School of Ecosystem and Forest 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.
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    Effects of adding nutrients on soil chemistry and tree growth in native Eucalyptus forests of south-eastern Australia
    Severino, Dean Christopher ( 2007)
    The decreasing area available for timber extraction in south-eastern Australia, due largely to social pressure to reserve greater areas of forest, has led to the consideration of fertiliser-application to increase wood output from the remaining available forest. Potentially deleterious effects of fertilising on water quality must be assessed before implementation on a wide scale. This is in accordance with relevant forest management policies. This study examined the effects of applying fertilisers containing nitrogen and phosphorus, on soil and soil-water chemistry in two pole-sized stands of mixed Eucalyptus spp in the Wombat Forest, in the Midlands Forest Management Area, Victoria, Australia. The findings are synthesised and discussed in relation to management of regenerating mixed-eucalypt forests in south-eastern Australia. Fertiliser treatments were none (R); 400 kg N ha-1 as ammonium-sulphate (N); or 400 kg ha-1 plus 202 kg P ha-1 as triple superphosphate coated with 10% sulphur (NP). It was calculated that incidental additions of S were 1371 kg ha -1 (N treatments), and 1696 kg ha-1 (NP treatments). It was expected that P would be principally adsorbed on soil surfaces; N immobilised in the soil organic pool and that metallic cations would enter the soil solution to varying degrees. Fertiliser-addition increased both plot-basal-area (BA) growth and the rate of stand self-thinning. In 3.8 years, BA in reference (R) plots at two sites increased by 7.3% and 23.4%. Where N alone was added, BA increased by 14.2% and 27.1%, while in NP plots BA increased by 17.1% and 42.7% respectively. Mortality was 9% in untreated plots compared to 14% in NP plots. Estimated increases in biomass growth equated to additional above-ground nutrient accumulation of 0.4 to 1.5 kg ha-1 of P, and 5.5 to 20.8 kg ha-1 of N. This represented only 0.2 to 0.7% of added P, and 1.4 to 5.2% of added N. Soil solution was extracted from 10 and 50 cm with porous-ceramic-cup tension-lysimeters (-0.6 kPa). Concentrations of P and N were low both before and after adding fertiliser. Across all treatments the maximum median PO43- concentration in soil-water at 50 cm was 0.12 ppm (mean 0.28 ppm). Typically PO43- concentrations were not higher than 0.03 ppm. The 400 kg ha-1 of added N was rapidly immobilised in the soil organic pool. The greatest mean NH4' concentration from a single sampling occasion was 1.1 ppm. The mean NO3 concentration at 50 cm was never higher than 0.26 ppm. After adding N in fertiliser the proportion of NO3- relative to NH4* in soil-water increased and was correlated with decreasing soil-water pH. Less than 1% of added P and N was recovered from soil solution at 50 cm. The largest pool of added P recovered was PO43- adsorbed to soil between 0 and 20 cm, due to the soil adsorption capacity being well in excess of the applied 202 kg P ha-1. Phosphate desorption using sequential extractions with a mild acid extractant (0.3M NH4F, 0.1M HCI) recovered between 25% and 116% of added P. Differences were attributed to both the amount of P added and the effect of time since treatment at different sites. Soil disturbance during sampler installation was found to be more likely to raise soil-water P concentrations at 50 cm than would adding up to 202 kg P ha-1. Among the ions in solution. SO42- and CI' were the dominant anions while Cat+ dominated the cation chemistry. In untreated forest 5042- in soil-water ranged from 7.7 to 16.0 ppm at 10 cm and 7.9 to 12.2 ppm at 50 cm. In fertilised plots up to 100.5 ppm SO42 was measured in soil-water at 50 cm depth. In the N treatment at 50 cm, SO42- in soil-water accounted for 9.4 % of applied S. compared to 14.0 % in NP. In untreated forest, soil-water Cl- and SO42- accounted for over 98% of the total soil-water anions, in roughly equal proportions at 10 cm, and CI- slightly higher at 50 cm. Following fertiliser-application soil-water pH at 10 cm fell from 6.3 in R to as low as 4.81 (N) and 4.45 (NP). At 50 cm pH never dropped below 6 and there were no visible departures from reference concentrations. Relative activities of K+ and Mg2+ in solution increased with decreasing pH, indicating increased leaching potential. Sulphate in soil-water increased total anion charge further in NP than in N. Total charge (cmolc L-1) for cations followed anions. A slight deficit in anion charge was likely due to the unquantified contribution of organic anions. These results confirm that despite the quantity of fertilisers added in this trial being double likely operational quantities, the forest and associated soils had the capacity to retain these nutrients through a variety of processes. The study validates the environmental sustainability of proposed intensive management practices including fertiliser-application in this forest type. It also emphasises the importance of understanding fundamental forest nutrient cycling processes when aiming to carry out intensive forest management practices in an environmentally sensitive manner.
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    Looking for expansins : a molecular approach to the investigation of tylose development and heartwood formation in Eucalyptus nitens
    Tonkin, Miriam Ruth ( 2006)
    Advanced stages of stem development in many tree species, including eucalypts, are marked by the transition of conductive sapwood to non-conductive heartwood. Heartwood formation follows a characteristic sequence of events involving the accumulation of phenolic compounds in ray parenchyma cells, the occlusion of vessels by tylose and/or gum formation, cell death and the subsequent release of phenolic compounds into the surrounding tissue. These events are dependent upon the activity of ray parenchyma cells, but the molecular processes involved, particularly with regard to tylose formation, remain largely unknown. The identification of molecular pathways leading to tylose formation might yield insights into heartwood formation. A molecular approach to the investigation of tylose formation is hampered by the paucity and inaccessibility of ray parenchyma cells and the asynchronous nature of tylose formation. Based on the assumption that wound-induced tyloses and those formed during the transition of sapwood to heartwood develop via a common mechanism, these difficulties were overcome by using the wounding response of the tree (a 12-year-old Eucalyptus nitens sapling) to induce extensive and simultaneous development of tyloses. Tylose formation involves the marked extension of a primary cell wall structure. Elsewhere, such wall extension has been closely associated with the activity of expansins. These constitute a large, multi-gene family of proteins which are widely distributed throughout higher plants and which have been shown to induce relaxation and extension of primary cell walls, often in a cell- and tissue-specific manner. It is proposed that expansins are likely to be involved in tylose formation. Ray and axial parenchyma cells are the only living cells found in sapwood, and primary cell wall extension is only possible through tylose formation. Thus, gene expression associated with wall extension occurring in sapwood is likely to be associated with tylose formation. Cellular material from outer sapwood showing extensive wound-induced tylose development was successfully harvested and partial cDNA sequences displaying significant homology with a-expansins were identified. This provides circumstantial evidence that expansin gene expression is associated with tylose formation and should encourage further investigation of the molecular pathways involved in this process.
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    Photosynthetic responses to light, nitrogen, phosphorus and pruning of Eucalyptus in south-eastern Australia
    Turnbull, Tarryn Louise ( 2005)
    Eucalypts frequently grow faster after additions of fertiliser, but more slowly in the shade or following `green pruning'. The coupling of rates of growth to environmental factors is at least partly due to acclimation of photosynthetic processes. Photosynthesis rarely proceeds at maximum rates in natural environments as photosynthetic processes and the supply of basic requirements of photosynthesis (CO2, H20, light, phosphorus and nitrogen) vary at both short (minutes to hours) and longer (days to months) time scales. Currently we lack mechanistic explanations for how these variables, alone and in combination underpin changed growth rates in Eucalyptus. This study examined growth and photosynthetic characteristics in glasshouse-grown seedlings and field-grown trees of Eucalyptus species that are commonly planted for forestry and revegetation purposes in central Victoria. Acclimation to light (among seedlings and within canopies), nutrient availability (phosphorus and nitrogen) and increased sink-strength for photosynthates were the primary foci of the study. In each instance I examined distribution of leaf nutrients within a canopy and allocation of N to Rubisco and chlorophyll to assess the degree to which nutrients limit photosynthesis in Eucalyptus. A novel technique was introduced to quantify the allocation of inorganic phosphorus within cells (cytoplasm versus vacuole), followed by an assessment of inorganic phosphorus allocation in response to a long-term reduction in phosphorus supply. In all circumstances, rates of growth were responsive to environmental conditions. Growth responses were underpinned by altered patterns of biomass partitioning and changed leaf morphology more than by rates of photosynthesis per se. There was little difference in adaptive strategies implemented by seedlings and trees: both were oriented towards the accumulation of nutrients rather than increasing rates of photosynthesis. Photosynthesis was reduced by shading (among different plants and within the canopy of a tree) and reduced phosphorus supply whereas N had little effect on photosynthesis. Analysis of pools of inorganic P revealed that adequate supplies were maintained for photosynthetic processes regardless of P supply, therefore reduced photosynthesis follows, rather than leads, a more general leaf-level response to reduced P. Similarly, changed partitioning of nitrogen between Rubisco and chlorophyll was unnecessary as leaf nitrogen concentrations were consistently maintained at well above published minimum levels. Hence, photosynthesis was not up-regulated following increased nitrogen or phosphorus supply; instead excess nutrients were accumulated and used to support increased biomass. One exception was after defoliation, when up-regulation of photosynthesis was observed, presumably to ensure the demand for photosynthates could be met by a reduced leaf area. Sensitivity analyses consistently revealed variation in photosynthetic rates owed more to altered biochemical activity (e.g. Jmax and Vcmax) rather than stomatal conductance regardless of growth condition (glasshouse versus field). Hence, whilst Eucalyptus has considerable photosynthetic potential, faster rates of carbon fixation are only exhibited in the short-term. In part, this is due to the multiplicity of factors involved in `optimisation' of photosynthesis and their individual and collective responses to environmental conditions. In the long term however, increased canopy photosynthetic capacity follows only an increased photosynthetic area.
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    Physiological and phylogenetic patterns in eucalyptus spp. responses to salinity and drought
    Merchant, Andrew ( 2004)
    Increasing aridity is regarded as a major driving force behind the evolution of the genus Eucalyptus. Previous investigations have identified both morphological and physiological characteristics whose properties, both in distribution and function, correlate strongly with adaptation to saline and arid environments. One such response is the regulation of osmotic potential via both acclimating and constitutive mechanisms. The identity of solutes contributing to these responses has hitherto remained unknown. Substantial evidence for the role(s) of cyclitols (cyclic polyols) during stressful conditions has been gleaned from previous investigations in both tree and herbaceous species. Here I uncover contrasting biochemical mechanisms for the regulation of osmotic potential among eucalypt species, with broad implications for taxonomy and evolution. For the first time, targeted biochemical profiling has uncovered a quantitative yet discrete link between eucalypt taxonomy and adaptation to arid environments. The distribution of cyclitols among eucalypt species correlates strongly with other demonstrated adaptations to arid conditions. Analysis of Eucalyptus leptophylla leaf tissues from trees growing along a topographic gradient encompassing hyper-saline lakes, suggests that cyclitols contribute significantly to osmotic adjustment induced by drought. The presence of substantial quantities of cyclitols in xylem sap of E. leptophylla suggests additional roles in the signalling of plant stress reponses. Determination of the contribution of low molecular weight carbohydrates, polyols and inorganic ions towards the regulation of osmotic potential in glasshouse grown E. spathulata (Hook.) show that salt and drought stress elicit contrasting mechanisms of solute concentration. Under salt stress, seedlings reduce osmotic potential by accumulating large quantities of inorganic ions in leaf tissues, presumably sequestrating them into the vacuole. Under drought stress, seedlings reduce osmotic potential through the concentration of constitutively present solutes - particularly that of the cyclitol quercitol. Targeted biochemical profiling of 13 species of Eucalyptus originating from contrasting rainfall environments revealed equally contrasting biochemical responses to drought stress. In species originating from low rainfall environments, the constitutive concentration of quercitol is the major mechanism of osmotic regulation whilst species originating from high rainfall environments osmotically adjust through the active accumulation of sucrose. These results suggest that the solutes governing both acclimating and constitutive regulation of osmotic potential in Eucalyptus species have finally been identified. The implications of these findings are discussed in the context of plant adaptation to saline and arid environments.
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    The Cell biology of eucalypt heartwood formation
    Wilson, Lawrence Francis ( 2001)
    Trees are among the most successful of organisms. Despite voracious enemies, diverse and treacherous habitats, individuals may thrive for hundreds of years, or even millennia. Their success is due to adaptability, and the development of powerful multi-faceted defensive systems. The least understood of these is the heartwood/tylosis system. Once thought to be merely a metabolic waste dump, or an irrelevant consequence of homeostatic processes, heartwood is now, with the benefit of new perspectives, emerging as a crucial and dynamic defensive element. This thesis proposes that ray parenchyma cells actively produce heartwood to provide a micro-organism resistant base for the vulnerable sapwood transpiration system and carbohydrate reserve. Statistical analyses of a 20-year-old Eucalyptus nitens tree showed that heartwood existed as a central pillar within the tree, and tyloses within heartwood were optimally distributed to seal the largest and most vulnerable vessels against invasion. Ultrastructural analysis showed that ray cells in the middle and inner sapwood were essentially dormant, but were reactivated in the heartwood transition zone. Ray cells forming tyloses showed evidence of a high level of activity with the nucleus directing expansion from within tyloses. The coordinated activity and subsequent deaths of these cells suggest the possibility of programmed death as their ultimate fate. Ray cell deaths result in release of phytotoxins into the surrounding tissue. This is in contrast to animal cell apoptosis where release of toxins into the surrounding tissue is actively prevented. Research into the microstructural and molecular basis of heartwood formation in vivo is limited by the reactivity of extractives within the transition zone, the inaccessibility of cells, and the transitory and asynchronous nature of heartwood transformation. In vitro systems, in which initiation and inhibition of heartwood-like transformation can be easily manipulated, may ultimately provide solutions to these problems. The development of two of these systems is discussed.
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    Effects of salinity on growth and wood and fibre properties in eucalypts
    Catchpoole, Stephen John ( 2001)
    Salinity, the presence of soluble salts in soils or waters, can be separated into naturally occurring primary salinity, and secondary salinity resulting from human activities such as land development and agriculture. Secondary salinity involving high, saline water-tables affects large areas, estimated between 4.7 and 6.1 'ha (Williamson 1990, Robertson 1996), of agricultural land in Australia, and tree planting is one approach to lower saline watertables. Such plantations could become a significant fibre source for the pulp and paper industry, but it is not known whether growth in salt-affected environments influences key fibre properties important in paper production. This work therefore examined the wood and fibre properties of Tasmanian blue gum, Eucalyptus globulus ssp. globulus, and river red gum, E. camaldulensis, grown under various conditions of soil salinity. Eucalyptus globulus was studied in trial plantations in the Shepparton region of north-central Victoria. The plantations were established in 1993, and field sampling was carried out from 1995 to 1997. Salinity in the top 20 cm of soil over the period of study (1994 to 1997), according to the soil salinity classes set lutin Marcar et al. (1995), ranged from non-saline at the fresh-channel water-irrigated plot to slightly saline for the saline groundwater-irrigated plots. Tree size generally did not differ significantly between plots at any age. Differences in foliar [Na±], {K±] and [Cl-] occurred between the field plots but were not consistent between years. The highest concentrations of foliar ions were also not always associated with the plot receiving the highest salinity irrigation water, suggesting that in only some years was the soil salt level sufficient to cause a plant response. Wood basic density differed between the plots, but it could not be attributed to salinity, and may have reflected other site-specific effects. Fibre morphology parameters did not differ significantly between the plots. There were some differences between the plots in the increase in fibre length from year to year but the differences were not consistent over the entire survey period and could not conclusively be attributed to differences in soil salinity. A pilot salinity pot trial was conducted on E. camaldulensis plants, as a precursor to a more elaborate experiment planned for potted E. globulus plants. The E. camaldulensis pot trial comprised a single concentration salt (NaCl) treatment and a control (freshwater) treatment applied over a 60 day period. A marked reduction of growth occurred with salt-treated seedlings relative to control seedlings. Concomitant with the reduction in growth, salt-treated seedlings produced significantly shorter, thicker-walled fibres than the control seedlings. The pot-trial on 18-month-old Eucalyptus globulus ssp. globulus trees applied different concentration salt (lRlaCi) solutions over a 10-week period. The salinity of the potting mixture increased markedly in the salt-treated trees relative to the controls. Foliar chloride and sodium were also significantly greater in trees on the higher salt treatments than in the control trees. Diameter growth decreased with the higher salt treatments, and five trees under high salt treatments had to be harvested prior to the planned completion of the experiment, due to their poor state of health. These results indicated the salt treatments had influenced some aspects of tree physiology. A wound made to the cambium allowed pre-treatment fibres (fibres formed prior to the start of the experiment) to be distinguished from post-treatment fibres (fibres formed during the experiment) in the E.globulus pot trial. Trees on higher salt treatments produced significantly longer, thinner-walled fibres compared to controls, but this pattern also occurred in fibres formed before treatments were imposed, implying that these differences were due to preexisting differences in the trees unrelated to the salt treatment. Statistical analysis of fibres formed during treatments, taking account of pre-existing differences, found that there was no significant effect of salt treatment on fibre length or wall thickness, although this was possibly because of the low sample size relative to the variation of the experimental material. The controlled application of salt for 10 weeks during the E. globulus pot trial thus had some effect on tree physiology, but no significant effects on fibre dimensions or wood formation. This was consistent with the observation in the field trial that fibre dimensions and wood formation were not influenced by factors that did not also reduce tree growth, at least in trees up to 4 years old. Higher levels of salt could cause rapid tree decline due to the inability of the trees to exclude the salt, and processes associated with fibre formation would then also cease. The combined results from the field and pot trials indicated that E. globulus, a slightly to moderately salt-tolerant species, suffered negligible or minor growth reductions on soils irrigated to a slightly saline level, and produced fibres of similar morphology to trees grown under non-saline conditions. If soil salinity increased above the moderate level, the trees would continue to grow provided sufficient water is still available, but internal salt levels would increase to the point where tree death would result. Based on the pot trial, where such internal salt levels were achieved, the decline and death of the trees would occur before the salt affects fibre morphology. Eucalyptus camaldulensis adopted a different strategy to cope with salt stress than E. globulus. Eucalyptus globulus continued to grow provided it was supplied with water, despite its saline nature. Finally, when salt levels within the plant reached a critical level, plant health rapidly declined. When E. camaldulensis was watered with solution of a similar salinity to the highest salt treatments in the E. globulus pot trial, there was a rapid cessation in extension growth, but there was no other sign of a deterioration in plant health. The mechanism by which E. camaldulensis was able to quickly cease shoot growth, which presumably allowed it to tolerate saline conditions by restricting salt uptake, was not investigated here. Material from Eucalyptus camaldulensis that had been growing for 14 years on a dryland plantation site in southwest Western Australia was also investigated. Trees from the high salinity area did not differ significantly in average height, diameter and volume from those from the low salinity area. Basic density were significantly greater in the high salinity group of trees than in the low salinity group, but no relationship with tree growth was established. The absence of a relationship between growth and basic density was not unusual, as natural variation in basic density makes it difficult to establish environmental or experimental effects (Downes and Raymond 1997). Fibre fractional wall coverage was greater in the high salinity group of trees than in the low salinity group, as was also the case for the E. camaldulensis pot trial. In the pot trial, however, a significant growth reduction due to salinity was recorded. There were no other differences in fibre morphology between the high and low salinity groups of the Western Australian plantation. Eucalyptus globulus is less salt and waterlogging tolerant than E. camaldulensis (Bennett and George 1995a; Bennett and George 1995b) but in the field studies the growth and wood and fibre properties for each species was similar across the range of salinities encountered. The exception was basic density and fibre fractional wall coverage in the 14-year-old E. camaldulensis, both of which were greater in the high salinity group of trees. It was expected that the growth of E. globulus would be adversely affected if irrigation with the saline groundwater continued for several more years, allowing a build up in soil salinity. Based on the results from the E. globulus pot trial, once soil salinity levels exceed the tolerance limits of this species, a rapid decline in tree health will occur, and fibre formation will cease. Eucalyptus camaldulensis, with its greater salt and waterlogging tolerance, will grow in areas where other commercial species, such s E. globulus, would not thrive. However, E. camaldulensis has disadvantages for farm forestry in Australia, due to low percentage pulp yields by comparison with E. globulus (Arnold et al. 1999), and poor growth rates and tree form (Mazanec 1999). In the USA, E. camaldulensis has equalled the pulp yield of the commercially proven E. globulus (Arnold et al. 1999). Further research into improving pulp yields, growth rates and tree form of E. camaldulensis in Australia, would allow expansion of eucalypt plantations for pulp and wood production, as well as land and water care, onto previously unsuitable land.
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    Chlorophyll fluorescence for rapid assessment of salt tolerance in eucalypts
    Corney, Helen Jane ( 1999)
    Chlorophyll fluorescence parameters were investigated, using the OPTISCIENCES 0S5-FL fluorometer. for their potential as non-invasive rapid indices of salt stress in Eucalypts. Cloned E. camaldulensis and E. urophvlla x grandis plants were subjected to salinity (0-650mM NaCI) and chlorophyll fluorescence parameters measured. Dark-adapted and quenching parameters did not show consistent responses to salinity stress. The light-adapted parameter Fds, measured by the Yield program, showed a predictable trend in response to salinity treatment. Measurements were influenced by diurnal changes, positional effects and environmental changes. It is therefore important to measure Fd, within a controlled environment, randomising and rearranging plants during the course of the experiment. Fully expanded, green, attached leaves should be used for measurement and salt concentration should be raised slowly (e.g. 50mM/5 days) after a concentration of 300mM NaCI has been reached. Statistical analysis should be performed using non-parametric methods as chlorophyll fluorescence parameters are not normally distributed. In most cases Fds declines before the onset of visible physical symptoms, however it is affected by sudden shock. It is therefore important that results show a consistent decline in Fds and/or Fds decreases markedly (e.g. 40-50% of the control). This method allows for the objective ranking for salt tolerance of both seedlings and clones. Fds may also have the potential to detect individual responses to salinity and assist the detection of individuals with uncharacteristic responses or unsatisfactory root systems within clones.