Dominant controls of transpiration along a hillslope transect inferred from ecohydrological measurements and thermodynamic limits

Abstract

We combine ecohydrological observations of sap flow and soil moisture with thermodynamically constrained estimates of atmospheric evaporative demand to infer the dominant controls of forest transpiration in complex terrain. We hypothesize that daily variations in transpiration are dominated by variations in atmospheric demand, while site-specific controls, including limiting soil moisture, act on longer timescales. We test these hypotheses with data of a measurement setup consisting of five sites along a valley cross section in Luxembourg. Both hillslopes are covered by forest dominated by European beech (Fagus sylvatica L.). Two independent measurements are used to estimate stand transpiration: (i) sap flow and (ii) diurnal variations in soil moisture, which were used to estimate the daily root water uptake. Atmospheric evaporative demand is estimated through thermodynamically constrained evaporation, which only requires absorbed solar radiation and temperature as input data without any empirical parameters. Both transpiration estimates are strongly correlated to atmospheric demand at the daily timescale. We find that neither vapor pressure deficit nor wind speed add to the explained variance, supporting the idea that they are dependent variables on land–atmosphere exchange and the surface energy budget. Estimated stand transpiration was in a similar range at the north-facing and the south-facing hillslopes despite the different aspect and the largely different stand composition. We identified an inverse relationship between sap flux density and the site-average sapwood area per tree as estimated by the site forest inventories. This suggests that tree hydraulic adaptation can compensate for heterogeneous conditions. However, during dry summer periods differences in topographic factors and stand structure can cause spatially variable transpiration rates. We conclude that absorption of solar radiation at the surface forms a dominant control for turbulent heat and mass exchange and that vegetation across the hillslope adjusts to this constraint at the tree and stand level. These findings should help to improve the description of land-surface–atmosphere exchange at regional scales

Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone

“…Note that we focus in our study on the PET dynamics and that the absolute values could vary depending on the aerodynamic and roughness parameter of different vegetation covers. We further did not partition PET into evaporation and transpiration fluxes, since PET was primarily used as a proxy for potential soil evaporation rates, and evaporation and transpiration usually show a linear relationship in temperate regions (Renner et al, 2016;Schwärzel et al, 2009). To understand the potential atmospheric drivers for the soil water isotopic composition, we investigated the effect of antecedent con-ditions.…”

Canopy-scale biophysical controls of transpiration and evaporation in the Amazon Basin

“…Although λE T and λE E estimates are interdependent on g C and g A (as shown in , the figures reflect the credibility of the conductances as well as transpiration estimates by realistically capturing the hysteretic behavior between biophysical conductances and water vapor fluxes, which is frequently observed in natural ecosystems (Zhang et al, 2014;Renner et al, 2016) (also Zuecco et al, 2016). These results are also compliant with the theories postulated earlier from observations that the magnitude of hysteresis depends on the radiation-vapor pressure deficit time lag, while the soil moisture availability is a key factor modulating the hysteretic transpiration-vapor pressure deficit relation as soil moisture declines (Zhang et al, 2014;O'Grady et al, 1999;Jarvis and McNaughton, 1986).…”

How Reliable Are Heat Pulse Velocity Methods for Estimating Tree Transpiration?

“…The parameters ρ s , C w , and C s are constants whereas ρ b and m c need to be measured and are commonly measured once at the end of a campaign (e.g., [39]). Although ρ b does not vary throughout the measurement period, m c can change up to 70% on a daily and seasonal basis [14,40,41]. An incorrect measurement of m c can lead to either an over or underestimation of transpiration.…”