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Green leaf volatiles (GLVs) are a diverse group of fatty acid-derived compounds emitted by all plants and are involved in a wide variety of developmental and stress-related biological functions. Recently, GLV emission bursts from leaves were reported following light–dark transitions and hypothesized to be related to the stress response while acetaldehyde bursts were hypothesized to be due to the ‘pyruvate overflow’ mechanism. In this study, branch emissions of GLVs and a group of oxygenated metabolites (acetaldehyde, ethanol, acetic acid, and acetone) derived from the pyruvate dehydrogenase (PDH) bypass pathway were quantified from mesquite plants following light–dark transitions using a coupled GC–MS, PTR-MS, and photosynthesis system. Within the first minute after darkening following a light period, large emission bursts of both C5 and C6 GLVs dominated by (Z)-3-hexen-1-yl acetate together with the PDH bypass metabolites are reported for the first time. We found that branches exposed to CO2-free air lacked significant GLV and PDH bypass bursts while O2-free atmospheres eliminated the GLV burst but stimulated the PDH bypass burst. A positive relationship was observed between photosynthetic activity prior to darkening and the magnitude of the GLV and PDH bursts. Photosynthesis under 13CO2 resulted in bursts with extensive labeling of acetaldehyde, ethanol, and the acetate but not the C6-alcohol moiety of (Z)-3-hexen-1-yl acetate. Our observations are consistent with (1) the “pyruvate overflow” mechanism with a fast turnover time (<1 h) as part of the PDH bypass pathway, which may contribute to the acetyl-CoA used for the acetate moiety of (Z)-3-hexen-1-yl acetate, and (2) a pool of fatty acids with a slow turnover time (>3 h) responsible for the C6 alcohol moiety of (Z)-3-hexen-1-yl acetate via the 13-lipoxygenase pathway. We conclude that our non-invasive method may provide a new valuable in vivo tool for studies of acetyl-CoA and fatty acid metabolism in plants at a variety of spatial scales.

Water availability limits plant growth and production in almost all terrestrial ecosystems. However, biomes differ substantially in sensitivity of aboveground net primary production (ANPP) to between-year variation in precipitation. Average rain-use efficiency (RUE; ANPP/precipitation) also varies between biomes, supposedly because of differences in vegetation structure and/or biogeochemical constraints. Here we show that RUE decreases across biomes as mean annual precipitation increases. However, during the driest years at each site, there is convergence to a common maximum RUE (RUEmax) that is typical of arid ecosystems. RUEmax was also identified by experimentally altering the degree of limitation by water and other resources. Thus, in years when water is most limiting, deserts, grasslands and forests all exhibit the same rate of biomass production per unit rainfall, despite differences in physiognomy and site-level RUE. Global climate models predict increased between-year variability in precipitation, more frequent extreme drought events, and changes in temperature. Forecasts of future ecosystem behaviour should take into account this convergent feature of terrestrial biomes.

Increases in the abundance or density of woody plants in historically semiarid and arid grassland ecosystems have important ecological, hydrological, and socioeconomic implications. Using a simplified water-balance model, we propose a framework for conceptualizing how woody plant encroachment is likely to affect components of the water cycle within these ecosystems. We focus in particular on streamflow and the partitioning of evapotranspiration into evaporation and transpiration. On the basis of this framework, we suggest that streamflow and evaporation processes are affected by woody plant encroachment in different ways, depending on the degree and seasonality of aridity and the availability of subsurface water. Differences in landscape physiography, climate, and runoff mechanisms mediate the influence of woody plants on hydrological processes. Streamflow is expected to decline as a result of woody plant encroachment in landscapes dominated by subsurface flow regimes. Similarly, encroachment of woody plants can be expected to produce an increase in the fractional contribution of bare soil evaporation to evapotranspiration in semiarid ecosystems, whereas such shifts may be small or negligible in both subhumid and arid ecosystems. This framework for considering the effects of woody plant encroachment highlights important ecological and hydrological interactions that serve as a basis for predicting other ecological aspects of vegetation change-such as potential changes in carbon cycling within an ecosystem. In locations where woody plant encroachment results in increased plant transpiration and concurrently the availability of soil water is reduced, increased accumulation of carbon in soils emerges as one prediction. Thus, explicitly considering the ecohydrological linkages associated with vegetation change provides needed information on the consequences of woody plant encroachment on water yield, carbon cycling, and other processes.

1. Ecosystem respiration varies substantially at short temporal intervals and identifying the role of coupled temperature- and precipitation-induced changes has been an ongoing challenge. To address this challenge we applied a metabolic ecological theory to identify pulses in ecosystem respiration following rain events. Using this metabolic framework, precipitation-induced pulses were described as a reduction in metabolic activation energy after individual precipitation events. 2. We used this approach to estimate the responses of 237 individual events recorded over 2 years at four eddy-covariance sites in southern AZ, USA. The sites varied in both community type (woody and grass dominated) and landscape position (riparian and upland). We used a nonlinear inversion procedure to identify both the parameters for the pre-event temperature sensitivity and the predicted response of the temperature sensitivity to precipitation. By examining multiple events we evaluated the consistency of pulses between sites and discriminated between hypotheses regarding landscape position, event distributions, and pre-event ecosystem metabolism rates. 3. Over the 5-day post-event period across all sites the mean precipitation effect was attributed to 6·1 g CO₂ m⁻² of carbon release, which represented a 21% increase in respiration over the pre-event steady state trajectory of carbon loss. Differences in vegetation community were associated with differences in the integrated magnitude of pulse responses, while differences in topographic position were associated with the initial peak pulse rate. In conjunction with the differences between sites, the individual total pulse response was positively related to the drying time interval and metabolic rates prior to the event. The quantitative theory presented provides an approach for understanding ecosystem pulse dynamics and helps characterized the dependence of ecosystem metabolism on both temperature and precipitation.

$\\bullet$ The influences of prior monsoon-season drought (PMSD) and the seasonal timing of episodic rainfall ('pulses') on carbon and water exchange in water-limited ecosystems are poorly quantified. $\\bullet$ In the present study, we estimated net ecosystem exchange of CO2 (NEE) and evapotranspiration (ET) before, and for 15 d following, experimental irrigation in a semi-arid grassland during June and August 2003. Rainout shelters near Tucson, Arizona, USA, were positioned on contrasting soils (clay and sand) and planted with native (Heteropogon contortus) or non-native invasive (Eragrostis lehmanniana) C4 bunchgrasses. Plots received increased ('wet') or decreased ('dry') monsoon-season (July-September) rainfall during 2002 and 2003. $\\bullet$ Following a June 2003 39-mm pulse, species treatments had similar NEE and ET dynamics including 15-d integrated NEE ($NEE_{pulse}$). Contrary to predictions, PMSD increased net C uptake during June in plots of both species. Greater flux rates after an August 2003 39-mm pulse reflected biotic activity associated with the North American Monsoon. Furthermore, August $NEE_{pulse}$ and ecosystem pulse-use efficiency ($PUE_{e} = NEE_{pulse}/ET_{pulse}$) was greatest in Heteropogon plots. $\\bullet$ PMSD and rainfall seasonal timing may interact with bunchgrass invasions to alter NEE and ET dynamics with consequences for $PUE_e$ in water-limited ecosystems.

1 Combining long-term observational studies with comparative physiological ecology can yield a deeper understanding of the contribution of individual function to population and community dynamics. Sonoran Desert winter annuals exhibit striking year-to-year variation in population dynamics that is driven by variable precipitation, but species differ in the strength of demographic response to precipitation and hence in the degree of temporal variance in population dynamics. To understand the physiological mechanisms of differing population dynamic responses to environmental variation, we investigated interspecific differences in functional traits that mediate responsiveness to precipitation. 2 We conducted sequential harvests throughout the growing season to examine relative growth rate and biomass allocation patterns. We then related growth parameters to leaf-level carbon isotope discrimination (a time-integrated measure of water-use efficiency) and long-term demographic variation. 3 We hypothesized that water-use efficiency should trade-off with rapid growth rates. Furthermore, we hypothesized that species having efficient water use should have buffered population dynamics in dry years but sacrifice high growth and fecundity in wet years, resulting in low long-term variance in demographic success. Conversely, species with high growth capacity should be very responsive to infrequent periods of high precipitation and thus exhibit high temporal variance. 4 Species differed in seasonal relative growth rate and allocation patterns. Species with the highest relative growth rates rapidly deployed large leaf area displays following mid-season rainfall. Species with intermediate relative growth rates exhibited high biomass assimilation rates per unit leaf area. Species with low relative growth rates exhibited low leaf area ratios and low assimilation rates per unit leaf area. 5 Relative growth rate was positively related to leaf carbon isotope discrimination, consistent with a trade-off between growth rate and water-use efficiency. 6 Seasonal relative growth rate did not predict long-term demographic variance. However, leaf area plasticity in response to precipitation was positively related to long-term demographic variance. Our results illustrate how morphological and physiological traits influence demographic tracking of environmental variability and demonstrate how species differences in functional strategies determine population and community dynamics.

Subsurface flow and storage dynamics at hillslope scale are difficult to ascertain, often in part due to a lack of sufficient high-resolution measurements and an incomplete understanding of boundary conditions, soil properties, and other environmental aspects. A continuous and extreme rainfall experiment on an artificial hillslope at Biosphere 2's Landscape Evolution Observatory (LEO) resulted in saturation excess overland flow and gully erosion in the convergent hillslope area. An array of 496 soil moisture sensors revealed a two-step saturation process. First, the downward movement of the wetting front brought soils to a relatively constant but still unsaturated moisture content. Second, soils were brought to saturated conditions from below in response to rising water tables. Convergent areas responded faster than upslope areas, due to contributions from lateral subsurface flow driven by the topography of the bottom boundary, which is comparable to impermeable bedrock in natural environments. This led to the formation of a groundwater ridge in the convergent area, triggering saturation excess runoff generation. This unique experiment demonstrates, at very high spatial and temporal resolution, the role of convergence on subsurface storage and flow dynamics. The results bring into question the representation of saturation excess overland flow in conceptual rainfall-runoff models and land-surface models, since flow is gravity-driven in many of these models and upper layers cannot become saturated from below. The results also provide a baseline to study the role of the co-evolution of ecological and hydrological processes in determining landscape water dynamics during future experiments in LEO.

1. In water-limited ecosystems, discrete precipitation events trigger brief but important episodes of biological activity. Differential responses of above- and below-ground biota to precipitation may constrain biogeochemical transformations at the ecosystem scale. 2. We quantified short-term dynamics of whole ecosystem response to 39 mm irrigation events (precipitation pulses) during June 2002 and 2003 using plant physiological and ecosystem gas-exchange measurements as state variables in a principal components analysis (PCA). Experimental plots consisted of either native (Heteropogon contortus L.) or non-native (Eragrostis lehmanniana Nees) bunchgrasses planted in monoculture on two distinct geomorphic surfaces in a semi-arid grassland. 3. For 15 days, treatments followed similar, non-linear trajectories through state variable space with measurement periods forming distinct clusters; PCA axes 1 and 2 combined to explain 80.7% of the variation during both 2002 and 2003. 4. During both years, bunchgrass species interacted with soil type such that there was a reduction in ecosystem functional resistance in plots planted with the non-native bunchgrass species on the fine-textured clay geomorphic surface. 5. System-level hysteresis, emerging as a result of independent responses of photosynthesis, respiration and evapotranspiration to precipitation, indicated the potential for alternative functional states. 6. Quantifying the frequency and duration of ecosystem alternative functional states in response to individual precipitation events within a season will provide insights into the controls of species, soils and climate on ecosystem carbon and water cycles.

Understanding ecosystem-atmosphere carbon exchanges in dryland environments has been more challenging than in mesic environments, likely due to more pronounced nonlinear responses of ecosystem processes to environmental variation. To better understand diurnal to interannual variation in gross primary productivity (GPP) variability, we coupled continuous eddy-covariance derived whole ecosystem gas exchange measurements with an ecophysiologic model based on fundamental principles of diffusion, mass balance, reaction kinetics, and biochemical regulation of photosynthesis. We evaluated the coupled data-model system to describe and understand the dynamics of 3 years of growing season GPP from a riparian grassland and woodland in southern Arizona. The data-model fusion procedure skillfully reproduced the majority of daily variation GPP throughout three growing seasons. While meteorology was similar between sites, the woodland site had consistently higher GPP rates and lower variability at daily and interannual timescales relative to the grassland site. We examined the causes of this variation using a new state factor model analysis that partitioned GPP variation into four factors: meteorology, physiology, leaf area, and water supply. The largest proportion of GPP variation was associated with physiological differences. The woodland showed a greater sensitivity than the grassland to water supply, while the grassland showed a greater sensitivity to leaf area. These differences are consistent with hypotheses of woody species using resistance mechanisms, stomatal regulation, and grassland species using resilience mechanisms, leaf area regulation, in avoiding water stress and have implications for future GPP sensitivity to climate variability following wood-grass transitions.

1. A major research focus in population and community ecology is to establish a mechanistic understanding of plant interactions and demographic responses. The first step towards this mechanistic approach relies on understanding the differences in stress caused by different environmental conditions. Leaf-level photosynthetic rate (A) within and among plant populations provides important insight into population and community processes, but is difficult to acquire with sufficient replication under field conditions. Instead, chlorophyll fluorescence (Fv/Fm) and predawn water potential (Ψpd) are often used in arid and semi-arid ecosystems. 2. Fv/Fm reflects the photoactivation status of photosystem II (PSII), whereas Ψpd indicates water availability in the rhizosphere. Here we compare these indices with A in two perennial C₄ grasses (native Heteropogon contortus and invasive Eragrostis lehmanniana) and in seedlings of the C₃ shrub Prosopis velutina growing on highly contrasting sandy loam and loamy clay soils in experimental plots. Measurements were made the day prior to and up to 7 days following a 39-mm rainfall pulse after 2 months of drought. 3. A was more sensitive across a broad range of environmental conditions, whereas Fv/Fm and Ψpd only responded to periods of protracted drought. The use of these measures was further complicated because their values varied daily and we observed different time-lags in their response to precipitation pulses. 4. We suggest sampling schemes and a priori measurements to capture the value that is representative for the question of interest, and that match the pulsed biological activity in these ecosystems. Finally, we suggest the use of these measures in combination with measurements providing integration over longer time periods, such as δ¹³C, δ¹⁸O and N concentration in bulk leaf tissue.