Contrasting foliar nitrogen nutrition of coexisting temperate and boreal trees across a modest temperature cline

The temperate-boreal forest ecotone represents a transition zone from temperate to boreal forest where nitrogen (N) is frequently limiting tree growth. However, the spatial patterns and potential drivers of plant N nutrition and soil N availability remain poorly understood. To address this, we conducted a field investigation along a temperate-boreal forest ecotone in northeastern China, characterized by a modest mean annual temperature gradient (∼1 °C) within the range of current climate warming. Our goal was to evaluate the spatial variation in foliar N nutrition and soil N availability, and the potential driving factors for Mongolian oak (Quercus mongolica) and Dahurian larch (Larix gmelinii), the dominant trees of the local temperate and adjacent boreal forests, respectively. Our results revealed no significant spatial trend in topsoil N availability across the sampling transect. Foliar N concentration was significantly higher, but foliar δ¹⁵N was lower, for Mongolian oak than Dahurian larch. Foliar N concentration for Mongolian oak increased significantly toward the boreal forest, driven by lower mean annual temperature and mean annual precipitation, with no significant trend observed for Dahurian larch. Moreover, foliar Δδ¹⁵N (foliar δ¹⁵N-soil δ¹⁵N) decreased significantly for Mongolian oak as it approached the boreal forest, while it increased significantly for Dahurian larch toward the temperate forest. Notably, foliar N concentration, δ¹⁵N, and Δδ¹⁵N for Dahurian larch increased significantly with an increasing basal area proportion of Mongolian oak. Our findings demonstrate contrasting patterns of foliar N nutrition between co-occurring temperate and boreal trees across a temperate-boreal forest ecotone with a modest climatic gradient. These results underscore the importance of incorporating interspecific interactions to enhance our understanding of future N cycling in southern boreal forests in the context of climate warming.