Browsing by Author "Novick, Kimberly A."
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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 Examining Ecosystem Drought Responses Using Remote Sensing and Flux Tower Observations(2022-09) Jiao, Wenzhe; Wang, Lixin; Novick, Kimberly A.; Filippelli, Gabriel; Wang, Honglang; Li, LinWater is fundamental for plant growth, and vegetation response to water availability influences water, carbon, and energy exchanges between land and atmosphere. Vegetation plays the most active role in water and carbon cycle of various ecosystems. Therefore, comprehensive evaluation of drought impact on vegetation productivity will play a critical role for better understanding the global water cycle under future climate conditions. In-situ meteorological measurements and the eddy covariance flux tower network, which provide meteorological data, and estimates of ecosystem productivity and respiration are remarkable tools to assess the impacts of drought on ecosystem carbon and water cycles. In regions with limited in-situ observations, remote sensing can be a very useful tool to monitor ecosystem drought status since it provides continuous observations of relevant variables linked to ecosystem function and the hydrologic cycle. However, the detailed understanding of ecosystem responses to drought is still lacking and 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. This dissertation addressed drought monitoring, ecosystem drought responses, trends of vegetation water constraint based on in-situ metrological observations, flux tower and multi-sensor remote sensing observations. This dissertation first developed a new integrated drought index applicable across diverse climate regions based on in-situ meteorological observations and multi-sensor remote sensing data, and another integrated drought index applicable across diverse climate regions only based on multi-sensor remote sensing data. The dissertation also evaluated the applicability of new satellite dataset (e.g., solar induced fluorescence, SIF) for responding to meteorological drought. Results show that satellite SIF data could have the potential to reflect meteorological drought, but the application should be limited to dry regions. The work in this dissertation also accessed changes in water constraint on global vegetation productivity, and quantified different drought dimensions on ecosystem productivity and respiration. Results indicate that a significant increase in vegetation water constraint over the last 30 years. The results highlighted the need for a more explicit consideration of the influence of water constraints on regional and global vegetation under a warming climate.Item High atmospheric demand for water can limit forest carbon uptake and transpiration as severely as dry soil(Wiley, 2016-09) Sulman, Benjamin N.; Roman, D. Tyler; Yi, Koong; Wang, Lixin; Phillips, Richard P.; Novick, Kimberly A.; Department of Earth Sciences, School of ScienceWhen stressed by low soil water content (SWC) or high vapor pressure deficit (VPD), plants close stomata, reducing transpiration and photosynthesis. However, it has historically been difficult to disentangle the magnitudes of VPD compared to SWC limitations on ecosystem-scale fluxes. We used a 13 year record of eddy covariance measurements from a forest in south central Indiana, USA, to quantify how transpiration and photosynthesis respond to fluctuations in VPD versus SWC. High VPD and low SWC both explained reductions in photosynthesis relative to its long-term mean, as well as reductions in transpiration relative to potential transpiration estimated with the Penman-Monteith equation. Flux responses to typical fluctuations in SWC and VPD had similar magnitudes. Integrated over the year, VPD fluctuations accounted for significant reductions of GPP in both nondrought and drought years. Our results suggest that increasing VPD under climatic warming could reduce forest CO2 uptake regardless of changes in SWC.Item The importance of cuticular permeance in assessing plant water–use strategies(Oxford, 2020-04) Lanning, Matthew; Wang, Lixin; Novick, Kimberly A.; Earth Sciences, School of ScienceAccurate understanding of plant responses to water stress is increasingly important for quantification of ecosystem carbon and water cycling under future climates. Plant water-use strategies can be characterized across a spectrum of water stress responses, from tight stomatal control (isohydric) to distinctly less stomatal control (anisohydric). A recent and popular classification method of plant water-use strategies utilizes the regression slope of predawn and midday leaf water potentials, σ, to reflect the coupling of soil water availability (predawn leaf water potential) and stomatal dynamics (daily decline in leaf water potential). This type of classification is important in predicting ecosystem drought response and resiliency. However, it fails to explain the relative stomatal responses to drought of Acer sacharrum and Quercus alba, improperly ranking them on the spectrum of isohydricity. We argue this inconsistency may be in part due to the cuticular conductance of different species. We used empirical and modeling evidence to show that plants with more permeable cuticles are more often classified as anisohydric; the σ values of those species were very well correlated with measured cuticular permeance. Furthermore, we found that midday leaf water potential in species with more permeable cuticles would continue to decrease as soils become drier, but not in those with less permeable cuticles. We devised a diagnostic parameter, Γ, to identify circumstances where the impact of cuticular conductance could cause species misclassification. The results suggest that cuticular conductance needs to be considered to better understand plant water-use strategies and to accurately predict forest responses to water stress under future climate scenarios.Item The increasing importance of atmospheric demand for ecosystem water and carbon fluxes(Nature, 2016-11) Novick, Kimberly A.; Ficklin, Darren L.; Stoy, Paul C.; Williams, Christopher A.; Bohrer, Gil; Oishi, A. Christopher; Papuga, Shirley A.; Blanken, Peter D.; Noormets, Asko; Sulman, Benjamin N.; Scott, Russell L.; Wang, Lixin; Phillips, Richard P.; Department of Earth Sciences, School of ScienceSoil moisture supply and atmospheric demand for water independently limit—and profoundly affect—vegetation productivity and water use during periods of hydrologic stress1, 2, 3, 4. Disentangling the impact of these two drivers on ecosystem carbon and water cycling is difficult because they are often correlated, and experimental tools for manipulating atmospheric demand in the field are lacking. Consequently, the role of atmospheric demand is often not adequately factored into experiments or represented in models5, 6, 7. Here we show that atmospheric demand limits surface conductance and evapotranspiration to a greater extent than soil moisture in many biomes, including mesic forests that are of particular importance to the terrestrial carbon sink8, 9. Further, using projections from ten general circulation models, we show that climate change will increase the importance of atmospheric constraints to carbon and water fluxes in all ecosystems. Consequently, atmospheric demand will become increasingly important for vegetation function, accounting for >70% of growing season limitation to surface conductance in mesic temperate forests. Our results suggest that failure to consider the limiting role of atmospheric demand in experimental designs, simulation models and land management strategies will lead to incorrect projections of ecosystem responses to future climate conditions.Item A new multi-sensor integrated index for drought monitoring(Elsevier, 2019-04) Jiao, Wenzhe; Wang, Lixin; Chang, Qing; Novick, Kimberly A.; Tian, Chao; Earth Sciences, School of ScienceDrought is one of the most expensive but least understood natural disasters. Remote sensing based integrated drought indices have the potential to describe drought conditions comprehensively, and multi-criteria combination analysis is increasingly used to support drought assessment. However, conventional multi-criteria combination methods and most existing integrated drought indices fail to adequately represent spatial variability. An index that can be widely used for drought monitoring across all climate regions would be of great value for ecosystem management. To this end, we proposed a framework for generating a new integrated drought index applicable across diverse climate regions. In this new framework, a local ordered weighted averaging (OWA) model was used to combine the Temperature Condition Index (TCI) from the Moderate-resolution Imaging Spectroradiometer (MODIS), the Vegetation Condition Index (VCI) developed using the Vegetation Index based on Universal Pattern Decomposition method (VIUPD), the Soil Moisture Condition Index (SMCI) derived from the Advanced Microwave Scanning Radiometer–Earth Observation System (AMSR-E), and the Precipitation Condition Index (PCI) derived from the Tropical Rainfall Measuring Mission (TRMM). This new index, which we call the “Geographically Independent Integrated Drought Index (GIIDI),” was validated in diverse climate divisions across the continental United States. Results showed that GIIDI was better correlated with in-situ PDSI, Z-index, SPI-1, SPI-3 and SPEI-6 (overall r-value = 0.701, 0.794, 0.811, 0.733, 0.628; RMSE = 1.979, 0.810, 0.729, 1.049 and 1.071, respectively) when compared to the Microwave Integrated Drought Index (MIDI), Optimized Meteorological Drought Index (OMDI), Scaled Drought Condition Index (SDCI), PCI, TCI, SMCI, and VCI. GIIDI also performed well in most climate divisions for both short-term and long-term drought monitoring. Because of the superior performance of GIIDI across diverse temporal and spatial scales, GIIDI has considerable potential for improving our ability to monitor drought across a range of biomes and climates.Item A new station-enabled multi-sensor integrated index for drought monitoring(Elsevier, 2019-07) Jiao, Wenzhe; Wang, Lixin; Novick, Kimberly A.; Chang, Qing; Earth Sciences, School of ScienceRemote sensing data are frequently incorporated into drought indices used widely by research and management communities to assess and diagnose current and historic drought events. The integrated drought indices combine multiple indicators and reflect drought conditions from a range of perspectives (i.e., hydrological, agricultural, meteorological). However, the success of most remote sensing based drought indices is constrained by geographic regions since their performance strongly depends on environmental factors such as land cover type, temperature, and soil moisture. To address this limitation, we propose a framework for a new integrated drought index that performs well across diverse climate regions. Our framework uses a geographically weighted regression model and principal component analysis to composite a range of vegetation and meteorological indices derived from multiple remote sensing platforms and in-situ drought indices developed from meteorological station data. Our new index, which we call the station-enabled Geographically Independent Integrated Drought Index (GIIDI_station), compared favorably with other common drought indices such as Microwave Integrated Drought Index (MIDI), Optimized Meteorological Drought Index (OMDI), Precipitation Condition Index (PCI), Temperature Condition Index (TCI), Soil Moisture Condition Index (SMCI), and Vegetation Condition Index (VCI). Using Pearson correlation analyses between remote sensing and in-situ drought indices during the growing season (April to October) from 2002 to 2011, we show that GIIDI_station had the best correlations with in-situ drought indices. Across the entire study region of the continental United States, the performance of GIIDI_station was not affected by common environmental factors such as precipitation, temperature, land cover and soil conditions. Taken together, our results suggest that GIIDI_station has considerable potential to improve our ability of monitoring drought at regional scales, provided local meteorological station data are available.Item Response of ecosystem intrinsic water use efficiency and gross primary productivity to rising vapor pressure deficit(IOP, 2019) Zhang, Quan; Ficklin, Darren L.; Manzoni, Stefano; Wang, Lixin; Way, Danielle; Phillips, Richard P.; Novick, Kimberly A.; Earth Sciences, School of ScienceElevated vapor pressure deficit (VPD) due to drought and warming is well-known to limit canopy stomatal and surface conductance, but the impacts of elevated VPD on ecosystem gross primary productivity (GPP) are less clear. The intrinsic water use efficiency (iWUE), defined as the ratio of carbon (C) assimilation to stomatal conductance, links vegetation C gain and water loss and is a key determinant of how GPP will respond to climate change. While it is well-established that rising atmospheric CO2 increases ecosystem iWUE, historic and future increases in VPD caused by climate change and drought are often neglected when considering trends in ecosystem iWUE. Here, we synthesize long-term observations of C and water fluxes from 28 North American FLUXNET sites, spanning eight vegetation types, to demonstrate that ecosystem iWUE increases consistently with rising VPD regardless of changes in soil moisture. Another way to interpret this result is that GPP decreases less than surface conductance with increasing VPD. We also project how rising VPD will impact iWUE into the future. Results vary substantially from one site to the next; in a majority of sites, future increases in VPD (RCP 8.5, highest emission scenario) are projected to increase iWUE by 5%–15% by 2050, and by 10%–35% by the end of the century. The increases in VPD owing to elevated global temperatures could be responsible for a 0.13% year−1 increase in ecosystem iWUE in the future. Our results highlight the importance of considering VPD impacts on iWUE independently of CO2 impacts.Item Water vapor δ2H, δ18O and δ17O measurements using an off-axis integrated cavity output spectrometer – sensitivity to water vapor concentration, delta value and averaging-time(Wiley, 2016-10) Tian, Chao; Wang, Lixin; Novick, Kimberly A.; Department of Earth Sciences, School of ScienceRationale High-precision analysis of atmospheric water vapor isotope compositions, especially δ17O values, can be used to improve our understanding of multiple hydrological and meteorological processes (e.g., differentiate equilibrium or kinetic fractionation). This study focused on assessing, for the first time, how the accuracy and precision of vapor δ17O laser spectroscopy measurements depend on vapor concentration, delta range, and averaging-time. Methods A Triple Water Vapor Isotope Analyzer (T-WVIA) was used to evaluate the accuracy and precision of δ2H, δ18O and δ17O measurements. The sensitivity of accuracy and precision to water vapor concentration was evaluated using two international standards (GISP and SLAP2). The sensitivity of precision to delta value was evaluated using four working standards spanning a large delta range. The sensitivity of precision to averaging-time was assessed by measuring one standard continuously for 24 hours. Results Overall, the accuracy and precision of the δ2H, δ18O and δ17O measurements were high. Across all vapor concentrations, the accuracy of δ2H, δ18O and δ17O observations ranged from 0.10‰ to 1.84‰, 0.08‰ to 0.86‰ and 0.06‰ to 0.62‰, respectively, and the precision ranged from 0.099‰ to 0.430‰, 0.009‰ to 0.080‰ and 0.022‰ to 0.054‰, respectively. The accuracy and precision of all isotope measurements were sensitive to concentration, with the higher accuracy and precision generally observed under moderate vapor concentrations (i.e., 10000–15000 ppm) for all isotopes. The precision was also sensitive to the range of delta values, although the effect was not as large compared with the sensitivity to concentration. The precision was much less sensitive to averaging-time than the concentration and delta range effects. Conclusions The accuracy and precision performance of the T-WVIA depend on concentration but depend less on the delta value and averaging-time. The instrument can simultaneously and continuously measure δ2H, δ18O and δ17O values in water vapor, opening a new window to better understand ecological, hydrological and meteorological processes.