New approaches to investigate the seasonal growth dynamics in forests
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2016
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Melbourne Authors
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Forest and Ecosystem Science
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PhD thesis
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© 2016 Dr. Anne Griebel
Abstract
Forests play a significant role in sequestering and storing atmospheric CO2. Temperate eucalypt forests are widely distributed in south-eastern Australia, but little is known about seasonal patterns of carbon allocations to crown or stem biomass pools, and how these patterns are related to variations in the ecosystem carbon balance.
My study is the first to combine tree-, plot- and ecosystem-scale assessments to examine the seasonal variability of carbon allocation patterns to crown and stem pools and concomitant ecosystem-scale carbon dynamics in a temperate eucalypt forest. In the first research chapter I addressed methodological gaps by examining the utility of a novel lidar scanner in assessing canopy growth dynamics in a mixed-species eucalypt forest. Lidar-derived canopy dynamics compared well with hemispherical images and I developed automated data filtering and processing routines for daily lidar scans. Moreover, I demonstrated the additional value of structural information to explain canopy growth dynamics in an evergreen eucalypt forest.
In the second research chapter, I combined the advances in canopy monitoring with measures of stem increments and ecosystem carbon fluxes to provide an integrated assessment of seasonal carbon allocation. Dominant eucalypts grew year-round, allocating carbon to crown expansion in summer and to stem growth in all other seasons, while stem growth of trees in the lower crown classes was more irregular. The plant area index was remarkably stable throughout the year but the three dimensional assessment of canopy volume indicated a biomass shift from lower to higher canopy positions. Seasonal patterns in stem increments and canopy volume alone or in combination were not closely associated with seasonal ecosystem-level carbon fluxes, indicating that carbon allocations to root growth and/or non-structural carbohydrates were influencing ecosystem carbon dynamics in temperate eucalypt forests.
A final component of my thesis assessed whether surface inhomogeneities in combination with directional variations of climate variables affected instantaneous carbon fluxes and thus carbon budget estimates from eddy covariance measurements. I demonstrated that air temperature, vapour pressure deficit and turbulence varied significantly with wind direction, which, in combination with variable surface characteristics, resulted in carbon sequestration hotspots that could affect annual budgets by ~25%. A normalisation of wind sector contributions allowed me to quantify and adjust seasonal and annual carbon budget estimates.
My research was the first integrated assessment of carbon sequestration and allocation dynamics in temperate eucalypt forests. My growth assessments were consistent with general understanding that eucalypts in temperate forests grow opportunistically. However, my data indicate that under favourable conditions eucalypts can grow more-or-less continuously (as either crown expansion or stem increments), but with a new qualifier that this was only true for trees in the dominant crown classes. My study highlighted that temperate eucalypt forests are continuous carbon sinks, but that carbon budgets are significantly affected by an inhomogeneous flux footprint and/or by significant directional variations of climate. As a whole, my research emphasises the importance of field-based experiments under ambient climate conditions for adequately capturing the considerable variability of carbon sequestration and allocation dynamics in temperate and structurally complex eucalypt forests.
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Keywords
canopy dynamics, carbon allocation, eddy covariance, eucalypt forest, stem growth, terrestrial lidar, time lags, footprint, carbon budget, complex terrain, correction, forest structure, leaf area index, plant area index, monitoring, VEGNET