2003; Sevanto et al., 2014; O’Brien et al., 2015). According to the time in the season and the environmental conditions the carbon source-sink balance may lead to periods of carbon surplus or deficit. For example, when soil water content becomes limiting, trees close their stomata to limit their water loss by transpiration but this impacts also negatively the photosynthesis (Bréda et al., 2006; Palacio et al., 2008; Hartmann et al., 2013; Piper and Fajardo, 2014; Dickman et al., 2015). Consequently, the carbon balance may become negative (McDowell et al., 2008; Rowland et al., 2015). To face to temporary negative carbon balance trees can use carbon stored in their perennials organs under form of starch and soluble sugars which constitute the non- structural carbohydrates (NSC). NSC quantity are dynamic within the seasons especially on deciduous trees, with increase starch concentrations in woody tissues at the end of the growing season (Barbaroux et al., 2003; Damesin and Lelarge, 2003; Dietze et al., 2014; Sevanto et al., 2014; Hartmann et al., 2016). These NSC are then used to maintain the winter metabolism during the non-leafy period (cold tolerance, maintenance respiration) and are also fundamental for new leaf growth during the next spring (Penning de Vries, 1975; Sakai and Larcher, 1987; Wanner and Junttila, 1999; Ogren, 2000).

When photosynthesis is altered by drought or by a loss of photosynthetic tissue through defoliation or branch mortality it has been hypothesized that trees are forced into C storage dependency to meet continuous carbon demand for osmoregulation, respiration and defense against pathogens (McDowell et al., 2008; Sala et al., 2012). In the long term, trees might not survive if carbon supply by photosynthesis and stored NSC does not meet overall carbon demands (Martinez-Vivalta et al., 2012). Several studies have focused of the role of carbon into mortality processes but this role need more investigations because contradictory results were observed about NSC dynamics under disturbances (Piper, 2011; Hartmann et al., 2013; Zhao et al., 2013) as summarized in a recent report by Alice Delaporte (2015, thesis). In the cohort of organs involved in carbon metabolisms under drought, leaves play a crucial role in balancing the risks of carbon starvation and hydraulic failure through the stomata regulation, since they are the site of carbon assimilation and sugar production, as well as the main source of water loss for trees. Consequently, by bearing leaves and being the more proximal perennial organs of the new photoassimilates production, we hypothesized that twigs have a critical role in carbon acquisition strategy. Indeed, with the study of twigs, we search have a good proxy of the metabolic adjustments made under stressful conditions but little is known about the specific response of twigs which supporting leaves (Salmon et al., 2015). Moreover, when disturbances such as drought or defoliation limit growth, change of NSC concentration should take place in tissues proximate to those where primary growth occurring, e.g twigs (Bazot et al., 2013; Piper et al., 2016) because trees usually cover their demand by first mobilizing stored resources from the closest possible storage sites (Landhausser and Lieffers 2003).