Carbon cycle processes in tropical savannas of far North Queensland, Australia

Davies, Kalu J.E. (2017) Carbon cycle processes in tropical savannas of far North Queensland, Australia. PhD thesis, James Cook University.

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Tropical savannas represent a diverse range of heterogenous ecosystems that play an important role in the global carbon cycle as they occupy approximately one fifth of the land surface. Despite their significance, there remains a paucity of studies that quantify the carbon stocks and uxes of these dynamic ecosystems, particularly within Australia (Chapter 1). This study contributes to the field of tropical savanna carbon cycle dynamics by quantifying the carbon stocks and fluxes at three study sites in northern Queensland and placing them in the context of climate change and the global carbon cycle.

Chapter 2 presents a review of the ecology and carbon dynamics of tropical savannas. The chapter addresses uncertainty regarding the definition and extent of tropical savannas, and the state of knowledge surrounding the drivers behind the extent of tropical savannas. In particular, stochastic disturbances such as fire play a crucial role in these ecosystems. Tropical savannas are also unique in that they consist of a mix of distinct and competing vegetation in the form of trees and grasses, that generally utilise contrasting photosynthetic strategies. As these photosynthetic pathways respond differently to atmospheric CO₂ and temperature, how these ecosystems will respond to climate change is also a matter of some uncertainty.

Chapter 3 presents a detailed description of the three sites used in this study; two of the sites (Davies Creek and Koombooloomba) are located near the rainforest-savanna boundary in a high rainfall zone (mean annual precipitation (MAP) ~1500mm) while the third site (Brooklyn Station) is both more arid (MAP ~900mm) and more seasonal.

In Chapter 4, aboveground biomass (AGB) and aboveground net primary productivity (ANPP) is quantified using an inventory-based assessment. These sites are some of the more densely wooded tropical savannas in Australia and basal area remained unchanged at all sites over the sample interval. While the carbon stocks in the AGB (40, 75, and 87 t C ha⁻¹ for Brooklyn Station, Davies Creek, and Koombooloomba, respectively) were dominated by the tree component, the grassy understorey was a significant contributor to ANPP (4.1, 5.1, and 9.2 t C ha⁻¹ y⁻¹ for Brooklyn Station, Davies Creek, and Koombooloomba, respectively) at all sites, consisting of almost three quarters of the total. The woody carbon stock was sensitive to disturbance with high mortality at two sites attributed to damage caused by fire and Tropical Cyclone (TC) Yasi. These results highlight the sensitivity of the carbon storage potential of tropical savannas to stochastic disturbance and their vulnerability to substantial shifts in both structure and composition as a result of climate change.

Chapter 5 presents data on soil carbon and nitrogen, as well as their stable isotopes. Soil carbon stocks (0-30cm) were 114 ± 25, 41 ± 14, and 38 ± 19 t C ha⁻¹ at Koombooloomba, Davies Creek, and Brooklyn Station, respectively. The carbon isotopes were used to partition the soil carbon into C3 and C4 sources; the grassy understorey contributed approximately half of the soil carbon at two of the sites (Brooklyn Station and Koombooloomba), but contributed only 27% at the third site. Soil carbon displayed a high level of spatial arrangement at the two wettest sites with higher soil carbon in areas adjacent to trees, but not at the driest site and redistribution of carbon inputs by termites may be responsible. Although considerable quantities of soil carbon are stored at these sites for now, climate change may result in vegetation shifts, changes to _re regimes, and exacerbated erosion that could result in the loss of soil carbon from these sites.

Chapter 6 provides the most comprehensive dataset of soil respiration in tropical savannas in Australia. Soil respiration was measured monthly for two years at the three sites. Soil respiration was temporally controlled by temperature and moisture availability, with high respiration rates (~4-8 μmol m⁻² s⁻¹) during the wet season (November - April) when plant growth is high, and low respiration rates (~1-4 μmol m⁻² s⁻¹) during the dry season (May - October) when plant growth slows. Soil respiration was spatially variable and controlled by proximity to woody vegetation with high soil respiration rates near trees and low soil respiration rates in open areas dominated by grasses. Soil respiration in these tropical savanna environments is thus both temporally and spatially heterogeneous suggesting that more complex models may be required to accurately estimate the soil respiration in such systems.

In Chapter 7, an experimental drought was created at Davies Creek over two years in order to examine the effects of drought on soil respiration and soil chemistry. The drought successfully reduced soil moisture without impacting significantly on other environmental variables. The drought delayed the growing season of the grasses in the first year and killed them in the second year. Soil respiration was correspondingly reduced in the first wet season and then increased in the second wet season, likely as a result of these changes to plant biomass availability. Higher soil carbon in the drought experiment suggests that there were additional carbon inputs, presumably due to the death of the grass and subsequent recolonisation of the soil by neighbouring grasses. Extended droughts could see shifts in species and ecosystems potentially accompanied by an increase in soil respiration before ecosystem stability is reached.

This study presents the first comprehensive assessment of carbon cycle dynamics in tropical savannas in Queensland. Furthermore, this is the first study in Australia to examine transition zone savannas that occur at the rainforest-savanna boundary and the first study in Australia to attempt to simulate drought in tropical savannas.

Item ID: 49914
Item Type: Thesis (PhD)
Keywords: Brooklyn Stations, carbon cycle, carbon sequestration, carbon stocks, climate change, Davies Creek, drought, Far North Queensland, Koombooloomba, rainforest-savanna boundary, soil carbon, soil respiration, transition zone savannas, tropical savannas
Date Deposited: 22 Aug 2017 04:31
FoR Codes: 05 ENVIRONMENTAL SCIENCES > 0503 Soil Sciences > 050301 Carbon Sequestration Science @ 50%
05 ENVIRONMENTAL SCIENCES > 0501 Ecological Applications > 050101 Ecological Impacts of Climate Change @ 25%
05 ENVIRONMENTAL SCIENCES > 0502 Environmental Science and Management > 050206 Environmental Monitoring @ 25%
SEO Codes: 96 ENVIRONMENT > 9603 Climate and Climate Change > 960307 Effects of Climate Change and Variability on Australia (excl. Social Impacts) @ 50%
96 ENVIRONMENT > 9614 Soils > 961403 Forest and Woodlands Soils @ 50%
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