Landscape-level vegetation recovery from herbivory: progress after four decades of invasive red deer control.
Abstract
Ungulates have reached such high densities in some natural ecosystems that culling is frequently used to reduce their impacts on vegetation. However, much is still unknown about the outcomes of landscape-level control, in part because monitoring vegetation recovery requires decades. We report long-term vegetation changes in permanent plots located in forest, shrubland and grassland communities across a mountain range in southern New Zealand. We test whether c. 92% reduction in the population of invasive non-indigenous red deer Cervus elaphus since 1964 has led to the recovery of deer-preferred species. Tree seedlings, saplings and the number of seedlings per adult tree increased over time. There was lower recruitment, however, of highly palatable forest species compared with less palatable species, and the recruitment of saplings was lower in browsed forest plots compared with deer exclosures. The total number of occurrences and absolute number of palatable species per plot increased over time in shrublands and grasslands respectively. The height of both shrublands and palatable grassland snow tussocks Chionochloa spp. increased, although the occurrences of most individual species remained unchanged over time. Vegetation recovery at our site in response to long-term and significant herbivore reductions may be limited by several factors, including the slow growth rates of New Zealand species, density-dependent diet switching by deer, altered successional trajectories and below-ground processes. Synthesis and applications. Our results suggest that after nearly four decades, even low densities of introduced herbivores may restrict ecosystem recovery, and therefore, restoring herbivore-disturbed ecosystems by solely manipulating herbivore population numbers may require a long-term perspective. Management strategies can accelerate recovery by protecting existing palatable plants within deer-exclosures, and planting or seeding palatable species within these refugia. However, in addition to increasing seed sources, restoration may only become apparent following large-scale disturbance events and canopy turnover.