Location, location, location: The influence of plant neighborhood configuration on grass-shrub interactions
State transition from perennial grassland to shrubland or woodland is often synonymous with land degradation and desertification in arid and semiarid ecosystems. These physiognomic transitions markedly alter rates and dynamics of ecological processes and the ability of ecosystems to provide services of value to society. Land managers and other stakeholders could benefit from research designed to identify vegetative structural properties that forecast when ecological sites might be at risk for state changes. We sought to determine such indicators via removal experiments conducted along an extant Bouteloua eriopoda-to-Prosopis glandulosa encroachment gradient at the Jornada Basin LTER site in the northern Chihuahuan Desert. Ninety plots centered upon B. eriopoda patches were established along the grassland-to-shrubland gradient. The size of and distance to all P. glandulosa individuals within a 5 m radius was recorded and aboveground shrub biomass was estimated using site- and species-specific allometric regression (R2 = 0.89). Peak season B. eriopoda biomass was quantified in 2010. P. glandulosa shrubs within 5 m of half of grass plots (n=45) were then killed via foliar herbicide in early summer 2011. B. eriopoda biomass was again quantified at the end of the subsequent growing seasons.
B. eriopoda cover ranged from 26.3% to 3.6% along the grassland-to-shrubland gradient, and declined exponentially as P. glandulosa cover and biomass increased linearly from 2.2% and <200 g to 17.9% and > 400 g m-2. In 2013, mean (SE) B. eriopoda productivity in plots with P. glandulosa neighbors removed was 94.8 (6.1) g m-2 y-1, compared to 52.9 (6.4) in plots with shrub neighbors intact (p < 0.0001). Grass response to shrub removal was significant in the grassland and shrubland portions of the gradient, more so in the latter [shrub removal vs. control = 111.8 (21.1) vs. 41.6 (8.2); p < 0.0001], than in the former [84.5 (6.6) vs. 60.8 (12.4) g m-2 y-1; p = 0.001]. However, B. eriopoda productivity did not differ between the control and shrub removal treatments in the ecotone (shrub biomass 200-400 g). This was due to high variability in grass production in the control plots (range 20.9 g – 212.2 g). Variability in grass ANPP may therefore be a more reliable indicator of impending state-transitions than mean ANPP. Forthcoming analyses will examine how the structure (e.g. size, number, density) of the shrub neighborhood influences grass productivity.