Ecological Thresholds as Constraints to the Growth and Survival of Woody Tree Species in Degraded Grassland in the South Island's Dryland Zone
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The native dryland zone in New Zealand’s South Island has been drastically altered by burning, grazing, and other anthropogenic activities since human first arrived some 700 years ago. Only 30% of its original native vegetation remains, with <2% of it legally protected. Preserving what is left of the remaining natural ecosystems is urgent, and ecological restoration can be an important part of the solution to increase the area by reclaiming some of the degraded landscape within the dryland zone. However, reintroducing native plants as seedlings is mostly ineffective if disturbances have pushed ecological processes over certain thresholds that now represent barriers to ecological succession and restoration. These ecological thresholds can be the exposure of seedlings/saplings to direct sunlight and strong winds, water stress, soil compaction, herbivory, or competition between the native and exotic species for resources, among others. The objective of the research described in this thesis was to identify management interventions that might allow restoration to overcome key ecological thresholds preventing the establishment of native woody vegetation. The research was undertaken at five study sites in Northern Canterbury and the Mackenzie Basin. A combination of ground cover manipulation and shading trial, together with irrigation and grazing exclusion, were used to investigate the options to overcome these thresholds for the establishment and growth of native woody tree species. The results showed that the native seedlings had higher probability of survival and growth rates in the shaded treatments, likely due to increased soil moisture and soil aeration. Removal of exotic grasses, irrigation, and fencing also increased native seedling establishment; however, the best results were detected when these treatments were combined with shade. Therefore, ecological restoration of degraded dryland areas on former agricultural/pastoral lands can be achieved if the effects of direct solar radiation on soil aeration, soil moisture, and microclimate are reduced through the creation of shelter for planted native seedlings. Additionally, exotic plant species must be removed, or at least reduced in density, and herbivores excluded in order for restoration efforts to be more successful.