Annual precipitation explains variability in dryland vegetation greenness globally but not locally.

Dryland vegetation productivity is strongly modulated by water availability. As precipitation patterns and variability are altered by climate change, there is a pressing need to better understand vegetation responses to precipitation variability in these ecologically fragile regions. Here we present a global analysis of dryland sensitivity to annual precipitation variations using long-term records of normalized difference vegetation index (NDVI). We show that while precipitation explains 66% of spatial gradients in NDVI across dryland regions, precipitation only accounts for <26% of temporal NDVI variability over most (>75%) dryland regions. We observed this weaker temporal relative to spatial relationship between NDVI and precipitation across all global drylands. We confirmed this result using three alternative water availability metrics that account for water loss to evaporation, and growing season and precipitation timing. This suggests that predicting vegetation responses to future rainfall using space-for-time substitution will strongly overestimate precipitation control on interannual variability in aboveground growth. We explore multiple mechanisms to explain the discrepancy between spatial and temporal responses and find contributions from multiple factors including local-scale vegetation characteristics, climate and soil properties. Earth system models (ESMs) from the latest Coupled Model Intercomparison Project overestimate the observed vegetation sensitivity to precipitation variability up to threefold, particularly during dry years. Given projections of increasing meteorological drought, ESMs are likely to overestimate the impacts of future drought on dryland vegetation with observations suggesting that dryland vegetation is more resistant to annual precipitation variations than ESMs project.

Rainfall statistics, stationarity, and climate change.

There is a growing research interest in the detection of changes in hydrologic and climatic time series. Stationarity can be assessed using the autocorrelation function, but this is not yet common practice in hydrology and climate. Here, we use a global land-based gridded annual precipitation (hereafter P) database (1940-2009) and find that the lag 1 autocorrelation coefficient is statistically significant at around 14% of the global land surface, implying nonstationary behavior (90% confidence). In contrast, around 76% of the global land surface shows little or no change, implying stationary behavior. We use these results to assess change in the observed P over the most recent decade of the database. We find that the changes for most (84%) grid boxes are within the plausible bounds of no significant change at the 90% CI. The results emphasize the importance of adequately accounting for natural variability when assessing change.

Identifying areas at risk of drought-induced tree mortality across South-Eastern Australia.

South-East Australia has recently been subjected to two of the worst droughts in the historical record (Millennium Drought, 2000-2009 and Big Dry, 2017-2019). Unfortunately, a lack of forest monitoring has made it difficult to determine whether widespread tree mortality has resulted from these droughts. Anecdotal observations suggest the Big Dry may have led to more significant tree mortality than the Millennium drought. Critically, to be able to robustly project future expected climate change effects on Australian vegetation, we need to assess the vulnerability of Australian trees to drought. Here we implemented a model of plant hydraulics into the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model. We parameterized the drought response behaviour of five broad vegetation types, based on a common garden dry-down experiment with species originating across a rainfall gradient (188-1,125 mm/year) across South-East Australia. The new hydraulics model significantly improved (~35%-45% reduction in root mean square error) CABLE's previous predictions of latent heat fluxes during periods of water stress at two eddy covariance sites in Australia. Landscape-scale predictions of the greatest percentage loss of hydraulic conductivity (PLC) of about 40%-60%, were broadly consistent with satellite estimates of regions of the greatest change in both droughts. In neither drought did CABLE predict that trees would have reached critical PLC in widespread areas (i.e. it projected a low mortality risk), although the model highlighted critical levels near the desert regions of South-East Australia where few trees live. Overall, our experimentally constrained model results imply significant resilience to drought conferred by hydraulic function, but also highlight critical data and scientific gaps. Our approach presents a promising avenue to integrate experimental data and make regional-scale predictions of potential drought-induced hydraulic failure.

Little change in global drought over the past 60 years.

Drought is expected to increase in frequency and severity in the future as a result of climate change, mainly as a consequence of decreases in regional precipitation but also because of increasing evaporation driven by global warming. Previous assessments of historic changes in drought over the late twentieth and early twenty-first centuries indicate that this may already be happening globally. In particular, calculations of the Palmer Drought Severity Index (PDSI) show a decrease in moisture globally since the 1970s with a commensurate increase in the area in drought that is attributed, in part, to global warming. The simplicity of the PDSI, which is calculated from a simple water-balance model forced by monthly precipitation and temperature data, makes it an attractive tool in large-scale drought assessments, but may give biased results in the context of climate change. Here we show that the previously reported increase in global drought is overestimated because the PDSI uses a simplified model of potential evaporation that responds only to changes in temperature and thus responds incorrectly to global warming in recent decades. More realistic calculations, based on the underlying physical principles that take into account changes in available energy, humidity and wind speed, suggest that there has been little change in drought over the past 60 years. The results have implications for how we interpret the impact of global warming on the hydrological cycle and its extremes, and may help to explain why palaeoclimate drought reconstructions based on tree-ring data diverge from the PDSI-based drought record in recent years.

Application of an ecological framework linking scales based on self-thinning.

Barnes and Roderick developed a generic, theoretical framework for vegetation modeling across scales. Inclusion of a self-thinning mechanism connects the individual to the larger-scale population and, being based on the conservation of mass, all mass flux processes are integral to the formulation. Significantly, disturbance (both regular and stochastic) and its impact at larger scales are included in the formulation. The purpose of this paper is to illustrate how this model can be used to predict patch and ecosystem dry mass, and consequently system carbon. Examples from pine plantations and mixed forests are considered, with these applications requiring estimates of system carrying capacity and the growth rates of individual plants. The results indicate that the model is relatively simple and straightforward to apply, and its predictions compare well with the data. A significant feature of this approach is that the impact of local scale data on the dynamics of larger patch and ecosystem scales can be determined explicitly, as we show by example. Further, the general formulation has an analytic solution based on characteristics of the individual, facilitating practical and predictive application.

A second pathway for gas out of the pressure chamber--what is being squeezed?

We report a qualitative description of the flows of gas that occur through a leaf when its balance pressure is measured in the pressure chamber. There are two distinct pathways: (a) a bulk flow of gas through the intercellular air spaces, and (b) a diffusion-driven pathway where gas is dissolved into solution under high pressure and comes out of solution at the liquid/atmosphere surface of the cut end where the pressure is atmospheric. The intercellular space flow is well known. It is argued that this flow shows to a reasonable approximation, that the externally supplied gas is squeezing the non-gaseous part of the leaf, and the outer boundary of the non-gaseous material is the boundary of the system that is being manipulated. The second pathway, the diffusion-driven flow, has not (we believe) been described before, and is analogous to a diver getting the bends. The diffusion-based flow demonstrates that gas spaces can and do form inside the outer boundary of the non-gaseous part of the leaf when a balance pressure is measured. These interior gas spaces alter the value recorded for the balance pressure, and complicate any interpretation of what this measurement tells us about the water status of the plant. A hypothesis is proposed that the diffusion-based flow from the xylem comes from vessels that are embolized, and that percentage embolisms might be measured by the proportion of vessels showing the diffusion-driven flow.

A mechanical interpretation of pressure chamber measurements--what does the strength of the squeeze tell us?

Argument still continues about what properties of a plant organ the pressure chamber measures. A mechanical (as opposed to a thermodynamic) analysis is made of the system squeezed by the pressurized gas, the non-gaseous part of the leaf. The boundary of the system is defined so that it remains at constant mass, and constant density is assumed, during the squeeze. This is equivalent to assuming constant volume. On those assumptions, it is shown that the liquid is brought to the cut surface by a change of shape of the system. Generic mechanical principles are then used to deduce a priori, a quantitative interpretation of the balance pressure. The formal mechanical interpretation involves two variables, the interfacial tension and the change in surface area, which cannot currently be measured. Instead of these, we used two related variables which can be measured, the mass fraction of water in the leaf (Q) and the maximum mass fraction of water at full saturation (Qx) to deduce an approximate mechanical interpretation. When Q is close to Qx, we deduced that the balance pressure (Pb) required for the shape change should be approximately proportional to the reduction in mass in changing from Qx to Q, a variable called the relative water loss (RWL). The constant of proportionality (kappa) is a basic characteristic of the type of leaf used, and the final relation, Pb=kappa (RWL) is called Relation A. We then deduce that the constant kappa should be an approximately linear function of Qx. The linear function is defined by limiting values, so that when Qx is 1, kappa is predicted to be 0 bar, and at the other extreme, when Qx is 0, kappa is predicted to be in the range 500-1000 bar. This is called Relation B. Experiments with 32 leaves from 10 species are used to test the mechanical interpretation. The results showed that Relation A was a reasonable approximation for most of the tested leaves. The data for 10 species, were used to estimate Relation B, confirming that as Qx approached 1, kappa did approach 0 bar as predicted, and that as Qx approached 0, kappa approached approximately 750 bar, consistent with the a priori prediction of 500-1000 bar. The relations were also successfully tested using independent published data. An estimate of Qx is shown to be of considerable practical value in (a) converting Pb to water status and vice versa; (b) characterizing leaf morphology and composition; and (c) rationalizing quantitatively the functional classes of xerophytes, mesophytes and hygrophytes. The assumption of constant density inside the outer boundary of the non-gaseous material cannot be guaranteed, and when this is violated, our (or any other) interpretation of Pb is unreliable. Investigation of the conditions under which this assumption is invalid should be a high priority.

Using an optimality model to understand medium and long-term responses of vegetation water use to elevated atmospheric CO2 concentrations.

Vegetation has different adjustable properties for adaptation to its environment. Examples include stomatal conductance at short time scale (minutes), leaf area index and fine root distributions at longer time scales (days-months) and species composition and dominant growth forms at very long time scales (years-decades-centuries). As a result, the overall response of evapotranspiration to changes in environmental forcing may also change at different time scales. The vegetation optimality model simulates optimal adaptation to environmental conditions, based on the assumption that different vegetation properties are optimized to maximize the long-term net carbon profit, allowing for separation of different scales of adaptation, without the need for parametrization with observed responses. This paper discusses model simulations of vegetation responses to today's elevated atmospheric CO2 concentrations (eCO2) at different temporal scales and puts them in context with experimental evidence from free-air CO2 enrichment (FACE) experiments. Without any model tuning or calibration, the model reproduced general trends deduced from FACE experiments, but, contrary to the widespread expectation that eCO2 would generally decrease water use due to its leaf-scale effect on stomatal conductance, our results suggest that eCO2 may lead to unchanged or even increased vegetation water use in water-limited climates, accompanied by an increase in perennial vegetation cover.

Linking wood density with tree growth and environment: a theoretical analysis based on the motion of water.

• An hydraulic model of a tree stem is presented to help understand how the carbon storage in ecosystems varies with changing environmental conditions. • The model is based on the assumption that a tree stem is a collection of parallel pipes and was used to (qualitatively) predict how the mass concentration of dry matter ([D]) would vary with water temperature (via changes in viscosity), nitrogen supply and atmospheric CO2 . • There was qualitative agreement between model predictions and observed gross trends. The model predicted that the flow rate would be relatively insensitive to variations in [D] in angiosperm stems; this was consistent with observations. It is concluded that other factors need to be considered to explain variations in [D] in angiosperm wood. The flow rate of water through gynmosperm stems was predicted to be very sensitive to variations in [D] and the model explained why [D]; decreases with decreases in water temperature, decreases with increases in nitrogen supply and increases with elevated CO2 . • The model captured some of the important underlying relations linking water transport with wood density and environment and qualitative testing of the model is recommended.

On the direct effect of clouds and atmospheric particles on the productivity and structure of vegetation.

The volume of shade within vegetation canopies is reduced by more than an order of magnitude on cloudy and/or very hazy days compared to clear sunny days because of an increase in the diffuse fraction of the solar radiance. Here we show that vegetation is directly sensitive to changes in the diffuse fraction and we conclude that the productivity and structure of vegetation is strongly influenced by clouds and other atmospheric particles. We also propose that the unexpected decline in atmospheric [CO2] which was observed following the Mt. Pinatubo eruption was in part caused by increased vegetation uptake following an anomalous enhancement of the diffuse fraction by volcanic aerosols that would have reduced the volume of shade within vegetation canopies. These results have important implications for both understanding and modelling the productivity and structure of terrestrial vegetation as well as the global carbon cycle and the climate system.

A canopy-scale test of the optimal water-use hypothesis.

Common empirical models of stomatal conductivity often incorporate a sensitivity of stomata to the rate of leaf photosynthesis. Such a sensitivity has been predicted on theoretical terms by Cowan and Farquhar, who postulated that stomata should adjust dynamically to maximize photosynthesis for a given water loss. In this study, we implemented the Cowan and Farquhar hypothesis of optimal stomatal conductivity into a canopy gas exchange model, and predicted the diurnal and daily variability of transpiration for a savanna site in the wet-dry tropics of northern Australia. The predicted transpiration dynamics were then compared with observations at the site using the eddy covariance technique. The observations were also used to evaluate two alternative approaches: constant conductivity and a tuned empirical model. The model based on the optimal water-use hypothesis performed better than the one based on constant stomatal conductivity, and at least as well as the tuned empirical model. This suggests that the optimal water-use hypothesis is useful for modelling canopy gas exchange, and that it can reduce the need for model parameterization.

Hypothesis. Air embolisms exsolving in the transpiration water - the effect of constrictions in the xylem pipes.

When water flows through a constriction, air can come out of solution (i.e. it can exsolve). This phenomenon is manifested in the transpiration stream of plants. Observations of gas in functioning xylem prompted a hypothesis predicting the daily balance between air and water in wood: a sudden fall in water content at sunrise, followed by an increase in water content during the day. An extended record by time domain reflectometry of volumetric water content (VWC) every 2 h throughout a summer shows the detailed pattern of change of VWC during 25 individual days, giving good agreement with the hypothesis. This hypothesis has wide-ranging consequences for experiments using cut plant parts. Perfusing aqueous solutions through excised xylem also can exsolve air from the water, causing declines in flow. The location of such air was investigated in cryo-fixed perfused vine stems by cryo-scanning electron microscopy. Bubbles formed at residual walls of perforation plates in small vessels, and filled many large vessels. The input surface is revealed as a major source of exsolved air. Precautions to reduce this effect are outlined and discussed.

A test of the optimality approach to modelling canopy properties and CO2 uptake by natural vegetation.

Photosynthesis provides plants with their main building material, carbohydrates, and with the energy necessary to thrive and prosper in their environment. We expect, therefore, that natural vegetation would evolve optimally to maximize its net carbon profit (NCP), the difference between carbon acquired by photosynthesis and carbon spent on maintenance of the organs involved in its uptake. We modelled N(CP) for an optimal vegetation for a site in the wet-dry tropics of north Australia based on this hypothesis and on an ecophysiological gas exchange and photosynthesis model, and compared the modelled CO2 fluxes and canopy properties with observations from the site. The comparison gives insights into theoretical and real controls on gas exchange and canopy structure, and supports the optimality approach for the modelling of gas exchange of natural vegetation. The main advantage of the optimality approach we adopt is that no assumptions about the particular vegetation of a site are required, making it a very powerful tool for predicting vegetation response to long-term climate or land use change.

The ever-flickering light.

To date, ecologists involved in global change have focused on the consequences of changes in air temperature. Concurrently, the amount of sunlight reaching the surface of the Earth has been declining, resulting in so-called 'global dimming'. Now, Wild et al. and Pinker et al. have reported a reversal in this trend in some regions that has occurred over the past 15 years or so. These new findings, combined with earlier work, show that the transparency of the atmosphere can vary substantially over periods of at least 20-50 years. Thus, the ecological consequences of sustained trends in the occurrence of sunlight at the surface of the Earth need a more careful assessment than was previously thought.

Plant-water relations and the fibre saturation point.

This review is about the behaviour of water in cell walls. The aim is to introduce to biologists the concept of the fibre saturation point (FSP), and the related research of material scientists and engineers on the thermodynamics and chemistry of water in timber and wood. In the review, we first summarise what the FSP is, why it is important and how the FSP is routinely used by engineers and material scientists to estimate the volume fractions of solid, liquid and gas phases in bulk timber. We then show that the FSP can be intuitively understood using equilibrium thermodynamics. That analysis shows that the FSP is based on the concept that a certain (and repeatable) amount of water is chemically bound to cellulose and other substances in wood. That water, sometimes called bound water, exists in a water-cell wall mixture. The noted physical chemist and wood scientist, A. J. Stamm, called this mixture a 'solid solution'. In timber, the 'solid solution' is considered a separate phase from adjacent water in either a pure liquid phase or a vapour phase. Following that, we examine the FSP and wood-water dynamics at the molecular and cellular level. Despite differences between timber and living trees, we conclude that the FSP-based framework long used by material scientists and engineers is likely to be useful to biologists.

The cause of decreased pan evaporation over the past 50 years.

Changes in the global water cycle can cause major environmental and socioeconomic impacts. As the average global temperature increases, it is generally expected that the air will become drier and that evaporation from terrestrial water bodies will increase. Paradoxically, terrestrial observations over the past 50 years show the reverse. Here, we show that the decrease in evaporation is consistent with what one would expect from the observed large and widespread decreases in sunlight resulting from increasing cloud coverage and aerosol concentration.

On the conservative nature of the leaf mass-area relationship.

In a previous empirical study, Hughes and colleagues showed that for several herbaceous species there is apparently a unique species-specific relationship between the area and mass of leaves. We tested this proposition using measurements from 15 broad-leaved species. We found that to a reasonable approximation, leaf area was proportional to leaf mass within a given species despite relatively large variations in both leaf thickness and the mass fraction of liquid matter. These observations show that the inverse density-thickness of leaves from a given species, which we call the Hughes constant, is approximately conserved. We conclude that the Hughes constant is likely to be more conservative than other traits traditionally used to describe leaves.

An ecological framework linking scales across space and time based on self-thinning.

Scaling up from measurements made at small spatial and short temporal scales is a central challenge in the ecological and related sciences, where predictions at larger scales and over long time periods are required. It involves two quite distinct aspects: a formulation of a theoretical framework for calculating space-time averages, and an acquisition of data to support that framework. In this paper, we address the theoretical part of the question, and although our primary motivation was an understanding of carbon accounting our formulation is more general. To that end, we adopt a dynamical systems approach, and incorporate a new dynamical formulation of self-thinning. We show how to calculate rates of change for total (and average) plant dry mass, volume, and carbon, in terms of the properties of the individual plants. The results emphasize how local scale statistics (such as, variation in the size of individuals) lead to nonlinear variation at larger scales. Further, we describe how regular and stochastic disturbance can be readily incorporated into this framework. It is shown that stochastic disturbance at patch-scales, results in (to first approximation) regular disturbance at ecosystem scales, and hence can be formulated as such. We conclude that a dynamical formulation of self-thinning can be used as a generic framework for scaling ecological processes in space and time.