Woody plant diversity before and after the Horseshoe Two Fire in the Chiricahua Mountains, Arizona, USA

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 90,000-ha wildfire on woody plant species diversity
in an arid mountain range in southern Arizona, USA. We tested recent
fire-diversity hypotheses by addressing the impacts of fire
severity, fire variability, historical 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 Fire in three vegetation types and across fire severity and
topographic gradients. We calculated gamma, alpha, and beta 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 severity, 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 a suite of drought-tolerant
shrubs signals the potential loss of functional diversity, a pattern that
may warrant restoration efforts to retain these important compositional
elements.