The semi-desert grasslands of North America have been converted to shrub dominance within the period of the historical record. Dynamics of these now shrub-dominated former grasslands are often viewed as relatively static – that is – slow to change over time. However, long-term quantitative studies of these plant communities are relatively rare. Here, we examine long-term dynamics using data from 39 plots located in the Chihuahuan semi-desert grassland zone of Big Bend National Park, Texas. These plots were established in 1955, soon after the Park’s establishment, to monitor the recovery of the semi-desert grasslands from the combined effects of heavy grazing (from ~1880 to 1944) followed by a severe, multi-year drought in the early 1950s.
The 0.3-m wide x 6.0-m long belt transects were photographed and measured in 1955, 1961, 1967, 1981, 1996, 2007, and 2014. Additionally, 3-m radius shrub plots were measured in 1955, 1981, and 2007. All plants present on both the belt transects and shrub plots were mapped to scale on each measurement date. Although the plot measurements were infrequent, they now span a 60 year period over which regime shifts have occurred in both the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO).
Results/Conclusions
Temperature and precipitation showed distinct shifts that aligned with regime shifts in the AMO. The cool AMO was characterized by cooler and wetter conditions; nearly all the increase in precipitation during this period resulted from more large summer rainstorms. Because these conditions persist over decades, they have large influences on vegetation dynamics. During the cool-wet period between 1967 and 1981, cover of native perennial bunch grasses, subshrubs, and shrubs increased markedly. Between 1981 and 2007 we observed fluctuations in species cover, but high cover of all functional groups was maintained.
Coupling of the cool phase of the PDO with the warm phase of the AMO tends to cause severe drought – conditions that prevailed through the 1950s and again since the late 1990s. In 2011, the driest year on record, growing season precipitation was only 20% of average. In 2014, after this drought, we observed nearly complete mortality of dominant native perennial bunch grasses and their replacement by the exotic Lehmann’s lovegrass, along with substantial declines in cover of succulents, subshrubs, and some shrubs.
The AMO and PDO strongly influenced the dynamics of these semi-desert grasslands and need to be considered when projecting likely future effects of climate change.