Resilience and alternative stable states after desert wildfires

Improving models of community change is a fundamental goal in ecology and
has renewed importance during global change and increasing human
disturbance of the biosphere.  Using the Mojave Desert
(southwestern USA) as a model system, invaded by non-native plants and
subject to wildfire disturbances, we examined models of resilience,
alternative stable states, and convergent-divergent trajectories for 36
years of plant community change after 31 wildfires in communities
dominated by the native shrubs Larrea tridentata or Coleogyne
ramosissima.  Perennial species richness on average was fully
resilient within 23 years after disturbance in both community
types.  Perennial cover was fully resilient within 25 years in
the Larrea community, but recovery was projected to require 52 years in
the Coleogyne community.  Species composition shifts were
persistent, and in the Coleogyne community, the projected compositional
recovery time of 550 years and increasing resembled a deflected trajectory
toward potential alternative states.  Disturbed sites contained a
perennial species composition of predominately short-statured forbs,
subshrubs, and grasses, contrasting with the larger-statured shrub and
tree structure of undisturbed sites.  Auxiliary datasets
characterizing species recruitment, annual plants including non-native
grasses, biocrust communities, and soils showed persistent differences
between disturbed and undisturbed sites consistent with positive feedbacks
potentially contributing to alternative stable states. 
Resprouting produced limited resilience for the large shrubs Larrea
tridentata and Yucca spp. important to population persistence, but did not
forestall long-term reduced abundance of the species.  The
non-native annual grass Bromus rubens increased on disturbed sites over
time, suggesting persistently abundant non-native plant fuels and reburn
potential.  Biocrust cover on disturbed sites was half and
species richness a third of amounts on undisturbed sites.  Soil
nitrogen was 30% greater on disturbed sites and no significant trend was
evident for it to decline on even the oldest burns.  Disturbed
desert plant communities simultaneously supported all three models of
resilience, alternative stable states, and convergent-divergent
trajectories among community measures (e.g., species richness,
composition), timeframes since disturbance, and spatial
resolutions.  Accommodating expression within ecosystems of
multiple models, including those opposing each other, may help broaden
theoretical models of ecosystem change.