Determinants of maximum tree height in Eucalyptus species along a rainfall gradient in Victoria, Australia

We present a conceptual model linking dry-mass allocational allometry, hydraulic limitation, and vertical stratification of environmental conditions to patterns in vertical tree growth and tree height. Maximum tree height should increase with relative moisture supply and both should drive variation in apparent stomatal limitation. Carbon isotope discrimination (δ) should not vary with maximum tree height across a moisture gradient when only hydraulic limitation or allocational allometry limit height, but increase with moisture when both hydraulic limitation and allocational allometry limit maximum tree height. We quantified tree height and D along a gradient in annual precipitation from 300 to 1600 mm from mallee to temperate rain forest in southeastern Australia; Eucalyptus on this gradient span almost the entire range of tree heights found in angiosperms worldwide. Maximum tree height showed a strong, nearly proportional relationship to the ratio of precipitation to pan evaporation. D increased with ln P/Ep, suggesting that both hydraulic limitation and allocational allometry set maximum tree height. Coordinated shifts in several plant traits should result in different species having an advantage in vertical growth rate at different points along a rainfall gradient, and in maximum tree height increasing with relative moisture supply, photosynthetic rate, nutrient supply, and xylem diameter

Phenotypic plasticity and water availability: responses of alpine herb species along an elevation gradient

Background Alpine regions are particularly vulnerable to the effects of climate change. The Australian Alps are potentially more so than other mountain regions, as they cover a very small geographic area (<0.05% of mainland Australia), with a low maximum elevation (2228 m). Therefore, response to climate change will be primarily determined by the ability of species to survive in-situ through local adaptation or phenotypic plasticity. Existing climate change models project not only warming but increasingly variable precipitation and snow cover across the Australian Alps. Thus, plasticity in water use traits may become increasingly important for the establishment and persistence of Australian alpine plants. Given that plants from lower elevations inhabit a more heterogeneous environment with more frequent frosts, greater temperature extremes, and higher evapotranspiration, we predict plasticity – and particularly adaptive plasticity – may be more common at low relative to high elevation. To test these predictions we investigated the extent of plasticity and the adaptive value thereof in water use traits in three herbaceous Australian alpine plant species. Seeds were collected from low and high elevation alpine sites and grown at ample and limiting water availability under common-garden conditions. For morphological and physiological traits, we compared both their means and phenotypic plasticity across treatments and elevations. Results Responses of morphological and physiological traits to water availability were in accord with many previous studies of water response. Although previous work in the same environment demonstrated greater plasticity in response to temperature for low elevation populations, plasticity in response to water availability in our study showed markedly little variation as a function of elevation. Rather, patterns of plasticity were highly variable among species and among traits within species, with few instances of adaptive plastic responses. Conclusion We discuss the difficulties in observing adaptive plasticity and the importance of microhabitat variation in shaping the persistence of these Australian alpine species.S.R.G was supported by an Australian Government Research Training Program (RTP) Scholarship. J.A.R-V was supported by a González Esparcia postdoctoral scholarship from the Technical University of Madrid. We also acknowledge an Australian Research Council fellowship to A.B.N, FT100100464

Contrasting hydraulic regulation in closely related forage grasses: implications for plant water use

Abstract. Plant traits that improve crop water use efficiency are highly sought after but difficult to isolate. Here, we examine the integrated function of xylem and stomata in closely related forage grasses to determine whether quantitative differences inwater transport properties could be used to predict plant performance under limitedwater conditions. Cultivars of two forage grass species with different drought tolerance ratings, Lolium multiflorum Lam. and Festuca arundinacea Schreb., were assessed for maximum hydraulic conductivity (Kmax), vulnerability of xylem to hydraulic dysfunction (P50) and stomatal sensitivity to leaf water potential. Species-specific differenceswere observed in several of these traits, and their effect on whole-plant performance was examined under well-watered and restricted watering conditions. It was shown that althoughP50was comparable between species, forF. arundinacea cultivars, therewas greater hydraulic risk associatedwith reduced stomatal sensitivity to leaf hydration. In contrast, L. multiflorum cultivars expressed a higher capacity for water transport, butmore conservative stomatal regulation. Despite different susceptibilities to leaf damage observed during acute drought, under the sustainedmoderate drought treatment, the two strategieswere balanced in termsofwater conservation and hydraulic utilisation, resulting in similar dry matter production. Characterisation of water use patterns according to the key hydraulic parameters is discussed in terms of implications to yield across different environmental scenarios as well as the applicability of water transport related traits to breeding programs

Estimation of Photorespiratory Fluxes by Gas Exchange

Photorespiratory fluxes can be easily estimated by photosynthetic gas exchange using an infrared gas analyzer and applying the Farquhar, von Caemmerer, and Berry (Farquhar et al. Planta 149:78–90, 1980) photosynthesis model. For a more direct measurement of photorespiratory CO2 release from glycine decarboxylation, infrared gas analysis can be coupled to membrane-inlet mass spectrometry, capable of separating the total CO2 concentration into its 12CO2 and 13CO2 components in a continuous online fashion. This chapter discusses how to calculate rates of photorespiration from Rubisco kinetics and describes in detail a method for measuring the CO2 release from glycine decarboxylation using 13CO2.We would therefore like to thank the Deutsche Forschungsgemeinschaft and the 12 project partners and 10 associate partners for financially supporting the Promics project (DFG FOR1186) and thus for making this book possible

Two-Source δ¹⁸O Method to Validate the CO¹⁸O-Photosynthetic Discrimination Model: Implications for Mesophyll Conductance

Theoretical models of photosynthetic isotopic discrimination of CO2 (13C and 18O) are commonly used to estimate mesophyll conductance (gm). This requires making simplifying assumptions and assigning parameter values so that gm can be solved for as the residual term. Uncertainties in gm estimation occur due to measurement noise and assumptions not holding, including parameter uncertainty and model parametrization. Uncertainties in the 13C model have been explored previously, but there has been little testing undertaken to determine the reliability of gm estimates from the 18O model (gm18). In this study, we exploited the action of carbonic anhydrase in equilibrating CO2 with leaf water and manipulated the observed photosynthetic discrimination (D18O) by changing the oxygen isotopic composition of the source gas CO2 and water vapor. We developed a two-source d18O method, whereby two measurements of D18O were obtained for a leaf with its gas-exchange characteristics otherwise unchanged. Measurements were performed in broad bean (Vicia faba) and Algerian oak (Quercus canariensis) in response to light and vapor pressure deficit. Despite manipulating the D18O by over 100‰, in most cases we observed consistency in the calculated gm18, providing confidence in the measurements and model theory. Where there were differences in gm18 estimates between source-gas measurements, we explored uncertainty associated with two model assumptions (the isotopic composition of water at the sites of CO2-water exchange, and the humidity of the leaf internal airspace) and found evidence for both. Finally, we provide experimental guidelines to minimize the sensitivity of gm18 estimates to measurement errors. The two-source d18O method offers a flexible tool for model parameterization and provides an opportunity to refine our understanding of leaf water and CO2 fluxes.This work was supported by Australian Research Council Discovery Grant DP15010058

Aciphylla glacialis mortality, growth and frost resistance: a field warming experiment

Decreasing snow depth and earlier snowmelt in alpine regions are expected to expose plants to a greater range of thermal extremes. Thus, paradoxically, in addition to increasing mean temperatures, aseasonal frost is likely to emerge as a major determinant of plant survival and development under a warming climate. Through the use of open top chambers, we assessed the effects of simulated warming on seedlings of the alpine herb Aciphylla glacialis (F.Muell.) Benth, sourced from provenances that vary in thermal characteristics. We hypothesised that seedlings grown at elevated temperature would have reduced survival and lower freezing resistance, but that individuals that did survive would show increased growth. Further, we hypothesised that seedlings sourced from the lower-elevation sites, where temperatures are more variable, would exhibit lower mortality at warmed conditions than those from sites with narrower thermal ranges. Warmed conditions significantly increased seedling mortality but those that survived grew slightly taller than their ambient counterparts, with no impact on leaf production or photosynthetic efficiency. Although the warming treatment did not influence temperature minimums, which would have allowed us to assess the impact of aseasonal frosts, there was no effect on freezing resistance. Contrary to our expectations, there was little evidence of variation among provenances. Our results indicate that a warmer climate with more extreme events may lead to a reduction in seedling establishment and survival; however, seedlings that do survive and establish are unlikely to express any lingering detrimental effects