Year

2021

Degree Name

Doctor of Philosophy

Department

School of Earth, Atmospheric and Life Sciences

Abstract

Intensified disturbance regimes, associated with climate change, will place selective pressure on forests via the survival of individual trees. Elevated atmospheric [CO2] (eCO2) will complicate responses, potentially enhancing growth and post-disturbance recovery under future climates. My thesis aimed to build an understanding of the drivers of mortality and regeneration in forests, such as drought, fire and eCO2, combining descriptive field studies, a glasshouse experiment and a remote sensing study. I focused on the disturbance-prone Eucalyptus forests of southern Australia, comparing responses between forest types, e.g. wet versus dry. I asked whether increases in drought and fire increased mortality of both mature trees and juveniles and assessed the potential for population declines. I also asked whether drought modified any eCO2 effects on biomass growth, including storage organs used in post-fire resprouting, and if remotely- sensed vegetation greenness was increasing across forests, potentially due to rising [CO2], once disturbance effects were accounted for. I found that mature trees were not sensitive to increases in drought or fire, although presence of previous fire damage and thin, non-compact bark increased the risk of mortality. Juvenile mortality was elevated when severe drought preceded fire. Drought negated eCO2 benefits to plant growth, although some drought-tolerant species developed enhanced coping strategies. Greening trends were divergent among forested bioregions. Thus, juveniles and populations of mature trees dominated by species with less effective resistance traits may be at elevated risk of declines in future. Drought is likely to offset eCO2 benefits to seedling growth and post-disturbance recovery. However, in some more resilient ecosystems, eCO2 benefits may outweigh the negative effects of disturbance over the long-term, resulting in widespread vegetation greening. eCO2 effects may vary across landscapes due to differences in climatic trends and ecosystem resilience to disturbances.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.