Understory community dynamics ten years after a mixed-severity wildfire in ponderosa pine and aspen stands in the Black Hills of South Dakota, USA

Abstract

Wildfires are important disturbances due to their ability to influence many ecosystem processes and functions. Following a mixed-severity wildfire, understory vegetation composition and structure may undergo both long- and short-term changes because of modified growing conditions, removal of overstory competition and changes in the amounts of available resources. While more rapid, short-term changes are easily observed and documented, understanding long-term changes is of critical importance for management purposes and allowing professionals to gain insights into forest composition following a major disturbance. Late in the summer of 2000, 34,000 ha of ponderosa pine (Pinus ponderosa Lawson & C. Lawson) forests in the Black Hills of South Dakota burned in what is now called the Jasper Fire; the largest wildfire recorded in the state's history. The Jasper Fire was classified as a mixed-severity wildfire resulting in a mosaic of areas burned at low- (25%), moderate- (48%) and high-severity (27%). Following the fire, plant communities appeared to recover rapidly leading to questions regarding how long various postfire communities would persist, how postfire community development varied by fire severity, and differences observed between zones of the Jasper Fire. Ultimately, many were interested in long-term postfire community dynamics. In this study we examined the understory vegetation composition and structure (relative abundance of graminoids, forbs and shrubs) and frequency of invasive species relative to fire severity (unburned, low, moderate and high) and zone (northern, central and southern) in ponderosa pine and aspen (Populus tremuloides Michx.) stands 10 years after the Jasper Fire in the Black Hills of South Dakota, USA. In both ponderosa pine and aspen sites, understory community composition differed by zone and severity simultaneously. In ponderosa pine stands, canopy cover of four species varied by only zone or severity and eleven varied by zone and severity simultaneously. In aspen stands, canopy cover of two species varied only by zone or severity and canopy cover of three species varied by zone and severity simultaneously. Grass and shrub cover were explained by the interaction of zone and severity in ponderosa pine stands while cover of forbs varied by zone and severity but not their interaction. In aspen stands grass, forb and shrub cover all varied by zone and severity simultaneously. Grass and forb cover values 10 years postfire were similar to the 5 year postfire levels, and were greatest in moderate and high-severity burned areas. Shrub cover was also similar 5 and 10 years postfire, with lower values in burned areas driven by the loss of common juniper (Juniperus communis L.). Although common juniper cover was drastically decreased by fire, other shrubs are beginning to appear across the landscape. Total plant cover appeared to be lower 10 years postfire than compared to 5 years postfire which might be driven by a shift from annual and biennial plants to perennial plants. Frequencies of invasive species reached 60-70%, however, canopy cover of individual invasive species never exceeded 5% in either ponderosa pine or aspen stands. Ten years postfire, burned areas support understory plant communities dominated by native perennial plants with very few invasive exotic species. Post-fire rehabilitation efforts need to be designed on a site-specific basis and invasive species monitoring should continue to ensure that these plants do not become a concern in the future.