Plant community patterns and population dynamics of coolibah woodlands

Abstract

Coolibah ('Eucalyptus coolabah' subsp. 'coolabah' Blakely & Jacobs) woodlands in the Darling Riverine Plains of New South Wales have been extensively cleared and modified since European occupation. Coolibah regenerated densely following floods in the 1970s and patches of dense regeneration are perceived to have negative effects on plant biodiversity although there is no documented evidence to support this notion. This thesis investigates aspects of the community and population dynamics of coolibah woodlands in order to investigate the potential negative effects of dense regeneration on plant biodiversity, and to assess the role of dense regeneration in the conservation of biodiversity and woodland persistence in the landscape. I found that patches of dense regeneration contained greater species richness and diversity of groundstorey plants compared to adjacent grasslands. When I compared alternative vegetation states - remnant woodlands, dense regeneration, derived grasslands and degraded derived grasslands - I found that groundstorey plant composition was not strongly associated with vegetation structure and that dense regeneration contained some woodland-associated groundstorey species that were uncommon in grassland states. Tree size distributions revealed that dense regeneration occurs where large trees are sparse, remnant woodlands appear to have regenerated episodically in the past and there is little ongoing recruitment within remnant woodlands. I investigated the ability of coolibah seedlings to establish in derived grasslands and found that tree seedling survival was affected more by seasonal conditions and herbivory than competition from grasses and that grasses actually facilitate seedling survival following germination. I propose that these results support a patch-dynamic model in which dense regeneration patches replace old-growth woodlands and contribute to the persistence of woodlands in the landscape. Further, these results suggest that dense regeneration occurs sporadically in response to rare climatic conditions, has no negative effects on plant diversity or composition and appears to be on a trajectory towards the remnant state.