Bridge to the future: Important lessons from 20 years of ecosystem observations made by the OzFlux network.

In 2020, the Australian and New Zealand flux research and monitoring network, OzFlux, celebrated its 20th anniversary by reflecting on the lessons learned through two decades of ecosystem studies on global change biology. OzFlux is a network not only for ecosystem researchers, but also for those 'next users' of the knowledge, information and data that such networks provide. Here, we focus on eight lessons across topics of climate change and variability, disturbance and resilience, drought and heat stress and synergies with remote sensing and modelling. In distilling the key lessons learned, we also identify where further research is needed to fill knowledge gaps and improve the utility and relevance of the outputs from OzFlux. Extreme climate variability across Australia and New Zealand (droughts and flooding rains) provides a natural laboratory for a global understanding of ecosystems in this time of accelerating climate change. As evidence of worsening global fire risk emerges, the natural ability of these ecosystems to recover from disturbances, such as fire and cyclones, provides lessons on adaptation and resilience to disturbance. Drought and heatwaves are common occurrences across large parts of the region and can tip an ecosystem's carbon budget from a net CO2 sink to a net CO2 source. Despite such responses to stress, ecosystems at OzFlux sites show their resilience to climate variability by rapidly pivoting back to a strong carbon sink upon the return of favourable conditions. Located in under-represented areas, OzFlux data have the potential for reducing uncertainties in global remote sensing products, and these data provide several opportunities to develop new theories and improve our ecosystem models. The accumulated impacts of these lessons over the last 20 years highlights the value of long-term flux observations for natural and managed systems. A future vision for OzFlux includes ongoing and newly developed synergies with ecophysiologists, ecologists, geologists, remote sensors and modellers.

The three major axes of terrestrial ecosystem function.

The leaf economics spectrum1,2 and the global spectrum of plant forms and functions3 revealed fundamental axes of variation in plant traits, which represent different ecological strategies that are shaped by the evolutionary development of plant species2. Ecosystem functions depend on environmental conditions and the traits of species that comprise the ecological communities4. However, the axes of variation of ecosystem functions are largely unknown, which limits our understanding of how ecosystems respond as a whole to anthropogenic drivers, climate and environmental variability4,5. Here we derive a set of ecosystem functions6 from a dataset of surface gas exchange measurements across major terrestrial biomes. We find that most of the variability within ecosystem functions (71.8%) is captured by three key axes. The first axis reflects maximum ecosystem productivity and is mostly explained by vegetation structure. The second axis reflects ecosystem water-use strategies and is jointly explained by variation in vegetation height and climate. The third axis, which represents ecosystem carbon-use efficiency, features a gradient related to aridity, and is explained primarily by variation in vegetation structure. We show that two state-of-the-art land surface models reproduce the first and most important axis of ecosystem functions. However, the models tend to simulate more strongly correlated functions than those observed, which limits their ability to accurately predict the full range of responses to environmental changes in carbon, water and energy cycling in terrestrial ecosystems7,8.

The validity of optimal leaf traits modelled on environmental conditions.

The ratio of leaf intercellular to ambient CO2 (χ) is modulated by stomatal conductance (gs ). These quantities link carbon (C) assimilation with transpiration, and along with photosynthetic capacities (Vcmax and Jmax ) are required to model terrestrial C uptake. We use optimization criteria based on the growth environment to generate predicted values of photosynthetic and water-use efficiency traits and test these against a unique dataset. Leaf gas-exchange parameters and carbon isotope discrimination were analysed in relation to local climate across a continental network of study sites. Sun-exposed leaves of 50 species at seven sites were measured in contrasting seasons. Values of χ predicted from growth temperature and vapour pressure deficit were closely correlated to ratios derived from C isotope (δ13 C) measurements. Correlations were stronger in the growing season. Predicted values of photosynthetic traits, including carboxylation capacity (Vcmax ), derived from δ13 C, growth temperature and solar radiation, showed meaningful agreement with inferred values derived from gas-exchange measurements. Between-site differences in water-use efficiency were, however, only weakly linked to the plant's growth environment and did not show seasonal variation. These results support the general hypothesis that many key parameters required by Earth system models are adaptive and predictable from plants' growth environments.

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

We present two comprehensive data sets that describe xylem vessel diameters and related sapwood traits for species of Eucalyptus from arid and semi-arid woodlands and forests in Australia. Between 2009 and 2014, sapwood of mature trees was sampled in south-western, south-eastern and eastern Australia. One additional species was sampled from tropical north-western Australia. The first data set describes samples collected from the basal stem section (130 cm above ground) of three individuals of 31 species of which eight species were replicated at sites that differed in climatic conditions. The second data set describes vessel characteristics of three trees from each of 10 species that were sampled at 8 m below the tree apex. The sampled trees of these 10 species are also part of the first data set. In total, we report diameters (D) for over 25 100 vessels, from 494 digital images taken from 117 trees. We also report vessel frequencies, void-to-wood ratios, sapwood densities and hydraulically weighted vessel diameters (Dh). Supporting data of the first data set include tree diameter at breast height (130 cm above ground), tree height, sample locations, and summary climate data. In this data set, diameter of individual vessels ranges from 10 to over 300 µm, and vessel frequency from 360 to 9070 vessels cm-2 . Wood density ranges from 0.47 to 0.96 g cm-3 . Void-to-wood ratio ranges from 6% to 27% and Dh ranges from 46 to 236 µm. Mean annual rainfall (P) at sample sites ranges from 246 to 2274 mm and FAO56 reference evaporation (E) from 777 to 2110 mm. The aridity index (E/P) ranges from 0.15 to 2.93 (dimensionless). Tree diameters range from 9 to 90 cm and tree heights range from 6 to 70 m. D and Dh in the second data set range from 11 to 271 and 68 to 205 µm, respectively. These datasets will make a valuable contribution to future continental-scale and global-scale studies of the relationship between xylem hydraulic architecture and climate. The data sets are unique in the sense that they are phylogenetically constrained, allowing in-depth assessment of plasticity of hydraulic attributes within a single tree genus.

The Australian SuperSite Network: A continental, long-term terrestrial ecosystem observatory.

Ecosystem monitoring networks aim to collect data on physical, chemical and biological systems and their interactions that shape the biosphere. Here we introduce the Australian SuperSite Network that, along with complementary facilities of Australia's Terrestrial Ecosystem Research Network (TERN), delivers field infrastructure and diverse, ecosystem-related datasets for use by researchers, educators and policy makers. The SuperSite Network uses infrastructure replicated across research sites in different biomes, to allow comparisons across ecosystems and improve scalability of findings to regional, continental and global scales. This conforms with the approaches of other ecosystem monitoring networks such as Critical Zone Observatories, the U.S. National Ecological Observatory Network; Analysis and Experimentation on Ecosystems, Europe; Chinese Ecosystem Research Network; International Long Term Ecological Research network and the United States Long Term Ecological Research Network. The Australian SuperSite Network currently involves 10 SuperSites across a diverse range of biomes, including tropical rainforest, grassland and savanna; wet and dry sclerophyll forest and woodland; and semi-arid grassland, woodland and savanna. The focus of the SuperSite Network is on using vegetation, faunal and biophysical monitoring to develop a process-based understanding of ecosystem function and change in Australian biomes; and to link this with data streams provided by the series of flux towers across the network. The Australian SuperSite Network is also intended to support a range of auxiliary researchers who contribute to the growing body of knowledge within and across the SuperSite Network, public outreach and education to promote environmental awareness and the role of ecosystem monitoring in the management of Australian environments.

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.

Phloem sap and leaf delta13C, carbohydrates, and amino acid concentrations in Eucalyptus globulus change systematically according to flooding and water deficit treatment.

Phloem is a central conduit for the distribution of photoassimilate, nutrients, and signals among plant organs. A revised technique was used to collect phloem sap from small woody plants in order to assess changes in composition induced by water deficit and flooding. Bled phloem sap delta(13)C and sugar concentrations were compared to delta(13)C of bulk material, soluble carbon extracts, and the neutral sugar fraction from leaves. Amino acid composition and inorganic ions of the phloem sap was also analysed. Quantitative, systematic changes were detected in phloem sap composition and delta(13)C in response to altered water availability. Phloem sap delta(13)C was more sensitive to changes of water availability than the delta(13)C of bulk leaf, the soluble carbon fraction, and the neutral soluble fraction of leaves. Changes in water availability also resulted in significant changes in phloem sugar (sucrose and raffinose), inorganic nutrient (potassium), and amino acid (phenylalanine) concentrations with important implications for the maintenance of phloem function and biomass partitioning. The differences in carbohydrate and amino acid composition as well as the delta(13)C in the phloem, along with a new model system for phloem research, offer an improved understanding of the phloem-mediated signal, nutrient, and photoassimilate transduction in relation to water availability.

Plant mitochondria electron partitioning is independent of short-term temperature changes.

We tested the hypotheses that relative activity of the less efficient alternative oxidase (AOX) path changes with diurnal temperature changes, and thus changes carbon use efficiency with temperature. The activities of the alternative and cytochrome oxidase (COX) paths in plant tissues of three species were determined by measuring 18O/16O discrimination and total respiration from 17 to 36 degrees C. A new, more accurate method for calculating oxygen uptake rate from the mass spectrometry data was developed. Total carbon use efficiency was calculated from the ratio of respiratory heat and CO2 rates measured from 10 to 35 degrees C. Oxygen isotope discrimination (22.9 +/- 0.4 per thousand) and AOX participation were invariant with temperature in leaf tissue of Cucurbita pepo, Nicotiana sativa and Vicia faba, thus falsifying the first part of the hypothesis. Stress responses of respiration at the temperature extremes limited the range for which carbon use efficiency could be accurately measured to 15-30 degrees C in N. sativa, to 10-25 degrees C in C. pepo and to 20-30 degrees C in V. faba. Carbon-use efficiency was invariant at these temperatures in these species, demonstrating that changes in other pathways that would vary carbon-use efficiency were also invariant with temperature.

Growth efficiency increases as relative growth rate increases in shoots and roots of Eucalyptus globulus deprived of nitrogen or treated with salt.

We used calorimetry to test whether there is a single general relationship between growth and respiration in shoots and roots of Eucalyptus globulus Labill. seedlings when stressed, irrespective of the type or severity of stress. We found that nitrogen (N) deprivation and salt treatment had no effect on the relationship between growth and respiration and little effect on absolute rates of respiration. Carbon-conversion efficiency (epsilonC) ranged from 0.7 to 0.9 for specific growth rates (R(SG)) greater than 0.3 day(-1). Above an R(SG) of 0.1 day(-1), epsilonC decreased gradually with decreasing R(SG) and between an R(SG) of 0- 0.1 day(-1), epsilonC decreased rapidly. We conclude that the relationship between epsilonC and R(SG) is not greatly affected by salt or N-deprivation stresses. Relationships between gross productivity and epsilonC may be generally applicable, in which case they could improve on the "flat-tax" approach to modeling net primary productivity from gross productivity while avoiding the complexity of more explicit models of plant respiration. However, the relationship between gross productivity and epsilonC was sensitive to temperature and the effect of temperature on epsilonC thus requires further investigation. Nitrogen deprivation caused large decreases in leaf area and shoot to root ratio, and mature leaves of N-deprived plants had lower intrinsic water-use efficiencies than leaves of plants well supplied with nutrients. Nitrogen deprivation increased apical dominance and most of the reduction in leaf area was the result of fewer secondary branches, although leaf size was also reduced. Our results suggest that N deprivation reduces productivity primarily by reducing sink size, rather than sink activity, and that apical dominance may be an important mechanism for maintaining adequate epsilonC in resource-limited conditions.

Comparison of cellulose extraction methods for analysis of stable-isotope ratios of carbon and oxygen in plant material.

The Jayme-Wise and diglyme-HCl methods for extracting cellulose from plant material for stable-isotope analysis differ considerably in ease of use, with the latter requiring significantly less time and specialized equipment. However, the diglyme-HCl method leaves a small lignin residue in the crude cellulose that may affect stable-isotope values, whereas alpha-cellulose produced by the Jayme-Wise method is relatively pure. We examined whether adding a bleaching step to the diglyme-HCl method could produce cellulose of comparable purity to alpha-cellulose by comparing the yield, percent carbon, and carbon (delta13C) and oxygen (delta18O) stable isotope ratios of the two celluloses. We tested each method on the wood of five species that differ in ease of delignification, Eucalyptus maculata Hook., E. botryoides Sm., E. resinifera Sm., Pinus pinaster Ait. and Callitris glaucophylla J. Thompson & L.A.S. Johnson, as well as the foliage of C. glaucophylla. For hardwoods such as the eucalypts, the diglyme-HCl method without bleaching produced cellulose with delta13C and delta18O ratios similar to alpha-cellulose. For the softwood, C. glaucophylla, 3 h of bleaching with acidified chlorite following treatment with diglyme-HCl produced cellulose with delta13C and delta18O ratios similar to alpha-cellulose. Bleached and unbleached crude celluloses and alpha-cellulose of P. pinaster were similar in delta18O, but not delta13C. Both types of crude cellulose produced from the foliage of C. glaucophylla had significantly different isotope ratios from alpha-cellulose. Overall, the diglyme-HCl method, with or without bleaching, appears to be a simple, fast method for extracting alpha-cellulose from hardwoods for stable-isotope analyses, but its suitability for softwoods and foliage needs to be evaluated depending on the species.

Application of an enthalpy balance model of the relation between growth and respiration to temperature acclimation of Eucalyptus globulus seedlings.

The enthalpy balance model of growth uses measurements of the rates of heat and CO(2) production to quantify rates of decarboxylation, oxidative phosphorylation and net anabolism. Enthalpy conversion efficiency (eta(H)) and the net rate of conservation of enthalpy in reduced biosynthetic products (R(SG)DeltaH(B)) can be calculated from metabolic heat rate (q) and CO(2) rate (R(CO2)). eta(H) is closely related to carbon conversion efficiency and the efficiency of conservation of available electrons in biosynthetic products. R(SG)DeltaH(B) and eta(H) can be used, together with biomass composition, to describe the rate and efficiency of growth of plant tissues. q is directly related to the rate of O(2) consumption and the ratio q:R(CO2) is inversely related to the respiratory quotient. We grew seedlings of Eucalyptus globulus at 16 and 28 degrees C for four to six weeks, then measured q and R(CO2) using isothermal calorimetry. Respiratory rate at a given temperature was increased by a lower growth temperature but eta(H) was unaffected. Enthalpy conversion efficiency - and, therefore, carbon conversion efficiency - decreased with increasing temperature from 15 to 35 degrees C. The ratio of oxidative phosphorylation to oxygen consumption (P/O ratio) was inferred in vivo from eta(H) and by assuming a constant ratio of growth to maintenance respiration with changing temperature. The P/O ratio decreased from 2.1 at 10-15 degrees C to less than 0.3 at 35 degrees C, suggesting that decreased efficiency was not only due to activity of the alternative oxidase pathway. In agreement with predictions from non-equilibrium thermodynamics, growth rate was maximal near 25 degrees C, where the calculated P/O ratio was about half maximum. We propose that less efficient pathways, such as the alternative oxidase pathway, are necessary to satisfy the condition of conductance matching whilst maintaining a near constant phosphorylation potential. These conditions minimize entropy production and maximize the efficiency of mitochondrial energy conversions as growing conditions change, while maintaining adequate finite rates of energy processing.