Elevated CO2 did not stimulate stem growth in 11 provenances of a globally important hardwood plantation species

Elevated atmospheric carbon dioxide (eCO2) often enhances rates of photosynthesis leading to increased productivity in trees. In their native habitats in Australia, eucalypts display considerable phenotypic plasticity in response to changes in environmental conditions. Little is known whether this plasticity can be harnessed effectively under future atmospheric eCO2 conditions and be used to identify provenances with superior growth. Here, we report two experiments that assessed the physiological and growth responses of Eucalyptus grandis—one of the world's most important hardwood plantation species—to eCO2. We used 11 provenances from contrasting climates. Our selection was based on site-specific information of long-term temperature and water availability. In Experiment 1, four provenances exhibited significant variation in light-saturated photosynthetic rates (Asat), stomatal conductance (gs), and concentrations of non-structural carbohydrates in leaves, stems and roots when grown under ambient CO2 (aCO2). Biomass of leaves, stems and roots varied significantly and were negatively correlated with mean annual temperature (MAT) at seed origin, indicating that provenances from cooler, wetter climates generally produced greater biomass. Yet, stem growth of these provenances was not stimulated by eCO2. Given the vast environmental gradient covered by provenances of E. grandis, we expanded the selection from four to nine provenances in Experiment 2. This allowed us to validate results from Experiment 1 with its small selection and detailed measurements of various physiological parameters by focusing on growth responses to eCO2 across a wider environmental gradient in Experiment 2. In Experiment 2, nine provenances also exhibited intraspecific differences in growth, but these were not related to climate of origin, and eCO2 had little effect on growth traits. Growth responses under eCO2 varied widely across provenances in both experiments, confirming phenotypic plasticity in E. grandis, though responses were not systematically correlated with climate of origin. These results indicate that selection of provenances for improved stem growth of E. grandis under future eCO2 cannot be based solely on climate of origin, as is common practice for other planted tree species

The effects of drought, heat and elevated atmospheric CO2 on physiology and growth of Eucalyptus : does climate-of-origin matter?

This thesis assessed the effects of future climate factors (i.e. [CO2], heat waves and soil water availability) on growth and physiology of Eucalyptus species originating from different climates-of-origin. The main aim was to test intra-specific variation of plant traits to climate change. Four tree species native to Australia were selected due to their national ecological and international economic importance: Eucalyptus camaldulensis, E. grandis, E. melliodora and E. coolabah. I tested the response of E. camaldulensis to elevated atmospheric [CO2] (eCO2), heat and drought stress; E. grandis to eCO2 and drought stress; and the acclimation response of E. melliodora and E. coolabah to wetting and drying cycles with final drought to mortality. Phenotypic plasticity in leaf gas exchange, growth and non-structural carbohydrate (NSC) reserves was significantly different in E. camaldulensis and E. grandis when subjected to heat and/or water stress. In E. grandis, the tallest trees from cool temperatures had the greatest growth reductions during stress. In E. camaldulensis, trees originating from semi-arid climates initiated leaf abscission early and conserved NSC, which led to faster stem and leaf area recovery than trees from more mesic climates. Moreover, eCO2 ameliorated stress responses related to photosynthesis when trees were either heat stressed or water-limited; time-to-leaf-death was extended in one provenance of E. camaldulensis in eCO2. There was no acclimation of leaf gas exchange to variable water availability during the series of droughts in E. melliodora and E. coolabah. Yet, species had contrasting water use strategies linked to their distributional range across Australia. E. coolabah originating from semi-arid climate reduced its leaf area to prevent hydraulic failure, while E. melliodora originating from mesic climate utilized NSC reserves to tolerate water limitation. These results highlight the importance of soil water availability for physiological functioning and growth, but also show that intra-specific differences exist in response to heat and drought. In conclusion, my PhD research extends information on inter- and intra-specific differences in phenotypic plasticity of trees to the main and interactive effects of climate factors, which can be used to identify plantation trees for future climate regimes

Insulation capacity of three bark types of temperate Eucalyptus species

Fire plays an increasingly important role in management of native forests and plantations around the world. Thick tree bark represents the most important defence against surface fires, although other bark traits (bark-type, moisture content, density) are also involved. The interplay of bark traits to reduce heat-induced cambium necrosis and related tree death remains poorly understood. Here we introduce a novel method using multiple sensors to accurately measure conductance of heat through the periderm and secondary phloem and to detect how individual bark traits influence the transfer of heat through those tissue types. We employed this method to document the capacity for bark insulation of three Eucalyptus species with different bark-type from temperate Australia, simulating a 'worst-case' scenario (750°C bark surface temperature for 900s). Thickness of these different bark types ranged from 3 to 65mm. Our results clearly show the importance of thickness and type of bark for prevention of cambium necrosis due to heat. Coefficients of determination that describe how bark thickness correlates with time to reach lethal temperatures (>60°C) at the cambium ranged from 0.61 (least effective: E. tricarpa, ironbark-type bark, average moisture content=34%, average bark density=0.58gcm-3) to 0.94 (most effective: E. leucoxylon, gum-type bark, average moisture content=54%, average bark density=0.42gcm-3) and both followed linear and curvilinear trajectories. The cooling effect of water in the periderm was found to slow conduction of heat towards the cambium. This effect has not previously been documented by empirical measurements and may have significant implications to survival of trees during "cooler" prescribed fires. Our study highlights between-species variation in ability to withstand heat from surface fires. Fire temperatures and duration thus have considerable capacity to change species composition of these Box-Ironbark forests via mortality

Elevated CO 2 Did Not Stimulate Stem Growth in 11 Provenances of a Globally Important Hardwood Plantation Species

International audienceElevated atmospheric carbon dioxide (eCO 2) often enhances rates of photosynthesis leading to increased productivity in trees. In their native habitats in Australia, eucalypts display considerable phenotypic plasticity in response to changes in environmental conditions. Little is known whether this plasticity can be harnessed effectively under future atmospheric eCO 2 conditions and be used to identify provenances with superior growth. Here, we report two experiments that assessed the physiological and growth responses of Eucalyptus grandis-one of the world's most important hardwood plantation species-to eCO 2. We used 11 provenances from contrasting climates. Our selection was based on site-specific information of long-term temperature and water availability. In Experiment 1, four provenances exhibited significant variation in light-saturated photosynthetic rates (A sat), stomatal conductance (g s), and concentrations of non-structural carbohydrates in leaves, stems and roots when grown under ambient CO 2 (aCO 2). Biomass of leaves, stems and roots varied significantly and were negatively correlated with mean annual temperature (MAT) at seed origin, indicating that provenances from cooler, wetter climates generally produced greater biomass. Yet, stem growth of these provenances was not stimulated by eCO 2. Given the vast environmental gradient covered by provenances of E. grandis, we expanded the selection from four to nine provenances in Experiment 2. This allowed us to validate results from Experiment 1 with its small selection and detailed measurements of various physiological parameters by focusing on growth responses to eCO 2 across a wider environmental gradient in Experiment 2. In Experiment 2, nine provenances also exhibited intraspecific differences in growth, but these were not related to climate of origin, and eCO 2 had little effect on growth traits. Growth responses under eCO 2 varied widely across provenances in both experiments, confirming phenotypic plasticity in E. grandis, though responses were not systematically correlated with climate of origin. These results indicate that selection of provenances for improved stem growth of E. grandis under future eCO 2 cannot be based solely on climate of origin, as is common practice for other planted 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)