School of Agriculture, Food and Ecosystem Sciences - Theses

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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.