Ecohydrological Controls on Grass and Shrub Above-ground Net Primary Productivity in a Seasonally Dry Climate

Seasonally dry, water‐limited regions are often co‐dominated by distinct herbaceous and woody plant communities with contrasting ecohydrological properties. We investigated the shape of the above‐ground net primary productivity (ANPP) response to annual precipitation (Pa) for adjacent grassland and shrubland ecosystems in Southern California, with the goal of understanding the role of these ecohydrological properties on ecosystem function. Our synthesis of observations and modelling demonstrates grassland and shrubland exhibit distinct ANPP‐Pa responses that correspond with characteristics of the long‐term Pa distribution and mean water balance fluxes. For annual grassland, no ANPP occurs below a ‘precipitation compensation point,’ where bare soil evaporation dominates the water balance, and ANPP saturates above the Pawhere deep percolation and runoff contribute to the modelled water balance. For shrubs, ANPP increases at a lower and relatively constant rate across the Pa gradient, while deep percolation and runoff account for a smaller fraction of the modelled water balance. We identify precipitation seasonality, root depth, and water stress sensitivity as the main ecosystem properties controlling these responses. Observed ANPP‐Paresponses correspond to notably different patterns of rain‐use efficiency (RUE). Grass RUE exceeds shrub RUE over a wide range of typical Pa values, whereas grasses and shrubs achieve a similar RUE in particularly dry or wet years. Inter‐annual precipitation variability, and the concomitant effect on ANPP, plays a critical role in maintaining the balance of grass and shrub cover and ecosystem‐scale productivity across this landscape

Ecohydrology: processes and implications for rangelands

This chapter is organized around the concept of ecohydrological processes that are explicitly tied to ecosystem services. Ecosystem services are benefits that people receive from ecosystems. We focus on (1) the regulating services of water distribution, water purification, and climate regulation; (2) the supporting services of water and nutrient cycling and soil protection and restoration; and (3) the provisioning services of water supply and biomass production. Regulating services are determined at the first critical juncture of the water cycle—on the soil surface, where water either infiltrates or becomes overland flow. Soil infiltrability is influenced by vegetation, grazing intensity, brush management, fire patterns, condition of biological soil crusts, and activity by fauna. At larger scales, water-regulating services are influenced by other factors, such as the nature and structure of riparian zones and the presence of shallow groundwater aquifers. Provisioning services are those goods or products that are directly produced from ecosystems, such as water, food, and fiber. Work over the last several decades has largely overturned the notion that water supply can be substantially increased by removal of shrubs. In riparian areas, surprisingly, removal of invasive, non-native woody plants appears to hold little potential for increasing water supply. Here, the primary factor appears to be that non-native plants use no more water than the native vegetation they displace. Clearly there is a close coupling between biota (both fauna and flora) and water on rangelands—which is why water-related ecosystem services are so strongly dependent on land management strategies.Fil: Wilcox, Bradford P.. Texas A&M University; Estados UnidosFil: Le Maitre, David. Council for Scientific and Industrial Research; SudáfricaFil: Jobbagy Gampel, Esteban Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Matemática Aplicada de San Luis "Prof. Ezio Marchi". Universidad Nacional de San Luis. Facultad de Ciencias Físico, Matemáticas y Naturales. Instituto de Matemática Aplicada de San Luis "Prof. Ezio Marchi"; ArgentinaFil: Wang, Lixin. Indiana University; Estados UnidosFil: Breshears, David D.. University of Arizona; Estados Unido

Climate forcings and the nonlinear dynamics of grassland ecosystems

The nonlinear interaction of climate forcings and ecosystem variables is instrumental in creating the temporal and spatial heterogeneity of grasslands. Ecosystem processes are a product of these interactions and vary in sensitivity to them across time. How forcings aggregate and shape ecosystem responses is an important aspect of grassland states and defines how they respond to changes in environmental conditions. Characterizing the relationship between climate drivers and ecosystem variables helps sharpen analysis of ecosystem flux dynamics during the growing season and identifies likely deviations from mean functioning. To address the question of how climate forcings and ecosystem variables interact to shape seasonal water and carbon dynamics in grasslands, this thesis is split into two analysis chapters. The first (Chapter 3) is a characterization of water and carbon flux responses to variable precipitation timing and magnitude. Particular focus is placed on temporal sensitivity to inputs, seasonality in water flux dynamics, and the linkage between precipitation, soil moisture, evapotranspiration, and potential evaporation. Chapter 4 extends International Panel on Climate Change (IPCC A1B) regional climate scenario projections for the Central Plains of the United States to assess mesic grassland responses. The specific focus is assessing the ecosystem response to increased precipitation variability, increased potential evaporation, and earlier growing season onset. Effects of these forcings are shaped by simulations of constant and seasonally-varying water-use efficiency to assess the role of vegetation on grassland carbon assimilation, and also to explore species-specific responses at the Konza Prairie in North Central Kansas, USA. Results from both chapters show variation in seasonal sensitivity of fluxes to precipitation, with varying relationships between drivers, variable conditions, and fluxes. This research provides for a better understanding of ecosystem processes and provides assessment of the magnitude and extent that forcing variation has on grassland function. Results from the second chapter show increased seasonal water and carbon flux variability and increased frequency of water stress conditions. Vegetation responses suggest climate change will impact species and habitat compositions through changing environmental conditions and partitioning of carbon assimilation periods. This illustrates potential effects to grassland functioning and growing season dynamics

Groundwater-dependent ecosystems: Recent insights from satellite and field-based studies

© 2015 Author(s). Groundwater-dependent ecosystems (GDEs) are at risk globally due to unsustainable levels of groundwater extraction, especially in arid and semi-arid regions. In this review, we examine recent developments in the ecohydrology of GDEs with a focus on three knowledge gaps: (1) how do we locate GDEs, (2) how much water is transpired from shallow aquifers by GDEs and (3) what are the responses of GDEs to excessive groundwater extraction? The answers to these questions will determine water allocations that are required to sustain functioning of GDEs and to guide regulations on groundwater extraction to avoid negative impacts on GDEs. We discuss three methods for identifying GDEs: (1) techniques relying on remotely sensed information; (2) fluctuations in depth-to-groundwater that are associated with diurnal variations in transpiration; and (3) stable isotope analysis of water sources in the transpiration stream. We then discuss several methods for estimating rates of GW use, including direct measurement using sapflux or eddy covariance technologies, estimation of a climate wetness index within a Budyko framework, spatial distribution of evapotranspiration (ET) using remote sensing, groundwater modelling and stable isotopes. Remote sensing methods often rely on direct measurements to calibrate the relationship between vegetation indices and ET. ET from GDEs is also determined using hydrologic models of varying complexity, from the White method to fully coupled, variable saturation models. Combinations of methods are typically employed to obtain clearer insight into the components of groundwater discharge in GDEs, such as the proportional importance of transpiration versus evaporation (e.g. using stable isotopes) or from groundwater versus rainwater sources. Groundwater extraction can have severe consequences for the structure and function of GDEs. In the most extreme cases, phreatophytes experience crown dieback and death following groundwater drawdown.We provide a brief review of two case studies of the impacts of GW extraction and then provide an ecosystem-scale, multiple trait, integrated metric of the impact of differences in groundwater depth on the structure and function of eucalypt forests growing along a natural gradient in depth-to-groundwater. We conclude with a discussion of a depth-to-groundwater threshold in this mesic GDE. Beyond this threshold, significant changes occur in ecosystem structure and function

Ecosystem processes at the watershed scale: extending optimality theory from plot to catchment

[1] The adjustment of local vegetation conditions to limiting soil water by either maximizing productivity or minimizing water stress has been an area of central interest in ecohydrology since Eagleson's classic study. This work has typically been limited to consider one-dimensional exchange and cycling within patches and has not incorporated the effects of lateral redistribution of soil moisture, coupled ecosystem carbon and nitrogen cycling, and vegetation allocation processes along topographic gradients. We extend this theory to the hillslope and catchment scale, with in situ and downslope feedbacks between water, carbon and nutrient cycling within a fully transient, distributed model. We explore whether ecosystem patches linked along hydrologic flow paths as a catena evolve to form an emergent pattern optimized to local climate and topographic conditions. Lateral hydrologic connectivity of a small catchment is calibrated with streamflow data and further tested with measured soil moisture patterns. Then, the spatial gradient of vegetation density within a small catchment estimated with fine-resolution satellite imagery and field measurements is evaluated with simulated vegetation growth patterns from different root depth and allocation strategies as a function of hillslope position. This is also supported by the correspondence of modeled and field measured spatial patterns of root depths and catchmentlevel aboveground vegetation productivity. We test whether the simulated spatial pattern of vegetation corresponds to measured canopy patterns and an optimal state relative to a set of ecosystem processes, defined as maximizing ecosystem productivity and water use efficiency at the catchment scale. Optimal carbon uptake ranges show effective compromises between multiple resources (water, light, and nutrients), modulated by vegetation allocation dynamics along hillslope gradient. Citation: Hwang, T., L. Band, and T. C. Hales (2009), Ecosystem processes at the watershed scale: Extending optimality theory from plot to catchment, Water Resour. Res., 45, W11425

Precipitation controls on carbon and water relations in two African ecosystems

Understanding the interaction between precipitation and vegetation growth in water-limited ecosystems is vital for various livelihoods that depend on water resources. Precipitation is the primary driver of vegetation growth in dry ecosystems, while fog deposition is essential for the microclimate at dry coastal ecosystems and cloud forests. The analysis of soil moisture, which incorporates the action of climate, soil, and vegetation, is the key to understanding the carbon and water relations and the interaction between precipitation and vegetation. This thesis examines the impacts of precipitation variability on carbon and water relations in African savannas and the similarities in rainfall and fog deposition. The ecosystem-scale transpiration was estimated from eddy covariance measurements based on annually fitted water use efficiency and optimality hypothesis. The soil moisture measurements were analyzed using a hierarchy of soil moisture models with precipitation, NDVI, and potential evapotranspiration (PET) variability. The statistics of fog and rainfall were analyzed using an analogy with self-organized criticality. The annual evapotranspiration (ET) was comparable to the annual precipitation at the grazed savanna grassland. While the annual precipitation was highly variable, the estimated annual transpiration was nearly constant 55 % of ET. The transpiration (T) was reduced only during the drought year due to grass dieback-regrowth and possibly due to other changes in soil surface properties that enhanced evaporation. The annual net CO2 exchange (NEE) had large variation ranging from –58 (sink) to 198 (source) gC m-2 yr-1. The annual NEE was related to the maximum of remotely sensed vegetation index (NDVI), and the annual ecosystem respiration was strongly correlated with early season rainfall amount. The analysis of measured soil moisture across savannas showed that NDVI and PET adjustments to daily maximum ET are necessary for modeling depth averaged soil moisture. The soil moisture memory timescale, a rough measure of the time it takes for a soil column to forget the initial soil moisture state, was linearly related to daily mean precipitation intensity at semi-arid savannas. Both rainfall and fog time series showed approximate power-law relations for dry period and event size distributions consistent with self-organized criticality prediction. The spectral exponents of the on-off time series of the fog and rainfall exhibited an approximate f(-0.8) scaling, but the on-off switching was not entirely independent from the amplitude intermittency in fog and rainfall. The results show the role of short and long-term variability in precipitation and its consequences for the carbon and water cycle of semi-arid savannas with significant tree cover. These findings can be used to develop minimalist water balance models to understand how vegetation state affects water resources.Sateen ja kasvien vuorovaikutuksen ymmärtäminen on tärkeää kuivissa ekosysteemeissä, joissa kasvien aktiivisuus riippuu vesivaroista. Sade määrittää ensisijaisesti kasvillisuuden kasvua kuivissa ekosysteemeissä. Sumu on myös tärkeä tekijä kuivien rannikkoekosysteemien ja pilvimetsien mikroilmastoille. Maankosteutta tutkimalla voidaan ymmärtää veden ja hiilen kiertoa, sekä sateen ja kasvillisuuden vuorovaikutusta. Tässä väitöskirjassa tarkastellaan sateen vaihtelun vaikutusta hiilen ja veden kiertoon Afrikan savanneilla, sekä sateen ja sumun samankaltaisuutta. Maankosteutta tutkittiin neljällä eri Afrikan savannilla käyttäen malleja, joissa sadanta, normalisoitu kasvillisuusindeksi (NDVI) ja potentiaalinen haihdunta (PET) otettiin asteittain huomioon. Etelä-Afrikan laidunnetulla heinäsavannilla ekosyysteemin transpiraatio arvioitiin mikrometeorologisista kovarianssimittauksista (EC-menetelmä) määrittämällä vuosittainen vedenkäytön tehokkuus ja olettamalla optimaalinen suhde hiilen ja veden diffuusiolle ilmaraoista. Sumun ja sateen mittauksia analysoitiin nojautumalla itseorganisoituvasti kriittiseen teoriaan (self organized criticality, SOC). Vuosittainen haihdunta (ET) oli saman suuruinen vuosisateen kanssa laidunnetulla heinäsavannilla. Vuosittainen sademäärä vaihteli suuresti, mutta arvioitu vuosittainen transpiraatio oli kaikkina vuosina noin 55% vuosittaisesta haihdunnasta. Transpiraatio väheni vain erityisen kuivana vuonna, jolloin heinien lakastuminen ja uudelleen kasvu sekä muut maanpinnan ominaisuuksien muutokset lisäsivät haihduntaa maaperästä. Ekosysteemi vaihteli hiilen nielusta (–58 gC m-2 yr-1) hiilen lähteeksi (198 gC m-2 yr-1). Vuosittainen nettohiilenvaihto korreloi kaukokartoitusaineistosta arvioidun maksimaalisen kasvillisuusindeksin (NDVI) kanssa, kun taas vuosittainen kokonaishengitys korreloi alkukauden sademäärän kanssa. Eri savannien maankosteusmittauksien analysointi osoitti, että NDVI ja PET muuttujien vaikutus päivän maksimaaliseen haihduntaan täytyy ottaa huomioon syvyyskeskiarvoistetun maankosteuden mallintamisessa. Maaperän kosteuden aikasarjalle laskettiin niin kutsuttu muisti, joka kuvaa keskimääräistä ajanjaksoa, jossa maankosteus poikkea alkuarvostaan. Tämä muisti korreloi lineaarisesti päiväkeskiarvoisen sateen intensiteetin kanssa kuivilla savanneilla. Sateen ja sumun aikasarjoissa havaittiin likimääräinen potenssilaki kuivien ajanjaksojen ja sadetapahtuman sademäärän jakaumissa. Tämä havainto on yhdenmukainen SOC mallin kanssa. Binäärisen sade/ei-sade ja sumu/ei-sumu aikasarjojen tehospektri noudatteli likimääräisesti f(-0.8) potenssilakia, jossa f on taajuus. Kuitenkaan binääriset aikasarjat eivät olleet täysin riippumattomia sateen ja sumun amplitudin epäsäännöllisyydestä. Väitöskirjan tulokset osoittavat sateen lyhyen ja pitkän aikavälin vaihtelun seuraukset hiilen ja veden kierrolle kuivilla savanneilla. Näitä tuloksia voidaan jatkossa käyttää minimaalisten vesitasemallien kehittämisessä ja arvioidessa kasvillisuuden vaikutusta vesivaroihin

Ecosystem processes at the watershed scale: Extending optimality theory from plot to catchment

The adjustment of local vegetation conditions to limiting soil water by either maximizing productivity or minimizing water stress has been an area of central interest in ecohydrology since Eagleson's classic study. This work has typically been limited to consider one-dimensional exchange and cycling within patches and has not incorporated the effects of lateral redistribution of soil moisture, coupled ecosystem carbon and nitrogen cycling, and vegetation allocation processes along topographic gradients. We extend this theory to the hillslope and catchment scale, with in situ and downslope feedbacks between water, carbon and nutrient cycling within a fully transient, distributed model. We explore whether ecosystem patches linked along hydrologic flow paths as a catena evolve to form an emergent pattern optimized to local climate and topographic conditions. Lateral hydrologic connectivity of a small catchment is calibrated with streamflow data and further tested with measured soil moisture patterns. Then, the spatial gradient of vegetation density within a small catchment estimated with fine-resolution satellite imagery and field measurements is evaluated with simulated vegetation growth patterns from different root depth and allocation strategies as a function of hillslope position. This is also supported by the correspondence of modeled and field measured spatial patterns of root depths and catchment-level aboveground vegetation productivity. We test whether the simulated spatial pattern of vegetation corresponds to measured canopy patterns and an optimal state relative to a set of ecosystem processes, defined as maximizing ecosystem productivity and water use efficiency at the catchment scale. Optimal carbon uptake ranges show effective compromises between multiple resources (water, light, and nutrients), modulated by vegetation allocation dynamics along hillslope gradient

Dynamic interactions of ecohydrological and biogeochemical processes in water-limited systems

Water is the essential reactant, catalyst, or medium for many biogeochemical reactions, thus playing an important role in the activation and deactivation of biogeochemical processes. The coupling b ..

Integration of spatio-temporal vegetation dynamics into a distributed ecohydrological model: application to optimality theory and real-time watershed simulations

Spatio-temporal vegetation dynamics are important drivers to characterize seasonal to annual water and carbon budgets. Spatial adjustment and evolution of the ecosystem is closely related to the geomorphic, climatic, and hydrologic settings. In particular, lateral hydrologic redistribution along flowpaths control the long-term joint adjustments of vegetation and soil over successional and quasi-geological time scales. For this reason, it is complex and challenging to incorporate the many relevant processes and feedbacks between ecological and hydrological systems for the full simulation of water, carbon, and nutrient cycling. Recent developments in remote sensing technology provide the potential to link dynamic canopy measurements with integrated process descriptions within distributed ecohydrological modeling frameworks. In this dissertation, three research studies are presented concerning estimation of spatio-temporal vegetation dynamics in application into a distributed ecohydrological model at the Coweeta Long Term Ecological Research site. In Chapter 2, we test whether the simulated spatial pattern of vegetation corresponds to measured canopy patterns and an optimal state relative to a set of ecosystem processes, defined as maximizing ecosystem productivity and water use efficiency at the catchment scale. A distributed ecohydrological model is simulated at a small catchment scale with various field measurements to see if the evolved pattern of vegetation density along the flowpaths leads to system-wide emergent optimality for carbon uptake over and above the individual patch. Lateral hydrological connectivity determines the degree of dependency on productivity and resource use with other patches along flowpaths, resulting in different system-wide carbon and water uptake by vegetation. In Chapter 3, phenological signals are extracted from global satellite products to find the topography-mediated controls on vegetation phenology in the study site. It provides a basis to understand spatial variations of local vegetation phenology as a function of microclimate, vegetation community types, and hillslope positions. In Chapter 4, near real-time vegetation dynamics are estimated by fusing multi-temporal satellite images, and integrated into the catchment scale distributed ecohydrological simulation. Integration of spatio-temporal vegetation dynamics into a distributed ecohydrological model helps to simulate ecohydrological feedbacks between vegetation patterns and lateral hydrological redistribution by reducing uncertainty related to state and flux variables