Neighbourhood canopy cover alleviates increased tree mortality after exceptionally dry summers at a climatic range limit

Populations situated at range margins are often at their environmental niche limit. The stress gradient hypothesis posits that facilitation effects should be more common in such conditions, but few studies have examined the joint effects of biotic interactions and climatic factors on vital rates at species range limits. We used eight years of annual unmanned aerial vehicle surveys to assess the sensitivity of tree mortality to summer moisture deficits across a landscape near the climatic moisture limit of forest cover in the Northern Great Plains. Trembling aspen (Populus tremuloides) and white spruce (Picea glauca), which were near the edge of their climatic niches, both experienced higher mortality in years following exceptionally dry summers. Mortality increases for these species were especially pronounced in open areas, as well as for tall trees and those on upper-slope positions. Individuals surrounded by neighbours appeared to better withstand high summer moisture deficits, possibly because microclimatic buffering reduced water loss. A third species, lodgepole pine (Pinus contorta var. latifolia), was resistant to summer moisture deficits and seemed to benefit somewhat from reduced competition at mid-slope positions. We conclude that facilitation from neighbourhood-scale canopy cover may help alleviate effects of climate-related environmental change at species’ range limits.