Publications by authors named "Lindsay B Hutley"

30 Publications

  • Page 1 of 1

Living on the edge: A continental-scale assessment of forest vulnerability to drought.

Glob Chang Biol 2021 08 20;27(15):3620-3641. Epub 2021 May 20.

Hawkesbury Institute for the Environment, Western Sydney University, Richmond, NSW, Australia.

Globally, forests are facing an increasing risk of mass tree mortality events associated with extreme droughts and higher temperatures. Hydraulic dysfunction is considered a key mechanism of drought-triggered dieback. By leveraging the climate breadth of the Australian landscape and a national network of research sites (Terrestrial Ecosystem Research Network), we conducted a continental-scale study of physiological and hydraulic traits of 33 native tree species from contrasting environments to disentangle the complexities of plant response to drought across communities. We found strong relationships between key plant hydraulic traits and site aridity. Leaf turgor loss point and xylem embolism resistance were correlated with minimum water potential experienced by each species. Across the data set, there was a strong coordination between hydraulic traits, including those linked to hydraulic safety, stomatal regulation and the cost of carbon investment into woody tissue. These results illustrate that aridity has acted as a strong selective pressure, shaping hydraulic traits of tree species across the Australian landscape. Hydraulic safety margins were constrained across sites, with species from wetter sites tending to have smaller safety margin compared with species at drier sites, suggesting trees are operating close to their hydraulic thresholds and forest biomes across the spectrum may be susceptible to shifts in climate that result in the intensification of drought.
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http://dx.doi.org/10.1111/gcb.15641DOI Listing
August 2021

Impact of an extreme monsoon on CO and CH fluxes from mangrove soils of the Ayeyarwady Delta, Myanmar.

Sci Total Environ 2021 Mar 4;760:143422. Epub 2020 Nov 4.

School of Biological Sciences, The University of Queensland, St Lucia, QLD 4067, Australia.

Mangrove ecosystems can be both significant sources and sinks of greenhouse gases (GHGs). Understanding variability in flux and the key factors controlling emissions in these ecosystems are therefore important in the context of accounting for GHG emissions. The current study is the first to quantify GHG emissions using static chamber measurements from soils in disused aquaculture ponds, planted mangroves, and mature mangroves from the Ayeyarwady Delta, Myanmar. Soil properties, biomass and estimated net primary productivity were also assessed. Field assessments were conducted at the same sites during the middle of the dry season in February and end of the wet season in October 2019. Rates of soil CO efflux were among the highest yet recorded from mangrove ecosystems, with CO efflux from the 8 year old site reaching 86.8 ± 17 Mg CO ha yr during February, an average of 862% more than all other sites assessed during this period. In October, all sites had significant rates of soil CO efflux, with rates ranging from 31.9 ± 4.4 Mg CO ha yr in a disused pond to 118.9 ± 24.3 Mg CO ha yr in the 8 year old site. High soil CO efflux from the 8 year old site in February is most likely attributable to high rates of primary production and belowground carbon allocation. Elevated CO efflux from all sites during October was likely associated with the extreme 2019 South Asian monsoon season which lowered soil pore salinity and deposited new alluvium, stimulating both autotrophic and heterotrophic activity. Methane efflux increased significantly (50-400%) during the wet season from all sites with mangrove cover, although was a small overall component of soil GHG effluxes during both measurement periods. Our results highlight the critical importance of assessing GHG flux in-situ in order to quantify variability in carbon dynamics over time.
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http://dx.doi.org/10.1016/j.scitotenv.2020.143422DOI Listing
March 2021

Net landscape carbon balance of a tropical savanna: Relative importance of fire and aquatic export in offsetting terrestrial production.

Glob Chang Biol 2020 Oct 13;26(10):5899-5913. Epub 2020 Aug 13.

Division of Earth Sciences, National Science Foundation, Alexandria, VA, USA.

The magnitude of the terrestrial carbon (C) sink may be overestimated globally due to the difficulty of accounting for all C losses across heterogeneous landscapes. More complete assessments of net landscape C balances (NLCB) are needed that integrate both emissions by fire and transfer to aquatic systems, two key loss pathways of terrestrial C. These pathways can be particularly significant in the wet-dry tropics, where fire plays a fundamental part in ecosystems and where intense rainfall and seasonal flooding can result in considerable aquatic C export (ΣF ). Here, we determined the NLCB of a lowland catchment (~140 km ) in tropical Australia over 2 years by evaluating net terrestrial productivity (NEP), fire-related C emissions and ΣF (comprising both downstream transport and gaseous evasion) for the two main landscape components, that is, savanna woodland and seasonal wetlands. We found that the catchment was a large C sink (NLCB 334 Mg C km  year ), and that savanna and wetland areas contributed 84% and 16% to this sink, respectively. Annually, fire emissions (-56 Mg C km  year ) and ΣF (-28 Mg C km  year ) reduced NEP by 13% and 7%, respectively. Savanna burning shifted the catchment to a net C source for several months during the dry season, while ΣF significantly offset NEP during the wet season, with a disproportionate contribution by single major monsoonal events-up to 39% of annual ΣF was exported in one event. We hypothesize that wetter and hotter conditions in the wet-dry tropics in the future will increase ΣF and fire emissions, potentially further reducing the current C sink in the region. More long-term studies are needed to upscale this first NLCB estimate to less productive, yet hydrologically dynamic regions of the wet-dry tropics where our result indicating a significant C sink may not hold.
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http://dx.doi.org/10.1111/gcb.15287DOI Listing
October 2020

Land transformation in tropical savannas preferentially decomposes newly added biomass, whether C or C derived.

Ecol Appl 2020 12 28;30(8):e02192. Epub 2020 Jul 28.

Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, North Territory, Australia.

As tropical savannas are undergoing rapid conversion to other land uses, native C -C vegetation mixtures are often transformed to C - or C -dominant systems, resulting in poorly understood changes to the soil carbon (C) cycle. Conventional models of the soil C cycle are based on assumptions that more labile components of the heterogenous soil organic C (SOC) pool decompose at faster rates. Meanwhile, previous work has suggested that the C -derived component of SOC is more labile than C -derived SOC. Here we report on long-term (18 months) soil incubations from native and transformed tropical savannas of northern Australia. We test the hypothesis that, regardless of the type of land conversion, the C component of SOC will be preferentially decomposed. We measured changes in the SOC and pyrogenic carbon (PyC) pools, as well as the carbon isotope composition of SOC, PyC and respired CO , from 63 soil cores collected intact from different land use change scenarios. Our results show that land use change had no consistent effect on the size of the SOC pool, but strong effects on SOC decomposition rates, with slower decomposition rates at C -invaded sites. While we confirm that native savanna soils preferentially decomposed C -derived SOC, we also show that transformed savanna soils preferentially decomposed the newly added pool of labile SOC, regardless of whether it was C -derived (grass) or C -derived (forestry) biomass. Furthermore, we provide evidence that in these fire-prone landscapes, the nature of the PyC pool can shed light on past vegetation composition: while the PyC pool in C -dominant sites was mainly derived from C biomass, PyC in C3-dominant sites and native savannas was mainly derived from C biomass. We develop a framework to systematically assess the effects of recent land use change vs. prior vegetation composition.
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http://dx.doi.org/10.1002/eap.2192DOI Listing
December 2020

Mangrove blue carbon stocks and dynamics are controlled by hydrogeomorphic settings and land-use change.

Glob Chang Biol 2020 05 24;26(5):3028-3039. Epub 2020 Mar 24.

Center for International Forestry Research, Bogor, Indonesia.

Globally, carbon-rich mangrove forests are deforested and degraded due to land-use and land-cover change (LULCC). The impact of mangrove deforestation on carbon emissions has been reported on a global scale; however, uncertainty remains at subnational scales due to geographical variability and field data limitations. We present an assessment of blue carbon storage at five mangrove sites across West Papua Province, Indonesia, a region that supports 10% of the world's mangrove area. The sites are representative of contrasting hydrogeomorphic settings and also capture change over a 25-years LULCC chronosequence. Field-based assessments were conducted across 255 plots covering undisturbed and LULCC-affected mangroves (0-, 5-, 10-, 15- and 25-year-old post-harvest or regenerating forests as well as 15-year-old aquaculture ponds). Undisturbed mangroves stored total ecosystem carbon stocks of 182-2,730 (mean ± SD: 1,087 ± 584) Mg C/ha, with the large variation driven by hydrogeomorphic settings. The highest carbon stocks were found in estuarine interior (EI) mangroves, followed by open coast interior, open coast fringe and EI forests. Forest harvesting did not significantly affect soil carbon stocks, despite an elevated dead wood density relative to undisturbed forests, but it did remove nearly all live biomass. Aquaculture conversion removed 60% of soil carbon stock and 85% of live biomass carbon stock, relative to reference sites. By contrast, mangroves left to regenerate for more than 25 years reached the same level of biomass carbon compared to undisturbed forests, with annual biomass accumulation rates of 3.6 ± 1.1 Mg C ha  year . This study shows that hydrogeomorphic setting controls natural dynamics of mangrove blue carbon stocks, while long-term land-use changes affect carbon loss and gain to a substantial degree. Therefore, current land-based climate policies must incorporate landscape and land-use characteristics, and their related carbon management consequences, for more effective emissions reduction targets and restoration outcomes.
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http://dx.doi.org/10.1111/gcb.15056DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7217146PMC
May 2020

Effect of elevated magnesium sulfate on two riparian tree species potentially impacted by mine site contamination.

Sci Rep 2020 02 19;10(1):2880. Epub 2020 Feb 19.

Charles Darwin University, Ellengowan Drive, Casuarina, Northern Territory, 0810, Australia.

Globally, mining activities have been responsible for the contamination of soils, surface water and groundwater. Following mine closure, a key issue is the management of leachate from waste rock accumulated during the lifetime of the mine. At Ranger Uranium Mine in northern Australia, magnesium sulfate (MgSO) leaching from waste rock has been identified as a potentially significant surface and groundwater contaminant which may have adverse affects on catchment biota. The primary objective of this study was to determine the effect of elevated levels of MgSO on two riparian trees; Melaleuca viridiflora and Alphitonia excelsa. We found that tolerance to MgSO was species-specific. M. viridiflora was tolerant to high concentrations of MgSO (15,300 mg l), with foliar concentrations of ions suggesting plants regulate uptake. In contrast, A. excelsa was sensitive to elevated concentrations of MgSO (960 mg l), exhibiting reduced plant vigour and growth. This information improves our understanding of the toxicity of MgSO as a mine contaminant and highlights the need for rehabililitation planning to mitigate impacts on some tree species of this region.
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http://dx.doi.org/10.1038/s41598-020-59390-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7031394PMC
February 2020

Australian vegetated coastal ecosystems as global hotspots for climate change mitigation.

Nat Commun 2019 10 2;10(1):4313. Epub 2019 Oct 2.

School of Science and Centre for Marine Ecosystems Research, Edith Cowan University, Joondalup, WA, 6027, Australia.

Policies aiming to preserve vegetated coastal ecosystems (VCE; tidal marshes, mangroves and seagrasses) to mitigate greenhouse gas emissions require national assessments of blue carbon resources. Here, we present organic carbon (C) storage in VCE across Australian climate regions and estimate potential annual CO emission benefits of VCE conservation and restoration. Australia contributes 5-11% of the C stored in VCE globally (70-185 Tg C in aboveground biomass, and 1,055-1,540 Tg C in the upper 1 m of soils). Potential CO emissions from current VCE losses are estimated at 2.1-3.1 Tg CO-e yr, increasing annual CO emissions from land use change in Australia by 12-21%. This assessment, the most comprehensive for any nation to-date, demonstrates the potential of conservation and restoration of VCE to underpin national policy development for reducing greenhouse gas emissions.
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http://dx.doi.org/10.1038/s41467-019-12176-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6773740PMC
October 2019

Effect of land-use and land-cover change on mangrove blue carbon: A systematic review.

Glob Chang Biol 2019 Dec 27;25(12):4291-4302. Epub 2019 Aug 27.

Research Institute for the Environment and Livelihoods (RIEL), Charles Darwin University, Darwin, NT, Australia.

Mangroves shift from carbon sinks to sources when affected by anthropogenic land-use and land-cover change (LULCC). Yet, the magnitude and temporal scale of these impacts are largely unknown. We undertook a systematic review to examine the influence of LULCC on mangrove carbon stocks and soil greenhouse gas (GHG) effluxes. A search of 478 data points from the peer-reviewed literature revealed a substantial reduction of biomass (82% ± 35%) and soil (54% ± 13%) carbon stocks due to LULCC. The relative loss depended on LULCC type, time since LULCC and geographical and climatic conditions of sites. We also observed that the loss of soil carbon stocks was linked to the decreased soil carbon content and increased soil bulk density over the first 100 cm depth. We found no significant effect of LULCC on soil GHG effluxes. Regeneration efforts (i.e. restoration, rehabilitation and afforestation) led to biomass recovery after ~40 years. However, we found no clear patterns of mangrove soil carbon stock re-establishment following biomass recovery. Our findings suggest that regeneration may help restore carbon stocks back to pre-disturbed levels over decadal to century time scales only, with a faster rate for biomass recovery than for soil carbon stocks. Therefore, improved mangrove ecosystem management by preventing further LULCC and promoting rehabilitation is fundamental for effective climate change mitigation policy.
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http://dx.doi.org/10.1111/gcb.14774DOI Listing
December 2019

Estimating the full greenhouse gas emissions offset potential and profile between rehabilitating and established mangroves.

Sci Total Environ 2019 May 8;665:419-431. Epub 2019 Feb 8.

Department of Geography, National University of Singapore, 1 Arts Link, Singapore 117570, Singapore.

Mangrove forests are extremely productive, with rates of growth rivaling some terrestrial tropical rainforests. However, our understanding of the full suite of processes underpinning carbon exchange with the atmosphere and near shore-waters, the allocation of carbon in mangroves, and fluxes of non-CO greenhouse gases (GHGs) are limited to a handful of studies. This constrains the scientific basis from which to advocate for greater support for and investment in mangrove restoration and conservation. Improving understanding is urgently needed given the on-going landuse pressures mangrove forests face, particularly throughout much of Southeast Asia. The current study reduces uncertainties by providing a holistic synthesis of the net potential GHG mitigation benefits resulting from rehabilitating mangroves and established forests. Rehabilitating sites from two contrasting locations representative of high (Tiwoho) and low (Tanakeke) productivity systems on the island of Sulawesi (Indonesia) were used as case studies to compare against established mangroves. A carbon budget, allocation and pathways model was developed to account for inputs (carbon sequestration) and outputs (GHG emissions of CO, NO and CH) to estimate Net Ecosystem Production (NEP) and Net Ecosystem Carbon Balance (NECB). Our results indicate that while Tiwoho's rehabilitating sites and established mangroves represent a significant carbon sink (-10.6 ± 0.9 Mg COe ha y and 16.1 Mg COe ha y respectively), the low productivity of Tanakeke has resulted in minimal reductions to date (0.7 ± 0.3 Mg COe ha y). Including NEP from mangrove-allied primary producer communities (e.g. benthic algae) and the portion of dissolved inorganic carbon exported from mangroves (EX) that remains within the water column may drive overall removals considerably upwards in established forests to -37.2 Mg COe ha y. These values are higher than terrestrial forests and strengthen the evidence base needed to underpin the use of forest carbon financing mechanisms for mangrove restoration.
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http://dx.doi.org/10.1016/j.scitotenv.2019.02.104DOI Listing
May 2019

Termite mounds mitigate half of termite methane emissions.

Proc Natl Acad Sci U S A 2018 12 26;115(52):13306-13311. Epub 2018 Nov 26.

School of Ecosystem and Forest Sciences, The University of Melbourne, Richmond, VIC 3121, Australia.

Termites are responsible for ∼1 to 3% of global methane (CH) emissions. However, estimates of global termite CH emissions span two orders of magnitude, suggesting that fundamental knowledge of CH turnover processes in termite colonies is missing. In particular, there is little reliable information on the extent and location of microbial CH oxidation in termite mounds. Here, we use a one-box model to unify three independent field methods-a gas-tracer test, an inhibitor approach, and a stable-isotope technique-and quantify CH production, oxidation, and transport in three North Australian termite species with different feeding habits and mound architectures. We present systematic in situ evidence of widespread CH oxidation in termite mounds, with 20 to 80% of termite-produced CH being mitigated before emission to the atmosphere. Furthermore, closing the CH mass balance in mounds allows us to estimate in situ termite biomass from CH turnover, with mean biomass ranging between 22 and 86 g of termites per kilogram of mound for the three species. Field tests with excavated mounds show that the predominant location of CH oxidation is either in the mound material or the soil beneath and is related to species-specific mound porosities. Regardless of termite species, however, our data and model suggest that the fraction of oxidized CH ( ) remains well buffered due to links among consumption, oxidation, and transport processes via mound CH concentration. The mean of 0.50 ± 0.11 (95% CI) from in situ measurements therefore presents a valid oxidation factor for future global estimates of termite CH emissions.
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http://dx.doi.org/10.1073/pnas.1809790115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6310829PMC
December 2018

Community structure dynamics and carbon stock change of rehabilitated mangrove forests in Sulawesi, Indonesia.

Ecol Appl 2019 01 26;29(1):e01810. Epub 2018 Nov 26.

Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, Northern Territory, Australia.

To date, discourse associated with the potential application of "blue carbon" within real-world carbon markets has focused on blue carbon as a mitigation strategy in the context of avoided deforestation (e.g., REDD+). Here, we report structural dynamics and carbon storage gains from mangrove sites that have undergone rehabilitation to ascertain whether reforestation can complement conservation activities and warrant project investment. Replicated sites at two locations with contrasting geomorphic conditions were selected, Tiwoho and Tanakeke on the island of Sulawesi, Indonesia. These locations are representative of high (Tiwoho, deep muds and silty substrates) and low (Tanakeke, shallow, coralline sands) productivity mangrove ecosystems. They share a similar management history of clearing and conversion for aquaculture before restorative activities were undertaken using the practice of Ecological Mangrove Rehabilitation (EMR). Species diversity and mean biomass carbon storage gains after 10 yr of regrowth from the high productivity sites of Tiwoho (49.2 ± 9.1 Mg C·ha ·yr ) are already almost of one-third of mean biomass stocks exhibited by mature forests (167.8 ± 30.3 Mg C·ha ·yr ). Tiwoho's EMR sites, on average, will have offset all biomass C that was initially lost through conversion within the next 11 yr, a finding in marked contrast to the minimal carbon gains observed on the low productivity, low diversity, coral atoll EMR sites of Tanakeke (1.1 ± 0.4 Mg C·ha ·yr ). These findings highlight the importance of geomorphic and biophysical site selection if the primary purpose of EMR is intended to maximize carbon sequestration gains.
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http://dx.doi.org/10.1002/eap.1810DOI Listing
January 2019

Hydroperiod, soil moisture and bioturbation are critical drivers of greenhouse gas fluxes and vary as a function of landuse change in mangroves of Sulawesi, Indonesia.

Sci Total Environ 2019 Mar 10;654:365-377. Epub 2018 Nov 10.

Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, Northern Territory, Australia.

The loss and degradation of mangroves can result in potentially significant sources of atmospheric greenhouse gas (GHG) emissions. For mangrove rehabilitation carbon projects, quantifying GHG emissions as forests regenerate is a key accounting requirement. The current study is one of the first attempts to systematically quantify emissions of carbon dioxide (CO), nitrous oxide (NO) and methane (CH) from: 1) aquaculture ponds, 2) rehabilitating mangroves, and 3) intact mangrove sites and frame GHG flux within the context of landuse change. In-situ static chamber measurements were made at three contrasting locations in Sulawesi, Indonesia. The influence of key biophysical variables known to affect GHG flux was also assessed. Peak GHG flux was observed at rehabilitating (32.8 ± 2.1 Mg COe ha y) and intact, mature reference sites (43.8 ± 4.5 Mg COe ha y) and a dry, exposed disused aquaculture pond (30.6 ± 1.9 Mg COe ha y). Emissions were negligible at low productivity rehabilitating sites with high hydroperiod (mean 1.0 ± 0.1 Mg COe ha y) and an impounded, operational aquaculture pond (1.1 ± 0.2 Mg COe ha y). Heterogeneity in biophysical conditions and geomorphic position exerted a strong influence on GHG flux, with the longer hydroperiod and higher soil moisture content of seaward fringing mangroves correlated with decreased fluxes. A greater abundance of Mud lobster mounds and root structures in landward mangroves correlated to higher flux. When viewed across a landuse change continuum, our results suggest that the initial conversion of mangroves to aquaculture ponds releases extremely high rates of GHGs. Furthermore, the re-institution of hydrological regimes in dry, disused aquaculture ponds to facilitate tidal flushing is instrumental in rapidly mediating GHG flux, leading to a significant reduction in baseline emissions. This is an important consideration for forest carbon project proponents seeking to maximise creditable GHG emissions reductions and removals.
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http://dx.doi.org/10.1016/j.scitotenv.2018.11.092DOI Listing
March 2019

Exotic grass invasion alters microsite conditions limiting woody recruitment potential in an Australian savanna.

Sci Rep 2018 04 26;8(1):6628. Epub 2018 Apr 26.

Faculty of Science, University of Western Australia, Crawley, Western Australia, 6009, Australia.

Andropogon gayanus Kunth. is a large African tussock grass invading Australia's tropical savannas. Invasion results in more intense fires which increases the mortality rate of adult woody plants. Invasion may also affect community structure by altering the recruitment potential of woody plants. We investigated the effects of A. gayanus invasion on ground-level microclimate, and the carbon assimilation potential and recruitment potential of two Eucalyptus species. We compared microclimatic variables from the early wet-season and into the mid-dry season to coincide with the period of growth of A. gayanus. We assessed Eucalyptus recruitment by monitoring seedling establishment, growth and survival of experimentally sown seed, and estimating seedling density resulting from natural recruitment. A. gayanus invasion was associated with increased grass canopy height, biomass and cover. Following invasion, the understorey microclimate had significantly reduced levels of photon flux density, increased air temperatures and vapour pressure deficit. The conditions were less favourable for woody seedling with aboveground biomass of seedlings reduced by 26% in invaded plots. We estimated that invasion reduced daily carbon assimilation of woody seedlings by ~30% and reduced survivorship of Eucalyptus seedlings. Therefore, A. gayanus invasion reduces recruitment potential, contributing to the transformation of savanna to a grassland ecosystem.
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http://dx.doi.org/10.1038/s41598-018-24704-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5920062PMC
April 2018

Seasonal, interannual and decadal drivers of tree and grass productivity in an Australian tropical savanna.

Glob Chang Biol 2018 06 28;24(6):2530-2544. Epub 2018 Feb 28.

School of Earth, Atmosphere and Environment, Monash University, Clayton, Vic., Australia.

Tree-grass savannas are a widespread biome and are highly valued for their ecosystem services. There is a need to understand the long-term dynamics and meteorological drivers of both tree and grass productivity separately in order to successfully manage savannas in the future. This study investigated the interannual variability (IAV) of tree and grass gross primary productivity (GPP) by combining a long-term (15 year) eddy covariance flux record and model estimates of tree and grass GPP inferred from satellite remote sensing. On a seasonal basis, the primary drivers of tree and grass GPP were solar radiation in the wet season and soil moisture in the dry season. On an interannual basis, soil water availability had a positive effect on tree GPP and a negative effect on grass GPP. No linear trend in the tree-grass GPP ratio was observed over the 15-year study period. However, the tree-grass GPP ratio was correlated with the modes of climate variability, namely the Southern Oscillation Index. This study has provided insight into the long-term contributions of trees and grasses to savanna productivity, along with their respective meteorological determinants of IAV.
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http://dx.doi.org/10.1111/gcb.14072DOI Listing
June 2018

Resource-use efficiency explains grassy weed invasion in a low-resource savanna in north Australia.

Front Plant Sci 2015 4;6:560. Epub 2015 Aug 4.

Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, NT Australia.

Comparative studies of plant resource use and ecophysiological traits of invasive and native resident plant species can elucidate mechanisms of invasion success and ecosystem impacts. In the seasonal tropics of north Australia, the alien C4 perennial grass Andropogon gayanus (gamba grass) has transformed diverse, mixed tree-grass savanna ecosystems into dense monocultures. To better understand the mechanisms of invasion, we compared resource acquisition and usage efficiency using leaf-scale ecophysiological and stand-scale growth traits of A. gayanus with a co-habiting native C4 perennial grass Alloteropsis semialata. Under wet season conditions, A. gayanus had higher rates of stomatal conductance, assimilation, and water use, plus a longer daily assimilation period than the native species A. semialata. Growing season length was also ~2 months longer for the invader. Wet season measures of leaf scale water use efficiency (WUE) and light use efficiency (LUE) did not differ between the two species, although photosynthetic nitrogen use efficiency (PNUE) was significantly higher in A. gayanus. By May (dry season) the drought avoiding native species A. semialata had senesced. In contrast, rates of A. gayanus gas exchange was maintained into the dry season, albeit at lower rates that the wet season, but at higher WUE and PNUE, evidence of significant physiological plasticity. High PNUE and leaf (15)N isotope values suggested that A. gayanus was also capable of preferential uptake of soil ammonium, with utilization occurring into the dry season. High PNUE and fire tolerance in an N-limited and highly flammable ecosystem confers a significant competitive advantage over native grass species and a broader niche width. As a result A. gayanus is rapidly spreading across north Australia with significant consequences for biodiversity and carbon and retention.
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http://dx.doi.org/10.3389/fpls.2015.00560DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4523779PMC
August 2015

Vulnerability of native savanna trees and exotic Khaya senegalensis to seasonal drought.

Tree Physiol 2015 Jul 1;35(7):783-91. Epub 2015 May 1.

School of Ecosystem and Forest Sciences, The University of Melbourne, 500 Yarra Boulevard, Richmond, VIC 3121, Australia.

Seasonally dry ecosystems present a challenge to plants to maintain water relations. While native vegetation in seasonally dry ecosystems have evolved specific adaptations to the long dry season, there are risks to introduced exotic species. African mahogany, Khaya senegalensis Desr. (A. Juss.), is an exotic plantation species that has been introduced widely in Asia and northern Australia, but it is unknown if it has the physiological or phenotypic plasticity to cope with the strongly seasonal patterns of water availability in the tropical savanna climate of northern Australia. We investigated the gas exchange and water relations traits and adjustments to seasonal drought in K. senegalensis and native eucalypts (Eucalyptus tetrodonta F. Muell. and Corymbia latifolia F. Muell.) in a savanna ecosystem in northern Australia. The native eucalypts did not exhibit any signs of drought stress after 3 months of no rainfall and probably had access to deeper soil moisture late into the dry season. Leaf water potential, stomatal conductance, transpiration and photosynthesis all remained high in the dry season but osmotic adjustment was not observed. Overstorey leaf area index (LAI) was 0.6 in the native eucalypt savanna and did not change between wet and dry seasons. In contrast, the K. senegalensis plantation in the wet season was characterized by a high water potential, high stomatal conductance and transpiration and a high LAI of 2.4. In the dry season, K. senegalensis experienced mild drought stress with a predawn water potential -0.6 MPa. Overstorey LAI was halved, and stomatal conductance and transpiration drastically reduced, while minimum leaf water potentials did not change (-2 MPa) and no osmotic adjustment occurred. Khaya senegalensis exhibited an isohydric behaviour and also had a lower hydraulic vulnerability to cavitation in leaves, with a P50 of -2.3 MPa. The native eucalypts had twice the maximum leaf hydraulic conductance but a much higher P50 of -1.5 MPa. Khaya senegalensis has evolved in a wet-dry tropical climate in West Africa (600-800 mm) and appears to be well suited to the seasonal savanna climate of northern Australia. The species exhibited a large phenotypic plasticity through leaf area adjustments and conservative isohydric behaviour in the 6 months dry season while operating well above its critical hydraulic threshold.
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http://dx.doi.org/10.1093/treephys/tpv037DOI Listing
July 2015

Natural abundance (δ¹⁵N) indicates shifts in nitrogen relations of woody taxa along a savanna-woodland continental rainfall gradient.

Oecologia 2015 May 13;178(1):297-308. Epub 2014 Dec 13.

Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, 14853, USA,

Water and nitrogen (N) interact to influence soil N cycling and plant N acquisition. We studied indices of soil N availability and acquisition by woody plant taxa with distinct nutritional specialisations along a north Australian rainfall gradient from monsoonal savanna (1,600-1,300 mm annual rainfall) to semi-arid woodland (600-250 mm). Aridity resulted in increased 'openness' of N cycling, indicated by increasing δ(15)N(soil) and nitrate:ammonium ratios, as plant communities transitioned from N to water limitation. In this context, we tested the hypothesis that δ(15)N(root) xylem sap provides a more direct measure of plant N acquisition than δ(15)N(foliage). We found highly variable offsets between δ(15)N(foliage) and δ(15)N(root) xylem sap, both between taxa at a single site (1.3-3.4 ‰) and within taxa across sites (0.8-3.4 ‰). As a result, δ(15)N(foliage) overlapped between N-fixing Acacia and non-fixing Eucalyptus/Corymbia and could not be used to reliably identify biological N fixation (BNF). However, Acacia δ(15)N(root) xylem sap indicated a decline in BNF with aridity corroborated by absence of root nodules and increasing xylem sap nitrate concentrations and consistent with shifting resource limitation. Acacia dominance at arid sites may be attributed to flexibility in N acquisition rather than BNF capacity. δ(15)N(root) xylem sap showed no evidence of shifting N acquisition in non-mycorrhizal Hakea/Grevillea and indicated only minor shifts in Eucalyptus/Corymbia consistent with enrichment of δ(15)N(soil) and/or decreasing mycorrhizal colonisation with aridity. We propose that δ(15)N(root) xylem sap is a more direct indicator of N source than δ(15)N(foliage), with calibration required before it could be applied to quantify BNF.
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http://dx.doi.org/10.1007/s00442-014-3176-3DOI Listing
May 2015

Climate change and long-term fire management impacts on Australian savannas.

New Phytol 2015 Feb 12;205(3):1211-1226. Epub 2014 Nov 12.

Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, NT, Australia.

Tropical savannas cover a large proportion of the Earth's land surface and many people are dependent on the ecosystem services that savannas supply. Their sustainable management is crucial. Owing to the complexity of savanna vegetation dynamics, climate change and land use impacts on savannas are highly uncertain. We used a dynamic vegetation model, the adaptive dynamic global vegetation model (aDGVM), to project how climate change and fire management might influence future vegetation in northern Australian savannas. Under future climate conditions, vegetation can store more carbon than under ambient conditions. Changes in rainfall seasonality influence future carbon storage but do not turn vegetation into a carbon source, suggesting that CO₂ fertilization is the main driver of vegetation change. The application of prescribed fires with varying return intervals and burning season influences vegetation and fire impacts. Carbon sequestration is maximized with early dry season fires and long fire return intervals, while grass productivity is maximized with late dry season fires and intermediate fire return intervals. The study has implications for management policy across Australian savannas because it identifies how fire management strategies may influence grazing yield, carbon sequestration and greenhouse gas emissions. This knowledge is crucial to maintaining important ecosystem services of Australian savannas.
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http://dx.doi.org/10.1111/nph.13130DOI Listing
February 2015

Fire in Australian savannas: from leaf to landscape.

Glob Chang Biol 2015 Jan 9;21(1):62-81. Epub 2014 Sep 9.

School of Earth and Environment, The University of Western Australia, Crawley, WA, 6009, Australia; School of Geography and Environmental Science, Monash University, Melbourne, Vic., 3800, Australia.

Savanna ecosystems comprise 22% of the global terrestrial surface and 25% of Australia (almost 1.9 million km2) and provide significant ecosystem services through carbon and water cycles and the maintenance of biodiversity. The current structure, composition and distribution of Australian savannas have coevolved with fire, yet remain driven by the dynamic constraints of their bioclimatic niche. Fire in Australian savannas influences both the biophysical and biogeochemical processes at multiple scales from leaf to landscape. Here, we present the latest emission estimates from Australian savanna biomass burning and their contribution to global greenhouse gas budgets. We then review our understanding of the impacts of fire on ecosystem function and local surface water and heat balances, which in turn influence regional climate. We show how savanna fires are coupled to the global climate through the carbon cycle and fire regimes. We present new research that climate change is likely to alter the structure and function of savannas through shifts in moisture availability and increases in atmospheric carbon dioxide, in turn altering fire regimes with further feedbacks to climate. We explore opportunities to reduce net greenhouse gas emissions from savanna ecosystems through changes in savanna fire management.
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http://dx.doi.org/10.1111/gcb.12686DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4310295PMC
January 2015

Savanna vegetation-fire-climate relationships differ among continents.

Science 2014 Jan;343(6170):548-52

Department of Biological Sciences, Macquarie University, New South Wales 2109, Australia.

Ecologists have long sought to understand the factors controlling the structure of savanna vegetation. Using data from 2154 sites in savannas across Africa, Australia, and South America, we found that increasing moisture availability drives increases in fire and tree basal area, whereas fire reduces tree basal area. However, among continents, the magnitude of these effects varied substantially, so that a single model cannot adequately represent savanna woody biomass across these regions. Historical and environmental differences drive the regional variation in the functional relationships between woody vegetation, fire, and climate. These same differences will determine the regional responses of vegetation to future climates, with implications for global carbon stocks.
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http://dx.doi.org/10.1126/science.1247355DOI Listing
January 2014

Changes in body fluids of the cocooning fossorial frog Cyclorana australis in a seasonally dry environment.

Comp Biochem Physiol A Mol Integr Physiol 2011 Nov 13;160(3):348-54. Epub 2011 Jul 13.

School of Environmental and Life Sciences, Charles Darwin University, Darwin, Northern Territory, Australia.

We investigated changes in the lymph (equivalent to plasma) and urine of the cocooning frog Cyclorana australis during the dry season in monsoonal northern Australia. Frogs in moist soil for two days were fully hydrated (lymph 220 mOsm kg(-1), urine 49 mOsm kg(-1)). From five weeks onwards the soil was dry (matric potential <-8000 kPa). Aestivating frogs at three and five months formed cocoons in shallow (<20 cm) burrows and retained bladder fluid (25-80% of standard mass). After three months, urine but not lymph osmolality was elevated. After five months, lymph (314 mOsm kg(-1)) and urine (294 mOsm kg(-1)) osmolality and urea concentrations were elevated. Urea was a major contributing osmolyte in urine and accumulated in lymph after five months. Lymph sodium concentration did not change with time, whereas potassium increased in urine after five months. Active animals had moderate lymph osmolality (252 mOsm kg(-1)), but urea concentrations remained low. Urine was highly variable in active frogs, suggesting that they tolerate variation in hydration state. Despite prolonged periods in dry soil, osmolality increase in C. australis was not severe. Aestivation in a cocoon facilitates survival in shallow burrows, but such a strategy may only be effective in environments with seasonally reliable rainfall.
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http://dx.doi.org/10.1016/j.cbpa.2011.06.028DOI Listing
November 2011

Termite mound emissions of CH4 and CO2 are primarily determined by seasonal changes in termite biomass and behaviour.

Oecologia 2011 Oct 12;167(2):525-34. Epub 2011 May 12.

Department of Forest and Ecosystem Science, The University of Melbourne, Richmond, VIC, Australia.

Termites are a highly uncertain component in the global source budgets of CH(4) and CO(2). Large seasonal variations in termite mound fluxes of CH(4) and CO(2) have been reported in tropical savannas but the reason for this is largely unknown. This paper investigated the processes that govern these seasonal variations in CH(4) and CO(2) fluxes from the mounds of Microcerotermes nervosus Hill (Termitidae), a common termite species in Australian tropical savannas. Fluxes of CH(4) and CO(2) of termite mounds were 3.5-fold greater in the wet season as compared to the dry season and were a direct function of termite biomass. Termite biomass in mound samples was tenfold greater in the wet season compared to the dry season. When expressed per unit termite biomass, termite fluxes were only 1.2 (CH(4)) and 1.4 (CO(2))-fold greater in the wet season as compared to the dry season and could not explain the large seasonal variations in mound fluxes of CH(4) and CO(2). Seasonal variation in both gas diffusivity through mound walls and CH(4) oxidation by mound material was negligible. These results highlight for the first time that seasonal termite population dynamics are the main driver for the observed seasonal differences in mound fluxes of CH(4) and CO(2). These findings highlight the need to combine measurements of gas fluxes from termite mounds with detailed studies of termite population dynamics to reduce the uncertainty in quantifying seasonal variations in termite mound fluxes of CH(4) and CO(2).
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http://dx.doi.org/10.1007/s00442-011-1991-3DOI Listing
October 2011

Stable isotopes reveal the contribution of corticular photosynthesis to growth in branches of Eucalyptus miniata.

Plant Physiol 2011 Jan 15;155(1):515-23. Epub 2010 Nov 15.

School of Environmental and Life Sciences, Charles Darwin University, Darwin, Northern Territory 0909, Australia.

The deciduous bark habit is widespread in the woody plant genus Eucalyptus. Species with deciduous bark seasonally shed a layer of dead bark, thereby maintaining smooth-bark surfaces on branches and stems as they age and increase in diameter. This has a significant cost in terms of fire protection, because smooth-barked species have thinner bark than rough-barked species that accumulate successive layers of dead bark. Eucalypts are closely associated with fire, suggesting that the smooth-bark habit must also provide a significant benefit. We suggest that this benefit is corticular photosynthesis. To test this, we quantified the contribution of corticular photosynthesis to wood production in smooth-barked branches of Eucalyptus miniata growing in tropical savanna in northern Australia. We covered branch sections with aluminum foil for 4 years to block corticular photosynthesis and then compared the oxygen and carbon stable isotope composition of foil-covered and uncovered branch sections. We developed theory to calculate the proportion of wood constructed from corticular photosynthate and the mean proportional refixation rate during corticular photosynthesis from the observed isotopic differences. Coverage with aluminum foil for 4 years increased wood δ(13)C by 0.5‰ (P = 0.002, n = 6) and wood δ(18)O by 0.5‰ (P = 0.02, n = 6). Based on these data, we estimated that 11% ± 3% of wood in the uncovered branch sections was constructed from corticular photosynthate, with a mean δ(13)C of -34.8‰, and that the mean proportional refixation rate during corticular photosynthesis was 0.71 ± 0.15. This demonstrates that corticular photosynthesis makes a significant contribution to the carbon economy of smooth-barked eucalypts.
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http://dx.doi.org/10.1104/pp.110.163337DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3075781PMC
January 2011

Which of these continents is not like the other? Comparisons of tropical savanna systems: key questions and challenges.

New Phytol 2009 ;181(3):508-11

School for Environmental Research, Charles Darwin University, Darwin NT 0909, Australia.

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http://dx.doi.org/10.1111/j.1469-8137.2009.02734.xDOI Listing
March 2009

Impacts of fire on forest age and runoff in mountain ash forests - RETRACTED.

Funct Plant Biol 2008 Aug;35(6):483-492

School of Geography and Environmental Science, Monash University, Vic. 3800, Australia.

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http://dx.doi.org/10.1071/FP08120DOI Listing
August 2008

A canopy-scale test of the optimal water-use hypothesis.

Plant Cell Environ 2008 Jan 30;31(1):97-111. Epub 2007 Oct 30.

School of Environmental Systems Engineering, The University of Western Australia, Perth, Australia.

Common empirical models of stomatal conductivity often incorporate a sensitivity of stomata to the rate of leaf photosynthesis. Such a sensitivity has been predicted on theoretical terms by Cowan and Farquhar, who postulated that stomata should adjust dynamically to maximize photosynthesis for a given water loss. In this study, we implemented the Cowan and Farquhar hypothesis of optimal stomatal conductivity into a canopy gas exchange model, and predicted the diurnal and daily variability of transpiration for a savanna site in the wet-dry tropics of northern Australia. The predicted transpiration dynamics were then compared with observations at the site using the eddy covariance technique. The observations were also used to evaluate two alternative approaches: constant conductivity and a tuned empirical model. The model based on the optimal water-use hypothesis performed better than the one based on constant stomatal conductivity, and at least as well as the tuned empirical model. This suggests that the optimal water-use hypothesis is useful for modelling canopy gas exchange, and that it can reduce the need for model parameterization.
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http://dx.doi.org/10.1111/j.1365-3040.2007.01740.xDOI Listing
January 2008

A test of the optimality approach to modelling canopy properties and CO2 uptake by natural vegetation.

Plant Cell Environ 2007 Dec 9;30(12):1586-98. Epub 2007 Oct 9.

School of Environmental Systems Engineering, The University of Western Australia, Perth, Western Australia, Australia.

Photosynthesis provides plants with their main building material, carbohydrates, and with the energy necessary to thrive and prosper in their environment. We expect, therefore, that natural vegetation would evolve optimally to maximize its net carbon profit (NCP), the difference between carbon acquired by photosynthesis and carbon spent on maintenance of the organs involved in its uptake. We modelled N(CP) for an optimal vegetation for a site in the wet-dry tropics of north Australia based on this hypothesis and on an ecophysiological gas exchange and photosynthesis model, and compared the modelled CO2 fluxes and canopy properties with observations from the site. The comparison gives insights into theoretical and real controls on gas exchange and canopy structure, and supports the optimality approach for the modelling of gas exchange of natural vegetation. The main advantage of the optimality approach we adopt is that no assumptions about the particular vegetation of a site are required, making it a very powerful tool for predicting vegetation response to long-term climate or land use change.
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http://dx.doi.org/10.1111/j.1365-3040.2007.01728.xDOI Listing
December 2007

Stem and leaf gas exchange and their responses to fire in a north Australian tropical savanna.

Plant Cell Environ 2006 Apr;29(4):632-46

School of Science and Primary Industries, Faculty of Education, Health and Science, Charles Darwin University, Darwin, NT 0909, Australia.

We measured stem CO2 efflux and leaf gas exchange in a tropical savanna ecosystem in northern Australia, and assessed the impact of fire on these processes. Gas exchange of mature leaves that flushed after a fire showed only slight differences from that of mature leaves on unburned trees. Expanding leaves typically showed net losses of CO2 to the atmosphere in both burned and unburned trees, even under saturating irradiance. Fire caused stem CO2 efflux to decline in overstory trees, when measured 8 weeks post-fire. This decline was thought to have resulted from reduced availability of C substrate for respiration, due to reduced canopy photosynthesis caused by leaf scorching, and to priority allocation of fixed C towards reconstruction of a new canopy. At the ecosystem scale, we estimated the annual above-ground woody-tissue CO2 efflux to be 275 g C m(-2) ground area year(-1) in a non-fire year, or approximately 13% of the annual gross primary production. We contrasted the canopy physiology of two co-dominant overstory tree species, one of which has a smooth bark on its branches capable of photosynthetic re-fixation (Eucalyptus miniata), and the other of which has a thick, rough bark incapable of re-fixation (Eucalyptus tetrodonta). Eucalyptus miniata supported a larger branch sapwood cross-sectional area in the crown per unit subtending leaf area, and had higher leaf stomatal conductance and photosynthesis than E. tetrodonta. Re-fixation by photosynthetic bark reduces the C cost of delivering water to evaporative sites in leaves, because it reduces the net C cost of constructing and maintaining sapwood. We suggest that re-fixation allowed leaves of E. miniata to photosynthesize at higher rates than those of E. tetrodonta, while the two invested similar amounts of C in the maintenance of branch sapwood.
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http://dx.doi.org/10.1111/j.1365-3040.2005.01442.xDOI Listing
April 2006

Viewpoint: Assessing the carbon sequestration potential of mesic savannas in the Northern Territory, Australia: approaches, uncertainties and potential impacts of fire.

Funct Plant Biol 2004 Jun;31(5):415-422

School of Science and Primary Industries, Charles Darwin University, Darwin, NT 0909, Australia. Current address: Department of Geography, University of Toronto, Canada.

Tropical savannas cover a quarter of the Australian landmass and the biome represents a significant potential carbon sink. However, these savannas are subject to frequent and extensive fire. Fire regimes are likely to affect the productivity and carbon sequestration potential of savannas, through effects on both biomass and carbon emissions. The carbon sequestration potential has been estimated for some savanna sites by quantifying carbon storage in biomass and soil pools, and the fluxes to these pools. Using different techniques, previous work in these savannas has indicated that net ecosystem productivity [NEP, net primary productivity (NPP) less heterotrophic respiration] was about -3 t C ha y (i.e. a carbon sink). However, the impacts of fire were not accounted for in these calculations. Estimates of NEP have been combined with remotely-sensed estimates of area burnt and associated emissions for an extensive area of mesic savanna in Arnhem Land, NT, Australia. Combining NEP estimates with precise fire data provides an estimate of net biome productivity (NBP), a production index that includes carbon loss through disturbance (fire), and is thus a more realistic indicator of sequestration rate from this biome. This preliminary analysis suggests that NBP is approximately -1 t C ha y (i.e. a carbon sink). A reduction in the annual area burnt is likely to increase the sink size. Uncertainties surrounding these estimates of NBP and the implications of these uncertainties for land management in these extensive landscapes are discussed.
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http://dx.doi.org/10.1071/FP03215DOI Listing
June 2004

Carbon balance of a tropical savanna of northern Australia.

Oecologia 2003 Nov 26;137(3):405-16. Epub 2003 Aug 26.

Cooperative Research Centre for the Sustainable Development of Tropical Savannas, Faculty of Science, Information Technology and Education, Northern Territory University, NT 0909, Darwin, Australia.

Through estimations of above- and below-ground standing biomass, annual biomass increment, fine root production and turnover, litterfall, canopy respiration and total soil CO(2) efflux, a carbon balance on seasonal and yearly time-scales is developed for a Eucalypt open-forest savanna in northern Australia. This carbon balance is compared to estimates of carbon fluxes derived from eddy covariance measurements conducted at the same site. The total carbon (C) stock of the savanna was 204+/-53 ton C ha(-1), with approximately 84% below-ground and 16% above-ground. Soil organic carbon content (0-1 m) was 151+/-33 ton C ha(-1), accounting for about 74% of the total carbon content in the ecosystem. Vegetation biomass was 53+/-20 ton C ha(-1), 39% of which was found in the root component and 61% in above-ground components (trees, shrubs, grasses). Annual gross primary production was 20.8 ton C ha(-1), of which 27% occurred in above-ground components and 73% below-ground components. Net primary production was 11 ton C ha(-1) year(-1), of which 8.0 ton C ha(-1) (73%) was contributed by below-ground net primary production and 3.0 ton C ha(-1) (27%) by above-ground net primary production. Annual soil carbon efflux was 14.3 ton C ha(-1) year(-1). Approximately three-quarters of the carbon flux (above-ground, below-ground and total ecosystem) occur during the 5-6 months of the wet season. This savanna site is a carbon sink during the wet season, but becomes a weak source during the dry season. Annual net ecosystem production was 3.8 ton C ha(-1) year(-1).
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http://dx.doi.org/10.1007/s00442-003-1358-5DOI Listing
November 2003
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