School of Agriculture, Food and Ecosystem Sciences - Research Publications

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    Frequent wildfires erode tree persistence and alter stand structure and initial composition of a fire-tolerant sub-alpine forest
    Fairman, TA ; Bennett, LT ; Tupper, S ; Nitschke, CR ; Ward, D (WILEY, 2017-11)
    QUESTION: Frequent severe wildfires have the potential to alter the structure and composition of forests in temperate biomes. While temperate forests dominated by resprouting trees are thought to be largely invulnerable to more frequent wildfires, empirical data to support this assumption are lacking. Does frequent fire erode tree persistence by increasing mortality and reducing regeneration, and what are the broader impacts on forest structure and understorey composition? LOCATION: Sub‐alpine open Eucalyptus pauciflora forests, Australian Alps, Victoria, Australia. METHODS: We examined tree persistence and understorey composition of E. pauciflora open forests that were unburned, burned once, twice or three times by high‐severity wildfires between 2003 and 2013. At each of 20 sites (five per fire frequency class) we assessed extent of top‐kill and mortality of eucalypt clumps, spatial configuration of surviving and dead clumps, densities of new and lignotuberous eucalypt seedlings, and shrub and grass cover. RESULTS: At least 2 yr after the last wildfire, proportions of top‐killed E. pauciflora stems were significantly higher, and densities of live basal resprouts significantly lower, at sites burned two or three times compared to once burned or unburned sites. Clump death increased to 50% of individuals at sites burned by three short‐interval wildfires, which led to changes in live tree patchiness, as indicated by nearest‐neighbour indices. Increased tree mortality was not offset by seedling recruitment, which was significantly lower at the twice‐ and thrice‐burned sites relative to single‐burn sites – although seedling recruitment was also influenced by topography and coarse woody debris. In addition to changes in the tree layer, the prominence of understorey shrubs was substantially reduced, and the frequency of grasses markedly increased, after two, and particularly three wildfires. CONCLUSIONS: Our study provides strong empirical evidence of ecologically significant change in E. pauciflora forests after short‐interval severe wildfires, namely, erosion of the persistence niche of resprouting trees, and a shift in understorey dominance from shrubs to grasses. Our findings highlight the need to consider the impacts of compounded perturbation on forests under changing climates, including testing assumptions of long‐term persistence of resprouter‐dominated communities.
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    Structural diversity underpins carbon storage in Australian temperate forests
    Aponte, C ; Kasel, S ; Nitschke, CR ; Tanase, MA ; Vickers, H ; Parker, L ; Fedrigo, M ; Kohout, M ; Ruiz-Benito, P ; Zavala, MA ; Bennett, LT ; Hickler, T (WILEY, 2020-05)
    Abstract Aim Forest carbon storage is the result of a multitude of interactions among biotic and abiotic factors. Our aim was to use an integrative approach to elucidate mechanistic relationships of carbon storage with biotic and abiotic factors in the natural forests of temperate Australia, a region that has been overlooked in global analyses of carbon‐biodiversity relations. Location South‐eastern Australia. Time period 2010–2015. Major taxa studied Forest trees in 732 plots. Methods We used the most comprehensive forest inventory database available for south‐eastern Australia and structural equation models to assess carbon‐storage relationships with biotic factors (species or functional diversity, community‐weighted mean (CWM) trait values, structural diversity) and abiotic factors (climate, soil, fire history). To assess the consistency of relationships at different environmental scales, our analyses involved three levels of data aggregation: six forest types, two forest groups (representing different growth environments), and all forests combined. Results Structural diversity was consistently the strongest independent predictor of carbon storage at all levels of data aggregation, whereas relationships with species‐ and functional‐diversity indices were comparatively weak. CWMs of maximum height and wood density were also significant independent predictors of carbon storage in most cases. In comparison, climate, soil, and fire history had only minor and mainly indirect effects via biotic factors on carbon storage. Main conclusions Our results indicate that carbon storage in our temperate forests was underpinned by tree structural diversity (representing efficient utilisation of space) and by CWM trait values (representing selection effects) more so than by tree species richness or functional diversity. Abiotic effects were comparatively weak and mostly indirect via biotic factors irrespective of the environmental range. Our study highlights the importance of managing forests for functionally important species and to maintain and enhance their structural complexity in order to support carbon storage.
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    Climate extreme variables generated using monthly time-series data improve predicted distributions of plant species
    Stewart, SB ; Elith, J ; Fedrigo, M ; Kasel, S ; Roxburgh, SH ; Bennett, LT ; Chick, M ; Fairman, T ; Leonard, S ; Kohout, M ; Cripps, JK ; Durkin, L ; Nitschke, CR (WILEY, 2021-04)
    Extreme weather can have significant impacts on plant species demography; however, most studies have focused on responses to a single or small number of extreme events. Long‐term patterns in climate extremes, and how they have shaped contemporary distributions, have rarely been considered or tested. BIOCLIM variables that are commonly used in correlative species distribution modelling studies cannot be used to quantify climate extremes, as they are generated using long‐term averages and therefore do not describe year‐to‐year, temporal variability. We evaluated the response of 37 plant species to base climate (long‐term means, equivalent to BIOCLIM variables), variability (standard deviations) and extremes of varying return intervals (defined using quantiles) based on historical observations. These variables were generated using fine‐grain (approx. 250 m), time‐series temperature and precipitation data for the hottest, coldest and driest months over 39 years. Extremes provided significant additive improvements in model performance compared to base climate alone and were more consistent than variability across all species. Models that included extremes frequently showed notably different mapped predictions relative to those using base climate alone, despite often small differences in statistical performance as measured as a summary across sites. These differences in spatial patterns were most pronounced at the predicted range margins, and reflect the influence of coastal proximity, continentality, topography and orographic barriers on climate extremes. Species occupying hotter and drier locations that are exposed to severe maximum temperature extremes were associated with better predictive performance when modelled using extremes. Understanding how plant species have historically responded to climate extremes may provide valuable insights into our understanding of contemporary distributions and help to make more accurate predictions under a changing climate.
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    Predicted consequences of increased rainfall variability on soil carbon stocks in a semiarid environment
    Forouzangohar, M ; Setia, R ; Wallace, DD ; Nitschke, CR ; Bennett, LT (INTER-RESEARCH, 2016)
    Research on the impacts of climate change on soil organic carbon (SOC) stocks has focused on the effects of changes in average climate, but the potential effects of increased climate variability, including more frequent extreme events, remain under-examined. In this study, set in a semiarid agricultural landscape in southeastern Australia, we used the Rothamsted carbon (RothC) model to isolate the effects of interannual rainfall variability on SOC stocks over a 50 yr period. We modelled SOC trends in response to 3 scenarios that had the same 50 yr average climate but different interannual rainfall distributions: non-changing average climate, historic variability (H), and increased variability due to more frequent extreme rainfall years (XH). Relative to the non-changing average climate, RothC simulations predicted net decreases in mean SOC stocks to 50 yr of 11% under the H scenario and 13% under the XH scenario. These decreases were the result of predicted SOC decreases (and increased CO2 emissions) in extreme wet years (ca. 0.26 Mg ha(-1) yr(-1)) that were not counterbalanced by SOC increases in extreme dry years (ca. 0.11 Mg ha(-1) yr(-1)). No significant difference in mean SOC stocks at 50 yr between the H and XH scenarios was likely due to an increase in both extreme wet and counterbalancing extreme dry years in the latter. Strong negative correlations were found between annual changes in SOC stocks and rainfall. Our modelled predictions indicate the potential for extreme rainfall years to influence SOC gains and losses in semiarid environments and highlight the importance of maintaining plant inputs in these environments, particularly during extreme wet years.
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    Psychological values and cues as a basis for developing socially relevant criteria and indicators for forest management
    Ford, RM ; Anderson, NM ; Nitschke, C ; Bennett, LT ; Williams, KJH (Elsevier BV, 2017-05-01)
    Criteria and indicators (C & I) have proven an essential tool for managers implementing sustainable forest management, but have been less effective for communication with the wider community. We demonstrate a new bottom-up approach to developing socially relevant C & I using social analysis and psychology-based concepts and methods. Our conceptual framework links the concepts of valued attributes and environmental cues with, respectively, criteria and indicators. We illustrate our approach using thirty-six semi-structured interviews of individual members of the general public and of stakeholder groups in Victoria, southern Australia. The interviews included a modified cognitive mapping task to identify attributes of forests valued by the interviewees, as well as cues used by them to know if a valued attribute was present or had changed. Seven broad valued attributes of forests were identified: Natural; Experiential; Productive; Setting; Social/Economic; Learning; and Cultural. Four broad categories of cues were identified: Biophysical; Social/Psychological; Economic; and Management/Planning. Cues were translated into a set of measurable ‘socially relevant’ indicators of forest management. Comparison with existing frameworks revealed some similarities, but that an important component of public evaluations, Experiential and Setting valued attributes, was largely absent from C & I used in Victoria, which are based on the Montreal Process framework. Some socially relevant indicators aligned with existing indicators, but others were poorly represented, particularly sensory indicators that are concerned with subjective experiences of forests. Our approach demonstrates a new way of developing C & I and has a strong conceptual basis that enables more explicit consideration and communication of a comprehensive range of social values and cues in environmental management systems.