The Role of Plant-Plant Interactions in Forest Response to Climate Change: Sessile Oak and Beech in the Western Pyrenees

Many models of forest response to climate change ignore biotic interactions. We conducted an in situ experiment along a 600 m elevation gradient in the western Pyrenees to measure interactions between sessile oak (Quercus petraea) and common beech (Fagus sylvatica) seedlings under adult canopies. Seedling survival data revealed that competition from beech increased sharply with elevation for oak, due to cooler, shaded microclimates limiting growing season length. Beech seedlings showed less sensitivity. Under projected warming, oak’s greater phenological response to heat should reduce this competitive pressure, accelerating its altitudinal migration and amplifying species-range shifts.