Transitions and invasion along a grazing gradient in experimental California grasslands

Abstract

Resilience-based frameworks, founded upon the existence of multiple attractors and regime shifts, have long been applied to complex dynamics of semiarid systems. Utilizing seed addition tests in experimental plantings along grazing gradients, we applied an increase-when-rare criterion to identify bidirectional (states can invade each other) and directional (only one state can invade) transitions among vegetation states characteristic of California grasslands over five years. Annual forage and medusahead grasslands were able to invade each other at all grazing intensities, indicating coexistence. Directional transitions involving invasion of native bunchgrass by other species occurred as grazing intensity increased; recovery (transitions to natives) did not occur at low grazing. While directional transitions at some grazing intensities were accompanied by state persistence at others, we found little evidence for persistence of alternative states at any grazing intensity. Our results suggest that grazing can affect resilience by causing hard-to-reverse transitions, but rarely produces alternative states. However, variation in precipitation seems to dominate grazing responses, supporting the applicability of the nonequilibrium concept in our study system.

Keywords: Elymus caput-medusae; annual exotic grasses; coexistence; livestock grazing; native perennial grasses; nonequilibrium dynamics; persistence; regime shifts; state and transition model.