Woody-plant encroachment has been occurring in US rangelands since early 1900s. The mix of herbaceous and woody plants in dryland plant communities has been shifting towards woody vegetation. Overgrazing has often been regarded as the primary causes, however, there is also evidence that changes in precipitation may also play a major role. Even though woody-plant encroachment has been studied extensively, little is known about how their seedlings establish in grass-dominated landscapes. To better understand this process, we focused on the seedling establishment phase and ask: How do precipitation, grass-shrub competition, and herbivory interact to influence the probability of woody-plant (Prosopis glandulosa) establishment in grassland? We hypothesized that unique combinations of grazing, seed and seedling predation, grass-seedling competition and water availability are required for successful shrub establishment. To test this hypothesis, we conducted a field-based germination experiment at Chihuahuan Desert Jornada LTER site. Using an Automatic Rainfall Manipulation System (ARMS) we established 30-2.5x5 m plots such that 10 received +80% of ambient precipitation (PPT), 10 received -80% of ambient PPT, and 10 received ambient PPT (controls). The vegetation (primarily the grass, Bouteloua eriopida) within half of each plots was clipped to 15cm height to simulate grazing and left intact the remainder of each plot to simulate no grazing. Scarified shrub seeds (n=5 per exclosure) added to clipped and unclipped plots were placed either in or outside of exclosures (n=16 per plot) designed to prevent access by ants, lagomorphs, or ant+lagomorphs, and quantify their germination and early establishment.
Our results show that Prosopis seedling emergence increased with PPT (F2,6237=83.6, P<0.0001), where PPT simulating extreme wet years had the highest density of seedlings/m2 (M=12.7, SE=0.6), while PPT simulating extreme drought had lowest densities (M=1.5, SE=0.6). For predation, Prosopis showed a significant difference (F3,6236=67.9, P<0.0001), where the highest density of seedlings/m2 occurred when only ants had access (M=19.5, SE=0.9), while lower density when only lagomorphs had access (M=3.5, SE=0.8). Shrub emergence/establishment on clipped (M=2.08, SE=0.4) and unclipped plots (M=1.01, SE=0.4) was significant different only on extreme drought conditions (t2078=-1.8, P<0.05). Our results suggest that, competition of grasses and shrubs lowers the seedlings density only when experiencing extreme drought. Different predators affect differently the seedling establishment; compared with ant’s seeds removal, lagomorph’s seedlings predation had a stronger negative effect on shrub seedling establishment. Results from this preliminary experiment helps inform us of the unique combination of drivers necessary for woody-plants to establish on grass-dominated rangelands.