Understanding long-term change in biological communities is a goal in ecology and is increasingly important for conserving unique communities and wilderness areas during a changing climate. The project is located in the Newberry Mountains of Southern Nevada which represent a unique elevation and ecotonal gradient. The establishment of long-term vegetation monitoring sites with reproducible methods can provide opportunities for insight into vegetation community change and the impacts of fire and other disturbance.
In comparing vegetation data collected in 1979 and repeated in 2008, we sought to measure if changes have occurred and if the degree of change differs with elevation, solar insolation, soil type, and disturbance. We expected most community change to occur in high-elevation plant communities. Due to fire and drought we expected reduction in coniferous woodlands and Blackbrush communities (Coleogyne ramosissima) and resultant incursion of species associated with lower elevations and the Sonoran Desert.
We resurveyed 103 transects from a vegetation community study established in 1979. Plots encompassed an elevation gradient of 120 m to 1,700 m from the hyper-arid Colorado River lowlands to Pinon-juniper woodlands. The original sampling methods were repeated in 2008 and included measures of density, frequency, and dominance within 600-m2 belt transects.
Results/Conclusions
Plots surveyed in 2008 exhibited a higher mean Simpson’s diversity index (SI) than plots surveyed in 1979 (1.82 vs. 1.69). These changes in species diversity were driven by greater diversity in 2008 within high-elevation plots. Plots at high elevation sites in 2008 had greater mean SI than high elevation sites in 1979 (p=0.0211). Species compositional changes also have occurred, such as density changes among long-lived shrubs in the absence of disturbance. Blackbrush (Coleogyne ramosissima) decreased from an average of 308 plants/ha in 1979 to 176 plants/ha in 2008. In contrast, Barrel Cactus (Ferocactus cylandraceus) increased from 39 plants/ha in 1979 to 90 plants/ha in 2008.
Our results reinforce the utility of long-term networks of monitoring plots for examining changes in ecological communities. The 29-year perspective was useful for detecting shifts in the distribution and abundance of plant species along environmental gradients during a time of climatic and land-use changes. Further analyzing the changes in individual species with specific climatic limitations will be useful in modeling potential directional shifts of the Mojave Desert ecotone as well as directional shifts of plant communities along elevation gradients. These data are useful for conservation purposes and developing management plans during a changing climate.
