There has been increasing concern that positive feedbacks between fire and invasive plants have become more common in the Mojave Desert, and that ecosystem transformations and alterations to fire regimes are now more frequent and widespread than in the past several decades. For the last ten years we have been conducting an ecoregion-scale study on fire and vegetation dynamics in the Mojave to: (1) determine the degree to which the annual area burned, number of fires, and burn severity has changed since 1972; (2) evaluate the influences of burn frequency and severity on vegetation and seed banks across a 40-year post-fire chronosequence; (3) analyze and model the individual and joint occurrence of four invasive plants known to alter fire regimes and/or be a significant component of post-fire communities; (4) relate contemporary patterns of burning to climate; and, (5) model intra-annual fine fuel loads and longer-term probabilities of ignition and burn severity. To do this we combined data from three sources: (1) historical photos and written accounts from 1930 to 1970; (2) remote sensing data on lightning, NDVI, and all fires from 1972 to 2012 that were > 400 ha; (3) climate data from 1950 – 2015; and, (4) 229 unburned reference and 578 burned vegetation plots (0.1 ha) sampled between 2009 and 2013 that were stratified by elevation zone, years post-burn (YPB; 3-40 years), and fire frequency.
Results/Conclusions
Sites that burned once but at high fire severity or burned multiple times were often dominated by non-native annuals, especially in the low elevation zone. However, structure in most burned sites was similar to that in unburned sites within several decades. Persistent changes in composition occurred after burning, but this was the case even in sites where invasive plants were uncommon. Moreover, many assemblages appeared to be transient and alternative states were often in close proximity with one another. The annual area burned and number of fires increased over the 40-year period, but the pattern was pulsed and related to precipitation. A large proportion of the ecoregion was predicted as highly suitable for one or more of the non-native species and areas with the highest predicted suitability values were areas with the highest fire frequency. Our analyses indicate burned sites in the Mojave are not truly “novel” but are, at most, “hybrid” systems. However, our ignition probability, burn severity, and non-native species models suggest substantial future potential for transitions to novel conditions.