Global warming has strongly modified the phenology, growth, and distribution of species throughout the world. In temperate European forests, an upward and northward shift in species distribution has been evidenced. Distributional changes are the consequences of the direct or indirect effects of rising temperatures on reproductive or vegetative traits that affect competitiveness over a species range. Despite its importance, the link between species range shifts and altered competitiveness is still poorly understood in a warming context. We hypothesized that growth is a key vegetative trait involved in the species range shift induced by climate warming. We investigated if recent warming had modified the growth of four tree species, such that growth decreases in the warmer part of their range and increases in the colder part of their range. Cores were collected from Abies alba (n=429 cores), Fagus sylvatica (n=239), Pinus sylvestris (n=21) and Acer pseudoplatanus (n=40) trees in 143 plots regularly distributed over a wide altitudinal range (from 330 to 1100 m) in northeast France. For each core, an index of mean radial growth was calculated, independent of tree age. Using a niche modeling approach, the ecological optimum for the mean annual temperature was computed for each species and used to define the warmer and colder parts of the species ranges. Recent growth trends were then analyzed for each species separately in the warmer and colder parts of their range.
Results/Conclusions
All four species exhibited lower radial growth in the warmer part of their distribution range than in the colder part during the period of recent warming. Notably, two species ̶ Pinus sylvestris and Fagus sylvatica ̶ exhibited a significant decrease in growth during this recent period in the warmer part of their range. No significant growth increases were observed in the colder part of the species ranges. The differential growth changes observed across the species ranges suggest a relative decrease of species competitiveness in the warmer part of their range, possibly linked to species regression at low elevation. However, the absence of a growth increase in the colder part of the distribution ranges cannot explain the expansion of distribution at a high altitude. Further investigations linking growth decline in the warmer part of the species ranges and species regressions could be carried out to determine how growth decline could be used as an early indicator of upcoming local extinction of tree species with climate warming.