Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Vegetation - water dynamics in the Australian landscape

Auteur : Donohue, Randall James

Université de soutenance : Australian National University

Grade : Doctor of Philosophy (PhD) 2009

Résumé partiel
The study of the interactions between vegetation and water is an important research field in water-limited environments such as typically occur across Australia. Vegetation functioning and water availability are intimately linked in drier environments, generally existing in a dynamic equilibrium. The research presented herein utilises the concept of vegetation equilibrium to examine whether satellite-derived vegetation cover information can be used to enhance current understanding of ecohydrological processes. Two research hypotheses are tested. The first hypothesis states that the observed changes in growing conditions across Australia over the past two and a half decades will have increased Australia’s average vegetation cover and that such changes should be evident in satellite-derived fPAR data (the fraction of Photosynthetically Active Radiation absorbed by vegetation, which is proportional to the fraction of green cover). A consistent, long-term record of remotely sensed fPAR data, spanning 1981-2006, is created using a novel method developed herein. In this method, Advanced Very High Resolution Radiometer red and near infra-red reflectance data are linearly transformed to ensure the position of the vegetation cover triangle is temporally stable in reflectance space. The first hypothesis is tested by identifying linear trends in total, persistent and recurrent fPAR (the latter two variables approximate the cover of perennial and annual vegetation types, respectively, and are derived here using a newly developed technique). Results show that an average increase in total fPAR has occurred (an increase of 8%), due to large increases in persistent fPAR (up 21%) and despite decreases in recurrent fPAR (down 7%). Results also show that increases in persistent fPAR were not always linked to changes in precipitation, leading to the possibility that some of the observed ‘greening’ may be due to higher concentrations of atmospheric CO2 . Overall, this research implies that Australia has, on average, become effectively wetter over the past 2-3 decades.

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