The infiltration of stormwater runoff for use by urban trees is a major co-benefit of green infrastructure for desert cities with limited water resources. However, the effects of this passive ...irrigation versus regular, controlled moisture inputs, or active irrigation, is largely unquantified. We monitored the ecohydrology of urban mesquite trees (Prosopis spp.) under these contrasting irrigation regimes in semiarid Tucson, AZ. Measurements included soil moisture, sap velocity, canopy greenness, and leaf-area index. We expected both irrigation types to provide additional deep (>20 cm) soil moisture compared to natural conditions, and that trees would depend on this deep moisture for transpiration and phenological activity. Results show that active irrigation supported higher soil moisture throughout the study than passive irrigation. Passive irrigation only provided additional deep moisture when green infrastructure features received impervious runoff from a city street. Sap velocity and greenness were similar under both irrigation types, outside of isolated periods of time. These differences occurred during the extremely wet summer 2017 when passively irrigated trees exhibited a greenness peak, and the dry conditions of spring when actively irrigated trees had higher sap flow and relative greenness. Finally, it was not determined that deep soil moisture had a stronger relationship with mesquite productivity than shallow moisture, but both relationships were stronger in the spring, before summer rains. This study aims to contribute empirical observations of green infrastructure performance for urban watershed management.
Global‐scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitude and interannual variability of the land CO2 sink. However, such analyses are poorly constrained by ...measured CO2 exchange in drylands. Here we address this observation gap with eddy covariance data from 25 sites in the water‐limited Southwest region of North America with observed ranges in annual precipitation of 100–1000 mm, annual temperatures of 2–25°C, and records of 3–10 years (150 site‐years in total). Annual fluxes were integrated using site‐specific ecohydrologic years to group precipitation with resulting ecosystem exchanges. We found a wide range of carbon sink/source function, with mean annual net ecosystem production (NEP) varying from ‐350 to +330 gCm−2 across sites with diverse vegetation types, contrasting with the more constant sink typically measured in mesic ecosystems. In this region, only forest‐dominated sites were consistent carbon sinks. Interannual variability of NEP, gross ecosystem production (GEP), and ecosystem respiration (Reco) was larger than for mesic regions, and half the sites switched between functioning as C sinks/C sources in wet/dry years. The sites demonstrated coherent responses of GEP and NEP to anomalies in annual evapotranspiration (ET), used here as a proxy for annually available water after hydrologic losses. Notably, GEP and Reco were negatively related to temperature, both interannually within site and spatially across sites, in contrast to positive temperature effects commonly reported for mesic ecosystems. Models based on MODIS satellite observations matched the cross‐site spatial pattern in mean annual GEP but consistently underestimated mean annual ET by ~50%. Importantly, the MODIS‐based models captured only 20–30% of interannual variation magnitude. These results suggest the contribution of this dryland region to variability of regional to global CO2 exchange may be up to 3–5 times larger than current estimates.
Global‐scale studies suggest that drylands dominate an increasing trend in the magnitude and interannual variability of the land CO2 sink, but direct measurements are lacking; 25 eddy covariance sites in the water‐limited southwest of North America showed wide‐ranging carbon sink/source function, contrasting with the persistent sink typically measured in mesic ecosystems. Interannual variability of CO2 exchange was larger than for mesic regions, and half the sites switched between functioning as C sinks/sources in wet/dry years. CO2 exchanges were negatively related to temperature, in contrast to positive effects commonly reported for mesic ecosystems. MODIS‐based models captured only 20–30% of interannual variation, suggesting this dryland region may contribute 3–5 times more variability to global carbon and water cycles than current estimates.
We present an observational analysis examining soil moisture control on surface energy dynamics and planetary boundary layer characteristics. Understanding soil moisture control on land‐atmosphere ...interactions will become increasingly important as climate change continues to alter water availability. In this study, we analyzed 4 years of data from the Santa Rita Creosote Ameriflux site. We categorized our data independently in two ways: (1) wet or dry seasons and (2) one of the four cases within a two‐layer soil moisture framework for the root zone based on the presence or absence of moisture in shallow (0–20 cm) and deep (20–60 cm) soil layers. Using these categorizations, we quantified the soil moisture control on surface energy dynamics and planetary boundary layer characteristics using both average responses and linear regression. Our results highlight the importance of deep soil moisture in land‐atmosphere interactions. The presence of deep soil moisture decreased albedo by about 10%, and significant differences were observed in evaporative fraction even in the absence of shallow moisture. The planetary boundary layer height (PBLh) was largest when the whole soil profile was dry, decreasing by about 1 km when the whole profile was wet. Even when shallow moisture was absent but deep moisture was present the PBLh was significantly lower than when the entire profile was dry. The importance of deep moisture is likely site‐specific and modulated through vegetation. Therefore, understanding these relationships also provides important insights into feedbacks between vegetation and the hydrologic cycle and their consequent influence on the climate system.
Key Points
Shallow and deep soil moisture influence surface energy dynamics
Soil moisture in the deep layer is linked with decreased ecosystem albedo
Soil moisture in the deep layer is linked with planetary boundary layer height
Plant chlorophyll fluorescence Cendrero-Mateo, M. Pilar; Moran, M. Susan; Papuga, Shirley A. ...
Journal of experimental botany,
01/2016, Letnik:
67, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Most studies assessing chlorophyll fluorescence (ChlF) have examined leaf responses to environmental stress conditions using active techniques. Alternatively, passive techniques are able to measure ...ChlF at both leaf and canopy scales. However, the measurement principles of both techniques are different, and only a few datasets concerning the relationships between them are reported in the literature. In this study, we investigated the potential for interchanging ChlF measurements using active techniques with passive measurements at different temporal and spatial scales. The ultimate objective was to determine the limits within which active and passive techniques are comparable. The results presented in this study showed that active and passive measurements were highly correlated over the growing season across nitrogen treatments at both canopy and leaf-average scale. At the single-leaf scale, the seasonal relation between techniques was weaker, but still significant. The variability within single-leaf measurements was largely related to leaf heterogeneity associated with variations in CO₂ assimilation and stomatal conductance, and less so to variations in leaf chlorophyll content, leaf size or measurement inputs (e.g. light reflected and emitted by the leaf and illumination conditions and leaf spectrum). This uncertainty was exacerbated when single-leaf analysis was limited to a particular day rather than the entire season. We concluded that daily measurements of active and passive ChlF at the single-leaf scale are not comparable. However, canopy and leaf-average active measurements can be used to better understand the daily and seasonal behaviour of passive ChlF measurements. In turn, this can be used to better estimate plant photosynthetic capacity and therefore to provide improved information for crop management.
Purpose
Detecting belowground chemical contamination is a challenging environmental problem due to subsurface heterogeneity and limited monitoring capabilities associated with labor-intensive, ...intrusive, and costly sampling techniques. With their extensive root systems, vascular plants are an easily accessible aboveground link to the subsurface. Therefore, plant tissues, particularly tree cores, are used in phytoscreening applications for detecting belowground chemical contaminants. While phytoscreening has been evolving as an alternative to traditional sampling, this method has caveats: (a) easier to harvest tissues such as leaves and twigs tend to exhibit lower concentrations than tree cores; (b) coring can negatively influence tree health; and (c) trees may be unavailable for sampling at all sites. Therefore, less invasive techniques that incorporate plants other than trees could be more effective phytoscreeners.
Methods
Here, we explore the use of insect-induced plant galls–that can be found on many vascular plant species–for their phytoscreening capabilities. Driven by observations that they can act as sinks for extraordinary high concentrations of nutrients during development, we tested the following hypothesis: galls will accumulate belowground chemical contaminants in higher concentrations compared to other aboveground plant tissues. Specifically, we measured and compared the concentrations of two contaminant types, inorganic heavy metals (HMs) and a volatile organic compound (VOC) and known human carcinogen, 1,4-dioxane, in four aboveground plant tissue types (leaf, twig, core, fruit) and insect-induced plant gall, across three plant types (shrub, tree, vine) sampled from contaminated areas.
Results
Twelve different HMs were present in shrubs, and 1,4–dioxane was present in both trees and vines; however, the concentrations of each contaminants varied quantitatively across plant tissue types. While galls accumulated the lowest concentrations of HMs, they accumulated the highest concentration of 1,4-dioxane, at five-times that of tree cores.
Conclusion
Given they can be found on many vascular plant species and are easy to collect when they are locally abundant, with further development, galls may be a powerful alternative for detecting belowground chemical contamination, particularly for VOCs in urban areas. We discuss the dichotomy in the ability of galls to detect inorganic HMs versus VOCs, and outline future questions that should be considered to further develop galls for phytoscreening application.
Chlorophyll molecules absorb photosynthetic active radiation (PAR). The resulting excitation energy is dissipated by three competing pathways at the level of photosystem: (i) photochemistry (and, by ...extension, photosynthesis); (ii) regulated and constitutive thermal energy dissipation; and (iii) chlorophyll-a fluorescence (ChlF). Because the dynamics of photosynthesis modulate the regulated component of thermal energy dissipation (widely addressed as non-photochemical quenching (NPQ)), the relationship between photosynthesis, NPQ and ChlF changes with water, nutrient and light availability. In this study we characterised the relationship between photosynthesis, NPQ and ChlF when conducting light-response curves of photosynthesis in plants growing under different water, nutrient and ambient light conditions. Our goals were to test whether ChlF and photosynthesis correlate in response to water and nutrient deficiency, and determine the optimum PAR level at which the correlation is maximal. Concurrent gas exchange and ChlF light-response curves were measured for Camelina sativa (L.) Crantz and Triticum durum (L.) Desf plants grown under (i) intermediate light growth chamber conditions, and (ii) high light environment field conditions respectively. Plant stress was induced by withdrawing water in the chamber experiment, and applying different nitrogen levels in the field experiment. Our study demonstrated that ChlF was able to track the variations in photosynthetic capacity in both experiments, and that the light level at which plants were grown was optimum for detecting both water and nutrient deficiency with ChlF. The decrease in photosynthesis was found to modulate ChlF via different mechanisms depending on the treatment: through the action of NPQ in response to water stress, or through the action of changes in leaf chlorophyll concentration in response to nitrogen deficiency. This study provides support for the use of remotely sensed ChlF as a proxy to monitor plant stress dynamics from space.
•Synthetic time series of albedo and EVI can capture land surface dynamics with high similarity to tower and field data.•The RMSE and bias of the synthetic albedos are <0.013 and within ±0.006, ...respectively as compared to Ameriflux field data.•Spatially representative NEON albedometer data will greatly improve moderate resolution satellite products validation.
Seasonal vegetation phenology can significantly alter surface albedo which in turn affects the global energy balance and the albedo warming/cooling feedbacks that impact climate change. To monitor and quantify the surface dynamics of heterogeneous landscapes, high temporal and spatial resolution synthetic time series of albedo and the enhanced vegetation index (EVI) were generated from the 500m Moderate Resolution Imaging Spectroradiometer (MODIS) operational Collection V006 daily BRDF/NBAR/albedo products and 30m Landsat 5 albedo and near-nadir reflectance data through the use of the Spatial and Temporal Adaptive Reflectance Fusion Model (STARFM). The traditional Landsat Albedo (Shuai et al., 2011) makes use of the MODIS BRDF/Albedo products (MCD43) by assigning appropriate BRDFs from coincident MODIS products to each Landsat image to generate a 30m Landsat albedo product for that acquisition date. The available cloud free Landsat 5 albedos (due to clouds, generated every 16days at best) were used in conjunction with the daily MODIS albedos to determine the appropriate 30m albedos for the intervening daily time steps in this study. These enhanced daily 30m spatial resolution synthetic time series were then used to track albedo and vegetation phenology dynamics over three Ameriflux tower sites (Harvard Forest in 2007, Santa Rita in 2011 and Walker Branch in 2005). These Ameriflux sites were chosen as they are all quite nearby new towers coming on line for the National Ecological Observatory Network (NEON), and thus represent locations which will be served by spatially paired albedo measures in the near future. The availability of data from the NEON towers will greatly expand the sources of tower albedometer data available for evaluation of satellite products. At these three Ameriflux tower sites the synthetic time series of broadband shortwave albedos were evaluated using the tower albedo measurements with a Root Mean Square Error (RMSE) less than 0.013 and a bias within the range of ±0.006. These synthetic time series provide much greater spatial detail than the 500m gridded MODIS data, especially over more heterogeneous surfaces, which improves the efforts to characterize and monitor the spatial variation across species and communities. The mean of the difference between maximum and minimum synthetic time series of albedo within the MODIS pixels over a subset of satellite data of Harvard Forest (16km by 14km) was as high as 0.2 during the snow-covered period and reduced to around 0.1 during the snow-free period. Similarly, we have used STARFM to also couple MODIS Nadir BRDF Adjusted Reflectances (NBAR) values with Landsat 5 reflectances to generate daily synthetic times series of NBAR and thus Enhanced Vegetation Index (NBAR-EVI) at a 30m resolution. While normally STARFM is used with directional reflectances, the use of the view angle corrected daily MODIS NBAR values will provide more consistent time series. These synthetic times series of EVI are shown to capture seasonal vegetation dynamics with finer spatial and temporal details, especially over heterogeneous land surfaces.
In semiarid regions, where water resources are limited and precipitation dynamics are changing, understanding land surface‐atmosphere interactions that regulate the coupled soil ...moisture‐precipitation system is key for resource management and planning. We present a modeling approach to study soil moisture and albedo controls on planetary boundary layer height (PBLh). We used Santa Rita Creosote Ameriflux and Tucson Airport atmospheric sounding data to generate empirical relationships between soil moisture, albedo, and PBLh. Empirical relationships showed that ∼50% of the variation in PBLh can be explained by soil moisture and albedo with additional knowledge gained by dividing the soil profile into two layers. Therefore, we coupled these empirical relationships with soil moisture estimated using a two‐layer bucket approach to model PBLh under six precipitation scenarios. Overall we observed that decreases in precipitation tend to limit the recovery of the PBL at the end of the wet season. However, increases in winter precipitation despite decreases in summer precipitation may provide opportunities for positive feedbacks that may further generate more winter precipitation. Our results highlight that the response of soil moisture, albedo, and the PBLh will depend not only on changes in annual precipitation, but also on the frequency and intensity of this change. We argue that because albedo and soil moisture data are readily available at multiple temporal and spatial scales, developing empirical relationships that can be used in land surface‐atmosphere applications have great potential for exploring the consequences of climate change.
Plain Language Summary
Soil moisture available at different depths triggers processes that change the planetary boundary layer (PBL) height. The PBL is the closest layer of the troposphere interacting with the land surface. For instance, rainfall can change the characteristics of the soil which influences the way energy is exchanged with the atmosphere, i.e., a light‐colored dry sandy soil reflects more energy (high albedo) than a darker wet sandy soil (low albedo). Using observations, we can better understand these interactions by generating models driven by empirically derived relationships. With these models, we can simulate how changes in precipitation frequency and amount could impact the dynamics of moisture in the soil and therefore the albedo of the land surface. Then, we can model how much the PBL grows and estimate the height of cloud formation. In semiarid regions, where water resources are limited and precipitation dynamics are changing, understanding these land surface‐atmosphere interactions is key for resource management and planning. We argue that because albedo and soil moisture data are readily available at multiple temporal and spatial scales, developing empirical relationships that can be used in land surface‐atmosphere applications have great potential for exploring the consequences of climate change.
Key Points
Both shallow and deep soil moisture influence planetary boundary characteristics
Planetary boundary characteristics can be estimated using empirical relationships based on soil moisture and albedo data
Empirical relationships are coupled with two‐layer soil moisture estimates to model PBLh under current and future precipitation regimes
Ecohydrological processes in semiarid shrublands and other dryland ecosystems are sensitive to discrete pulses of precipitation. Anticipated changes in the frequency and magnitude of precipitation ...events are expected to impact the spatial and temporal distribution of soil moisture in these drylands, thereby impacting their ecohydrological processes. Recent field studies have shown that in dryland ecosystems, transpiration dynamics and plant productivity are largely a function of deep soil moisture available after large precipitation events, regardless of where the majority of plant roots occur. However, the strength of this relationship and how and why it varies throughout the year remains unclear. We present eddy covariance, soil moisture, and sap flow measurements taken over an 18‐month period in conjunction with an analysis of biweekly precipitation, shallow soil, deep soil, and stem stable water isotope samples from a creosotebush‐dominated shrubland ecosystem at the Santa Rita Experimental Range in southern Arizona. Within the context of a hydrologically defined two‐layer conceptual framework, our results support that transpiration is associated with the availability of deep soil moisture and that the source of this moisture varies seasonally. Therefore, changes in precipitation pulses that alter the timing and magnitude of the availability of deep soil moisture are expected to have major consequences for dryland ecosystems. Our findings offer insights that can improve the representation of drylands within regional and global models of land surface atmosphere exchange and their linkages to the hydrologic cycle.
Key Points
Shallow and deep soil moisture are isotopically distinct at this warm semiarid shrubland
Deep moisture tends to be isotopically similar to precipitation from previous season
Shrubs use deep moisture all year round
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