Landscape‐level vegetation recovery from herbivory: progress after four decades of invasive red deer control

Abstract

Summary 1.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. 2. 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. 3. 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. 4. 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. 5. 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, densitydependent diet switching by deer, altered successional trajectories and below-ground processes. 6. 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.

Effects of grazing exclusion on soil carbon and nitrogen storage in semi-arid grassland in Inner Mongolia, China

“…The reduced perturbation had a direct effect on the aboveground biomass, demonstrating that grazing exclusion is an effective practice to improve vegetation productivity in degraded grassland (Tanentzap et al, 2009;Zhou et al, 2011). The substantial increase of aboveground biomass in our study sites was mainly due to the significant recovery of gramineous and sedge species after grazing exclusion, which is in agreement with other studies (Wu et al, 2009).…”

Decreasing deer browsing pressure influenced understory vegetation dynamics over 30 years

“…In addition, several species associated with high browsing intensity are known for their toxicity (Atropa bella-donna L. and Agrimonia eupatoria L.); this unpalatability provides them with a competitive advantage within the plant community (Augustine and McNaughton 1998). The negative correlation between browsing pressure and both tree and high shrub cover suggests that deer browsing contributes to open and to maintain open forest ecosystems by limiting the recruitment of shrubs over the browse line (Tanentzap et al 2009), thereby impacting the composition of ground flora. Indeed, species associated with high deer browsing pressure are also mainly light-demanding species.…”

“…However, our results reveal a lag in plant community response to a reduction in deer browsing pressure. Recently published studies of forest ecosystem monitoring following the reduction of (over-)abundant deer populations generally highlight the slowness of the recovery process (Tanentzap et al 2009;Royo et al 2010;Wright et al 2012). In their review, Tanentzap et al (2012) explain this slow recovery of forest understory through (i) the long-term consequences of plant biomass consumption that reduces individual plant species' ability to react to reduced deer densities, (ii) consumption of reproductive structures and propagules that limit plants' ability to reproduce and colonize and (iii) the formation of recalcitrant understories that prevent from the recolonization of other species, particularly palatable species.…”

Achieving landscape‐scale deer management for biodiversity conservation: The need to consider sources and sinks

“…However, as impacts are context-dependent and nonlinearly related to densities (Koda and Fujita 2011, Putman et al 2011, their assessment does not translate readily into cull targets. When deer numbers are unknown, reducing impacts may require progressive increases in the annual numbers killed and is further hindered by slow ecosystem recovery (e.g., Tanentzap et al 2009Tanentzap et al , 2011. Crucially, when enhanced culls have reduced numbers from an unknown high, to an unknown lesser level, the subsequent control level needed to constrain impacts within desired bounds is unknown, unless population assessment is undertaken.…”