School of Ecosystem and Forest 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, 2017-11-01)
    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: Subâ 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 highâ severity wildfires between 2003 and 2013. At each of 20 sites (five per fire frequency class) we assessed extent of topâ 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 topâ 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 shortâ interval wildfires, which led to changes in live tree patchiness, as indicated by nearestâ neighbour indices. Increased tree mortality was not offset by seedling recruitment, which was significantly lower at the twiceâ and thriceâ burned sites relative to singleâ burn sites â 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 shortâ 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 longâ term persistence of resprouterâ dominated communities.
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    The role of topography and the north Indian monsoon on mean monthly climate interpolation within the Himalayan Kingdom of Bhutan
    Stewart, SB ; Choden, K ; Fedrigo, M ; Roxburgh, SH ; Keenan, RJ ; Nitschke, CR (WILEY, 2017-08-01)
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    Nutrient uptake and use efficiency in co-occurring plants along a disturbance and nutrient availability gradient in the boreal forests of the southwest Yukon, Canada
    Nitschke, CR ; Waeber, PO ; Klaassen, JW ; Dordel, J ; Innes, JL ; Aponte, C ; Gilliam, F (Wiley, 2017-01-01)
    Aim In boreal forest ecosystems plant productivity is typically constrained by mineral nutrient availability. In some boreal regions changes in nutrient availability have led to limited changes in productivity but large changes in plant composition. To determine the impact that a change in nutrient availability has on the plant communities it is important to understand how species use nutrients. Here we explore how plant species and functional types in a cold‐dry boreal forest community use available nutrients by quantifying their respective nutrient utilization and response efficiency. Location Boreal forests in the southwest corner of the Yukon Territory, Canada. Methods We collected soil samples and total plant biomass from 29 plots from nine locations subjected to fire, harvesting or bark beetle disturbances. Nutrient analysis of all vegetation and soil samples were conducted to determine the concentration of macro‐ and micronutrients from both plant biomass and soils collected. Nutrient pools between stands with different disturbance histories are compared. Nutrient uptake, use and response efficiencies were then calculated and nutrient response profiles were developed for each species/functional type. Results We found few differences between nutrient pools in plots with different disturbance histories. A clear separation of species and functional groups in elemental hyperspace suggesting divergent nutrient use in co‐occurring species was identified. The use efficiency analysis highlighted that the species with the highest uptake efficiency have lowest use efficiency and vice versa. Species showed either a monotonic or constant relationship between nutrient response efficiency and N, P, K, reflecting a lack of relationship between plant productivity and resource availability or a linear increase in productivity with increasing nutrient availability, respectively. Conclusions Our findings indicate that species are maximizing nutrient use along different parts of the resource gradient, which has implications for understanding how species respond to changes in nutrient availability. Our findings also show that nutrient use by some species may be governed more by uptake efficiency than use efficiency, allowing them to respond to increases in resource availability by increasing uptake rather than use.
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    An ecoclimatic framework for evaluating the resilience of vegetation to water deficit
    Mitchell, PJ ; O'Grady, AP ; Pinkard, EA ; Brodribb, TJ ; Arndt, SK ; Blackman, CJ ; Duursma, RA ; Fensham, RJ ; Hilbert, DW ; Nitschke, CR ; Norris, J ; Roxburgh, SH ; Ruthrof, KX ; Tissue, DT (WILEY, 2016-05-01)
    The surge in global efforts to understand the causes and consequences of drought on forest ecosystems has tended to focus on specific impacts such as mortality. We propose an ecoclimatic framework that takes a broader view of the ecological relevance of water deficits, linking elements of exposure and resilience to cumulative impacts on a range of ecosystem processes. This ecoclimatic framework is underpinned by two hypotheses: (i) exposure to water deficit can be represented probabilistically and used to estimate exposure thresholds across different vegetation types or ecosystems; and (ii) the cumulative impact of a series of water deficit events is defined by attributes governing the resistance and recovery of the affected processes. We present case studies comprising Pinus edulis and Eucalyptus globulus, tree species with contrasting ecological strategies, which demonstrate how links between exposure and resilience can be examined within our proposed framework. These examples reveal how climatic thresholds can be defined along a continuum of vegetation functional responses to water deficit regimes. The strength of this framework lies in identifying climatic thresholds on vegetation function in the absence of more complete mechanistic understanding, thereby guiding the formulation, application and benchmarking of more detailed modelling.
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    Combining optimization and simulation modelling to measure the cumulative impacts of prescribed fire and wildfire on vegetation species diversity
    Chick, MP ; York, A ; Sitters, H ; Di Stefano, J ; Nitschke, CR ; Driscoll, D (WILEY, 2019-03-01)
    Growth‐stage optimization (GSO) offers a new approach to biodiversity conservation in fire‐prone regions by estimating the optimal distribution of vegetation growth stages that maximize a species diversity index. This optimal growth‐stage structure provides managers an operational goal explicitly linked to a positive conservation outcome but does not define the fire regime needed to achieve it. We paired GSO with LANDIS II, a landscape succession and disturbance simulation model, to (a) estimate the optimal growth‐stage structure that maximized vegetation diversity in a south‐east Australian heathy woodland, (b) define the fire regime needed to achieve it, and (c) determine the cumulative effects of different fire‐regime scenarios on vegetation diversity over a 60‐year period. Scenarios included 0%, 2%, 5%, and 10% of the landscape burnt per year by prescribed fire only, or in combination with three alternative wildfire regimes. Furthermore, we investigated the differences in the optimal growth‐stage structure relating to above‐ground, soil seedbank, and total (above and soil seedbank) diversity datasets. The growth‐stage structure that maximized total vegetation diversity comprised approximately even proportions of all stages. In contrast, separately analysed above‐ground and soil seedbank data resulted in a greater proportion of younger and older growth‐stages, respectively. Scenarios including 5% prescribed burning per year (with and without wildfire) resulted in diversity values within 1.5% of the theoretical maximum value. Scenarios including 2% and 10% prescribed fire resulted in diversity values 8%–12% and 1.5%–5% lower than the maximum, respectively. Scenarios without prescribed fire caused diversity to fall 30%–70%. Trends across the 60 years showed that wildfire depressed diversity and subsequent prescribed fire drove recovery within 15 years. The largest threat to vegetation diversity was the absence of fire. Synthesis and applications. Combining growth‐stage optimization and simulation modelling is a powerful way of defining a conservation‐based fire management goal and identifying the prescribed fire regime needed to achieve it. We demonstrated that vegetation diversity in heathy woodland was increased by prescribed fire, with and without the cumulative effect of wildfire, and declined sharply when fire was excluded. Our method provides a flexible platform for developing long‐term fire management strategies that seek to balance human safety and biodiversity conservation. Including both plants and animals in GSO will help land managers meet the needs of multiple taxa.
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    Factors influencing above-ground and soil seed bank vegetation diversity at different scales in a quasi-Mediterranean ecosystem
    Chick, MP ; Nitschke, CR ; Cohn, JS ; Penman, TD ; York, A ; Tanentzap, A (WILEY, 2018-07-01)
    QUESTIONS: Are factors influencing plant diversity in a fire‐prone Mediterranean ecosystem of southeast Australia scale‐dependent? LOCATION: Heathy woodland, Otways region, Victoria, southeast Australia METHODS: We measured patterns of above‐ground and soil seed bank vegetation diversity and associated them with climatic, biotic, edaphic, topographic, spatial and disturbance factors at multiple scales (macro to micro) using linear mixed effect and generalized dissimilarity modelling. RESULTS: At the macro‐scale, we found species richness above‐ground best described by climatic factors and in the soil seed bank by disturbance factors. At the micro‐scale we found species richness best described above‐ground and in the soil seed bank by disturbance factors, in particular time‐since‐last‐fire. We found variance in macro‐scale β‐diversity (species turnover) best explained above‐ground by climatic and disturbance factors and in the soil seed bank by climatic and biotic factors. CONCLUSIONS: Regional climatic gradients interact with edaphic factors and fire disturbance history at small spatial scales to influence species richness and turnover in the studied ecosystem. Current fire management regimes need to incorporate key climatic–disturbance–diversity interactions to maintain floristic diversity in the studied system.
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    Environmental effects on germination phenology of co-occurring eucalypts: implications for regeneration under climate change
    Rawal, DS ; Kasel, S ; Keatley, MR ; Nitschke, CR (SPRINGER, 2015-09-01)
    Germination is considered one of the important phenological stages that are influenced by environmental factors, with timing and abundance determining plant establishment and recruitment. This study investigates the influence of temperature, soil moisture and light on the germination phenology of six Eucalyptus species from two co-occurring groups of three species representing warm-dry and cool-moist sclerophyll forests. Data from germination experiments were used to calibrate the germination module of the mechanistic model TACA-GEM, to evaluate germination phenology under a range of climate change scenarios. With the exception of E. polyanthemos, the optimal niche for all species was characterised by cool-moist stratification, low light, cool temperatures and high soil moisture. Model results indicated that of the warm-dry species, Eucalyptus microcarpa exhibited greater germination and establishment under projected changes of warmer drier conditions than its co-occurring species Eucalyptus polyanthemos and Eucalyptus tricarpa which suggests that E. microcarpa could maintain its current distribution under a warmer and drier climate in southeastern Australia. Among the cool-moist species, Eucalyptus radiata was the only species that established under projected climate change of the 2080s but at such a low probability that its persistence compared to Eucalyptus obliqua and Eucalyptus sieberi cannot be posited. For all cool-moist species, germination did not benefit from the phenological shifts they displayed. This study successfully demonstrated environmental effects on germination phenology and how a shift in climate can influence the timing and success of recruitment.
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    Environmental effects on growth phenology of co-occurring Eucalyptus species
    Rawal, DS ; Kasel, S ; Keatley, MR ; Aponte, C ; Nitschke, CR (SPRINGER, 2014-05-01)
    Growth is one of the most important phenological cycles in a plant's life. Higher growth rates increase the competitive ability, survival and recruitment and can provide a measure of a plant's adaptive capacity to climate variability and change. This study identified the growth relationship of six Eucalyptus species to variations in temperature, soil moisture availability, photoperiod length and air humidity over 12 months. The six species represent two naturally co-occurring groups of three species each representing warm-dry and the cool-moist sclerophyll forests, respectively. Warm-dry eucalypts were found to be more tolerant of higher temperatures and lower air humidity than the cool-moist eucalypts. Within groups, species-specific responses were detected with Eucalyptus microcarpa having the widest phenological niche of the warm-dry species, exhibiting greater resistance to high temperature and lower air humidity. Temperature dependent photoperiodic responses were exhibited by all the species except Eucalyptus tricarpa and Eucalyptus sieberi, which were able to maintain growth as photoperiod shortened but temperature requirements were fulfilled. Eucalyptus obliqua exhibited a flexible growth rate and tolerance to moisture limitation which enables it to maintain its growth rate as water availability changes. The wider temperature niche exhibited by E. sieberi compared with E. obliqua and Eucalyptus radiata may improve its competitive ability over these species where winters are warm and moisture does not limit growth. With climate change expected to result in warmer and drier conditions in south-east Australia, the findings of this study suggest all cool-moist species will likely suffer negative effects on growth while the warm-dry species may still maintain current growth rates. Our findings highlight that climate driven shifts in growth phenology will likely occur as climate changes and this may facilitate changes in tree communities by altering inter-specific competition.
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