The temporal response to drought in a Mediterranean evergreen tree: comparing a regional precipitation gradient and a throughfall exclusion experiment

Abstract

Like many midlatitude ecosystems, Mediterranean forests will suffer longer and more intense droughts with the ongoing climate change. The responses to drought in long-lived trees differ depending on the time scale considered, and short-term responses are currently better understood than longer term acclimation. We assessed the temporal changes in trees facing a chronic reduction in water availability by comparing leaf-scale physiological traits, branch-scale hydraulic traits, and stand-scale biomass partitioning in the evergreen Quercus ilex across a regional precipitation gradient (long-term changes) and in a partial throughfall exclusion experiment (TEE, medium term changes). At the leaf scale, gas exchange, mass per unit area and nitrogen concentration showed homeostatic responses to drought as they did not change among the sites of the precipitation gradient or in the experimental treatments of the TEE. A similar homeostatic response was observed for the xylem vulnerability to cavitation at the branch scale. In contrast, the ratio of leaf area over sapwood area (LA/SA) in young branches exhibited a transient response to drought because it decreased in response to the TEE the first 4 years of treatment, but did not change among the sites of the gradient. At the stand scale, leaf area index (LAI) decreased, and the ratios of stem SA to LAI and of fine root area to LAI both increased in trees subjected to throughfall exclusion and from the wettest to the driest site of the gradient. Taken together, these results suggest that acclimation to chronic drought in long-lived Q. ilex is mediated by changes in hydraulic allometry that shift progressively from low (branch) to high (stand) organizational levels, and act to maintain the leaf water potential within the range of xylem hydraulic function and leaf photosynthetic assimilation.

Comparative Drought Responses of Quercus ilex L. and Pinus sylvestris L. in a Montane Forest Undergoing a Vegetation Shift

“…Our PLC values were estimated from xylem vulnerability curves obtained using appropriate methods for each species-air injection for P. sylvestris and bench dehydration for Q. ilex [48,60]. However, none of these methods is free of potential artifacts [61], and estimated branch P50 values for Q. ilex in the study site (−2 MPa; [47]) are low compared to the range of values reported in other studies (between −3.8 and −6.6 MPa; [27,48,62,63]). If we use the vulnerability curves recently reported for a climatically similar Q. ilex population (−4.70 MPa; [48]) to estimate branch PLC, Q. ilex would keep PLC below ~20% except for the drought period in 2011, when PLC reached ~80% (Figure 8).…”

The effects of thinning intensity and tree size on the growth response to annual climate in Cedrus atlantica: a linear mixed modeling approach

“…Indeed, 60 to 80 % of the French metropolitan territory may experience a Mediterranean type climate by the end of the twenty-first century (Roman-Amat 2007). The acclimation of existing forest stands to increasing drought relies on different physiological adjustments, among which leaf area reduction seems of particular importance over the long term (Martin-StPaul et al 2013), and may be related to changes in stem density (Barbeta et al 2013). Thinning operation, which modulates stem density and stand vertical structure, can thus accelerate this stand acclimation and has been reported to improve tree resistance to drought stress (Misson et al 2003) and post-drought resilience (Martín-Benito et al 2008;Sohn et al 2013).…”

Earth System Model Needs for Including the Interactive Representation of Nitrogen Deposition and Drought Effects on Forested Ecosystems

“…Over short time scales, stomata regulate water loss, but, over longer time scales, changes in allometry will occur to optimize hydraulic conductance [22]. Plant traits may change similarly across short and long drought intervals; for example, leaf area may decrease with decreasing precipitation, while other processes may be delayed (i.e., partitioning changes between the leaf and roots) or homeostatic (e.g., xylem vulnerability) [22]. Figure 2 shows some of the effects forests may experience under drought.…”

“…Figure 2 shows some of the effects forests may experience under drought. may be delayed (i.e., partitioning changes between the leaf and roots) or homeostatic (e.g., xylem vulnerability) [22]. Figure 2 shows some of the effects forests may experience under drought.…”

“…For example, increased N availability generally results in an increase in aboveground woody biomass and a decrease in root biomass, but experiments with drought indicate an increase in root biomass at the expense of aboveground tissue [20][21][22][23]. This has led to the conclusion that N deposition may increase ecosystem vulnerability to drought [24], but some studies find that N deposition may enhance the recovery of some species post drought [25].…”