Doctor of Philosophy
School of Biological Sciences
Mott, Beth, Valuing native plantation monocultures in Australia: the relationship between faunal richness and environment in Eucalypt plantations, Doctor of Philosophy thesis, School of Biological Sciences, University of Wollongong, 2013. http://ro.uow.edu.au/theses/4240
Native forests throughout the world are disappearing at rate of approximately 13 million hectares a year. One of the ways forest managers are attempting to halt deforestation is by growing trees in plantations to provide wood products. Industrial tree plantations will occupy 10-20% of the total forested land area in the world by 2024. Research into the suitability of plantations for wildlife has grown rapidly in the last fifteen years, but it has focused primarily on exotic plantations. Comparatively little work has focused on native tree plantations, which may better benefit biodiversity in forested landscapes if they act like forests. A literature review of existing work identified that age, size, level of management and the type of forest adjacent to plantations can all affect the diversity of faunal assemblages using native plantations, and that monocultures are generally species poor. This thesis extends the small existing body of work on managed native plantations by comparing faunal use of young and old native plantations with logged and old growth native forests in the North Coast Bioregion of sub-tropical south-eastern Australia. This thesis used trapping to identify whether birds, mammals, and herpetofauna used plantations differently from forests, and ecophysiology, landscape and community ecology theories to explain why they do so.
Plantations collectively, differed from forests in vegetation structure and available microclimate at the ground level. The open canopy and lack of sub canopy complexity in young and old plantations resulted in higher insolation at the ground in plantations, which increased environmental temperature up to 200C above that of old growth native forests in summer during the day. Young plantations particularly presented a challenging environment to potential colonizers sensitive to microclimate. Large daily fluctuations in temperature and irradiance, coupled with the residual effects of management-associated soil compaction may have explained the loss of forest organisms and the persistence of simple understorey in old plantations, despite 30 years of growth and more than six years since disturbance by fire or harvesting.
Early maturity (spar) plantations differed from young and old plantations in having similarly simple understorey structure, but closed canopies, producing a cool thermal environment with low light penetration. Although spar plantations are a short-lived stage in the plantation cycle before the canopy is opened by subsequent harvesting, they could represent a valuable habitat resource for forest fauna that prefer cool thermal environments but are tolerant of simple structure. From a management perspective if this stage of plantation is promoted within the landscape mosaic at forest-plantation boundaries, it may provide forest species with additional habitat, and buffer forest from the microclimatic extremes experienced in young and old plantations.
Despite large thermal and structural differences herpetofauna, birds and mammals were equally speciose in old growth and logged native forests and young and old plantations. Assemblage structure differed significantly, and plantations were dominated by generalists and woodland species. Small ground mammals were less abundant in young plantations. Thus while afforested plantations provided habitat where habitat was previously absent, in both young and old plantations the ecological resilience of communities may have been compromised by lower species richness. From a landscape perspective plantations offered habitat to declining woodland birds, and increased the functional range of woodland species in agricultural environments.
Using small lizards as a model and conducting feeding and behavioral choice experiments, this thesis demonstrated that the thermal environment in young plantations was physiologically limiting to forest species. While temperatures optimal for digestive efficiency could be achieved, they required the species using this forest type to exhibit flexibility in both activity time and substrate choice. The forest species that did not possess this flexibility could not use young plantations. Those species that had the physiological capacity to exploit plantations were behaviourally constrained, and would only be likely to use young plantations if targeted understorey enrichment were provided.
The low abundance of insectivorous ground fauna identified from young plantations in this research was not driven by a lack of invertebrate prey, which was equally available in young and old plantations and native forests. Low mammal abundance was likely to have been related to the simplicity of the plantation environment rather than a decline in forest matrix quality associated with cattle grazing. However for herpetofauna, grazing may have increased the permeability of the forest matrix, with a resultant influx of thermophilic species into plantations.
Collectively the results of this research stress that: 1) young and old native plantations can support faunal assemblages as diverse as those in native forests, but for small mammals in particular, abundance in plantations is low; 2) cooler spar plantations have the potential to act as thermal stepping stone which may promote incursions of forest species into plantation mosaics; 3) young and old plantations can support an invertebrate prey base as diverse as that in adjacent native forests, with positive benefits on the species richness of insectivores; and 4) the role of grazing in the forest matrix requires further investigation in order to clarify its role in influencing species exchange between plantations and forests.