Contrasting responses of forest growth and carbon sequestration to heat and drought in the Alps

Projections of future climate change indicate that extreme events will be larger in frequency and intensity, with an increased risk of ecosystem transition from carbon sinks to carbon sources. In particular, warming is occurring at a higher rate in the Alps, with important impacts for tree species acclimated to a strong climate seasonality and a short growing season.

In this study, we investigated the ecosystem responses to heatwave and drought at a high-altitude Larix decidua (Mill.) forest in the western Italian Alps (IT-Trf, 2050 m asl), by coupling direct measurements of ecosystem-scale surface-atmosphere fluxes and tree-based observations. Ecosystem fluxes were monitored by means of the eddy covariance technique, measuring water and carbon fluxes (i.e., gross primary production, net ecosystem exchange, and evapotranspiration). From 2015 to 2017 additional observations were carried out at tree level, including stem growth and its duration, direct phenological observations, sap flow, and tree water deficit.

Results showed that the warm spells observed in 2015 and 2017, caused the advance of the larch phenological development and, thus, of the seasonal trajectories of many processes. However, we did not observe significant quantitative changes in the C sequestration at the ecosystem level, whereas in 2017 we found a reduction of 18% in larch stem growth and a contraction of 45% of the stem growth period. The growing season in 2017 was indeed characterized by different drought events and by the highest water deficit during the study years. By combining tree- and ecosystem-based observations, we demonstrated that larch growth decrease was not driven by a reduction of the photosynthetic activity.

We formulate two contrasting hypotheses to explain our results: i) a shift in C allocation within the plants towards the prioritization of NSC storage within leaves and roots over growth processes, which question the C-source limitation hypothesis, usually applied in vegetation modeling; ii) the ‘Insurance Hypothesis’, which can be used to explain the stability of the whole ecosystem gas exchanges, where the negative effects of climatic fluctuations on larch growth might have been buffered by the asynchrony responses of the understory species that can benefit from the higher temperatures.