Non-Harvest Pinus radiata Plantations for Forest Restoration in New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
New Zealand has lost ca. 71% of its former indigenous forest cover and this has contributed to the profound impacts and declining trends that have been reported for the state of New Zealand’s indigenous biodiversity. Today ca. 7% of the land area is occupied by exotic plantation forest, 90% of which comprises Pinus radiata monocultures. Despite its exotic identity, these Pinus plantations are known to facilitate the regeneration of shadetolerant indigenous forest flora. However, above-ground successional development is lost in commercial plantations when clear-fell forest harvest occurs. Over the life time of a plantation forest, for a range of social, economic, or environmental reasons management objectives might change meaning that plantation owners no longer intend to harvest their forest. Here I introduce the concept that “non-harvest” P. radiata plantations provide one option for the long-term restoration of indigenous forest cover in New Zealand. This research addresses three research questions: (1) What are the long-term forest regeneration patterns in New Zealand’s non-harvest P. radiata plantations? (2) Which ecological processes act to constrain indigenous forest regeneration processes? (3) How can management intervene to address these constraints, and thereby promote indigenous forest regeneration in exotic non-harvest P. radiata plantations? To answer these questions, firstly, I surveyed a ca. ninety-year chronosequence of P. radiata plantations to examine patterns in plantation understorey regeneration and determine the main factors influencing forest regeneration. Secondly, I conducted an experimental canopy gap study within ca. 18-year-old P. radiata plantation forest to determine the potential for creation of small-scale artificial canopy gaps as a means of accelerating seedling growth rates of mature forest canopy species. Thirdly, I carried out experimental thinning of dense tree fern dominated P. radiata plantation understories to determine the potential of understorey thinning interventions as a means of accelerating seedling growth rates of mature forest canopy species. Fourthly, I surveyed a ca. 50-year-old podocarp underplanting trial to determine the potential for accelerating and directing forest succession by underplanting nonharvest Pinus stands with mature forest canopy species. The findings of this research illustrate that the long-term structure and composition of P. radiata plantation understory regeneration is strongly influenced by both stand age and proximity to indigenous forest seed sources. The reliable natural establishment of largefruited, bird-dispersed mature forest canopy species requires the sheltered conditions provided by mature stands, proximal seed sources, and seed dispersal vectors. Creation of small-scale canopy gaps provides a means of increasing light transmission to the plantation understorey, and thereby optimising light levels for the growth of indigenous mature forest canopy species. Plantation understories comprising dense tree fern stands were found to significantly reduce both transmitted light and seedling growth rates of mature forest canopy species. Underplanting a degraded and ecologically isolated Pinus plantation with indigenous conifers accelerated successional development and provided a means of achieving dominance by mature forest canopy species within only 50 years. This research shows that the sheltered conditions and long-term stability of “nonharvest” P. radiata plantations provide one means of restoring indigenous forest communities in New Zealand. This method of forest restoration is particularly relevant for lowland, commercially-productive, landscapes where indigenous forest loss has been greatest and where opportunities to restore indigenous forest are limited. The research also illustrates the important role of indigenous forest remnants as natural seed sources in the landscape, and thus the importance of maintaining indigenous remnants amongst commercially-productive landscapes. This research shows that while constraints on indigenous forest regeneration within P. radiata plantations do exist in the form of competition for light and limited indigenous seed dispersal, management interventions to address these limitations are possible and can successfully accelerate and direct indigenous forest regeneration in non-harvest P. radiata plantations. Thus, this research establishes that exotic “non-harvest” plantations can provide an important opportunity for the management and conservation of indigenous biodiversity in New Zealand.