True water constraint under a rainfall interception experiment in a Mediterranean shrubland (Northern Tunisia): confronting discrete measurements with a plant-soil water budget model

International audienceIncreased drought length and intensity is expected in theMediterranean basin under anthropogenic increase in atmospheric CO2, leading to extreme events not yet encountered in the present climate variability. Understanding ecosystems responses and capturing peculiar ecophysiological processes related to these events have been investigated in the field by rainfall manipulation experiments. Quantifying the actual drought faced by the ecosystem under control and dry plots, or among experiments remain a key challenge for explaining functional impacts on plant growth. Fullprofile soil water content can be tricky to assess in rocky soils, and time-consuming plant water potential measurements remain a discrete information unable to capture short rainfall pulses. We propose here to fully investigate the water budget of a total rainfall interception manipulation on a Mediterranean shrubland, coupled with a plant-soil water balance model. We could accurately simulate the seasonal course of plant water status, including small rainfall pulses. We then derived yearly estimates of water stress integral for each water treatment, leading to an estimate of 66-86 %increase of drought intensity for the dry treatment compared to the control. Comparing actual and expected plant water budget from simulations in the dry plots allowed to identify and quantify the impact of methodological issues related to rainfall interception experiments as side effects for intrusive rain drops and subsurface lateral water flow

Living with extremes : the dark side of global climate change

Increased drought length and intensity is expected in the Mediterranean basin under anthropogenic increase in atmospheric CO2, leading to extreme events not yet encountered in the present climate variability. Understanding ecosystems responses and capturing peculiar ecophysiological processes related to these events have been investigated in the field by rainfall manipulation experiments. Quantifying the actual drought faced by the ecosystem under control and dry plots, or among experiments remain a key challenge for explaining functional impacts on plant growth. Full-profile soil water content can be tricky to assess in rocky soils, and time-consuming plant water potential measurements remain a discrete information unable to capture short rainfall pulses. We propose here to fully investigate the water budget of a total rainfall interception manipulation on a Mediterranean shrubland, coupled with a plant–soil water balance model. We could accurately simulate the seasonal course of plant water status, including small rainfall pulses. We then derived yearly estimates of water stress integral for each water treatment, leading to an estimate of 66–86 % increase of drought intensity for the dry treatment compared to the control. Comparing actual and expected plant water budget from simulations in the dry plots allowed to identify and quantify the impact of methodological issues related to rainfall interception experiments as side effects for intrusive rain drops and subsurface lateral water flow

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

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