Reproductive consequences of fire disturbance on Joshua tree in the Mojave Desert

Background/Question/Methods

Climate warming coupled with the successful invasion of alien grasses into desert ecosystems is changing the occurrence and size of fire events in warm desert regions of the Southwestern United States. Fires in desert landscapes have historically been sporadic and smaller in size when compared to recent fire data in these regions. Poor adaptability to fire related stresses makes fire effects more extensive in desert areas. Fires generally burn landscapes heterogeneously, leaving unburned plant habitat islands. Successful plant community regeneration is often pollinator dependent and while pollinator communities generally demonstrate resilience to fire, the impacts of fire on the reproductive success of desert plant communities is poorly understood. Our objective was to elucidate the impacts fragmentation, specifically isolation and size, and how they impact the reproductive success of native plants in the Mojave Desert region of Southern Utah, USA.  We initially counted the number of inflorescences per branch and later returned and counted the number of fruits set per branch on various Joshua Trees (Yucca brevifolia) in adjacent burned and unburned areas.  

Results/Conclusions

Joshua tree survivors that were sampled in burned zones were not different than individuals sampled in unburned areas for the average inflorescences per branch counts, but varied significantly in the average fruit set per branch counts. Individuals in burned zones showed significantly lower fruit set. Commensurate inflorescence production within burned and unburned areas suggests that observed discrepancies in fruit set is not the result of resource limitations to theses plants, but that some other factor(s) may be responsible for low fruit set in burned areas. All species of the Yucca genus are pollination dependent on specific coevolved yucca moth pollinators (Tegeticula spp.). Our data suggests that highly specialized plant-pollinator relationships, like that of the Joshua tree system, are highly sensitive to fire fragmentation and isolation. Evaluating the effects of isolation on a pollinator generalist species like creosote (Larrea tridentata) which boasts >100 native bee pollinators, will provide valuable insight as to the sensitivity of highly specialized plant pollinator relationships versus generalist pollinator systems, enabling land managers to accurately assess the reproductive sensitivity of surviving vegetation in unburned habitat islands in response to changing desert fire regimes.