Year

2016

Degree Name

Doctor of Philosophy

Department

School of Biological Sciences

Abstract

The contraction and aging of stands of overstory tree species in rangelands is well documented worldwide and largely reflects anthropogenic pressures such as agricultural land clearing and increased grazing leading to increased mortality and reduced recruitment. Without recruitment, stands that largely comprise old and senescent plants may soon go locally extinct. Complicating assessments of plant population health however, is the diversity of dynamics in populations of plant species; what constitutes reproductive failure and population contraction in one species can simply represent a natural cycle in another. In far western New South Wales (NSW), several Acacia species subject to intense grazing by domestic and feral herbivores display prolonged recruitment failure. Surveys over the past two decades have also failed to detect fruit set suggesting they are trending to extinction. Hypotheses explaining the failure of these fragmented populations to reproduce sexually have included restrictions to mating systems, insufficient genetic diversity, prolonged drought period, and the widely supported claim that extant plants are senescent. In stark contrast, some shorter lived co-occurring Acacia species such as A. ligulata and A. victoriae are thriving and reproducing regularly under the same conditions. It is not understood why this difference exists. Reproductive effort has not yet been monitored outside a prolonged period of drought, demographic surveys to date have only been qualitative, and little is known about their mating systems. Without this information it is impossible to know which of the competing hypotheses explain their decline, or to recommend conservation strategies for the future. Here I use a multidisciplinary and comparative approach combining surveys, genetic analysis and manual pollination and growth experiments to gain this information.

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