Decoupling between ecosystem photosynthesis and transpiration: a last resort against overheating

Abstract

Ecosystems are projected to face extreme high temperatures more frequently in the near future. Various biotic coping strategies exist to prevent heat stress. Controlled experiments have recently provided evidence for continued transpiration in woody plants during high air temperatures, even when photosynthesis is inhibited. Such a decoupling of photosynthesis and transpiration would represent an effective strategy ("known as leaf or canopy cooling") to prevent lethal leaf temperatures. At the ecosystem scale, continued transpiration might dampen the development and propagation of heat extremes despite further desiccating soils. However, at the ecosystem scale, evidence for the occurrence of this decoupling is still limited. %One of the reasons is that photosynthesis and transpiration are difficult to isolate from other fluxes. Here, we aim to investigate this mechanism using eddy-covariance data of thirteen woody ecosystems located in Australia and a causal graph discovery algorithm. Working at half-hourly time resolution, we find evidence for a decoupling of photosynthesis and transpiration in four ecosystems which can be classified as Mediterranean woodlands. The decoupling occurred at air temperatures above 35\C. At the nine other investigated woody sites, we found that vegetation \coo\ exchange remained coupled to transpiration at the observed high air temperatures. Ecosystem characteristics suggest that the canopy energy balance plays a crucial role in determining the occurrence of a decoupling. Our results highlight the value of causal-inference approaches for the analysis of complex physiological processes. With regard to projected increasing temperatures and especially extreme events in future climates, further vegetation types might be pushed to threatening canopy temperatures. Our findings suggest that the coupling of leaf-level photosynthesis and stomatal conductance, common in land surface schemes, may need be re-examined when applied to high-temperature events.

Disentangling the Role of Forest Structure and Functional Traits for the Thermal Balance in the Mediterranean-Temperate Ecotone

“…In contrast, ∆𝑇 and 𝑇 were disproportionately reduced by VPD during hot spells in drought-resistant conifer species, such as P. halepensis (Sánchez-Costa et al, 2015). This traitmediated disparity in species-specific (and intra-specific) response to VPD during hot spells is in agreement with the observations of high transpiration during extremely high VPD conditions in Australian Mediterranean woodlands (Krich et al, 2022), but only for certain ecosystems with access to groundwater. The capacity to tap on deep water sources in Mediterranean ecosystem may be key to sustain transpiration under high VPD.…”

“…In temperate regions, the deeper root systems of forests compared to grasslands also result in more transpirational cooling and lower temperatures (Zhang et al, 2020), emphasizing the importance of root water uptake. The cooling effect associated with this deeper soil water access is due to the heat dissipation occurring during the evaporation of water from the leaves, that avoids overheating and decreases 𝑇 and sensible heat fluxes (Krich et al, 2022).…”

Rising water-use efficiency in European grasslands is driven by increased primary production

“…shortwave radiation→GPP in deciduous forests is given in summer but not in winter 68 . We further acknowledged changes in the causal relationships between the variables during special conditions, such as extreme high temperatures 69 , and in the long-term over the 24-year period but we argue that they are negligible in the 8-day timely resolution in our data. Time series stationarity is satisfied by detrending and using seasonal anomalies of the time series of each variable that have already been masked to the summer months, before inputting into the PCMCI + algorithm of the python package tigramite (https:// github.com/jakobrunge/tigramite).…”

Disentangling the Role of Forest Structure and Functional Traits in the Thermal Balance of the Mediterranean–Temperate Ecotone

“…The interaction between VPD and CWB was significant in the mixed models. through evaporation from the leaves, thus avoiding overheating, decreasing T can and sensible heat fluxes (Krich et al, 2022). Under wet soils and high VPD, our model predicted that ∆T can−air would become negative, that is, T can could be cooler than T air (Figure 5a).…”