Seedling drought responses governed by root traits, site-soil moisture regimes and overstory competition-facilitation

Abstract

Periodic drought may interact with spatially patterned soil moisture regimes, canopy disturbance and species seedling traits to influence tree regeneration processes in northern temperate forests. In northern Michigan, USA, we assessed the effects of regional upland soil moisture regimes (six sites classified from dry to mesic) and tree canopy shading (% light) on planted-seedling dry mass, probability of mortality and functional traits for eight tree species varying in shade tolerance and natural distribution over soil moisture gradients (i.e., drought tolerance): Quercus velutina, Quercus alba, Quercus rubra, Prunus serotina, Acer rubrum, Acer saccharum, Fraxinus americana, and Betula alleghaniensis. The assessment period included droughts during the seedlings’ 2nd and 3rd growing seasons. High mortality (especially on drier-classified sites and in more open, high-light environments) and negative associations of soil water minima with mortality collectively suggest drought-driven mortality across sites. In contrast, canopy shade driven mortality occurred only on mesic sites at <10 % light. Overstory canopies facilitated seedling survival relative to more open environments, with survival maxima ranging from 3 to 20 % light among sites. Among species, probability of survival was 1) highest for Quercus species (∼0.70) and lowest for B. alleghaniensis (0.10), 2) positively and generally associated with drought tolerance, but not associated with shade tolerance, and 3) positively and strongly associated with species seed-size related differences in total dry mass, root dry mass and rooting depth. In turn, rooting depth (range 3–68 cm) was positively related to seedling water status, and water status to survival during drought. In summary, site soil moisture regimes, facilitation vs. competition via variation in canopy density, and size-related species traits drove patterns of young seedling mortality during an interval that included droughts. Results suggests current drought regimes likely have strong influence on canopy composition across the region’s spatially patterned dry to mesic soil gradients, and may have larger impacts if drought intensifies with climate change. Our results can aid development of planting strategies for assisted migration and other emerging needs, including planting 1) species more drought adapted than those dominating current overstories, 2) large stock with well-developed root systems, and 3) in microsites that maximize canopy facilitation.