Future northeastern U.S. forests: Seedling survival and growth is shaped by species-specific responses to climate, canopy openness and soil quality

Seedling regeneration in temperate forests is influenced by both climatic conditions and the microenvironment, but the interacting effects of soil type, canopy openness, and species identity remain poorly understood. We examined how interactions among climate, soil type, canopy openness, and species influence the seedling survival and growth of twelve tree species representing temperate broadleaved, northern conifer, southern broadleaved, and southern conifer taxa within northeastern U.S. forests. In a common garden experiment across six sites spanning 4° of latitude, five U.S.D.A. plant hardiness zones, and approximately a 3 °C summer temperature gradient, we compared seedling growth and survival in relation to soil types (native versus enriched garden soils), canopy openness (forest edge versus forest interior), and local climate, linking tree performance to species biogeographic origins, distribution range, and shade tolerance. Seedling survival was generally greater at cooler, wetter sites, particularly for temperate broadleaved and northern conifer species. Southern conifer species experienced lower overall survival, except for eastern red cedar, which had higher survival at the warmest and driest sites. Survival was higher in native soils under high light conditions, whereas enriched soils tended to enhance the relative height growth of surviving trees. Growth responses varied with canopy openness, with shade-tolerant species often exhibiting greater height growth in forest interiors. Collectively, our study suggests that forest regeneration under a climate warming scenario will rely on maintaining canopy openness, conserving hydric microsites, and preserving soil integrity. Management strategies should prioritize sustaining canopy heterogeneity and favoring species with broad ecological tolerances. Simultaneously, it is important to recognize that warmer, drier conditions may constrain regeneration of many native species, and that the cooler northern sites may not yet provide suitable conditions for migration of more southerly taxa.