Abstract
Eastern Joshua tree (Yucca jaegeriana) plays a central role in the ecology of the Mojave Desert ecosystem. However, the emergence of invasive grass fire-cycles in the last several decades brings into question Joshua trees’ tolerance and resilience to changing fire regimes. This study’s objective was to examine the effects of wildfires on the structure and regeneration potential of Joshua trees forests. We examined the density, size class structure, and regeneration response of Joshua tree populations on a network of one kilometer transects along the boundaries of four independent wildfires and the interior of the largest fire that occurred 15 years ago (2005) in the northeast Mojave Desert. Burned edge and burned interior transects had 23- and 4.1-fold lower Joshua tree stand densities than unburned transects. The more robust recovery of stand density along burned interior transects compared to burned edge transects appears to be primarily driven by more prolific vegetative sprouting. Our data show that Joshua trees can sprout vegetatively following fire, but it is not a strong or consistent post-fire resprouter. Limiting the spread of invasive annual grasses and novel fire regimes will be critical to maintaining healthy Joshua tree populations into the future, particularly on the edge of its ecological range.
Highlights
Wildfire activity is changing across earth’s ecosystems due to human activity resulting in novel fire regimes that can have lasting effects on vegetation (Bowman et al, 2009; Crotteau et al, 2013)
We addressed the following questions: (1) What are the characteristics of Joshua tree stands 15 years after wildfire occurrence? (2) Is there evidence of post-fire Joshua tree regeneration, and does it vary spatially between burned edges along fire boundaries to locations deeper into the fire’s interior? We hypothesized greater Joshua tree regeneration along burned edges due to higher seed dispersal from adjacent unburned vegetation and lower burn severity
Joshua tree stand density was dramatically lower along burned edge (23-fold) and burned interior transects (4.1-fold) compared to unburned transects (P < 0.0001) (Figure 2)
Summary
Wildfire activity is changing across earth’s ecosystems due to human activity resulting in novel fire regimes that can have lasting effects on vegetation (Bowman et al, 2009; Crotteau et al, 2013). Wildfires are increasing in deserts of North America, primarily driven by invasive grass fire cycles (Brooks et al, 2004; Balch et al, 2013). Red brome (Bromus rubens L.), an invasive annual grass, has played a significant role in increasing fire activity in the Mojave Desert by increasing plant fuels’ continuity and flammability upon drying at the end of the growing season (Brooks and Matchett, 2006). Fire risk is especially high in years with early fall precipitation and warm winter temperatures that produces longer growing seasons that fuel red brome growth (Horn et al, 2015; Horn and St. Clair, 2017). There is concern that increasing fire activity in North American deserts may favor the establishment and spread of exotic alien species to the exclusion of the native plant communities (Klinger and Brooks, 2017)
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