The demography of tree species response to climate: seedling recruitment and survival

Seedling recruitment and survival are critical bottlenecks in tree population dynamics and are likely to play central roles in shifts in species distributions under climate change. We use data from the Forest Inventory and Analysis program to quantify the relationships between two key climate variables—mean annual temperature and growing season water deficit—and rates of seedling recruitment and survival for the 50 most common tree species in the eastern United States. Our statistical models include the positive effects of conspecific adult abundance on recruitment and the potentially negative competitive effects of total canopy abundance on seedling survival. The tradeoff between these two effects creates a range from positive to negative conspecific density dependence, depending on the absolute and relative abundance of conspecific vs. heterospecific adults in a plot. Variation along the climate gradients mirrors patterns found previously in adult distributions. The clearest signal is in variation in the presence/absence of seedlings, while seedling density when present is only weakly related to local climate. The relatively narrow niche breadths for the presence of both seedlings and adults suggest that the frequency of occurrence of species within the landscape, rather than their relative abundance when present, will show the greatest response to climate change. Our analyses predict seedling survival as a function of mean annual temperature independent of the effects of competition and water deficit and thus provide an indication of the fundamental niche for seedling distribution along the temperature gradient. For more than half of the 50 species, their realized seedling niches are displaced to warmer climates. This reflects the prevalence of species in which survival declines with increasing temperature across a significant portion of at least their southern range. Our results show that when the effects of warmer climates are taken into account, seedling survival generally increases with increasing water deficit in the generally humid climates of the eastern United States. This result is consistent with recent global surveys of the relationship between net primary productivity and forest turnover rates, but contrasts with recent studies highlighting the potential impact of drought stress on tree mortality in more arid climates.