Future woodland composition in a changing climate: Abiotic and biotic controls on tree regeneration in piñon-juniper woodlands

Background/Question/Methods

Globally, rising temperatures and associated increases in water deficits have increased tree mortality rates over the past several decades. These tree mortality events have dramatically changed forest and woodland structure, altering water and energy fluxes, carbon cycling, and ecosystem services. However, long-term changes in these wooded ecosystems will be dependent upon the ability of species to regenerate following these mortality events under a warmer, drier climate. Within the spatially extensive semi-arid piñon-juniper woodlands of the southwestern United States, widespread piñon mortality during the multi-year drought of 2002 and 2004 shifted the woodland structure and composition to a less dense, more savannah-type ecosystem that is dominated by juniper. In this study, we use long-term (10+ years), spatially-explicit data to examine piñon-juniper recruitment dynamics across an elevational gradient in San Francisco Peaks, Arizona. We ask the following questions: 1.) What are the effects of recent drought-induced overstory tree mortality on tree regeneration dynamics? and 2.) Are these effects moderated by local climatic differences, soil properties, and/or vegetation structure?

Results/Conclusions

There was a significant decline in both piñon and juniper juvenile densities across our study sites from 1999-2002 (pre-drought) to 2010-2014 (post-drought). Due to a combination of greater piñon juvenile mortality and a lack of new recruitment, piñon juvenile densities decreased more than juniper juvenile densities, resulting in a more juniper-dominated juvenile community. Post-drought (2010-2014) piñon juvenile densities were strongly positively associated with post-drought live adult piñon basal area and live adult juniper basal area. Over 80% of these juveniles were located beneath the canopy of a tree or shrub, highlighting the importance of these nurse plants in facilitating tree recruitment. Overall, the shift in the juvenile tree community across our study sites to a more juniper dominated stand suggests that these woodlands will become increasingly juniper dominated following drought-induced tree mortality events. Further, the reduction in both piñon and juniper juvenile densities 10 years post drought suggests that these woodlands may continue to be a less-dense, more savannah-type ecosystem.