Cover of woody and herbaceous functional groups in burned and unburned plots, Mojave Desert, 2009-2013
Description
Arid ecosystems are often vulnerable to transformation to invasive-dominated states following fire, but data on persistence of these states are sparse. The grass/fire cycle is a feedback process between invasive annual grasses and fire frequency that often leads to the formation of alternative vegetation states dominated by the invasive grasses. However, other components of fire regimes, such as burn severity, also have the potential to produce long-term vegetation transformations. Our goal was to evaluate the influence of both fire frequency and burn severity on the transformation of woody-dominated communities to communities dominated by invasive grasses in major elevation zones of the Mojave Desert of western North America.
We used a chronosequence design to collect data on herbaceous and woody cover at 229 unburned reference plots and 578 plots that burned between 1972 and 2010. We stratified the plots by elevation zone (low, mid, high), fire frequency (1 to 3 times), and years postfire (YPF; 1 - 5, 6 - 10, 11 - 20, and 21 - 40 YPF). Burn severity for each plot was estimated by the difference normalized burn ratio (dNBR). A Geographic Positioning System (GPS) was used to match the plots as closely as possible with the corner of a dNBR pixel. Each plot was 0.1 ha (32 m x 32 m) and contained three randomly positioned 25-m transects. Point intercept sampling was conducted at 0.5 m intervals along each transect (N = 50 points per transect, 150 points per plot). All plants intercepted by a wooden rod (1mm diameter) were recorded to species at each point. Species cover in each plot was estimated as the sum of point intercepts for each species divided by 150.