Abstract
The impacts of fire on creosote bush scrub vegetation have received attention recently as fire has become locally common throughout the Mojave and Sonoran deserts. One area of particular concern is western Coachella Valley, which forms the northwestern extent of the Colorado Desert. This is a major wildland-urban interface area that has been significantly impacted by atmospheric nitrogen deposition concomitant with fuel alterations from invasive annual grasses and increased ignition frequencies from human activities. Creosote bush scrub takes much longer than more mesic vegetation types to re-establish after fire, and the majority of desert species lack traits associated with resiliency to fire disturbance. Previous research in this area has only investigated once-burned stands for up to five years since fire. This study documents perennial vegetation from seven sites that represent a 2- to 28-year-old fire chronosequence. Our surveys revealed that fire significantly reduced shrub richness and diversity regardless of time since fire. Total shrub cover and density returned to or exceeded unburned levels at least 20 years after fire, although species composition was almost entirely brittlebush (Encelia farinosa), a shortlived shrub. Longer-lived shrubs indicative of unburned vegetation, such as creosote (Larrea tridentata), white bursage (Ambrosia dumosa), and white ratany (Krameria grayi), failed to recover. Shrub plus cacti richness and diversity were lower in burned stands regardless of time since fire. Encelia shrublands may form an alternate stable state following fire in this region.
Highlights
Fires have been relatively rare in creosote bush scrub of the Colorado and Mojave deserts, but the frequency and extent have increased significantly during the last 40 years as alien annual grasses and human ignition sources have become increasingly more common (Brooks and Esque 2002, Brooks and Matchett 2006)
Since most native shrub components of creosote bush scrub lack traits associated with fire resiliency (Brooks and Minnich 2006), fires can result in long-lasting alterations to vegetation
The study area was located in western Coachella Valley, situated on the far western edge of the Colorado Desert, a subdivision of the Sonoran Desert, at its nexus with the San Jacinto Mountains, San Bernardino Mountains, and the inland valleys of cismontane southern California, USA (Figure 1)
Summary
Rubens [L.] Durin), Mediterranean grasses (Schismus spp.), and redstem filaree (Erodium cicutarium [L.] L’Hér ex Aiton), may either be aggressive fire followers in the immediate post-fire growing season or will initially suffer set-backs, but can regain or exceed pre-fire abundance within a few years after fire (Brown and Minnich 1986, Minnich and Dezzani 1998, Brooks 2002, Brooks and Matchett 2003, Keeley et al 2005) Since these invasive annual species are competitive with the dominant shrub species in this vegetation type (Melgoza et al 1990, Holzapfel and Mahall 1999, DeFalco et al 2007, Steers and Allen 2010), the combination of fire and the particular suite of invasive plants is a threat to desert ecosystems.
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