Drivers of understory plant communities in Sierra Nevada mixed conifer forests with pyrodiversity

Background Fire suppression in western North America increased and homogenized overstory cover in conifer forests, which likely affected understory plant communities. We sought to characterize understory plant communities and their drivers using plot-based observations from two contemporary reference sites in the Sierra Nevada, USA. These sites had long-established natural fire programs, which have resulted in restored natural fire regimes. In this study, we investigated how pyrodiversity—the diversity of fire size, severity, season, and frequency—and other environment factors influenced species composition and cover of forest understory plant communities. Results Understory plant communities were influenced by a combination of environmental, plot-scale recent fire history, and plot-neighborhood pyrodiversity within 50 m. Canopy cover was inversely proportional to understory plant cover, Simpson’s diversity, and evenness. Species richness was strongly influenced by the interaction of plot-based fire experience and plot-neighborhood pyrodiversity within 50 m. Conclusions Pyrodiversity appears to contribute both directly and indirectly to diverse understory plant communities in Sierra Nevada mixed conifer forests. The indirect influence is mediated through variability in tree canopy cover, which is partially related to variation in fire severity, while direct influence is an interaction between local and neighborhood fire activity.

The effect of forest land allocation on understory plant species associations

Species association is a reciprocal relationship among species within a community and can estimate the community composition. This research aimed to obtain information regarding the level of dominance and plant species association in each forest land allocation. Data analysis tested in this study constituted an analysis of the importance value index and association analysis performed on the main constituent species with INP > 10% using a 2x2 Contingency Table. The research revealed that species association in understorey plants in plantation forests contained one pair with real associations, five with unreal associations, six with positive associations, and none with negative associations. Meanwhile, there were no real associations in protected forest areas, ten pairs with unreal associations, four pairs with positive associations, and six pairs with negative ones. There were two pairs with real associations in natural forests outside the company’s concessions, 13 species with unreal associations, four pairs with positive associations, and 11 pairs with negative associations.