Repeated spring precipitation shortage alters individual growth patterns in Scots pine forests in the Western Alps

Key message: In the inner-Alpine Aosta Valley, severe growth reductions occurred in Scots pine stands during 1987–1993 and 2003–2005. Trees more sensitive to May precipitation exhibited stronger growth decline during these periods. Abstract: Over the last decades, Scots pine (Pinus sylvestris L.) decline has involved large areas in the European Alps. Although the species is supposed to be drought resistant, increased temperatures and droughts are often indicated as predisposing causes of the decline. Nevertheless, the exact climate conditions that initiate the decline, and the reasons why they differentially affect individual trees, are largely unknown. Our aims were to identify climate constraints on Scots pine growth, and elucidate the effect of individual characteristics, such as tree age, size, crown condition, mistletoe occurrence, competition, and sensitivity to climate, on tree growth decline and recovery after repeated climatic stress. We analysed 232 trees in four second-growth stands located at 985–1350 m a.s.l. in the inner-Alpine Aosta Valley, NW Italy. Multi-year growth declines, occurred in 1987–1993 and 2003–2005, were related to repeated May precipitation shortage, while temperatures and summer precipitations played a minor role. Growth decline was stronger in the stand at lower elevation and with higher competition intensity. At the individual scale, trees more sensitive to May precipitation exhibited lower growth rates during both dry periods, but not a faster recovery. In the lower elevation stand, tree growth decline was significantly related to both crown transparency and mistletoe abundance, which was almost absent in the other stands. We conclude that future variations in the spring precipitation regime could threaten Scots pine more than warming per se, in inner-Alpine valleys around 1000–1400 m a.s.l. Still, different individual sensitivity to precipitation will likely result in patchy patterns of healthy and declining trees within the same stand.