School of Agriculture, Food and Ecosystem Sciences - Theses

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    Environmental filtering shapes plant turnover and species occurrence in post‐logging regrowth forest in southeastern Australia
    Singh, Anu ( 2021)
    Environmental factors play a more influential role in shaping plant community composition, while disturbance shapes plant community composition in southeastern Australian temperate forests. Plant communities in forests subjected to timber harvesting have been found to differ from wildfire sites in the montane forests of the Central Highlands in Victoria; however, a quantitative understanding of the factors that shape post harvesting plant communities is lacking. Quantifying the factors that shape plant community composition in post logging regrowth forest is important for understanding how timber harvesting influences plant biodiversity. Here, I aimed to explore the role environmental filtering species turnover and composition of the species in post logging regrowth forests. I focused my studies on the forested landscapes of southeastern Australia, where bushfires and timber harvesting are the primary catalysts for regeneration in Eucalyptus regnans, E. delegatensis, and high elevation mixed species forests. I investigated the post disturbance regeneration dynamics in these forests and sought to determine the direct impact of climate variability on regeneration and the interactive effects of climate, topography, and edaphic factors on the regeneration success of Eucalyptus. Untangling the roles of climate, topography and edaphic conditions on plant regeneration is important for understanding current and future risks of climate change to plant species richness. To test the influence of climatic, topographic, and edaphic variables on the occurrence and abundance of Eucalyptus regeneration, I used machine learning models. Declines in number of seedling regeneration of eucalypt during the period of drought were greater in E. regnans and E. delegatensis than HEMS forests, suggesting that regeneration in the HEMS forests is more resistant to drought. I furthermore found that seasonal precipitation and temperature had the greatest influence on regeneration success of Eucalyptus. My findings highlight the importance of seasonal and annual climate variability on Eucalyptus regeneration and portend potential declines in regeneration success in a warmer and drier future, particularly for E. regnans and E. delegatensis. A fundamental requirement of sustainable forest management is that stands are adequately regenerated after harvesting. To date most research has focused on the regeneration of the dominant timber species and to a lesser degree on plant communities. Relatively few studies have explored the impact of regeneration success of the dominant tree species on plant community composition and diversity. Therefore, I quantified the influence of environmental filtering on plant species diversity in montane regrowth forests dominated by Eucalyptus regnans in mainland southeastern Australia. I found that Acacia density shaped plant biodiversity more than Eucalyptus density. I also found that edaphic factors, in particular soil nutrition and moisture availability, played a significant role in shaping species turnover and occurrence. My findings suggest that the density of Acacia is a key biotic filter that influences the occurrence of many understorey plant species and shapes plant community turnover. This should be considered when assessing the impacts of both natural and anthropogenic disturbances on plant biodiversity. In this thesis, I also explore the role of soil seedbank as a source of plant propagules in these forests. Our ecological understanding of plant community response to disturbance and environmental variation is largely restricted to the above ground species pool. Plant community composition often changes dramatically after disturbance due to mortality of above ground vegetation and recruitment of species that respond to a change in resource availability. To quantify the relative importance of environmental gradients on individual species occurrence and community composition, I used a joint analysis approach. In total there were 113 plant species in the combined species pool. A total of 39 species were shared between above ground and soil seedbank pools. There were 41 species exclusive to the above ground vegetation. Aridity was the main environmental covariate explaining plant community across all pools of plant diversity and across non woody and woody life forms. Environmental covariates explained more than 59 percent of the variance for 43 species in the combined species pool. The composition of the soil seedbank and above ground diversity was distinct, with low similarity 14 percent, which highlights the importance of the soil seedbank as a reservoir for plant diversity not captured in above ground vegetation. Finally, I aimed to quantify the influence of Acacia and Eucalyptus composition and configuration on species turnover to provide an important tool for mapping patterns of plant diversity in post disturbance forests. To achieve this, I combined remotely sensed UAS imagery with ground survey data of plant composition from post logging regrowth forests. I found that spatial predictions of forest configurations providing Eucalyptus and Acacia cover metrics such as spatial aggregation were useful in estimating understorey plant beta diversity. Significant relationships between the aggregation metrics derived from UAS imagery as well as site aridity and beta diversity were observed. Increasing aggregation of Acacia, aridity and number of Acacia patches had a significant negative effect on plant beta diversity, whereas number of patches of Eucalyptus had a positive influence. This research highlights how remote sensing can provide and improve measures of forest plant biodiversity in regrowth forests which can support forest managers and conservation efforts to quantify and map patterns of plant diversity at the stand scale and beyond. Overall, my findings highlight that post logging regrowth forests are systematically shaped by soil and climatic factors while also being filtered by stand structure and composition. I demonstrate the role of climate, topography, soil, and light availability in shaping plant communities in post logging regrowth forests. The success of eucalypt regeneration in the stand reinitiation phase influences overstorey composition and structure. I found that that soil nutrition and moisture availability played a significant role in shaping plant community composition at fine scales and aridity at broad scales. I further found that Acacia density shaped plant biodiversity more than Eucalyptus density. My study highlights the role of environmental filtering on plant community composition in post logging regrowth and how it must be considered when assessing the impacts of anthropogenic disturbances on plant biodiversity in the temperate forest of southeastern mainland Australia.
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    Investigation of bark properties and cambium cell viability of Eucalyptus in relation to heat exposure
    Subasinghe Achchige, Yasika Medhavi ( 2021)
    Fire is integral to many temperate forest ecosystems. Given increasing occurrence of wildfires around the world, forest management applications such as low and moderate intensity burnings are required to reduce fuel loads to decrease the severity of wildfires. However, little is known about the effect of low to moderate intensity fires on vascular cambium necrosis in trees. During a fire, heat is transferred through the tree bark towards the vascular cambium (i.e., a vital tissue layer inside a tree stem which ensures the perennial growth of a tree) potentially increasing cambium temperature to lethal levels. As tree bark shields the vascular cambium from thermal damage, a better understanding of the bark traits that protect the vascular cambium during fires is required. Genus Eucalyptus is broadly distributed in fire-prone ecosystems thus, exhibits different fire adaptive traits such as post-fire regeneration strategies (i.e., resprouting via epicormic strands) and has a wide range of different bark types. As a native plant genus and the dominant species in open forests of southern Australia, Eucalyptus species present a great opportunity to investigate bark properties in relation to cambium cell viability. In this study, firstly, cambium sections were exposed to heat treatments in vitro to determine the best method to estimate a cell viability index (CVI) to allow a detailed investigation of heat degradation of cellular function in relation to fires. A tetrazolium reduction method (TTC method) was compared to a Neutral Red method applied to different tissue sizes to quantitatively determine CVI and to derive a critical temperature threshold for cambial cell viability in vitro (Chapter 2). The interactive effect of temperature and exposure time on cambium cell viability in vitro was investigated in the third Chapter. Based on findings of Chapters 2 and 3, properties of the bark i.e., bark thickness, moisture content, bark density, thermal diffusivity, and thermal conductivity of the three Eucalyptus species of contrasting bark types (E. obliqua - stringy bark, E. radiata - Fibrous bark and E. ovata - Smooth bark) were investigated in Chapter 4. In Chapter 4, stem sections of freshly felled trees were exposed to a fixed heat flux which simulated conditions of low to moderate intensity fires; thermocouples were inserted into sapwood, cambium and bark to measure the temperature and time to reach critical temperature of 60oC was recorded. Cell viability was measured against the untreated control samples. Bark properties of three species were measured and analyzed against cell damage. The key results of this study were: (i) Tetrazolium reduction method (TTC method) is the preferred method to assess cell viability of Eucalyptus species, while Neutral Red method can be used to cross check the results of the TTC method; (ii) Critical temperature for cambium cell viability is 60oC; (iii) A prolonged exposure to sublethal temperatures (40-50oC) causes similar effect as a short exposure to lethal temperatures (>50oC); (iv) Critical exposure time in-vitro for cambium cell viability of Eucalyptus species is 1-5 minutes; (v) Bark moisture and thickness play the major roles in regulating heat transfer through bark; (vi) A thicker, dryer, lower density and lower thermal conductivity stringy bark of E. obliqua shows greater insulation ability than the other two bark types tested; (vii) Critical exposure time for cambium cell viability in-vivo may vary between 20 to 40 minutes depending on bark type and bark thickness; (viii) Among the trees tested the radiant energy required for the cambium-phloem cells to reach critical temperature ranged between 3.5 and 13.6 MJ m-2; (ix) Prolonged exposure to low heat flux like 10 kW m-2 can also cause significant cambium damage. Findings of this study have provided significant insights in relation to properties of tree bark, to better understand the heat tolerance levels of Eucalyptus species during low to moderate intensity fires. The study developed a novel method to assess the cambium cell viability of Eucalyptus species following heat exposure. Overall, this study provides a better understanding for land managers to perform low intensity fuel reduction burns to avoid tree damage. Findings of this work will guide and expand future research on stem heat transfer models and fire behavior models to improve tree survival following fires.