Clumped planting arrangements can improve the ecological function of revegetated eucalypt woodlands

Abstract

Revegetation is a key conservation activity in areas that have been extensively cleared and is undertaken in the hope it will prevent further species losses, mitigate land degradation and return functional ecosystems to degraded areas. Although revegetation has the potential to achieve these outcomes, the field is still relatively young and actively developing in terms of standards and best practice. As a result, the long-term viability, functionality and resilience of many re-planted systems remains uncertain. There have been calls for revegetation to move towards more ecologically informed designs and one way to achieve this is for plantings to mimic the composition and structure of natural vegetation. However, the outcomes of failing to undertake such practice is still poorly understood. The spatial arrangements of plants are central to natural communities and influence the majority of ecological processes that occur. Consequently, the position of plants within revegetated sites may affect the long-term viability and resilience of these restored systems. Despite this, planting arrangements are rarely considered an important feature of revegetated communities, especially for variables other than overall planting density and this may limit the ecological value of revegetated communities. The primary aim of this thesis was to examine how planting arrangements influence the ecological processes occurring within revegetated sites, with a focus on reproduction in woodland systems. I first review the available literature and synthesise information from natural ecosystems, plantation communities, and experimental plantings to identify ways plant arrangements may influence the ecological function of revegetated systems and highlight key knowledge gaps. The data chapters of my thesis then evaluate how planting arrangement influences pollination, seed production, plant mating patterns and patterns of gene flow in a revegetated eucalypt woodland in southern Australia. Following this, I document the arrangement of plants within remnant eucalypt woodlands and identify key features that can potentially be incorporated into revegetation design if projects seek to re-create more natural woodland plant arrangements. I found that plant arrangements have the potential to influence a range of ecological processes, from those at the individual plant level (survival, growth), the population and community level (pollination, seed dispersal) and the ecosystem level more generally (habitat provision, erosion). My experimental results support these expectations and although plant reproduction was highly variable, the spacing between conspecifics and the degree of aggregation influenced seed production and plant mating patterns in the Eucalyptus species studied, whereas population abundance had little influence. Taken together, these findings suggest that woodland revegetation should consider not only the number of each species to be planted, but also the fine-scale arrangement (conspecific spacing, aggregation) of those species, if reproductively productive populations are to be established. One way to achieve this is to re-create more natural plant arrangements, where aggregation is common and large distances between conspecifics are rare. The challenge is now to find ways to effectively incorporate spatially designed revegetation into the planning and planting phases of revegetation and then monitor the outcomes of this approach.