Restoration of canopy tree species in degraded eastern South Island ecosystems
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The history of ecosystem disturbances has altered most of the original forest ecosystems in New Zealand especially those associated with the eastern zone of the South Island where fire and pastoral farming have transformed much of the forestlands into degraded grasslands and shrublands. While these communities are viable conservation areas for biodiversity in restoration schemes, they require active human intervention to facilitate natural regeneration and the success of restoration planting project. Thus, in the context of managing early-successional communities for forest restoration, understanding the effects of any intervention will have important implications to the sustainability of any restoration measure as restoration outcomes may vary with target species, vegetation, and disturbance regimes involved.
This research addresses three key research questions: (1) Whether long-term protection alone (e.g. grazing exclusion) is sufficient to facilitate the establishment of canopy trees and other native woody species in regenerating communities; (2) Whether canopy disturbance regime can be a viable intervention to accelerate forest succession and/or establishment of canopy species in degraded early-successional communities; and (3) Whether early-successional communities can be managed as nurse vegetation for canopy species restoration in degraded and stressful environment. To answer these questions, I examined the succession in an intact canopy of regenerating kānuka (Kunzea robusta, Myrtaceae) forest in abandoned pastureland by utilizing long-term data from permanent vegetation plots to describe the changes in species composition, density, and growth of native woody species over time. Two separate restoration trials were then established within the regenerating kānuka forest to examine respectively the succession of native woody species in response to artificial canopy gaps and the establishment of canopy species in response to different canopy manipulation methods. Nurse-protégé or plant-to-plant interaction between canopy species and early-successional shrubland communities with different canopy interventions were also assessed in a degraded high-country environment to determine whether regenerating shrub communities can be managed to serve as nurse plants to restoration planting in post-disturbance grasslands.
The findings of this research demonstrate that canopy intervention, particularly small-scale canopy gaps, is a viable management option to facilitate the regeneration and establishment of canopy species within degraded and dense early-successional communities. Small-scale canopy gaps can provide favourable growing conditions for canopy species including the mitigation of the intensity of competition, improvement of species tolerance to the impacts of herbivory by ungulates, and the provision of opportunities for co-existence with other regenerating native woody species with different shade tolerance traits. The findings, however, show that while succession can benefit from canopy intervention, regeneration can be slow even in gap environments especially in degraded seral communities with inadequate seed sources or with seed dispersal limitation. Moreover, results from the nurse-protégé interactions between canopy species and shrub communities in post-disturbance grasslands indicated that dense early-successional communities can be managed to serve as a nurse vegetation to facilitate the initial growth and survival of canopy species in stressful environment.
These restoration benefits from canopy interventions are relevant to the management goals in New Zealand Canterbury region that include conservation of native biodiversity and restoration of representative habitats and ecosystems that have been lost or severely degraded. While the focus of this research was the eastern South Island, the findings are likely to be relevant as well to other areas in New Zealand and in many parts of the world as the dominance of early-successional communities such as grassland and shrubland in stressful or degraded conditions are typical over large areas especially in tropical and subtropical environments.