Disentangling the response of forest and grassland energy exchange to heatwaves under idealized land–atmosphere coupling

Abstract

Abstract. This study investigates the difference in land-atmosphere interactions between grassland and forest during typical heatwave conditions in order to understand the controversial results of Teuling et al. (2010) (hereafter T10), who found the systematic occurrence of higher sensible heat fluxes over forest than over grassland during heatwaves. With a simple but accurate coupled land-atmosphere model, we show that existing parametrizations are able to reproduce the findings of T10 for normal summer and heatwave conditions. Furthermore, we demonstrate the sensitivity of the coupled system to changes in incoming radiation and early-morning temperature typical for European heatwaves.Our results suggest that the fast atmospheric control of stomatal resistance can explain the observed differences between grassland and forest. The atmospheric boundary layer has a buffering function therein: increases in stomatal resistance are largely compensated for by increases in the potential evaporation due to atmospheric warming and drying.In order to disentangle the contributions of differences in several static and dynamic properties between forest and grassland, we have performed a virtual experiment with artificial land-use types that are equal to grassland, but with one of its properties replaced by that of forest. From these, we confirm the important role of the fast physiological processes that lead to the closure of stomata. Nonetheless, for a full explanation of T10's results, the other properties (albedo, roughness and the ratio of minimum stomatal resistance to leaf-area index) play an important but indirect role; their influences mainly consist of strengthening the feedback that leads to the closure of the stomata by providing more energy that can be converted into sensible heat. The model experiment also confirms that, in line with the larger sensible heat flux, higher atmospheric temperatures occur over forest.As our parametrization for stomatal resistance is empirical rather than mechanical, our study stresses the demand for a better mechanistic understanding of physiological processes in plants.

Observational evidence for cloud cover enhancement over western European forests

“…Such differences are large enough to impact the growth and properties of the ABL. Differences in the partitioning of available energy also drive ABL dynamics and the timing of the onset of shallow cumulus formation11, but they might be of less importance than albedo differences9. Nonetheless, our results are consistent with observed higher sensible heat fluxes over temperate forest4, leading to a growing ABL and a forest breeze.…”

“…We can reconcile our results with studies over Amazonia1819 by recognizing that in both cases higher sensible heat fluxes trigger preferred cloud formation423. Although this may seemingly contradict the cooler LSTs over forest, differences in roughness prevent LST from being a direct measure of sensible heat flux and temperature-sensitive flux partitioning over forest49 might not be independent of cloud cover conditions. It should be noted that both a lifting mechanism (provided by sensible heat flux) and sufficient moisture (provided by latent heat flux) are required for development of convective clouds.…”

“…Forest canopies also provide a rougher surface8, thus promoting an efficient exchange of heat, moisture and momentum between the land surface and the atmosphere, whereas tempering the surface temperature and further increasing the net available energy. The resulting excess energy is either used for evaporation or transpiration of water (together referred to as evapotranspiration), or for heating of the well-mixed atmospheric boundary layer (ABL)9. Indirect biophysical effects of forests are more uncertain.…”

Atmospheric Aridity and Apparent Soil Moisture Drought in European Forest During Heat Waves

“…This model simulates how vegetation and the diurnal boundary layer interact by making use of idealized atmospheric profiles. We maintained early morning relative humidity across a range of early morning temperatures, such that the specific humidity profile changed with temperature (van Heerwaarden & Teuling, ). A latitude of 50°N was used as representative for European midlatitude heat waves.…”

“…By controlling the opening of their leaf stomata in response to several environmental variables (including soil moisture and VPD), plants can either enhance and reduce evapotranspiration in response to an increased VPD depending on air dryness and temperature (Figure ). Using the simplified land‐atmosphere model, van Heerwaarden and Teuling () showed that the observational results of Teuling et al () could be reproduced by assuming that forest stomata close in response to high VPD during heat waves, whereas grasses do not. Sulman et al () similarly found that observed evaporation in a mixed forest responded to both VPD and soil moisture anomalies with similar magnitudes, but the timescales of VPD response (hours) was much shorter than the soil moisture response (multiple days or weeks).…”