Severe drought effects on ecosystem CO₂ and H₂O fluxes at three Mediterranean evergreen sites: revision of current hypotheses?

Abstract

Eddy covariance and sapflow data from three Mediterranean ecosystems were analysed via top‐down approaches in conjunction with a mechanistic ecosystem gas‐exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO2/H2O exchange. The three sites include two nearly monospecific Quercus ilex L. forests – one on karstic limestone (Puéchabon), the other on fluvial sand with access to ground water (Castelporziano) – and a typical mixed macchia on limestone (Arca di Noè). Estimates of ecosystem respiration were derived from light response curves of net ecosystem CO2 exchange. Subsequently, values of ecosystem gross carbon uptake were computed from eddy covariance CO2 fluxes and estimates of ecosystem respiration as a function of soil temperature and moisture. Bulk canopy conductance was calculated by inversion of the Penman‐Monteith equation. In a top‐down analysis, it was shown that all three sites exhibit similar behaviour in terms of their overall response to drought. In contrast to common assumptions, at all sites ecosystem respiration revealed a decreasing temperature sensitivity (Q10) in response to drought. Soil temperature and soil water content explained 70–80% of the seasonal variability of ecosystem respiration. During the drought, light‐saturated ecosystem gross carbon uptake and day‐time averaged canopy conductance declined by up to 90%. These changes were closely related to soil water content. Ecosystem water‐use efficiency of gross carbon uptake decreased during the drought, regardless whether evapotranspiration from eddy covariance or transpiration from sapflow had been used for the calculation. We evidence that this clearly contrasts current models of canopy function which predict increasing ecosystem water‐use efficiency (WUE) during the drought. Four potential explanations to those results were identified (patchy stomatal closure, changes in physiological capacities of photosynthesis, decreases in mesophyll conductance for CO2, and photoinhibition), which will be tested in a forthcoming paper. It is suggested to incorporate the new findings into current biogeochemical models after further testing as this will improve estimates of climate change effects on (semi)arid ecosystems' carbon balances.

Seasonal variation in carbon dioxide exchange over a Mediterranean annual grassland in California

“…Davidson et al (1998) reported that Q 10 for soil respiration ranged from 3.4 to 5.6 for different study sites in a temperate mixed hardwood forest. Considering variations in soil moisture, Reichstein et al (2002b) reported a significant decline in Q 10 (from 0.5 to 2.6) when the soil moisture dropped for three Mediterranean evergreen sites.…”

Forest Carbon Sequestration: The Impact of Forest Management

“…To a certain extent, carbon storage in forests buys time, until we find more definitive solutions to our dependency on energy from fossil fuel. However, forests can easily become carbon sources rather than carbon sinks (Kurz and Apps, 1999, Gracia et al, 2001, Reichstein et al, 2002. Changes in the regimes of natural disturbances such as fire, pests or drought (Fuhrer et al, 2006, Sohngen et al, 2005, Ciais et al, 2005, can affect major forest functions, forestry outputs and forest stability.…”

Modeling permafrost thaw and ecosystem carbon cycle under annual and seasonal warming at an Arctic tundra site in Alaska

“…The function f 1 is based on the work of Tjoelker et al [2001], and f 2 is based on the work of Reichstein et al [2002], where b 1 is a biome-dependent model parameter (μmol m À2 s À1 ) and the empirical constants b 2 , b 3 , T s0 , and θ s0 are equal to 52.4 (K), 285 (K), 227.2 (K), and 0.16, respectively Reichstein et al [2002]. T avg is the annual mean soil temperature (K) and is assumed to be equal to the temperature of the deepest soil layer (T s,6 ).…”