The effect of climate variables on sapwood anatomy of eucalyptus

This thesis considerably expands our understanding of hydraulic architecture in the genus Eucalyptus. The major finding is that xylem vessels in eucalypts taper at variable rates from the base of the stem to the top of the tree, depending on species and environment. The systematic assessment of changes in the structure of vessel tapering (reduction in xylem vessel diameter per unit length of stem, from the base to the apex) across a gradient of increasing aridity uncovers that the degree of taper is not a function of plant height but for coping with prevalent limitations like water or light. In some tall species from mesic climates (e.g. Eucalyptus regnans F. Muell.) vessel taper may be close to zero for the great majority of the stem, before increasing rapidly within the apical region of the canopy. For other species, such as those from semi-arid environments (e.g. Eucalyptus gracilis F. Muell.), tapering begins much further towards the base of the stem. These findings are highly novel and contradict some major theories (e.g. Metabolic Scaling Theory, MST). Yet, they are entirely in keeping with the general thrust of the ‘cohesion-tension’ theory of water movement in trees. This thesis includes several supporting studies for the above. A glasshouse study suggested thateven at seedling stage, environmental conditions such as temperature and moisture and nutrient availability, play roles in xylem formation (xylogenesis). Despite these suggestions, research in the glasshouse was inconclusive. This was most likely due to the relatively slow response of major biophysical processes, such as the development of structural entities like xylem vessels, when compared to faster responses to environmental conditions of biochemical processes such as those involved in photosynthesis and respiration. A field study of the potential role of water storage in heartwood (i.e. capacitance) in water transport, was hampered by prevailing environmental conditions and uncertainty around the sources of water being used by the study tree. Nonetheless, the data and the knowledge gained by both experiments add to current understanding how functionality of xylem tissues can be maintained under different environmental conditions, including elevated temperatures and water shortage. However, the major body of work in this thesis rests with analysis of xylem vessels at a microscopic scale. This work required significant development of techniques suitable for use with eucalypts that contain some of the hardest wood of all trees. The research also required development of software scripts capable of quantification of properties in large numbers (>150,000) of vessels across a dozen or so species, and multiple field sites. Additionally, the work reported here includes a rigorous assessment of climate across field sites and then use of that to interpret xylem structure. The resultant phase analysis of rates of tapering within trees, is both an Australian and world first. Adopting methodology widely used in other fields of biology, this thesis employs a phase analysis of tapering of xylem vessels to highlight: 1. That the insertion point of vessel taper towards the top of trees differs largely among eucalypt species. 2. That regardless of species and location, vessel diameter at the apex does not differ widely among species – this, at least, accords with MST. 3. That rates of taper within the apical region of canopies where the risk of cavitation is greatest are closely related to environmental conditions, particularly the availability of water and competition for light

The effect of climate variables on sapwood anatomy of eucalyptus

This thesis considerably expands our understanding of hydraulic architecture in the genus Eucalyptus. The major finding is that xylem vessels in eucalypts taper at variable rates from the base of the stem to the top of the tree, depending on species and environment. The systematic assessment of changes in the structure of vessel tapering (reduction in xylem vessel diameter per unit length of stem, from the base to the apex) across a gradient of increasing aridity uncovers that the degree of taper is not a function of plant height but for coping with prevalent limitations like water or light. In some tall species from mesic climates (e.g. Eucalyptus regnans F. Muell.) vessel taper may be close to zero for the great majority of the stem, before increasing rapidly within the apical region of the canopy. For other species, such as those from semi-arid environments (e.g. Eucalyptus gracilis F. Muell.), tapering begins much further towards the base of the stem. These findings are highly novel and contradict some major theories (e.g. Metabolic Scaling Theory, MST). Yet, they are entirely in keeping with the general thrust of the ‘cohesion-tension’ theory of water movement in trees. This thesis includes several supporting studies for the above. A glasshouse study suggested thateven at seedling stage, environmental conditions such as temperature and moisture and nutrient availability, play roles in xylem formation (xylogenesis). Despite these suggestions, research in the glasshouse was inconclusive. This was most likely due to the relatively slow response of major biophysical processes, such as the development of structural entities like xylem vessels, when compared to faster responses to environmental conditions of biochemical processes such as those involved in photosynthesis and respiration. A field study of the potential role of water storage in heartwood (i.e. capacitance) in water transport, was hampered by prevailing environmental conditions and uncertainty around the sources of water being used by the study tree. Nonetheless, the data and the knowledge gained by both experiments add to current understanding how functionality of xylem tissues can be maintained under different environmental conditions, including elevated temperatures and water shortage. However, the major body of work in this thesis rests with analysis of xylem vessels at a microscopic scale. This work required significant development of techniques suitable for use with eucalypts that contain some of the hardest wood of all trees. The research also required development of software scripts capable of quantification of properties in large numbers (>150,000) of vessels across a dozen or so species, and multiple field sites. Additionally, the work reported here includes a rigorous assessment of climate across field sites and then use of that to interpret xylem structure. The resultant phase analysis of rates of tapering within trees, is both an Australian and world first. Adopting methodology widely used in other fields of biology, this thesis employs a phase analysis of tapering of xylem vessels to highlight: 1. That the insertion point of vessel taper towards the top of trees differs largely among eucalypt species. 2. That regardless of species and location, vessel diameter at the apex does not differ widely among species – this, at least, accords with MST. 3. That rates of taper within the apical region of canopies where the risk of cavitation is greatest are closely related to environmental conditions, particularly the availability of water and competition for light

Carbon Sequestration and Carbon Stock of Agroforestry Tree Species Around Cyamudongo Isolated Rain Forest and Arboretum of Ruhande, Rwanda

Agroforestry (AF) is widely considered the most important tool to mitigate climate change-related issues by removing Carbon (C) Dioxide (CO2) from the atmosphere and storing C. Therefore, this study aims to broaden current knowledge on the impact of sustainable Agroforestry (AF) on the C sequestration rate and C stock in the surroundings of Cyamudongo isolated rain forest and Ruhande Arboretum. To understand this, the permanent sample plots (PSPs) were established mainly in the four designed transects of four km long originating on the Cyamudongo isolated rain forest boundary following the slope gradient ranging from 1286 to 2015 m asl. A total number of 73 PSPs were established in the Cyamudongo study area while 3 PSPs were established in the Ruhande AF plot. The Arc Map GIS 10.4 was used to design and map the sampling areas while GPS was used for the localization of the plots. Statistical significance was analyzed through R-software. The estimated quantity of sequestrated C for 2 years and 34 years of AF species was 13.11 t C ha -1 yr-1 (equivalent to 48 t CO2 ha -1 yr-1) and 6.85 t ha-1 yr-1 (equivalent to 25.1 t CO2 ha -1 yr-1) in Cyamudongo and Ruhande respectively. The estimated quantity of C stored by the Ruhande AF plot is 232.94 t ha-1. In Cyamudongo, the overall C stored by the AF systems was 823 t ha-1 by both young tree species established by the Cyamudongo Project (35.84 t ha-1) and C stored by existing AF species before the existence of the Project (787.12 t ha-1). In all study areas, the Grevillea robusta contributed more to overall stored C. The correlation coefficients between tree diameter and living biomass ranged from moderate to very strong due to differences in terms of age, stage of growth, and tree species

Climate determines vascular traits in the ecologically diverse genus Eucalyptus

Current theory presumes that natural selection on vascular traits is controlled by a trade-off between efficiency and safety of hydraulic architecture. Hence, traits linked to efficiency, such as vessel diameter, should show biogeographic patterns; but critical tests of these predictions are rare, largely owing to confounding effects of environment, tree size and phylogeny. Using wood sampled from a phylogenetically constrained set of 28 Eucalyptus species, collected from a wide gradient of aridity across Australia, we show that hydraulic architecture reflects adaptive radiation of this genus in response to variation in climate. With increasing aridity, vessel diameters narrow, their frequency increases with a distribution that becomes gradually positively skewed and sapwood density increases while the theoretical hydraulic conductivity declines. Differences in these hydraulic traits appear largely genotypic in origin rather than environmentally plastic. Data reported here reflect long-term adaptation of hydraulic architecture to water availability. Rapidly changing climates, on the other hand, present significant challenges to the ability of eucalypts to adapt their vasculature

Vessel diameter and related hydraulic traits of 31 Eucalyptus species arrayed along a gradient of water availability

Theory predicts a trade-off between efficiency and safety of water transport in trees (e.g. Tyree and Zimmermann, 2002; Sperry et al., 2008; Meinzer et al., 2010), manifested through changes in tree hydraulic architecture. It is widely observed that average diameters of xylem vessels gradually narrow with decreasing water availability (Carlquist, 2012; Pfautsch et al., 2016). The associated trade-off with narrowing vessel diameter – despite at a higher frequency – is reduced rates of transpiration, lower stomatal conductance and consequently lower foliar uptake of atmospheric CO2 (Santiago et al., 2004; Poorter et al. 2009). Sapwood in such trees is likely to be dense (Chave et al., 2009), arguably due to greater investment in fibre wall thickness that provides increased mechanical strength against the collapse of vessels under high negative pressures (Poorter et al. 2009). Hence, while sapwood of trees in arid environments would consist mostly of narrow vessels, one would expect the hydraulic architecture of trees in mesic environments to feature fewer but wider vessels so as to transport larger quantities of water. This would in turn support high rates of stomatal conductance and uptake of CO2, which fuels rapid growth when synthesizing low-density sapwood (Poorter et al. 2009)