Drought controls over conductance and assimilation of a Mediterranean evergreen ecosystem: scaling from leaf to canopy

Abstract

Drought control over conductance and assimilation was assessed using eddy flux and meteorological data monitored during four summer periods from 1998 to 2001 above a closed canopy of the Mediterranean evergreen oak tree Quercus ilex. Additional discrete measurements of soil water content and predawn leaf water potential were used to characterize the severity of the drought.Canopy conductance was estimated through the big-leaf approach of Penman-Monteith by inverting latent heat fluxes. The gross primary production (GPP) was estimated by adding ecosystem respiration to net ecosystem exchange. Ecosystem respiration was deduced from night flux when friction velocity (u * ) was greater than 0.35 m s À 1 . Empirical equations were identified that related maximal canopy conductance and daily ecosystem GPP to relative soil water content (RWC), the ratio of current soil water content to the field capacity, and to the predawn leaf water potential. Both variables showed a strong decline with soil RWC for values lower than 0.7. The sharpest decline was observed for GPP. The curves reached zero for RWC 5 0.41 and 0.45 for conductance and GPP, respectively. When the predawn leaf water potential was used as a surrogate for soil water potential, both variables showed a hyperbolic decline with decreasing water potential.These results were compared with already published literature values obtained at leaf level from the same tree species. Scaling up from the leaf to ecosystem highlighted the limitation of two big-leaf representations: Penman-Monteith and Sellers' P factor. Neither held completely for comparing leaf and canopy fluxes. Tower measurements integrate fluxes from foliage elements clumped at several levels of organization: branch, tree, and ecosystem. The Q. ilex canopy exhibited non-random distribution of foliage, emphasizing the need to take into account a clumping index, the factor necessary to apply the Lambert-Beer law to natural forests.Our results showed that drought is an important determinant in water losses and CO 2 fluxes in water-limited ecosystems. In spite of the limitations inherent to the big-leaf representation of the canopy, the equations are useful for predicting the influence of environmental factors in Mediterranean woodlands and for interpreting ecosystem exchange measurements.

Process-based simulation of seasonality and drought stress in monoterpene emission models

“…The models implemented can be applied alternatively or in addition to each other (for more information see Grote, 2007;Grote et al, 2009a, b;Holst et al, 2010). In the current context the short-term isoprenoid emission models MEGAN , GUENTHER (Guenther et al, 1993), NIINEMETS (Niinemets et al, 2002a), and BIM2 (Grote et al, 2006) are coupled with models describing the canopy micro-climate within the canopy (ECM, Grote et al, 2009a) and the soil (DNDC, Li et al, 1992), photosynthesis (FARQUHAR, Farquhar and Von Caemmerer, 1982), phenology (SIM, Lehning et al, 2001;Grote, 2007), and soil hydrological conditions (QUERCUS, Rambal et al, 1993Rambal et al, , 2003) (see Fig. 1 and model descriptions below).…”

Temperate forest trees and stands under severe drought: a review of ecophysiological responses, adaptation processes and long-term consequences

“…Mediterranean and dry-tropical vegetations adapt to severe and frequent drought episodes by adjusting species composition, leaf area duration, leaf area index, root-to-shoot ratio, leaf thickness and through physiological acclimation processes. However, the primary productivity of such drought adapted ecosystems is usually very low [54,113,116,117].…”

Carbon exchange and water use efficiency of a growing, irrigated olive orchard

“…Whenever 2 or more night-time hours with measured 7 friction velocity u *  0.35 m s -1 (e.g. Rambal et al, 2003) were present (around 80% 8 of the nights), the average of these F c fluxes was adopted as R eco for the whole 9 night-time. In very stable nights, when the above-mentioned conditions were not met, 10 the night-time R eco value was obtained interpolating the R eco values of the previous 11 and next well mixed nights.…”