Simulating the drought response of water and carbon cycle in European forests with a dynamic vegetation model

The forest carbon sink is crucial to climate change mitigation efforts. At the same time forests are threatened by climate-change induced extremes. In Europe, increasingly frequent and severe droughts are one of the main culprits endangering the forest carbon sink. Understanding how droughts may alter water- and carbon-cycle dynamics of forests is essential to preparing for an almost inevitably hotter and drier future. Here, dynamic vegetation models (DVMs) serve as useful tools for studying how extremes can impact the carbon and water cycles. In recent years, many DVMs have consequently begun including mechanistic representations of plant hydraulic architecture.

 

We use a version of LPJ-GUESS with plant hydraulic architecture, water-potential regulation strategies, and hydraulic failure mortality (LPJ-GUESS-HYD) and extend its capabilities by including aspects of turgor-driven growth dynamics to better simulate the impact of drought on the water and carbon cycles for common European forest tree species. We evaluate the performance of LPJ-GUESS for 12 European tree species across a network of 37 eddy-covariance flux sites covering a wide climatic and geographic gradient.

 

Our results indicate that LPJ-GUESS-HYD is able to simulate observed patterns of evapotranspiration more accurately than the standard version of LPJ-GUESS. Additionally, we show that LPJ-GUESS-HYD can simulate a wide range of species-specific evapotranspiration and canopy conductance in response to increasing VPD in line with established theories on the isohydric-anisohydric continuum. Lastly, our results suggest that the currently implemented model processes responsible for governing the response of water fluxes to drought are not as crucial in regulating the simulated carbon response to drought. This indicates that a shift toward more sink-driven model processes may be necessary to capture the full effect of drought on both the water and carbon cycles.

 

Given these results, future model development should focus on the interplay of source and sink processes in regulating tree response to extreme events such as drought. In particular, our results suggest that reliably modeling drought impacts entails improving the representation of water limitation not only on photosynthesis but, independently, also on growth. Here, we present results for the impact of drought on the modeled water cycle and discuss concepts and ideas related to improving the simulated effect of drought on the carbon cycle.