Warming alters the variability and trajectories of tree growth around the world by intensifying or alleviating energy and water limitation. This insight from regional to global-scale research emphasizes the susceptibility of forest ecosystems and resources to climate change. However, globally-derived estimates are not necessarily meaningful for local nature conservation or management considerations, if they lack specific information on present or prospective tree species habitats. This is particularly the case towards the edge of their distribution, where shifts in growth trajectories may be imminent or already occurring. Importantly, the geographic space occupied by a tree species is not only constrained by climate, but often reflects biotic pressure such as competition for resources with other species. Hence, distinguishing climatic from competitive niche boundaries becomes a central challenge to identifying areas where tree species are most susceptible to climate change. Here, we employ a novel concept to characterize each position within a species’ bioclimatic niche based on two criteria: a climate sensitivity index (CSI) and a habitat occupancy index (HOI). The CSI is derived from step-wise multiple linear regression models that explain variability in annual radial tree growth as a function of monthly climate anomalies. The HOI is based on an ensemble of five species distribution models calculated from a combination of observed species occurrences and twenty-five bioclimatic variables.
We calculated these two indices for 11 widespread tree species across the Northern Hemisphere. The combination of climate sensitivity and habitat suitability indicated hotspots of change, where tree growth is mainly limited by competition (low HOI and low CSI), as well as areas that are particularly sensitive to climate variability (low HOI and high CSI). In the former, we expect that forest management geared towards adjusting the competitive balance between several candidate species will be most effective under changing environmental conditions. In the latter areas, selecting particularly drought-tolerant accessions of a given species may reduce forest susceptibility to the predicted warming and drying.