Wildfire disturbance reveals evidence of ecosystem resilience and precariousness in a forest-grassland mosaic

Forest and grassland ecosystems are sometimes located adjacently within
the same climate. In interior British Columbia, Canada, there are complex
forest-grassland mosaics within the Interior Douglas-fir biogeoclimatic
zone. Historically, both grassland and forest ecosystems experienced
high-frequency, low-severity fire regimes. Since European settlement and
introduction of livestock grazing and fire exclusion, trees have
encroached on grasslands, and tree densities in forests have increased. In
this study, we characterize plant communities and near-surface soil
moisture in forest and grassland sites, and in historical grassland sites
affected by tree encroachment. We hypothesized that spatial and temporal
patterns of near-surface soil moisture are reflected in above-ground plant
community composition and structure. After initial sampling of soil
moisture and plant communities, the study area was burned in a wildfire.
Applying a multifactorial approach to comparing adjacent grassland and
forest sites, we treated the wildfire event as a natural experiment,
sampling post-wildfire plant species composition and soil moisture, and
measuring the severity and spatial heterogeneity of surface burn
conditions. Evidence supports the concept of mutually exclusive
fire-reinforced bi-stable grassland and forest states, with greater
spatial heterogeneity of soil moisture and burn severity in forests, and
highly uniform patterns of soil moisture, vegetation, and burn severity in
grasslands. Areas of forest encroachment on grasslands had understory
plant communities dominated by exotic species, while restored grasslands
had native bunchgrass cover like typical grasslands of the region.
Additionally, there was post-wildfire divergence of forest- and grassland-
associated plant communities. Viewed through a resilience theory
conceptual framework, we suggest that ecosystem legacies are reinforcing
post-wildfire ecosystem identity and associated native plant communities.
External factors - particularly past heavy livestock grazing and fire
suppression - have caused ecosystem precariousness that can be addressed
with management actions.