Divergent responses of mountain forests and grasslands to heat and drought events

Both natural and managed ecosystems contribute to mitigating climate change through the process of CO₂ sequestration from the atmosphere. However, projections of future global climate indicate that extreme weather events will become more frequent and more intense in the coming years. And since heat waves and droughts can alter the ecosystems functioning, and increase the vulnerability of carbon sinks, this trend represents a potential risk for their contribution to reaching global climate change mitigation goals. Further efforts are therefore needed to assess the resistance and resilience of different ecosystems and land uses to climate change. In this context, Alpine mountain ecosystems face a double challenge, on the one hand, warming in the Alps is occurring twice as fast as in other regions of the planet and drought events are increasingly frequent, on the other, socio-economic changes have led to partial land abandonment, with effects on the composition and distribution of plant species.

The main objective of this study is to analyze the impacts of extreme heat and drought events on the functioning of two different ecosystems in the Alps, a European larch forest (Larix decidua Mill.) and an abandoned subalpine pasture dominated by Nardus stricta, both located in the Aosta Valley region (Italy) at about 2100 m asl and thus experiencing the same climate conditions. The eddy covariance method was used to measure the carbon and water fluxes between the ecosystem and the atmosphere. Radiometric vegetation indices (eg. NDVI), and field observations related to plant phenology were used to explain the role of timing in determining the carbon and water fluxes impacts. Finally, functional traits of plant species were used to interpret the divergent ecosystem responses from an adaptive perspective. The results show that the different heat and drought events observed during the ten-year study period (2012-2022) had a variable impact on the different ecosystems monitored, also based on the timing of the extreme event in relation to the phenology of the vegetation and the presence/absence of the snowpack, with impacts generally more severe for the grassland compared to the forest. The contrasting responses observed will be discussed by exploring the linkage between the functioning of the whole ecosystem and the adaptive strategies of individual plant species.