Partitioning overstory and understory evapotranspiration in a semiarid savanna woodland from the isotopic composition of water vapor

Abstract The relative contributions of overstory and understory plant transpiration and soil evaporation to total evapotranspiration (ET) in a semiarid savanna woodland were determined from stable isotope measurements of atmospheric water vapor. The savanna overstory was dominated by the deeply rooted, woody legume Prosopis velutina ("mesquite"), and the understory was dominated by a perennial C 4 grass, Sporobolus wrightii. "Keeling plots" (turbulent mixing relationships) were generated from isotope ratios (␦D and ␦ 18 O) of atmospheric water vapor collected within the tree (3-14 m) and understory (0.1-1 m) canopies during peak (July) and post-monsoon (September) periods of 2001. The unique regression intercepts from upper and lower profiles were used to partition the ET flux from the understory layer separately from that of the whole ecosystem. Although ET partitioning was problematic during the first sampling period in July, our results in September provided support to the validity of this method for measuring and understanding the dynamic behavior of water balance components in this semiarid savanna woodland. During the post-monsoon period (22nd September), transpiration accounted for 85% of ecosystem ET. Transpiration by the grass layer accounted for 50% of the understory ET over the same period. The total ecosystem ET estimated by eddy covariance (EC) on 22nd September was 3.5 mm per day. Based on partitioning by the isotope method, 2.5 mm per day (70%) was from tree transpiration and 0.5 mm per day (15%) was from transpiration by the grass layer. Independent estimates of overstory and understory ET partitioning from distributed understory EC measurements were remarkably consistent with our isotope approach

Evaluating theories of drought-induced vegetation mortality using a multimodel– experiment framework

Model–data comparisons of plant physiological processes provide an understanding of mechanisms underlying vegetation responses to climate. We simulated the physiology of a pi~non pine–juniper woodland (Pinus edulis–Juniperus monosperma) that experienced mortality during a 5 yr precipitation-reduction experiment, allowing a framework with which to examine our knowledge of drought-induced tree mortality. We used six models designed for scales ranging from individual plants to a global level, all containing state-of-the-art representations of the internal hydraulic and carbohydrate dynamics of woody plants. Despite the large range of model structures, tuning, and parameterization employed, all simulations predicted hydraulic failure and carbon starvation processes co-occurring in dying trees of both species, with the time spent with severe hydraulic failure and carbon starvation, rather than absolute thresholds per se, being a better predictor of impending mortality. Model and empirical data suggest that limited carbon and water exchanges at stomatal, phloem, and below-ground interfaces were associated with mortality of both species. The model–data comparison suggests that the introduction of a mechanistic process into physiology-based models provides equal or improved predictive power over traditional process-model or empirical thresholds. Both biophysical and empirical modeling approaches are useful in understanding processes, particularly when the models fail, because they reveal mechanisms that are likely to underlie mortality. We suggest that for some ecosystems, integration of mechanistic pathogen models into current vegetation models, and evaluation against observations, could result in a breakthrough capability to simulate vegetation dynamics.Keywords: carbon starvation, hydraulic failure, process-based models, cavitation, dynamic global vegetation models (DGVMs), die off, photosynthesi

Sources and Dynamics of Carbon Dioxide Exchange and Evapotranspiration in Semiarid Environments

Precipitation, more than any other environmental factor, controls patterns of ecosystem production and biogeochemical cycling in arid and semiarid environments. Growing-season rains in these regions are highly unpredictable as they come in intermittent pulses varying in size, frequency and spatial extent, thereby producing unique hydrological patterns that constrain the location and residence time of soil water available for biological activity. In order to understand how arid and semiarid ecosystems respond to inputs of precipitation within the context of ecosystem science and global change studies, knowledge is needed on how plants and other organisms respond as an integrated system to such environmental control. The focus of my research was to understand how the distribution of precipitation events influences the dynamics of carbon cycling in semiarid ecosystems. At a semiarid riparian woodland, measurements of CO2 exchange and evapotranspiration revealed that following precipitation events occurring soon after prolonged dry periods the efficiency of rain-use (amount of carbon gain per unit of precipitation over a specific period time) was low. Precipitation did not readily stimulate primary productivity, water was mainly lost as soil evaporation and large respiratory CO2 effluxes were observed. This commonly observed features in seasonally dry ecosystems might have profound consequences for the seasonal and annual carbon balance. In this woodland, 47% of the precipitation within a single growing season (May-October) was returned to atmosphere as soil evaporation and the CO2 efflux observed just during the first rainy month (July) was equivalent to almost 50% of the net carbon gain observed over the six-month growing season. Results from experimental irrigations in understory plots of riparian mesquite woodland revealed that the magnitude and duration of the large CO2 fluxes occurring soon after rainfall was higher in plots located under tree canopies where, relative to intercanopy plots, the amount of plant litter was higher, soil evaporation and plant photosynthetic rates were lower. Efficiency of rain-use in semiarid ecosystems during the growing season apparently was determined by the degree of coupling between gross photosynthesis and ecosystem respiration, by the fraction of precipitation lost as soil evaporation and by the water-use efficiency of the component vegetation

Data from: Convergence in resource use efficiency across trees with differing hydraulic strategies in response to ecosystem precipitation manipulation

1. Plants are expected to respond to drought by maximizing the efficiency of the most limiting resource, the water use efficiency (WUE), at the expense of nitrogen and carbon use efficiencies (NUE and CUE). Therefore, plants resource use efficiencies are viewed as indicators of species drought tolerance. 2. We tested these predictions by measuring leaf-level intrinsic WUE (WUEi, the ratio of net assimilation to stomatal conductance), photosynthetic NUE (PNUE, the ratio of daily maximum net assimilation to leaf nitrogen content) and leaf-scale CUE (approached by the ratio of nighttime respiration to daytime net assimilation, Rd/An) in piñon pine and juniper, two tree species that differ in drought tolerance and vulnerability to drought-induced mortality. Variations in resource use efficiency in the two species were measured in response to seasonal drought and in response to an ecosystem-scale precipitation manipulation experiment comprising three precipitation treatments: ambient, irrigation (+30%) and partial rainfall exclusion (-45%). 3. Increasing water limitation, either seasonally or across treatments, resulted in increased WUE and decreased PNUE and CUE in both species. WUE, PNUE and CUE varied more strongly in response to water limitation than across species and converged to the same relationships against precipitation for piñon and juniper. 4. Plasticity in WUE, PNUE and CUE in response to water limitation was associated, in both species, with low carbon acquisition during drought. Our results exhibited a convergence in resource use efficiency across piñon and juniper which contradicts the paradigm that resource use efficiencies are indicators of species drought tolerance and ecological strategy

Global warming potential of intensive wheat production in the Yaqui Valley, Mexico: a resource for the design of localized mitigation strategies

© 2016 Elsevier Ltd A reduction in greenhouse gas (GHG) emissions from productive activities can contribute to climate change mitigation by diminishing the future impacts on natural and socioeconomic systems. Nitrous oxide is one of the most important GHGs and agriculture represents its main anthropogenic source. Using a standardized life cycle assessment (LCA) methodology, this study aims to identify and quantify the GHG emissions associated with the different stages of wheat production using local information to develop localized climate change mitigation strategies in one of the most intensive agricultural areas in the world. A set of mitigation scenarios created based on inputs and information obtained directly from producer\u27s associations and farmers were evaluated. These scenarios range from the traditional approaches to the more innovative strategies currently being applied. They are considered to maintain the same yields considering changes mainly in fertilization, tillage and machinery efficiency. We found that the main source of GHGs in wheat production in the Yaqui Valley is fertilizing, with an average of 83% of the life cycle emissions in all the production scenarios proposed. The second contributing activity is tillage, accounting for 13% of Global Warming Potential (GWP) in conventional systems and 1% with ‘no tillage’ strategies. Results show that the manufacture of fertilizers accounted for 42% of the fertilizing emissions and 35% of the total life cycle emissions of wheat. In addition, by using more efficient tractors that decreased diesel inputs, emissions from conventional tillage can be reduced by 33% and emissions from no tillage can be reduced by 24%. The application of the LCA methodology allowed providing a more detailed quantification of the GHG and environmental impacts of different wheat production processes. Compared to other studies, the mitigation strategies developed from this work have a better chance of being adopted by producers because there were developed based on the actual practices proposed by the farmers and consider existing approaches currently being promoted by producer\u27s associations for cost reduction purposes. In this sense, the results of this LCA suggest that implementation of innovation strategies in fertilizing, tillage, and machinery efficiency can both reduce costs and mitigate GHG emissions in intensive wheat production systems all over the world

Initial response of phenology and yield components of wheat (Triticum durum L., CIRNO C2008) under experimental warming field conditions in the Yaqui Valley

"This work evaluates the experimental warming effects on phenology and grain yield components of wheat in the Yaqui Valley, Sonora, México, using CIRNO C2008 variety from Triticum durum L., as a model during the cropping cycle of 2016–2017 (December to April). Infrared radiators were deployed to induce experimental warming by 2 °C above ambient crop canopy temperature, in a temperature free-air controlled enhancement system. Temperature was controlled by infrared temperature sensors placed in eight plots which covered a circle of r = 1.5 m starting five days after germination until harvest. The warming treatment caused a reduction of phenophases occurrence starting at the stem extension phenophase. Such phenological responses generated a significant biological cycle reduction of 14 days. Despite this delay, CIRNO C2008 completed its biological cycle adequately. However, plant height under the warming treatment was reduced significantly and differences were particularly observed at the final phenophases of the vegetative cycle. Plant height correlated negatively with spikes length, spikes mass, and number of filled grains. Warming also reduced grain yield in 33%. The warming treatment caused a stress intensity (SI = 1-yield warming/yield control) of 39.4% and 33.2% in biomass and grain yield, respectively. The differences in stress intensities between biomass and grain yield were based on plant height reduction. Grain mass was not affected, demonstrating the crop capability for remobilization and adequate distribution of elaborated substances for the spikes under warming conditions.

Water regime and osmotic adjustment under warming conditions on wheat in the Yaqui Valley, Mexico

An experiment was carried out to evaluate the effect of increased temperature on roots and leaf water and osmotic potential, osmotic adjustment (OA) and transpiration on Triticum durum L. (CIRNO C2008 variety) during growth (seedling growth), tillering and heading phenophases. Wheat was sown under field conditions at the Experimental Technology Transfer Center (CETT-910), as a representative wheat crop area from the Yaqui Valley, Sonora México. Thermal radiators were placed at 1.20 m from the crop canopy. Treatments included warmed plots (2 °C) and ambient canopy temperature with five replicates. Temperature treatment was controlled using a (proportional, integrative, derivative) feedback control system on plots covering a circular area of r = 1.5 m. Results indicated a significant decrease in the osmotic potential of roots and leaves for the warmed plots. Water potential, under warming treatment, also experienced a significant reduction and a potential gradient was observed in both, roots and leaves, while the phenophases were delayed. Such results demonstrate that, under warmer conditions, plants increase water absorption for cooling. Hence, transpiration experienced a significant increase under warming in all phenophases that was related to the low root and leaf water potential. CIRNO C2008 also experienced OA in all phenophases with glycine betaine as the osmolyte with major contribution

Data_FE_Limousin et al 2015

The data table shows the ecophysiological measurements performed during 6 field campaigns in 2010 and 6 field campaigns in 2011 within the ecosystem scale rainfall manipulation experiment at the Sevilleta National Wildlife refuge, New Mexico (N 34°23'11"; W 106°31'46"; elevation 1911 m asl)

Carbon dioxide and water vapour exchange in a tropical dry forest as influenced by the North American Monsoon System (NAMS)

a b s t r a c t To better understand the effects and relationship between precipitation, net ecosystem carbon dioxide (NEE) and water vapor exchange (ET), we report a study conducted in the tropical dry forest (TDF) in the northwest of Mexico. Ecosystem gas exchange was measured using the eddy correlation technique during the presence of North American Monsoon System (NAMS) in 2006. Patterns in NEE and ET were different in wet and dry periods. Three markedly defined periods were found during the six-month study period. A pre-monsoon period, where gas exchange was close to zero. A monsoon period, divided in two stages: 1) early monsoon: a strong increase in the respiratory rate marked by a peak of positive values, with a maximum of 22 g CO 2 m À2 day À1 , and, 2) late monsoon: an assimilation period occurred in the peak of the monsoon period, with sustained values around À20 g CO 2 m À2 day À1 . The final was a postmonsoon period, where ecosystems returned to dormancy. NEE and ET trends in the TDF were similar to other seasonally dry ecosystems influenced by the NAMS. During the study period the TDF of Northwest Mexico acted as a sink capturing 374 g CO 2 m 2 with an ecosystem water use efficiency (-NEE/ET) comparable to other ecosystems in the region. Mechanistic information about biological and environmental variables controlling gas exchange dynamics is still necessary to predict how seasonally dry ecosystems would respond to climate change