Autumnal senescence in a poplar plantation in response to elevated carbon dioxide, from cell to canopy

Abstract

Global data sets derived from remote sensing of terrestrial vegetation and phonological observations of bud set, leaf colour change and leaf drop have provided evidence for the recent extension of the growing season.  Atmospheric carbon dioxide concentration has risen by ~39% since pre-industrial times and is considered a strong driver for a mean global temperature rise.  This increased temperature is generally thought to be the cause of the extension in the growing seasons.  In this thesis the influence increased atmospheric carbon dioxide concentration may have on the autumnal phenology of a poplar plantation was examined.

Following up to six years growth in an atmosphere enriched with carbon dioxide (CO₂) using free air CO₂ enrichment technology, the autumnal phenology of two poplar genotypes was examined.  Using remote sensing technology, at spatial and spectral resolutions varying from leaf level to airborne sensors, the changes in canopy spectral reflectance during senescence were monitored.  These changes were associated with a delayed autumnal decline in canopy leaf area and leaf level chlorophyll concentration for the trees exposed to elevated CO₂.  Associated with this was a decrease in both specific leaf area (leaf area per unit mass) and leaf nitrogen content (on a leaf mass basis).  The extension of autumnal senescence in this plantation resulting from atmospheric CO₂ enrichment was estimated to contribute approximately 2% to the annual gross primary production.,

The change in gene expression associated with this delay was studied using microarray technology.  Delayed senescence in elevated CO₂ was also evident at the level of gene expression, confirming the remotely-sensed observations.  For the first time, an up-regulation of genes encoding enzymes within the pathways of phenylpropanoid metabolism were identified during autumnal senescence in elevated CO₂ inferring increased stress tolerance.

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