Summer droughts limit tree growth across 10 temperate species on a productive forest site

Summary

Studies on climate impacts on tree annual growth are mainly restricted to marginal sites. To date, the climate effects on annual growth of trees in favorable environments remain therefore unclear despite the importance of these sites in terms of forest productivity. Because species respond differently to climate, comparing a multitude of species further enhances our knowledge on climate impacts on tree growth and forest productivity. We present a first study that reveals to what extent radial growth is limited by climate and the groundwater table across 10 temperate tree species growing on a uniform, productive soil (i.e. high nutrient and water contents) in the Netherlands. We ranked our study species according to their shade tolerance, which is associated with species’ resource requirements and growth rates, and examined their annual growth using tree-ring analysis. This allowed us to investigate how these species with diverse ecological backgrounds differ in their growth response to precipitation, temperature, irradiance, potential evapotranspiration (PET), and the groundwater table, when growing under similar and favorable site conditions. Nine out of 10 species had strikingly similar radial growth rates (on average 3.9–4.8 mm yr-1), which contradicts the widely established trade-off between shade tolerance and growth. Populus trichocarpa, the least shade-tolerant of our species, however grew much faster (on average 6.8 mm yr-1). Trees of all species reduced their growth significantly during dry summers, driven by low rainfall and high PET. Yet, the magnitude of their growth responses to climate conditions and especially the groundwater table differed across species. Receding groundwater tables significantly and strongly reduced radial growth of the shade-intolerant species, but not of the shade-tolerant species. The climate impacts on growth were not associated with variation in shade tolerance across our species. Our study demonstrates that even on a productive forest site, summer droughts reduced radial growth across a multitude of common tree species, whereas lower groundwater tables only affected the shade-intolerant species significantly. Thus, the productivity of forests in favorable environments may be seriously affected when summers in north western Europe become hotter and drier as predicted by climate models.