Proxy For Water Availability Research Articles

Water limitation regulates positive feedback of increased ecosystem respiration.

Terrestrial ecosystem respiration increases exponentially with temperature, constituting a positive feedback loop accelerating global warming. However, the response of ecosystem respiration to temperature strongly depends on water availability, yet where and when the water effects are important, is presently poorly constrained, introducing uncertainties in climate-carbon cycle feedback projections. Here, we disentangle the effects of temperature and precipitation (a proxy for water availability) on ecosystem respiration by analysing eddy covariance CO2 flux measurements across 212 globally distributed sites. We reveal a threshold precipitation function, determined by the balance between precipitation and ecosystem water demand, which separates temperature-limited and water-limited respiration. Respiration is temperature limited for precipitation above that threshold function, whereas in drier areas water limitation reduces the temperature sensitivity of respiration and its positive feedback to global warming. If the trend of expansion of water-limited areas with warming climate over the last decades continues, the positive feedback of ecosystem respiration is likely to be weakened and counteracted by the increasing water limitation.

Preserving isohydricity: vertical environmental variability explains Amazon forest water use strategies.

Increases in hydrological extremes, including drought, are expected for Amazon forests. A fundamental challenge for predicting forest responses lies in identifying ecological strategies which underlie such responses. Characterization of species-specific hydraulic strategies for regulating water use, thought to be arrayed along an 'isohydric-anisohydric' spectrum, is a widely used approach. However, recent studies have questioned the usefulness of this classification scheme, because its metrics are strongly influenced by environments, hence can lead to divergent classifications even within the same species. Here, we propose an alternative approach positing that individual hydraulic regulation strategies emerge from the interaction of environments with traits. Specifically, we hypothesize that the vertical forest profile represents a key gradient in drought-related environments (atmospheric vapor pressure deficit, soil water availability) that drives divergent tree water use strategies for coordinated regulation of stomatal conductance (gs) and leaf water potentials (ΨL) with tree rooting depth, a proxy for water availability. Testing this hypothesis in a seasonal eastern Amazon forest in Brazil, we found that hydraulic strategies indeed depend on height-associated environments. Upper canopy trees, experiencing high VPD, but stable soil water access through deep rooting, exhibited isohydric strategies, defined by little seasonal change in the diurnal pattern of gs and steady seasonal minimum ΨL. By contrast, understory trees, exposed to less variable VPD but highly variable soil water availability, exhibited anisohydric strategies, with fluctuations in diurnal gs that increased in the dry season along with increasing variation in ΨL. Our finding that canopy height structures the coordination between drought-related environmental stressors and hydraulic traits provides a basis for preserving the applicability of the isohydric-to-anisohydric spectrum, which we show here may consistently emerge from environmental context. Our work highlights the importance of understanding how environmental heterogeneity structures forest responses to climate change, providing a mechanistic basis for improving models of tropical ecosystems.

Population dynamics of the threatened Oregon spotted frog before and after drought mitigation

AbstractAmphibians are among the most sensitive taxa to climate change, and species inhabiting arid and semiarid landscapes at the extremes of their range are especially vulnerable to drought. The Jack Creek, Oregon, USA, population of Oregon spotted frogs (Rana pretiosa) faces unique challenges because it occupies the highest elevation site in the species' extant range and one that has been transformed by loss of American beavers (Castor canadensis), which historically maintained open water. We evaluated the effects of drought mitigation (addition of excavated ponds) on relationships between local and regional water availability, inactive legacy beaver dams, and Oregon spotted frog population dynamics in the Jack Creek system. We conducted egg mass surveys and capture‐mark‐recapture sampling at a treatment reach with excavated ponds and 3 reference reaches over 13 years; surveys spanned a period before and after pond excavation at the treatment and 1 primary comparison reference reach. We analyzed data using a combination of robust design capture‐mark‐recapture estimators and generalized linear mixed models to characterize population dynamics. Adult Oregon spotted frog survival was approximately 19.5% higher at the treatment reach than the primary reference reach during the study period. Annual survival was most strongly associated with late summer vegetation greenness, a proxy for water availability, and males had higher survival than females. Among the 4 study reaches, the treatment reach consistently had higher late summer vegetation greenness, and the hydrology functioned more independently of regional precipitation patterns relative to the reference reaches; however, these dynamics were not linked to pond excavation. Breeding was concentrated in 2 legacy beaver ponds that were deepened by excavation during the study compared to an unexcavated beaver pond, 2 excavated ponds without legacy beaver dams, and 9 reference ponds. These results point to the benefit of enhancing existing beaver structures and indicate that management actions aimed at maintaining surface water for breeding in spring and saturated soils and ponded water for adults in late summer would benefit this unique population of Oregon spotted frogs in the face of drought.

Phytoliths as indicators of plant water availability: The case of millets cultivation in the Indus Valley civilization

The interpretation of crop water management practices has been central to the archeological debate on agricultural strategies and is crucial where the type of water strategy can provide fundamental explanations for the adoption and use of specific crops. Traces of water administration are difficult to detect and are mostly indirect, in the form of water harvesting or distribution structures. Attempts have been made to infer plant water availability directly from archaeobotanical remains. Current evidence suggests that the ratio of sensitive to fixed phytolith morphotypes can be used as a proxy for water availability in C₃ crops, as well as in sorghum and maize. Nevertheless, the controversy on whether genetically and environmentally controlled mechanisms of biosilica deposition are directly connected to water availability in C₄ crops is open, and several species remain to be tested for their phytolith production in relation to water levels. This research aims at clarifying whether leaf phytolith assemblages and concentration, silica skeleton size and ratio of sensitive to fixed morphotypes can be related to different water regimes in Eleusine coracana Gaertn., Pennisetum glaucum (L.) R. Br., and Sorghum bicolor (L.) Moench. We cultivated 5 traditional landraces for each species in lysimeters, under different watering conditions and analyzed their phytolith content/production in leaves. Results show higher proportions of long cells, bulliforms and stomata produced in well watered conditions. The model built on the basis of phytolith composition has been then applied to interpret archeological phytolith assemblages recovered from a single phase at four different sites of the Indus Civilisation: Harappa, Kanmer, Shikarpur and Alamgirpur. The results show that most probably C4 crops grew under water stress conditions, providing new data on the interpretation of ancient agricultural management in the Indus Valley.

Local dynamic variation of lianas along topography maintains unchanging abundance at the landscape scale in central Amazonia

AbstractQuestionHave liana density and biomass increased in central Amazonia over the last 10 years? Can a spatially explicit consideration of liana mortality and recruitment rates across hydro‐edaphic and tree turnover gradients at the landscape scale explain changes in liana density and biomass?LocationDucke Forest Reserve, 26 km north of Manaus, Amazonas, Brazil.MethodsData were collected on 30–1 ha permanent plots in a central Amazonia undisturbed old‐growth rain forest 10 years after the first census. We measured lianas at 1.3 cm above their rooting point, with a diameter (D) ≥ 5 cm and subsampled lianas ≥1 cm diameter in 0.25 ha per plot. We estimated above‐ground density and biomass changes, mortality rate, recruitment and diameter increase. Soil cations and available P were reduced to two dimensions with PCA and the first axis used as the descriptor of soil fertility. Height above the nearest drainage, a proxy for water availability, tree turnover (D ≥ 10 cm) and soil fertility were used as predictors of liana dynamics.ResultsNo significant change in liana density and biomass, averaged over the 30‐km2 landscape, was observed over the last 10 years. In 2014, liana density was generally higher in more fertile soils, and it increased in areas closer to the water table and with higher tree turnover in the valleys. This pattern resulted from the higher liana recruitment rates in valley plots closer to the water table. Liana mortality rates were uniform across plots, similar among the diameter classes and, on average, higher than recruitment.ConclusionWe did not find any evidence that liana density and biomass have been increasing in this Neotropical site over the last 10 years. These findings suggest that the current knowledge on liana increase trends in the Neotropics should be reviewed if supported by further tropical studies.

Rates of woody encroachment in African savannas reflect water constraints and fire disturbance

AbstractAimThe aims of this study were to (1) estimate current rates of woody encroachment across African savannas; (2) identify relationships between change in woody cover and potential drivers, including water constraints, fire frequency and livestock density. The found relationships led us to pursue a third goal: (3) use temporal dynamics in woody cover to estimate potential woody cover.LocationSub‐Saharan African savannas.MethodsThe study used very high spatial resolution satellite imagery at sites with overlapping older (2002–2006) and newer (2011–2016) imagery to estimate change in woody cover. We sampled 596 sites in 38 separate areas across African savannas. Areas with high anthropogenic impact were avoided in order to more clearly identify the influence of environmental factors. Relationships between woody cover change and potential drivers were identified using linear regression and simultaneous autoregression, where the latter accounts for spatial autocorrelation.ResultsThe mean annual change in woody cover across our study areas was 0.25% per year. Although we cannot explain the general trend of encroachment based on our data, we found that change rates were positively correlated with the difference between potential woody cover and actual woody cover (a proxy for water availability; p < .001), and negatively correlated with fire frequency (p < .01). Using the relationship between rates of encroachment and initial cover, we estimated potential woody cover at different rainfall levels.Main conclusionsThe results indicate that woody encroachment is ongoing and widespread across African savannas. The fact that the difference between potential and actual cover was the most significant predictor highlights the central role of water availability and tree–tree competition in controlling change in woody populations, both in water‐limited and mesic savannas. Our approach to derive potential woody cover from the woody cover change trajectories demonstrates that temporal dynamics in woody populations can be used to infer resource limitations.

Multimodel assessment of sensitivity and uncertainty of evapotranspiration and a proxy for available water resources under climate change

Partitioning of precipitation (P) into actual evapotranspiration (ET) and runoff affects a proxy for water availability (P-ET) on land surface. ET accounts for more than 60% of global precipitation and affects both water and energy cycles. We study the changes in precipitation, air temperature, ET, and P-ET in seven large basins under the RCP 2.6 and 8.5 scenarios for the projected future climate. While a majority of studied basins is projected to experience a warmer and wetter climate, uncertainty in precipitation projections remains large in comparison to the temperature projections. Due to high uncertainty in ET, uncertainties in fraction of precipitation that is evaporated (ET/P) and a proxy for available water (P-ET) are also large under the projected future climate. Our assessment showed that under the RCP 8.5 scenario, global climate models are major contributors to uncertainties in ET (P-ET) simulations in the four (six) basins, while uncertainty due to hydrological models is prevailing or comparable in the other three (one) basins. The simulated ET is projected to increase under the warmer and wetter future climates in all the basins and periods under both RCPs. Regarding P-ET, it is projected to increase in five out of seven basins in the End term (2071–2099) under the RCP 8.5 scenario. Precipitation elasticity and temperature sensitivity estimated for ET were found to be positive in all the basins under the RCP 8.5 scenario. In contrast, the temperature sensitivity estimated for (P-ET) was found to be negative for all the basins under the RCP 8.5 scenario, indicating the role of increased energy availability and limited soil moisture. Our results highlight the need for improvements in climate and hydrological models with better representation of soil, vegetation, and cold season processes to reduce uncertainties in the projected ET and P-ET.

GPP and maximum light use efficiency estimates using different approaches over a rotating biodiesel crop

This paper presents: (a) results of gross primary production (GPP) 8-d estimated values using a light use efficiency model (LUE) in a non-irrigated rotating rapeseed crop in the upper Spanish plateau, and (b) inter-comparison results of observed GPP with those concurrently retrieved by MODIS. The rotation scheme over the four-year study comprised rapeseed, wheat, peas and rye. Rapeseed, peas and, in part, rye grew under well-watered conditions whereas wheat was dominated by drought.Input data for the LUE model were the fraction of PAR absorbed (FPAR) 8-d products supplied by MODIS (FPARMODIS), in situ photosynthetic active radiation (PAR) measurements and a scalar f varying between 0 and 1, to take into account the reduction of the maximum PAR conversion efficiency (ɛ0LUE) under limiting environmental conditions. In this study, f values were assumed to be dependent on air temperature (T) and the evaporative fraction which was considered a proxy of water availability. ɛ0LUE, a key parameter in LUE models, which varied according to land use, was derived through the results of a linear regression fit between observed GPP and concurrent GAPAR estimates defined as the product of PAR, FPARMODIS and f. Overall, the LUE model provided satisfactory results, R2=86.3%, significantly improving GPP MODIS estimates (GPPMODIS), R2=71.8%. GPPMODIS uncertainties have primarily been attributed to differences in the f stress factor involved in its formulation (fMODIS) depending on vapour pressure deficit and T which did not fully describe the environmental stress conditions at the measuring site.Overall, ɛ0LUE yielded 3.33±0.10gCMJ−1 although this varied depending on crop architecture, phenology and prevailing meteorological conditions. Crop-to-crop ɛ0LUE ranged from 2.74±0.17 to 3.95±0.19gCMJ−1 for peas and rye, respectively, yielding intermediate values for rapeseed and wheat, 2.92±0.18 and 2.86±0.23gCMJ−1, respectively. ɛ0MODIS, derived from the linear fit of GPP versus GPPMODIS estimates, yielded 2.13±0.10gCMJ−1 and crop-to-crop ranged from 1.28±0.17 to 2.41±0.12gCMJ−1 for wheat and rapeseed, respectively. The best linear fits corresponded to crops growing under well-watered conditions, rapeseed and peas, and the worst fits were for wheat, affected by drought. GPP annuals were 1680, 710, 730 and 1410gCm−2 for rapeseed, wheat, peas and rye, respectively.

Using an Unmanned Aerial Vehicle (UAV) to capture micro-topography of Antarctic moss beds

Mosses, the dominant flora of East Antarctica, show evidence of drying in recent decades, likely due to the regional effects of climate change. Given the relatively small area that such moss beds occupy, new tools are needed to map and monitor these fragile ecosystems in sufficient detail. In this study, we collected low altitude aerial photography with a small multi-rotor Unmanned Aerial Vehicle (UAV). Structure from Motion (SfM) computer vision techniques were applied to derive ultra-high resolution 3D models from multi-view aerial photography. A 2cm digital surface model (DSM) and 1cm orthophoto mosaic were derived from the 3D model and aerial photographs, respectively. The geometric accuracy of the orthophoto and DSM was 4cm. A weighted contributing upstream area was derived with the D-infinity algorithm, based on the DSM and a snow cover map derived from the orthophoto. The contributing upstream area was used as a proxy for water availability from snowmelt, one of the key environmental drivers of moss health. A Monte Carlo simulation with 300 realisations was implemented to model the impact of error in the DSM on runoff direction. Significant correlations were found between these simulated water availability values and field measurements of moss health and water content. In the future ultra-high spatial resolution DSMs acquired with a UAV could thus be used to determine the impact of changing snow cover on the health and spatial distribution of polar vegetation non-destructively.

Impact of root fragment dimension, weight, burial depth, and water regime on Cirsium arvense emergence and growth

Sciegienka, J. K., Keren, E. N. and Menalled, F. D. 2011. Impact of root fragment dimension, weight, burial depth, and water regime on Cirsium arvense emergence and growth. Can. J. Plant Sci. 91: 1027–1036. Cirsium arvense is an aggressive, introduced, perennial invasive weed that flourishes in a wide variety of environments including conventional and organic agricultural systems as well as disturbed non-crop habitats. Even though much research has been conducted on the chemical, biological, and cultural management of C. arvense, less information is available on how pre-emergence factors affect its reproductive biology and growth. This research assessed the combined impact of root fragment size, root fragment biomass, burial depth, and water regime (a proxy of water availability) on C. arvenseemergence and growth in fallow conditions. In field experiments, root burial depth was the most important factor determining C. arvenseemergence and growth, with roots at the 10-cm depth having the greatest average emergence (51.2±2.0% in 2007 and 43.5±7.2% in 2008; mean±SEM) compared to roots at the 2 cm (8.9±7.4% in 2007 and 38.1±8.3% in 2008) or 20 cm (12.8±4.0% in 2007 and 17.6±2.7% in 2008) depth. In greenhouse experiments, water regime was the overriding variable determining C. arvense emergence as well as above-ground and below-ground biomass. These results could improve decision-aid models and enhance the efficacy of site-specific C. arvense management practices.

Simultaneous thin section and phytolith observations of finely stratified deposits from Neolithic Çatalhöyük, Turkey: implications for paleoeconomy and Early Holocene paleoenvironment

AbstractSimultaneous thin section and phytolith observations of finely stratified anthropogenic deposits from the Neolithic settlement of Çatalhöyük, Turkey, dated between 7400 and 6000 BC provide evidence for both the depositional context and phytolith assemblage of these deposits. Although extracted phytoliths provide a general picture of vegetation that supports existing evidence of a local wet marshland environment, comparisons with observations of phytoliths in situ indicate a diverse range of microcontexts, as well as depositional and post‐depositional processes that influence phytolith size. This has implications for studies that use conjoined phytolith size as a proxy for water availability and early agricultural practices. Observations indicate a significant background noise of phytoliths and micro‐charcoal in the deposits, linked to the frequent use of fire, which has implications for interpreting assemblages where phytolith counts are low, such as from floors of buildings. This study confirms the usefulness of phytoliths in providing information on human plant use and environment where the taphonomy of the deposits is clear, and provides new evidence for wet farming of at least some of the wheat found at the site. It also suggests there needs to be greater consideration of phytolith taphonomy, which can be provided to an extent by combining phytolith analysis with thin section micromorphology. Copyright © 2011 John Wiley & Sons, Ltd.

Reproductive biomass allocation in the dioecious perennial Acanthosicyos horrida

In a Namib population of the dioecious perennial Acanthosicyos horrida Welw. ex Hook. f. in which fruit production is declining, we investigated the effects of plant gender, plant size, and subpopulation elevation (a proxy of water availability) on a plant's biomass allocation to sexual reproduction. While males invested more heavily in buds and flowers than females, females allocated 10 times more total biomass to reproduction per m3 of vegetation than males during our survey period. Also, smaller plants, particularly females, allocated more biomass to reproduction per m3 of vegetation than larger plants. This result suggests that plant fecundity per m3 of vegetation decreases with plant size. Aging of the population could thus be a cause of the reduced fruit production; however, we found that the population was skewed towards the smallest size classes. On the other hand, the fact that plants in the subpopulation at higher elevation above the river were only six percent as productive as plants in the subpopulation at low elevation suggests that the depth of the water table may be critical to fruit production and that a reduced groundwater level caused by nearby urbanisation could have long term effects on harvests.