Wildfire and topography drive woody plant diversity in a Sky Island
mountain range in the Southwest USA
Abstract
Aim: Drastic changes in fire regimes are altering plant communities,
inspiring ecologists to better understand the relationship between fire
and plant species diversity. We examined the impact of a 2011 megafire
on woody plant species diversity in an arid mountain range in southern
Arizona, USA. We tested recent fire-diversity hypotheses by addressing
the impact of the fire severity, fire variability, historic fire
regimes, and topography on diversity. Location: Chiricahua National
Monument, Chiricahua Mountains, Arizona. USA., part of the Sky Islands
of the US-Mexico borderlands. Taxon: Woody plant species. Methods: We
sampled woody plant diversity in 138 plots before (2002-2003) and after
(2017-2018) the 2011 Horseshoe Two Megafire in three vegetation types
and across fire severity and topographic gradients. We calculated gamma,
beta, and alpha diversity and examined changes over time in burned vs.
unburned plots and the shapes of the relationships of diversity with
fire severity and topography. Results: Alpha species richness declined
and beta and gamma diversity increased in burned but not unburned plots.
Fire-induced enhancement of gamma diversity was confined to low fire
severity plots. Alpha diversity did not exhibit a clear continuous
relationship with fire severity. Beta diversity was enhanced by fire
severity variation among plots and increased with fire severity up to
very high diversity, where it declined slightly. Main Conclusions: The
results reject the intermediate disturbance hypothesis for alpha
diversity but weakly support it for gamma diversity. Spatial variation
in fire severity promoted variation among plant assemblages, supporting
the pyrodiversity hypothesis. Long-term drought probably amplified
fire-driven diversity changes. Despite the apparent benign impact of the
fire on diversity, the replacement of two large conifer species with
shrubs signals the potential loss of functional diversity, emphasizing
the importance of intervention to direct the transition to a novel
vegetation mosaic.