Identifying thresholds in pattern-process relationships: a new cross-scale interactions experiment at the Jornada Basin LTER

Interactions among ecological patterns and processes at multiple scales play a significant role in threshold behaviors in arid systems. Black grama grasslands and mesquite shrublands are hypothesized to operate under unique sets of feedbacks: grasslands are maintained by fine-scale biotic feedbacks promoting the retention of plant-available water that promotes grass establishment and persistence, while shrublands are maintained by broader-scale abiotic feedbacks promoting an increase in bare ground connectivity and redistribution of material by wind that promote shrub expansion. We recently established a large-scale, long-term manipulative experiment to test these hypotheses in order to identify the threshold distribution of bare gaps beyond which plant-scale processes are overwhelmed by abiotic drivers to shift grassland system into shrublands. In a 2 x 2 factorial design at 15 experimental blocks arrayed along an existing gradient of bare ground connectivity, we manipulated a plant-scale biotic process and a patch-scale abiotic process to measure the relative importance of these controls on primary production, soil water dynamics, and other ecological responses. All mesquite shrubs were killed to manipulate plant-scale competition for resources. We installed small fence-like structures (connectivity modifiers, or ConMods) to manipulate bare ground connectivity of surface soil and plant material by wind at a patch scale. This experiment will improve understanding of the role of cross-scale interactions that lead to threshold dynamics across mixed lifeform ecotones.