Antagonistic effects of large- and small-scale disturbances on exotic tree invasion in a native tussock grassland relict

Abstract

It is generally accepted that disturbances increase community invasibility. Yet the role of disturbance in plant invasions may be less predictable than often assumed, due to the influence of environmental stochasticity and interactions between disturbance regimes. We evaluated the single and interactive effects of prescribed burning (large-scale, infrequent event) and animal diggings (small-scale, frequent events) on the invasion success of Gleditsia triacanthos L. in a tussock grassland relict of the Inland Pampa, Argentina. Tree seedling emergence and survival were monitored over 4 years, after adjusting for propagule pressure through copious seed addition to all disturbance treatments. Burning altered community structure by suppressing tussock grasses and promoting exotic forbs, whereas simulated, armadillo-like diggings had little impact on herbaceous composition. Overall, seedling emergence rather than survival represented the main demographic bottleneck for tree invasion. Tree establishment success varied among seedling cohorts emerged in different climatic years. In a dry year, emergence was only slightly affected by disturbances. In contrast, for two consecutive wet years, initial burning and armadillo-like diggings exerted strong, antagonistic effects on tree recruitment. Whereas fire alone increased recruitment, the simulated burrowing regime prevented seedling emergence in both burned and unburned plots. The latter effect might be explained by reduced soil moisture, and increased seed burial and predation in excavated patches. Thus, the impact of a single, large-scale perturbation promoting woody plant invasion was overridden by a regime of small-scale, frequent disturbances. Our results show that grassland invasibility was contingent on inter-annual climatic variation as well as unexpected interactions between natural and anthropogenic disturbance agents.