Drought effects on water relations in beech: The contribution of exchangeable water reservoirs

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Abstract

The exceptional soil drought and heat wave that occurred in Europe in summer 2003 provided a good opportunity to analyze the response of forest ecosystems to extreme climatic conditions. This work aimed at studying the functioning of exchangeable water reservoirs of beech trees under climatic and edaphic constraints, in a 37-year-old beech stand located in north-eastern France. We characterised the impact of drought on seasonal variations in water fluxes at the tree (sap flow measurements) and the forest (eddy covariance measurements) scales and estimated (i) the daily water storage capacity of beech trees as the difference between stand-scaled sap flow and water vapour flux over the stand and (ii) the contribution of exchangeable water in tree reservoirs to the total tree transpiration, under non-limiting soil water (summer 2002) or severe soil drought conditions (summer 2003). In parallel, daily variations in trunk circumference were analyzed to estimate the contribution of elastic tissues as a compartment for water storage to tree transpiration.

Stand transpiration was strongly reduced by soil water shortage (up to 80% at the peak of drought in August 2003). From the beginning of August 2003, we observed daytime contraction of stem circumference, with only partial, or even no night recovery, suggesting that trees were less and less able to refill the elastic and strongly depleted reservoirs. Even if those elastic reservoirs were active throughout the season, the corresponding volume of water withdrawn for tree transpiration remained very low (maximum 1% of the daily transpiration). Thus, elastic tissue reservoirs play a minor role in the total water budget of beech trees.

The amount of water depleted daily from the whole reservoirs was much higher than the water extracted from elastic tissues. Furthermore, the contribution of total exchangeable water within trees to the transpiration stream increased during the dry period (from a few percent to 67% at the peak of the drought) and was positively correlated with soil water shortage. Our results thus clearly demonstrated the strong sensitivity of beech to both climate and drought and the major role of whole tree water reservoirs to maintain leaf transpiration under severe drought.

Research highlights

▶ We studied the functioning of the exchangeable water reservoirs of beech trees under climatic and edaphic constraints. ▶ We characterised the strong impact of drought on water fluxes at the tree and forest scales and showed the strong sensitivity of beech to both climate and drought. ▶ The elastic tissue reservoirs played a minor role in the total water budget of beech trees. ▶ We highlighted the major role of total tree water reservoirs to maintain leaf transpiration under severe drought.

Introduction

Global change is expected to increase the frequency and severity of soil drought events in the northern hemisphere, especially during spring and summer (International Panel on Climate Change, IPCC, 2007). Thus, the exceptional soil drought and heat wave that occurred in Europe in summer 2003 (Stott et al., 2004, Ciais et al., 2005, Bréda et al., 2006, Granier et al., 2007, Gartner et al., 2009) should not be considered as an isolated extreme event, but representative of what might occur with increasing frequency in the near future. The influence of such extreme events on the dynamics and functioning of forest ecosystems is still poorly documented because of their low occurrence (Saxe et al., 2001, Bréda et al., 2006). The 2003 summer drought thus provided a good opportunity to analyze the response of these ecosystems to extreme climatic conditions.

Water and carbon fluxes at the tree or at the forest ecosystem level are strongly influenced by soil drought. Repeated episodes of drought induce a decline in gross primary productivity (Law et al., 2002, Hoff and Rambal, 2003, Ciais et al., 2005, Granier et al., 2007) and may induce tree decline at large scales (Bréda et al., 2006). Soil water shortage impacts different stages of the water transfer along the soil–tree–atmosphere continuum. It alters both soil–root and leaf–atmosphere interfaces and threatens the integrity of the liquid phase continuum in trees. In terrestrial ecosystems exposed to drought, different strategies of resistance, avoidance, or tolerance to water deficit have been observed (Raftoyannis and Radoglou, 2002, Bréda et al., 1993, Leuzinger et al., 2005, Zweifel et al., 2007, Gartner et al., 2009) and are driven by either structural or physiological adjustments, or by a combination of both. Notable adaptive processes are stomatal regulation and the capacity of trees to store and use water from reservoirs. The latter of these processes has been the least studied.

Elastic tissues (phloem, cambium and the subero-phellodermic layer) of the different tree organs (roots, trunks, branches and leaves), as well as sapwood of stems and branches, can be more or less important water storage compartments, depending on species and tree size (Domec et al., 2006, Scholz et al., 2008). Water withdrawn from trunk elastic tissues was reported to be equal to the total amount of water withdrawn from trunk reservoirs during the day for Picea abies (Zweifel et al., 2001), while several studies reported that sapwood is the major source of stored water that can be withdrawn and recharged on a daily (e.g. Logullo and Salleo, 1992, Loustau et al., 1996, Meinzer et al., 2003, Cermak et al., 2007, Scholz et al., 2007) or seasonal basis (Waring and Running, 1978, Waring et al., 1979).

The contribution of water from elastic tissues to tree transpiration never exceeded 15% in coniferous species (Zweifel et al., 2001, on P. abies; Cermak et al., 2007, on Pseudotsuga menziesii), whereas the contribution of stored water in sapwood was reported to range from 2% to as high as 50% (Waring and Running, 1978, Phillips et al., 2003, Meinzer et al., 2004, Scholz et al., 2008). The contribution of water stored within trees to the transpiration stream depends on the soil water deficit experienced by the tree: the lower the soil water content is, the greater the reservoir depletion will be (Hinckley and Bruckerhoff, 1975, Borchert, 1994, Goldstein et al., 1998, Intrigliolo and Castel, 2005, Deslauriers et al., 2007, Giovannelli et al., 2007).

European forest managers are concerned by the response of beech (Fagus sylvatica L.) to water shortage because this species, which usually grows in temperate and humid environments, is sensitive to drought (Epron and Dreyer, 1993, Magnani and Borghetti, 1995, Backes and Leuschner, 2000, Cochard et al., 2001, Leuschner et al., 2001) and is thus potentially vulnerable to future climatic changes. Studies on the effect of soil drought on beech transpiration, as estimated from sap flow measurements (T), and on stand evapotranspiration (ET), as measured by eddy covariance, have already been performed (e.g., Irvine et al., 1998, Granier et al., 2000, Barbour et al., 2005). These studies showed a fast and strong negative response of this species to soil water deficit, but the contribution of the different water reservoirs to tree transpiration under natural drought conditions has not yet been examined.

This work aimed at studying water reservoir functioning of beech trees under climatic and edaphic constraints in a 37-year-old beech stand located in north-eastern France. We took advantage of the severe climatic conditions during summer 2003 to characterise the impact of soil drought on water fluxes at the tree and the forest scales and to estimate the contribution of exchangeable water in reservoirs to total tree transpiration, under non-limiting soil water or soil drought conditions.

Section snippets

Site and stand characteristics

This study was carried out in an experimental plot located in the state forest of Hesse, France (48°40.453 N, 7°03.877 E, elevation 300 m), which was mainly composed of beech (F. sylvatica L.) (90% of the total basal area) (Granier et al., 2000, Granier et al., 2008). Other tree species (10% of the total basal area) were Carpinus betulus L., Betula pendula (Roth), Quercus petraea (Matt.) Liebl. and Larix decidua (Mill.). The experimental plot covering 0.6 ha was located in the centre of a 65 ha

Climatic and soil conditions

The year 2002 was taken as a reference year for climatic conditions, as the cumulated rainfall from June to September (357 mm) was close to the long-term average (290 mm; 1950–2002), with no water shortage during the summer.

In 2003, the summer was exceptionally warm (Fig. 1a). From June to September, average air temperature (19.6 °C) was much higher than in 2002 and than the long-term average (15.7 °C; 1950–2002). A 15-day heat wave (1st to 15th August) (Fig. 1a) was particularly severe, with mean

Reservoir characteristics and contribution under non-limiting conditions

In our study, the daily contribution of elastic tissues to total tree transpiration under non-limiting water conditions (summer 2002 and May and June 2003) represented a maximum of 2%. This amount is rather low and much lower than previous studies on beech (Steppe and Lemeur, 2004) or other species (cf. Table 3). The discrepancy between our results and that of Steppe and Lemeur (2004) could be due to the age of the trees studied (2-year-old beech trees as compared to 37 years old in this

Conclusion

In our study, beech showed a strong and direct response to climatic demand and to soil water depletion. Furthermore, during dry periods, every rain event induced increase in both sap flow and radial water flow. Additionally, as drought developed, we highlighted a strong increase of water withdrawal from tree reservoirs. Leuschner et al. (2001) also showed that F. sylvatica was a drought-sensitive tree species, exhibiting large drought effects in all tree organs (leaves, stem, roots). We

Acknowledgements

This work was supported by 3 successive European programmes: Euroflux (ENV4-CT95-0078), Carboeuroflux (EVK2-1999-00229), Carboeurope-IP (505572), by the French Forest Service (ONF) and by GIP-Ecofor. The authors thank all the technical staff who performed the measurements at Hesse (soil water content, tree circumference, biomass, litter collection, etc.). We thank particularly Patrick Gross, Bernard Clerc, Pascal Courtois, and Jean-Marie Gioria.

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