Pine forests constitute a key component of the rich biodiversity found in the Madrean Sky Islands of northwestern Mexico and southwestern United States. Located at watershed headwaters, pine forests provide vital ecosystem goods and services to human communities in the arid lowlands. Pine habitats are inherently both unique and threatened due to isolation by distance between mountain ranges. Given this geographic isolation, the ability of Madrean pine forests to persist as climates and fire regimes change is unknown. We asked: How does spatial heterogeneity in recent burned areas function to facilitate persistence and migration of pines in the Madrean Sky Islands?
To answer our question, we mapped pine species distributions across 15 islands using field measurements of canopy volume and spatial predictors including Landsat NDVI, bioclimate and terrain. We then characterized fire regimes across species distributions using a remotely sensed database of contemporary (1985-2011) fires. We predicted potential seed dispersal within and across burned landscapes that varied in post-fire species canopy volume using 2-dimensional dispersal models parameterized for variable canopy shape and seed position. Finally, we developed spatial models to evaluate the potential for birds moving between patches of differing fire histories to disperse seed between island populations of pines.
Results/Conclusions
Low- to moderate-fire effects dominated burn mosaics, with areas of high severity at upper elevations and latitudes. Northern populations of P. arizonica and populations of P. discolor south of the international border were notably affected by higher severity fire. However, spatial configurations of post-fire openings resulted in relatively short distances to seed sources, and mean distance was low even where maximum distances were >300 m (Chiricahua and Pinaleno, mean < 50 m). The potential for seed dispersal was limited at maximum distances for all species. In mixed forests, presence of spherical (e.g., P. arizonica, P. discolor) and cone-shaped (P. strobiformis) canopies resulted in diversified seed abundances at short compared to longer distances with implications for composition and structure of regenerating forests.
Spatial heterogeneity of burned areas in the Sky Islands supports persistence of legacy trees and potential migration of pines into post-fire openings with some exceptions. Wide-ranging birds that favor open woodlands (e.g., Mexican jay, Aphelocoma wollweberi) and pine forest dependent species (e.g., thick-billed parrot, Rhynchopsitta pachyrhyncha) show promise as long-distance seed dispersers. Comparing theoretical dispersal kernels with observed seedling density will enable evaluation of predictions under specific climate-environments and fire return intervals that may challenge subsequent germination and growth.