Browsing by Author "Bowman, DMJS"

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    Age and growth of a fire prone Tasmanian temperate old-growth forest stand dominated by Eucalyptus regnans, the world's tallest angiosperm.
    (Elsevier, 2010-07-15) Wood, SW; Hua, Q; Allen, KJ; Bowman, DMJS
    Forests are key components of the global carbon cycle, with deforestation being an important driver of increased atmospheric carbon dioxide. Temperate old-growth forests have some of the highest above ground stores of carbon of any forest types on Earth. Unlike tropical forests, the ecology of many temperate forests is dominated by episodic disturbance, such as high intensity fire. An exemplar of a particularly carbon dense temperate forest system adapted to infrequent catastrophic fires is the Eucalyptus regnans forests of south eastern Australia. Knowledge of the growth and longevity of old-growth trees is crucial to understanding the carbon balance and fire regimes of these forest systems. In an old-growth E. regnans stand in the Styx Valley in southern Tasmania we used dendrochronological techniques and radiocarbon dating to determine the age and stem growth of E. regnans and Phyllocladus aspleniifolius, an understorey rainforest conifer. Our analysis revealed that an even-aged cohort of E. regnans and P. aspleniifolius established in 1490–1510AD, apparently after a stand-replacing fire. The stem growth rates of E. regnans in the first 100 years were very rapid compared to the co-occurring P. aspleniifolius. That the longevity of E. regnans is >500 years challenges the suggested 350–450 year timeframe proposed for the widely held model of succession from eucalypt to rainforest. These forests not only have the potential to store vast amounts of carbon, but can also maintain these high carbon densities for a long period of time. Estimates of the capacity of these forests to sequester and store carbon should explicitly consider past harvesting and fire regimes and the potential increases in the risk of fire associated with climate change. © 2010, Elsevier Ltd.
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    Centennial-scale trends in the Southern Annular Mode revealed by hemisphere-wide fire and hydroclimatic trends over the past 2400 years
    (Geological Society of America, 2018-02-15) Fletcher, MS; Benson, B; Bowman, DMJS; Gadd, PS; Heijnis, H; Mariani, M; Saunders, KM; Wolfe, BB; Zawadzki, A
    Millennial-scale latitudinal shifts in the southern westerly winds (SWW) drive changes in Southern Ocean upwelling, leading to changes in atmospheric CO2 levels, thereby affecting the global climate and carbon cycle. Our aim here is to understand whether century-scale shifts in the SWW also drive changes in atmospheric CO2 content. We report new multiproxy lake sediment data from southwest Tasmania, Australia, that show centennial-scale changes in vegetation and fire activity over the past 2400 yr. We compare our results with existing data from southern South America and reveal synchronous and in-phase centennial-scale trends in vegetation and fire activity between southwest Tasmania and southern South America over the past 2400 yr. Interannual to centennial-scale rainfall anomalies and fire activity in both these regions are significantly correlated with shifts in the SWW associated with the Southern Annular Mode (SAM; atmospheric variability of the Southern Hemisphere). Thus, we interpret the centennial-scale trends we have identified as reflecting century-scale SAM-like shifts in the SWW over the past 2400 yr. We identify covariance between our inferred century-scale shifts in the SWW and Antarctic ice core CO2 values, demonstrating that the SWW-CO2 relationship operating at a millennial scale also operates at a centennial scale through the past 2400 yr. Our results indicate a possible westerly-driven modulation of recent increases in global atmospheric CO2 content that could potentially exacerbate current greenhouse gas–related warming. © 2021 Geological Society of America
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    Continental-scale climatic drivers of growth ring variability in an Australian conifer
    (Springer, 2011-10-01) Bowman, DMJS; Prior, LD; Tng, D; Hua, Q; Brodribb, TJ
    Callitris is Australia's most successful and drought tolerant conifer genus. Callitris species are distributed across a huge geographical range from rainforest to arid zones, and hence they provide a rare opportunity to view plant growth trends across the continent. Here, we make a continental-scale examination of how climate influences basal diameter growth in Callitris. We sampled a total of five species but focused effort (23 of 28 samples) on the most widespread species, C. columellaris. Cores from a total of 23 trees were sampled from 15 sites that spanned a gradient in mean annual rainfall from 225 to 2117 mm and mean annual temperature from 11.5 to 28.2 degrees C. Ring production is not annual across much of the distribution of the genus, so C-14-AMS dating was used to establish the frequency of ring production for each core. Ring width, tracheid lumen diameter and number of tracheids per ring were also measured on each core. Ring production was close to annual at mesic sites with reliable alternation of rainfall or temperature regimes but was more erratic elsewhere. For C. columellaris, ring width significantly increased with mean annual rainfall (r(2) = 0.49) as a result of wider and more tracheids per ring. For this species tracheid lumen diameter was correlated with annual rainfall (r(2) = 0.61), with a threefold increase from the driest to the wettest sites, lending support to the hypothesis that conifers growing at drier sites will have narrow lumen diameters to maximise mechanical strength of the xylem. © 2011, Springer. The original publication is available at www.springerlink.com
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    Effect of landscape fires on the demography of the endangered New Caledonian conifer Callitris sulcata
    (Elsevier, 2015-11-01) Haverkamp, C; Prior, LD; Fogliani, B; L’Huillier, L; Anquez, M; Hua, Q; Bowman, DMJS
    New Caledonia is a global biodiversity hotspot and an epicentre for Gondwanan conifers, many of which are threatened by mining and by altered fire regimes. We studied the distribution, abundance and demography of the endangered Callitris sulcata. The largest populations are restricted to one river system in the south-east of the island, with satellite populations in adjoining rivers. The local distribution is controlled by the fire protection afforded by terrain features such as scree slopes, creeklines and small cliffs. Adult trees, which have comparatively thick bark, are able to tolerate and recover from infrequent surface fires, but severe fires kill trees and the seeds they store, a pattern similar to that in many Australian Callitris species. Radiocarbon dating revealed the species is slower growing than Australian Callitris species, possibly due to the extreme infertility of the ultramafic soils. The species is of high cultural value to the indigenous population who also prizes the durable and aromatic timber, and harvests have been traditionally regulated. Illegal cutting of trees has become a problem, but uncontrolled fires, which have caused substantial population declines, dwarf this threat. Given these threats, conservation of the species hinges on ensuring some populations remain remote and rarely visited by humans. © 2015, Elsevier Ltd.
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    Fire-patterned vegetation and the development of organic soils in the lowland vegetation mosaics of south-west Tasmania
    (CSIRO Publishing, 2011-03-28) Wood, SW; Hua, Q; Bowman, DMJS
    Two contrasting ecological models have been proposed for the forest-moorland vegetation mosaics of southwest Tasmania that stress different interactions between fire, soils, vegetation and the physical environment to produce either stable or dynamic vegetation patterns. We investigated aspects of these models by sampling organic soil profiles across vegetation mosaics to determine variation in soil depth, organic carbon (C) content, nutrient capital, stable C isotope composition (delta(13)C) and (14)C radiocarbon age in two contrasting landscape settings. (14)C basal ages of organic soils ranged from recent (<400 calibrated (cal.) years BP) to mid Holocene (similar to 7200 cal. years BP), with a tendency for older soils to be from poorly drained moorlands and younger soils from the forest. The long-term net rate of C accumulation ranged from 2.7 to 19.2 gCm(-2) year(-1), which is low compared with northern hemisphere peatland systems. We found that delta(13)C in organic soil profiles cannot be used to infer Holocene vegetation boundary dynamics in these systems. We found a systematic decrease of phosphorus from rainforest through eucalypt to moorland, but estimated that phosphorus capital in moorland soils was still sufficient for the development of forest vegetation. Our results suggest that the characteristics of organic soils across the landscape are the result of interactions between not only vegetation and fire frequency, but also other factors such as drainage and topography. © 2011, CSIRO Publishing
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    The legacy of mid-Holocene fire on a Tasmanian montane landscape
    (Wiley Blackwell, 2014-03-01) Fletcher, MS; Wolfe, BB; Whitlock, C; Pompeani, DP; Heijnis, H; Haberle, SG; Gadd, PS; Bowman, DMJS
    AimTo assess the long-term impacts of landscape fire on a mosaic of pyrophobic and pyrogenic woody montane vegetation. LocationSouth-west Tasmania, Australia. MethodsWe undertook a high-resolution multiproxy palaeoecological analysis of sediments deposited in Lake Osborne (Hartz Mountains National Park, southern Tasmania), employing analyses of pollen, macroscopic and microscopic charcoal, organic and inorganic geochemistry and magnetic susceptibility. ResultsSequential fires within the study catchment over the past 6500years have resulted in the reduction of pyrophobic rain forest taxa and the establishment of pyrogenic Eucalyptus-dominated vegetation. The vegetation change was accompanied by soil erosion and nutrient losses. The rate of post-fire recovery of widespread rain forest taxa (Nothofagus cunninghamii and Eucryphia spp.) conforms to ecological models, as does the local extinction of fire-sensitive rain forest taxa (Nothofagus gunnii and Cupressaceae) following successive fires. Main conclusionsThe sedimentary analyses indicate that recurrent fires over several centuries caused a catchment-wide transition from pyrophobic rain forest to pyrophytic eucalypt-dominated vegetation. The fires within the lake catchment during the 6500-year long record appear to coincide with high-frequency El Nino events in the equatorial Pacific Ocean, signalling a potential threat to these highly endemic rain forests if El Nino intensity amplifies as predicted under future climate scenarios. © 2014, Wiley-Blackwell.
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    Validating putatively cross-dated Callitris tree-ring chronologies using bomb-pulse radiocarbon analysis
    (CSIRO Publishing, 2011-02-10) Pearson, S; Hua, Q; Allen, KJ; Bowman, DMJS
    Nuclear weapons testing during the 1950s generated an atmospheric pulse of the carbon isotope, (14)C. Worldwide, trees growing during that period and in subsequent decades assimilated (14)C-enriched CO(2), leaving a distinct isotopic signature that can be used to precisely date tree rings. Thirty single-ring samples were extracted for AMS (14)C analysis from cores taken from living trees of five different Callitris species [C. endlicheri (Parl.) F. M. Bailey, C. glaucophylla Joy Thomps. & L.A.S. Johnson, C. intratropica Benth., C. preissii Miq., and C. rhomboidea R.Br. ex Rich. & A. Rich] at 13 sites. The ages of individual tree rings were determined by both (14)C bomb-pulse dating and cross-dating (based on 20-30 cores from the same site) in order to (1) provide independent verification of tree-ring dates, (2) detect false or missing rings from sites with otherwise good chronologies, and (3) test whether growth rings were annual for cores from sites where cross-dating was not possible. Our approach confirmed dates on chronologies from monsoon tropical sites, provided checked chronologies in subtropical and temperate sites, and improved dating control on arid-zone ring counts. It was found that Callitris are more likely to form regular annual rings when growing in seasonally dry environments than in more arid sites with highly variable precipitation patterns. © 2011, CSIRO Publishing
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    Vegetation, fire and soil feedbacks of dynamic boundaries between rainforest, savanna and grassland
    (John Wiley & Sons, Inc, 2016-08-23) MacDermott, HJ; Fensham, RJ; Hua, Q; Bowman, DMJS
    At fine spatial scales, savanna-rainforest-grassland boundary dynamics are thought to be mediated by the interplay between fire, vegetation and soil feedbacks. These processes were investigated by quantifying tree species composition, the light environment, quantities and flammability of fuels, bark thickness, and soil conditions across stable and dynamic rainforest boundaries that adjoin grassland and eucalypt savanna in the highlands of the Bunya Mountains, southeast Queensland, Australia. The size class distribution of savanna and rainforest stems was indicative of the encroachment of rainforest species into savanna and grassland. Increasing dominance of rainforest trees corresponds to an increase in woody canopy cover, the dominance of litter fuels (woody debris and leaf), and decline in grass occurrence. There is marked difference in litter and grass fuel flammability and this result is largely an influence of strongly dissimilar fuel bulk densities. Relative bark thickness, a measure of stem fire resistance, was found to be generally greater in savanna species when compared to that of rainforest species, with notable exceptions being the conifers Araucaria bidwillii and Araucaria cunninghamii. A transect study of soil nutrients across one dynamic rainforest – grassland boundary indicated the mass of carbon and nitrogen, but not phosphorus, increased across the successional gradient. Soil carbon turnover time is shortest in stable rainforest, intermediate in dynamic rainforest and longest in grassland highlighting nutrient cycling differentiation. We conclude that the general absence of fire in the Bunya Mountains, due to a divergence from traditional Aboriginal burning practices, has allowed for the encroachment of fire-sensitive rainforest species into the flammable biomes of this landscape. Rainforest invasion is likely to have reduced fire risk via changes to fuel composition and microclimatic conditions, and this feedback will be reinforced by altered nutrient cycling. The mechanics of the feedbacks here identified are discussed in terms of landscape change theory. © Ecological Society of Australia