Heat and drought in the Alps: comparative tree-ring study between European larch and Norway spruce growing in dry and wet conditions

Climate change is expected to increase both the frequency and the intensity of climate extremes, such as drought events in mountain ecosystems. Thus, in a climate change perspective, the resilience of Alpine forests is directly linked to their capacity to adapt to extreme events and cope with water scarcity and high temperatures. Investigating the response of different tree species is essential to understand the complexity of forest ecosystem adaptation, resistance and resilience to severe drought periods and the role of forests in mountainous areas. Besides, in high-altitude forests, plant species growing in wetter terrains have a smaller safety hydraulic margin and are possibly less resistant than plant species raising in dry environments because of their differences in physiological responses and evapotranspiration processes.

Therefore, it is interesting to focus on a comparative study between two areas differing in terms of climate and ecology, a dry and a wet site, in order to analyse which environment is more capable to cope with extreme conditions. In the context of the Agile Arvier project, supported by funding from the European Union’s economic recovery plan, we carried out dendrochronological analyses by assessing climate-growth relationships and applying drought ‘resilience indices’ (RRR) based on tree-ring width. The drought severity was defined by the Standardised Precipitation Evapotranspiration Index (SPEI).

In this survey, the monitoring sites are located in the western Alps, (Italy, Aosta Valley region). One site, situated in Torgnon is characterized by dry conditions while the other site, located in Champorcher displays wet conditions. In both sites, two tree species, Larix decidua Mill. and Picea abies (L.) H.Karst., were sampled for tree-ring analyses at four different altitudes: 1500, 1800, 2000 and 2200 m a.s.l.

Our results highlight the contrasting water use strategies between larch and spruce and show differences in physiological and anatomical responses to drought stress. Specifically, we show that species responses vary with elevation and site conditions (dry versus wet), and that these differences become particularly evident during specific anomalous years. However, analyses across different altitudes introduce some uncertainties, making it difficult to draw a definitive conclusion about which species exhibits a more efficient recovery from extreme heat and drought events.

Furthermore, these changes are occurring rapidly in the Alps, with important consequences for tree species adapted to strong climate seasonality and short growing season, altering the role of Alpine European larch and Norway spruce forests in carbon sequestration and mitigation of climate change.