Establishment and growth of piñon pine regeneration vary by nurse type along a soil substrate age gradient in northern Arizona

Future climate predictions suggest that recent patterns of increased piñon pine (Pinus edulis) mortality related to drought in the southwestern U.S. will continue. However, the importance of facilitation for young piñon pine growing at different edaphic sites is not well understood. We measured the density and stem elongation of juvenile piñon pine that established under nurse plants or in open areas in northern Arizona, USA that ranged in soil substrate age from 0.93 to 750. ky. We found: 1) 4-9 times greater juvenile piñon pine density under nurse plants compared to open areas; 2) 1-2 times greater densities at the two older substrate sites compared to the younger substrate site; 3) nurse plant effects on juvenile piñon density changed with substrate age, 4) 14-19% greater growth in nursed conditions than open areas across all substrate ages; 5) 32-35% greater growth at the two older substrate age sites; and 6) growth beneath dead piñon nurse plants were approximately 20% greater than under live piñon nurse plants, though differences were not statistically significant (P=0.07). As a result, we propose that projections of piñon pine distributions need to consider influences of edaphic conditions and the abundance and types of nurse plants on regeneration.