Effects of changing evapotranspiration and precipitation patterns on Scots pine vitality and mortality

Forest decline has been attributed to climatic changes in many parts of the world. Although climate conditions are an undisputed crucial factor affecting tree vitality, open questions remain regarding the relative roles of evaporative demand versus precipitation and the relative importance of individual climate variables.

In recent decades, there was a pronounced decline of Scots pines (Pinus sylvestris) at lower elevations in the inner-alpine Rhône valley in Switzerland. Similar observations were made in other inner-alpine valleys. Tree vitality was not continuously decreasing: single events of strong decrease in tree vitality and high mortality rates were observed in between phases of largely constant vitality levels. However, trees were hardly able to recover from such events in recent decades, resulting in a pronounced decrease in living Scots pines.

Climate-trend signals in the Rhône valley from 1981 to 2018 vary between the seasons. The clearest changes occurred in spring, when a strong climatic shift towards drier conditions was detected with significantly increasing evapotranspiration, decreasing precipitation sums and frequency of precipitation events, increasing duration of dry spells at lower elevations, and increasing diurnal temperature ranges. Relative trends of evapotranspiration are elevation dependent with the highest increase at low elevations. Temperature trends are the main driver towards higher evapotranspiration rates, but humidity and sunshine duration are important drivers too. For seasonal evapotranspiration anomalies, anomalies of temperature, humidity, and sunshine duration are of similar importance.

In previous works on Scots pine mortality in the Rhône valley, mortality events were attributed to prolonged periods of water deficits. However, the occurrence and magnitude of mortality events cannot be explained by droughts only. In case of Scots pines at low elevations in the Rhône valley, factors such as insect infestation and spring frost may strongly impact tree vitality and increase mortality rates.

In summary, climatic conditions changed markedly in the Swiss Rhône valley within the last approximately 40 years, especially in spring and at lower elevations. Hence, the less favorable climatic background conditions result in decreased resilience and recuperative power of the Scots pines to various disturbances, leading to the observed forest decline.