Long-term demographics of dominant shrubs in semi-arid grasslands of the Chihuahuan desert region

Background/Question/Methods

Dynamics of semi-arid plant communities and the plant species that compose them are often viewed as relatively static – that is – slow to change over time. We explored the long-term demographics of dominant shrubs in semi-arid grasslands in the Chihuahuan desert ecoregion using data from long-term monitoring plots in Big Bend National Park, Texas. Plots, consisting of permanent photo-points and 0.3-m wide x 6.0-m long belt transects, were measured in 1955, 1961, 1967, 1981, 1996, and 2007. Additionally, 3-m radius shrub plots were measured in 1955, 1981, and 2007. All plants present on the plots were mapped to scale on each measurement date. Measurements track the long-term dynamics of these semi-arid grassland communities in response to two major disturbances: 1) intense grazing from approximately 1880 through 1946, prior to establishment of the National Park, and 2) a 7-year long intense drought that dominated regional weather patterns from 1951 through 1957. The data show that shrubs increased in cover and density over the 52 year study. However, because all individual plants were charted at each measurement date, we can identify the specific patterns of recruitment, growth, and death that drive these long-term dynamics.

Results/Conclusions

Shrubs proved to be highly dynamic. Some species were long lived whereas others were short lived with nearly complete replacement occurring over the 52-year study. For example, only 47 Larrea tridentata individuals were present on the 26 piedmont slope plots in 1955 for an average density of 0.062 #/m2 and an average cover of 1.5%. By 2007, density more than doubled to 0.142 #/m² and cover had increased to 16.7%. There was little mortality so that approximately half of the increase in cover resulted from growth of established individuals and half resulted from recruitment and growth of new individuals. Very similar patterns were observed for Flourensia cernua and Prosopis glandulosa. The density of flat-stemmed Opuntia species also doubled over this same period and cover increased from 2.5% to 12.8%. However Opuntia proved relatively short lived so that most individuals present in 1955 had died and been replaced with new individuals by 2007. Similarly, density of the small shrub Parthenium incanum was 0.069 #/m² in 1955 and increased 12-fold to 0.820 #/m² by 2007. Cover increased from 0.4% to 19%, nearly all of which resulted from recruitment and growth of new individuals.