Browsing by Subject "ecohydrology"
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Item Canopy isotopic investigation reveals different water uptake dynamics of maples and oaks(Elsevier, 2020-07) Lanning, Matthew; Wang, Lixin; Benson, Michael; Zhang, Quan; Novick, Kimberly A.; Earth Sciences, School of ScienceVariations in drought responses exhibited by cohabiting tree species such as Acer sacharrum and Quercus alba have often been attributed to differences in rooting depth or water accessibility. A. sacharrum is thought to be a shallow rooted species, and is assumed to not have access to the deep and stable water resources available to Q. alba. As such, A. sacharrum conserves water by minimizing stomatal conductance under drought conditions whereas Q. alba does not. However, detailed records of sufficient temporal resolution which integrate water accessibility, meteorological drivers, and leaf level parameters (e.g., photosynthesis, stomatal conductance) are lacking, making such assumptions—though plausible— largely untested. In this study, we investigated the water accessibility of both maples (A. sacharrum) and oaks (Q. alba) during the late growing season using novel canopy stable isotope measurements. Our results showed that maples can draw from the same water pool as cohabitating oaks, but can also switch to a shallow water source in response to available moisture in the shallow soil profile. We also found that maples tended to use a deep water source under high vapor pressure deficit even when shallow soil water was available. On the other hand, oaks had consistent deep water access during our study period. It is noted that our measurements do not cover the whole growing season and should be extrapolated with caution. Such findings indicate that differences in leaf functions during drought between maples and oaks may be due to both soil water accessibility and atmospheric water demand.Item Comprehensive quantification of the responses of ecosystem production and respiration to drought time scale, intensity and timing in humid environments: A FLUXNET synthesis(Wiley, 2022-05) Jiao, Wenzhe; Wang, Lixin; Wang, Honglang; Lanning, Matthew; Chang, Qing; Novick, Kimberly A.; Earth Sciences, School of ScienceDrought is one of the most important natural hazards impacting ecosystem carbon cycles. However, it is challenging to quantify the impacts of drought on ecosystem carbon balance and several factors hinder our explicit understanding of the complex drought impacts. First, drought impacts can have different time dimensions such as simultaneous, cumulative, and lagged impacts on ecosystem carbon balance. Second, drought is not only a multiscale (e.g., temporal and spatial) but also a multidimensional (e.g., intensity, time-scale, and timing) phenomenon, and ecosystem production and respiration may respond to each drought dimension differently. In this study, we conducted a comprehensive drought impact assessment on ecosystem productivity and respiration in humid regions by including different drought dimensions using global FLUXNET observations. Short-term drought (e.g., 1-month drought) generally did not induce a decrease in plant productivity even under high severity drought. However, ecosystem production and respiration significantly decreased as drought intensity increased for droughts longer than one month in duration. Drought timing was important, and ecosystem productivity was most vulnerable when drought occurred during or shortly after the peak vegetation growth. We found that lagged drought impacts more significantly affected ecosystem carbon uptake than simultaneous drought, and that ecosystem respiration was less sensitive to drought time scale than ecosystem production. Overall, our results indicated that temporally-standardized meteorological drought indices can be used to reflect plant productivity decline, but drought timing, antecedent, and cumulative drought conditions need to be considered together.Item Contribution of recycled moisture to local precipitation in the inland Heihe River Basin(Elsevier, 2019-06) Zhao, Liangju; Liu, Xiaohong; Wang, Ninglian; Kong, Yanlong; Song, Yaoxuan; He, Zhibin; Liu, Quanyu; Wang, Lixin; Earth Sciences, School of ScienceRecycled moisture contributed by continental evaporation and transpiration plays an important role in regulating the hydrological processes and atmospheric humidity budget in arid inland river basins. However, knowledge of moisture recycling within many large inland basins and the factors that control moisture recycling is generally lacking. Based on a three-component isotopic mixing model, we assessed the characteristics of moisture recycling in China’s semi-arid Heihe River Basin. During the active growing season, almost half of the precipitation in the upper reaches was provided by local moisture recycling, and the main contribution came from transpiration. In the middle reaches, almost half of the precipitation in the artificial oasis and the desert-oasis ecotone was also provided by local moisture recycling, and the transpiration fraction (fTr) and evaporation fraction (fEv) of the artificial oasis differed from those of the desert-oasis ecotone. In the lower reaches, less than 25% of the precipitation was provided by local moisture recycling. Mean fTr values were relatively low in the Gobi (15.0%) in the middle reaches and in the riparian forest at Ejina (25.6%) in the lower reaches. The positive correlations between fTr and both precipitation and relative humidity suggest that higher precipitation and relative humidity promote transpiration fraction, whereas higher vapor pressure deficit reduces transpiration fraction. The positive correlation between fEv and temperature and vapor pressure deficit, and the negative correlation between fEv and relative humidity indicate that higher temperature and vapor pressure deficit promotes evaporation fraction, whereas higher relative humidity reduces the evaporation fraction. Our results show that contributions of recycled moisture (especially transpiration) to local precipitation play an important role in regional water resource redistribution in the arid and semi-arid region of northwestern China.Item Dew formation reduction in global warming experiments and the potential consequences(Elsevier, 2021-02) Feng, Tianjiao; Zhang, Lixu; Chen, Qian; Ma, Zhiyuan; Wang, Hao; Shangguan, Zijian; Wang, Lixin; He, Jin-Sheng; Earth Sciences, School of ScienceDew, as an important contribution of non-rainfall water (NRW), plays a vital role in ecosystem processes in arid and semi-arid regions and is expected to be affected by climate warming. Infrared heater warming systems have been widely used to simulate climate warming effects on ecosystems. However, how this warming system affects dew formation has been long ignored and rarely addressed. In a typical alpine grassland ecosystem on the northeast of the Tibetan Plateau, we measured dew amount and duration using three independent methods: artificial condensing surfaces, leaf wetness sensors and in situ dew formation on plants from 2012 to 2017. We also measured plant traits related to dew conditions. The results showed that (1) warming reduced the dew amount by 41.6%-91.1% depending on the measurement method, and reduced dew duration by 32.1 days compared to the ambient condition. (2) Different plant functional groups differed in dew formation. (3) Under the infrared warming treatment, the dew amount decreased with plant height, while under the ambient conditions, the dew amount showed the opposite trend. We concluded that warming with an infrared heater system greatly reduces dew formation, and if ignored, it may lead to overestimation of the effects of climate warming on ecosystem processes in climate change simulation studies.Item Dryland ecohydrology and climate change: critical issues and technical advances(2012-08) Wang, Lixin; D'Odorico, P.; Evans, J. P.; Eldridge, D. J.; McCabe, M. F.; Caylor, K. K.; King, E. G.Drylands cover about 40% of the terrestrial land surface and account for approximately 40% of global net primary productivity. Water is fundamental to the biophysical processes that sustain ecosystem function and food production, particularly in drylands where a tight coupling exists between ecosystem productivity, surface energy balance, biogeochemical cycles, and water resource availability. Currently, drylands support at least 2 billion people and comprise both natural and managed ecosystems. In this synthesis, we identify some current critical issues in the understanding of dryland systems and discuss how arid and semiarid environments are responding to the changes in climate and land use. The issues range from societal aspects such as rapid population growth, the resulting food and water security, and development issues, to natural aspects such as ecohydrological consequences of bush encroachment and the causes of desertification. To improve current understanding and inform upon the needed research efforts to address these critical issues, we identify some recent technical advances in terms of monitoring dryland water dynamics, water budget and vegetation water use, with a focus on the use of stable isotopes and remote sensing. These technological advances provide new tools that assist in addressing critical issues in dryland ecohydrology under climate change.Item Dynamic interactions of ecohydrological and biogeochemical processes in water-limited systems(Wiley, 2015-08) Wang, Lixin; Manzoni, Stefano; Ravi, Sujith; Riveros-Iregui, Diego; Caylor, Kelly; Department of Earth Sciences, School of ScienceWater 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 between hydrological and biogeochemical processes is particularly evident in water-limited arid and semi-arid environments, but also in areas with strong seasonal precipitation patterns (e.g., Mediterranean) or in mesic systems during droughts. Moreover, this coupling is apparent at all levels in the ecosystems—from soil microbial cells to whole plants to landscapes. Identifying and quantifying the biogeochemical “hot spots” and “hot moments”, the underlying hydrological drivers, and how disturbance-induced vegetation transitions affect the hydrological-biogeochemical interactions are challenging because of the inherent complexity of these interactions, thus requiring interdisciplinary approaches. At the same time, a holistic approach is essential to fully understand function and processes in water-limited ecosystems and to predict their responses to environmental change. This article examines some of the mechanisms responsible for microbial and vegetation responses to moisture inputs in water-limited ecosystems through a synthesis of existing literature. We begin with the initial observation of Birch effect in 1950s and examine our current understanding of the interactions among vegetation dynamics, hydrology, and biochemistry over the past 60 years. We also summarize the modeling advances in addressing these interactions. This paper focuses on three opportunities to advance coupled hydrological and biogeochemical research: (1) improved quantitative understanding of mechanisms linking hydrological and biogeochemical variations in drylands, (2) experimental and theoretical approaches that describe linkages between hydrology and biogeochemistry (particularly across scales), and (3) the use of these tools and insights to address critical dryland issues of societal relevance.Item Evaluating ecohydrological modelling framework to link atmospheric CO2 and stomatal conductance(Wiley, 2018) Lu, Xuefei; Wang, Lixin; Earth Sciences, School of ScienceThe establishment of an accurate stomatal conductance (gs) model in responding to CO2 enrichment under diverse environmental conditions remains an important issue as gs is the key to understand the plant–water–atmosphere interactions. A better representation of gs is important to reduce uncertainties in predicting the climate change impacts on various ecosystem functions. In this study, we evaluated three most commonly used gs formulations for the estimation of the stomatal response to environmental factors using in situ measurements under different environmental conditions. The three gs models were Leuning's modified Ball–Berry model and two specific cases of the optimization models (i.e., Rubisco limitation model and RuBP regeneration limitation model). On the basis of an analysis of 234 data points obtained from experiments under instantaneous, semicontrolled, and the free‐air CO2 experiment conditions, we found that Leuning's modified Ball–Berry model and RuBP‐limited optimization model showed similar performance, and both performed better than Rubisco limitation model. Functional groups (e.g., C3 vs. C4 species) and life form (e.g., annual vs. perennial species) play an important role in determining the gs model performance and thus pose a challenge for gs predictions in mixed vegetation communities.Item Fog Spatial Distributions over the Central Namib Desert - An Isotope Approach(2018) Kaseke, Kudzai Farai; Tian, Chao; Wang, Lixin; Seely, Mary; Vogt, Roland; Wassenaar, Theo; Mushi, Roland; Earth Sciences, School of ScienceFog is a characteristic feature of the Namib Desert and is essential to life in this fog dependent system. It is often acknowledged that advective fog from the ocean is the dominant fog type over the Namib Desert fog-zone but recent evidence suggests that other fog types occur in this area. Knowledge of the existence and spatial distribution of different fog types will enhance the mechanistic understanding of fog formation and potential changes in this region, but such knowledge is limited in literature. In this study, we investigated fog spatial variations within the Namib Desert fog-zone by applying stable isotope (δ18O and δ2H) techniques to differentiate various fog types and identify their source waters. Isotope based results showed that at least three types of fog (advective, radiation and mixed) occurred in this region and what appears as a single fog event may include all three types. Results suggest that radiation fog was the dominant fog type during our study period. The results also suggest that advective fog (with Atlantic Ocean origins) either dissipated 30–50 km inland and the residual humidity combined with locally derived moisture to form mixed fog or advective fog incorporated local moisture along its trajectory inland resulting in mixed fog. Fog in the Namib Desert was consistently depleted in 18O and 2H compared to rainfall and this was attributed to sub-cloud evaporation of the rainfall as well as different sources of fog and rainfall. Sub-cloud evaporation led to enrichment of 18O and 2H in rainfall beyond that of the first stage condensate, fog. Advective fog is often considered the architect of the fog-zone in the Namib Desert, but our results demonstrated multiple dominant fog types during the study period, suggesting knowledge of both fog frequency and fog type is needed to better predict climate change impacts on the fog-zone.Item The impact of fog on soil moisture dynamics in the Namib Desert(Elsevier, 2018-03) Li, Bonan; Wang, Lixin; Kaseke, Kudzai F.; Vogt, Roland; Li, Lin; Seely, Mary; Earth Science, School of ScienceSoil moisture is a crucial component supporting vegetation dynamics in drylands. Despite increasing attention on fog in dryland ecosystems, the statistical characterization of fog distribution and how fog affects soil moisture dynamics have not been seen in literature. To this end, daily fog records over two years (Dec 1, 2014–Nov 1, 2016) from three sites within the Namib Desert were used to characterize fog distribution. Two sites were located within the Gobabeb Research and Training Center vicinity, the gravel plains and the sand dunes. The third site was located at the gravel plains, Kleinberg. A subset of the fog data during rainless period was used to investigate the effect of fog on soil moisture. A stochastic modeling framework was used to simulate the effect of fog on soil moisture dynamics. Our results showed that fog distribution can be characterized by a Poisson process with two parameters (arrival rate λ and average depth α (mm)). Fog and soil moisture observations from eighty (Aug 19, 2015–Nov 6, 2015) rainless days indicated a moderate positive relationship between soil moisture and fog in the Gobabeb gravel plains, a weaker relationship in the Gobabeb sand dunes while no relationship was observed at the Kleinberg site. The modeling results suggested that mean and major peaks of soil moisture dynamics can be captured by the fog modeling. Our field observations demonstrated the effects of fog on soil moisture dynamics during rainless periods at some locations, which has important implications on soil biogeochemical processes. The statistical characterization and modeling of fog distribution are of great value to predict fog distribution and investigate the effects of potential changes in fog distribution on soil moisture dynamics.Item The impact of rainfall and fog on soil moisture dynamics in the Namib Desert(2017-07) Li, Bonan; Wang, LixinSoil moisture is a key variable in dryland ecosystems. Knowing how and to what extent soil moisture is influenced by rainfall and non-rainfall waters (e.g., dew, fog, and water vapor) is essential to understand dryland dynamics. The hyper-arid environment of the Namib Desert with its frequent occurrence of fog events provides an ideal place to conduct research on the rainfall and non-rainfall effects on soil moisture dynamics. Rainfall and soil moisture records was collected from three locations (gravel plain at Gobabeb (GPG), sand dune at Gobabeb (SDG), and gravel plain at Kleinberg (GPK)) within the Namib Desert using CS655 Water Content Reflectometer and tipping-buckets, respectively. The fog data was collected from the FogNet stations. Field observations of rainfall and soil moisture from three study sites suggested that soil moisture dynamics follow rainfall patterns at two gravel plain sites, whereas no significant relationships was observed at the sand dune site. The stochastic modeling results showed that most of soil moisture dynamics can be simulated except the rainless periods. Model sensitivity in response to different soil and vegetation parameters was investigated under diverse soil textures. Sensitivity analyses suggested that soil hygroscopic point (sh), field capacity (sfc) were two main parameters controlling the model output. Despite soil moisture dynamics can be partially explained by rainfall, soil moisture dynamics during rainless period still poorly understood. In addition, characterization of fog distribution in the Namib Desert is still lacking. To this end, nearly two years’ continuous daily records of fog were used to derive fog distribution. The results suggested that fog is able to be well - characterized by a Poisson process with two parameters (arrival rate and average depth). Field observations indicated that there is a moderate positive relationship between soil moisture and fog at GPG and the relationship tend to be less significant at the other two sites. A modified modeling results suggested that mean and general patterns of soil moisture can be captured by the modeling. This thesis is of practical importance for understanding soil moisture dynamics in response to the rainfall and fog changing conditions.