Temporal and among‐site variability of inherent water use efficiency at the ecosystem level

Abstract

Half‐hourly measurements of the net exchanges of carbon dioxide and water vapor between terrestrial ecosystems and the atmosphere provide estimates of gross primary production (GPP) and evapotranspiration (ET) at the ecosystem level and on daily to annual timescales. The ratio of these quantities represents ecosystem water use efficiency. Its multiplication with mean daylight vapor pressure deficit (VPD) leads to a quantity which we call “inherent water use efficiency” (IWUE*). The dependence of IWUE* on environmental conditions indicates possible adaptive adjustment of ecosystem physiology in response to a changing environment. IWUE* is analyzed for 43 sites across a range of plant functional types and climatic conditions. IWUE* increases during short‐term moderate drought conditions. Mean annual IWUE* varied by a factor of 3 among all sites. This is partly explained by soil moisture at field capacity, particularly in deciduous broad‐leaved forests. Canopy light interception sets the upper limits to canopy photosynthesis, and explains half the variance in annual IWUE* among herbaceous ecosystems and evergreen needle‐leaved forests. Knowledge of IWUE* offers valuable improvement to the representation of carbon and water coupling in ecosystem process models.

Mechanisms of water supply and vegetation demand govern the seasonality and magnitude of evapotranspiration in Amazonia and Cerrado

“…Higher (lower) iWUE implies vegetation is photosynthesizing at a lower (higher) internal to ambient CO 2 ratio (Lloyd et al, 2002;Beer et al, 2009), and its variation across sites in Amazonia and Cerrado may reflect site differences in soil fertility, vegetation composition, or both. It is an important diagnostic for modeled E in addition to g s because it governs how the light response of photosynthesis is translated into evaporative losses.…”

“…For this reason, we did not attempt to estimate the transpiration fraction of evapotranspiration, though it is likely a large fraction for the forest sites (Jasechko et al, 2013). The slope of leaf-level stomatal conductance versus photosynthesis is the parameter m in the Ball-Woodrow-Berry-Collatz (BWBC) semi-empirical model of stomatal conductance (Collatz et al, 1991) (see Table A3), the inverse of which we refer to as intrinsic water use efficiency of photosynthesis (iWUE), following the definition of Beer et al (2009). Even though not all models use the BWBC model, we could estimate m and iWUE at the canopy scale for those which simulate GPP in addition to E. We estimated m as the slope of the best fit line between g s and GPP norm = GPP × (h/c a ) and took iWUE = 1/m, where c a and h are ambient CO 2 mole fraction and relative humidity, respectively.…”

“…Canopy stomatal conductance and intrinsic water use efficiency (iWUE) are two key mechanisms controlling vegetation demand for water, respectively, in relation to atmospheric vapor pressure deficit (D) and ecosystem photosynthesis (GPP) arising from the 'photosynthesis-transpiration' compromise (Lloyd et al, 2002;Beer et al, 2009). The degree to which stomata regulate transpiration (E t ) independent of environmental conditions in the Amazon has been the topic of debate (Avissar and Werth, 2004;Costa et al, 2004).…”

Spatiotemporal variations of T/ET (the ratio of transpiration to evapotranspiration) in three forests of Eastern China

“…where k was extinction coefficient, which was set to 0.5 in this study according to previous study (Beer et al, 2009) as the forests were needle forest or mixed forest. The calculations of each resistances were described in the previous study (Shuttleworth and Wallace, 1985) in detail.…”

“…Daily LAI was estimated from integrating the absorbed PAR with the relationship between the absorbed PAR and LAI, which was established by the absorbed PAR at 12:00 pm and the measured LAI (Beer et al, 2009;Gower et al, 1999;Law et al, 2001). …”