Implementing plant hydraulics in an Earth System Model and the implications for the global carbon and water cycles.

Uncertainty in the representation of vegetation in Earth System Models is a major contributor to the intermodel spread in climate projections under global warming. Empirical soil moisture stress parameterizations to model drought effects on photosynthesis have been identified as a major driver of this uncertainty, leading to a call to develop more mechanistic models that leverage the principles of soil and plant hydraulic theory. The goal of this dissertation is to develop and install a simplified plant hydraulics representation within a major Earth System Model, compare its dynamics with a non-hydraulic model, and refine methods to use transient leaf water potential observations to infer vegetation water-use strategy. Chapter 1 presents the full model description of Plant Hydraulic Stress (PHS), which we developed to implement plant hydraulics within the Community Land Model (CLM). PHS has since been adopted as the default representation of vegetation water use in version 5 of the CLM. PHS updates vegetation water stress and root water uptake to better reflect plant hydraulic theory, advancing the physical basis of the modeled vegetation hydrodynamics. Point simulations of a tropical forest site (Caxiuanã, Brazil) under ambient conditions and partial precipitation exclusion highlight the differences between PHS and the previous CLM implementation. Model description and simulation results are contextualized with a list of benefits and limitations of the new model formulation, including hypotheses that were not testable in previous versions of the model. Key results include reductions in transpiration and soil moisture biases relative to a control model under both ambient and exclusion conditions, correcting excessive dry season soil moisture stress in the control model. The new model structure, which bases water stress on leaf water potential, could have significant implications for vegetation-climate feedbacks, including increased sensitivity of photosynthesis to atmospheric vapor pressure deficit. Chapter 2 extends the analysis of PHS to the global scale. Historical simulations with and without plant hydraulics are compared to understand the influence on interannual soil moisture and photosynthesis dynamics. The focus of this chapter is on analyzing model dynamics across the semi-arid tropics. The PHS simulation yields longer soil moisture memory and increases interannual photosynthesis variability as compared to the non-hydraulic model. With an analytical derivation and analyses of soil moisture dynamics, we demonstrate the importance of the root water uptake parameterization for soil moisture memory and carbon cycle variability. Chapter 3 investigates methods to use transient leaf water potential observations to infer vegetation water-use strategy. We use a set of soil-plant-atmosphere models, ranging in complexity, to investigate the underlying meaning of three isohydricity metrics and identify potential classification errors. The model-based approach allows us to derive analytical expressions for the three metrics and to more methodically sample both environmental space and trait space to generate idealized experiments to test the fidelity of the resulting water-use strategy classifications. We consider two previously defined metrics, isohydricity slope and hydroscape area, in comparison to a third metric, relative isohydricity, defined herein. We describe classification challenges resulting from trait coordination and environmental variability, suggest practical recommendations for metric retrieval, and discuss the value and limitations of isohydricity and the broader pursuit of response-based metrics of vegetation traits. Our results indicate that the major limitations of the isohydricity slope and hydroscape area metrics can be corrected with the relative isohydricity methods described here

Toward estimation of seasonal water dynamics of winter wheat from ground-based L-band radiometry: a concept study

peer reviewedThe vegetation optical depth (VOD) variable contains information on plant water content and biomass. It can be estimated alongside soil moisture from currently operating satellite radiometer missions, such as SMOS (ESA) and SMAP (NASA). The estimation of water fluxes, such as plant water uptake (PWU) and transpiration rate (TR), from these earth system parameters (VOD, soil moisture) requires assessing water potential gradients and flow resistances in the soil, the vegetation and the atmosphere. Yet water flux estimation remains an elusive challenge especially on a global scale. In this concept study, we conduct a field-scale experiment to test mechanistic models for the estimation of seasonal water fluxes (PWU and TR) of a winter wheat stand using measurements of soil moisture, VOD, and relative air humidity (RH) in a controlled environment. We utilize microwave L-band observations from a tower-based radiometer to estimate VOD of a wheat stand during the 2017 growing season at the Selhausen test site in Germany. From VOD, we first extract the gravimetric moisture of vegetation and then determine the relative water content (RWC) and vegetation water potential (VWP) of the wheat field. Although the relative water content could be directly estimated from VOD, our results indicate this may be challenging for the phenological phases, when rapid biomass and plant structure development take place within the wheat canopy. We estimate water uptake from the soil to the wheat plants from the difference between the soil and vegetation potentials divided by the flow resistance from soil into wheat plants. The TR from the wheat plants into the atmosphere was obtained from the difference between the vegetation and atmosphere water potentials divided by the flow resistances from plants to the atmosphere. For this, the required soil matric potential (SMP), the vapor pressure deficit (VPD), and the flow resistances were obtained from on-site observations of soil, plant, and atmosphere together with simple mechanistic models. This pathfinder study shows that the L-band microwave radiation contains valuable information on vegetation water status that enables the estimation of water dynamics (up to fluxes) from the soil via wheat plants into the atmosphere, when combined with additional information of soil and atmosphere water content. Still, assumptions have to be made when estimating the vegetation water potential from relative water content as well as the water flow resistances between soil, wheat plants, and atmosphere. Moreover, direct validation of water flux estimates for the assessment of their absolute accuracy could not be performed due to a lack of in situ PWU and TR measurements. Nonetheless, our estimates of water status, potentials, and fluxes show the expected temporal dynamics, known from the literature, and intercompare reasonably well in absolute terms with independent TR estimates of the NASA ECOSTRESS mission, which relies on a Priestly-Taylor type of retrieval model. Our findings support that passive microwave remote-sensing techniques qualify for the estimation of vegetation water dynamics next to traditionally measured stand-scale or plot-scale techniques. They might shed light on future capabilities of monitoring water dynamics in the soil-plant-atmosphere system including wide-area, remote-sensing-based earth observation data

Entropy production of soil hydrological processes and its maximisation

Hydrological processes are irreversible and produce entropy. Hence, the framework of non-equilibrium thermodynamics is used here to describe them mathematically. This means flows of water are written as functions of gradients in the gravitational and chemical potential of water between two parts of the hydrological system. Such a framework facilitates a consistent thermodynamic representation of the hydrological processes in the model. Furthermore, it allows for the calculation of the entropy production associated with a flow of water, which is proportional to the product of gradient and flow. Thus, an entropy budget of the hydrological cycle at the land surface is quantified, illustrating the contribution of different processes to the overall entropy production. Moreover, the proposed Principle of Maximum Entropy Production (MEP) can be applied to the model. This means, unknown parameters can be determined by setting them to values which lead to a maximisation of the entropy production in the model. The model used in this study is parametrised according to MEP and evaluated by means of several observational datasets describing terrestrial fluxes of water and carbon. The model reproduces the data with good accuracy which is a promising result with regard to the application of MEP to hydrological processes at the land surfac

Factors influencing spatial variability in soil nitrogen (N) dynamics in N-treated and untreated watersheds of the Fernow Experimental Forest, West Virginia

The central Appalachian region of the United States receives some of the highest inputs of nitrogen (N) due to acidic deposition in the nation. It is believed that these high could levels contribute to a decline in forest soils within the next 50 to 70 yrs. This study examines factors that influence spatial variability in N-treated and untreated watersheds of the Fernow Experimental Forest, Parsons, West Virginia. Within each of the two watersheds [WS4 untreated control, \u3e 100 yr.; WS3 N-treated, acidified, clear cut, ~ 31 yr.], two 0.04 ha plots, one high N and one low N, were selected for study. Three subplots were chosen from within each of the two sample plots, for a total of 12 subplots. Soil samples were collected with a hand trowel. Nitrogen extractions were performed using 10 g of soil and 100 ml of 1 N KCl. Litter samples were ground using a Wiley Mill and analyzed for foliar lignin concentration, C:N, and %N. Nitrogen extracts were analyzed with a Bran + Luebbe TrAAcs 2000 automatic analysis system. Bacterial DNA was analyzed using primer sets. The primer sets (1-6) were designed for the specific detection of ammonia oxidizing bacteria in forest soils by PCR. DNA was purified and amplified by PCR. All plots detected AMO gene groups during pre-incubation, but by day 7 only WS4/low N had detectable gene groups present. On day 14 all plots, with the exception of WS4/high N, had detectable groups present. At days 21 and 28 only one plot for each day had detected AMO genes. On day 21 WS4/low N had group 2 present and on day 28 WS3/high N had group 2 present. Overall, it is clear that the nitrifying bacterial community is very dynamic. AMO genes were readily detected at pre-incubation, but were nearly absent after seven days of incubation. By fourteen days of incubation the communities had shifted. Only AMO genes of the group 2 were detected after three and four weeks of incubation. The data suggest either that incubation conditions were not suitable for nitrifying bacteria, or that previously uncharacterized AMO genes were dominant after prolonged incubation. The first hypothesis posed was that soil incubation temperature and net mineralization and nitrification were correlated. This hypothesis was supported. The lowest rates for both net mineralization and nitrification were seen at the 10 C incubation temperature. The 30 C incubation temperature allowed the highest rates. This was true of all the study plots within WS3 and WS4. The second hypothesis, that the lack of net nitrification on WS4/low N was caused by a lack of nitrifying bacteria at that site, was rejected. The opposite was found to be true. The high N plot of WS4 did have bacterial communities present. The lack of nitrification be attributed to the inactivity of the bacterial communities due to an unknown environmental limitation

Unlocking drought-induced tree mortality : physiological mechanisms to modeling

Drought-related tree mortality has become a major concern worldwide due to its pronounced negative impacts on the functioning and sustainability of forest ecosystems. However, our ability to identify the species that are most vulnerable to drought, and to pinpoint the spatial and temporal patterns of mortality events, is still limited. Model is useful tools to capture the dynamics of vegetation at spatiotemporal scales, yet contemporary land surface models (LSMs) are often incapable of predicting the response of vegetation to environmental perturbations with sufficient accuracy, especially under stressful conditions such as drought. Significant progress has been made regarding the physiological mechanisms underpinning plant drought response in the past decade, and plant hydraulic dysfunction has emerged as a key determinant for tree death due to water shortage. The identification of pivotal physiological events and relevant plant traits may facilitate forecasting tree mortality through a mechanistic approach, with improved precision. In this review, we (1) summarize current understanding of physiological mechanisms leading to tree death, (2) describe the functionality of key hydraulic traits that are involved in the process of hydraulic dysfunction, and (3) outline their roles in improving the representation of hydraulic function in LSMs. We urge potential future research on detailed hydraulic processes under drought, pinpointing corresponding functional traits, as well as understanding traits variation across and within species, for a better representation of drought-induced tree mortality in models

Forest fluxes and mortality response to drought: model description (ORCHIDEE-CAN-NHA r7236) and evaluation at the Caxiuanã drought experiment

Extreme drought events in Amazon forests are expected to become more frequent and more intense with climate change, threatening ecosystem function and carbon balance. Yet large uncertainties exist on the resilience of this ecosystem to drought. A better quantification of tree hydraulics and mortality processes is needed to anticipate future drought effects on Amazon forests. Most state-of-the-art dynamic global vegetation models are relatively poor in their mechanistic description of these complex processes. Here, we implement a mechanistic plant hydraulic module within the ORCHIDEE-CAN-NHA r7236 land surface model to simulate the percentage loss of conductance (PLC) and changes in water storage among organs via a representation of the water potentials and vertical water flows along the continuum from soil to roots, stems and leaves. The model was evaluated against observed seasonal variability in stand-scale sap flow, soil moisture and productivity under both control and drought setups at the Caxiuanã throughfall exclusion field experiment in eastern Amazonia between 2001 and 2008. A relationship between PLC and tree mortality is built in the model from two empirical parameters, the cumulated duration of drought exposure that triggers mortality, and the mortality fraction in each day exceeding the exposure. Our model captures the large biomass drop in the year 2005 observed 4 years after throughfall reduction, and produces comparable annual tree mortality rates with observation over the study period. Our hydraulic architecture module provides promising avenues for future research in assimilating experimental data to parameterize mortality due to drought-induced xylem dysfunction. We also highlight that species-based (isohydric or anisohydric) hydraulic traits should be further tested to generalize the model performance in predicting the drought risks.</p

Structure, productivité et régime hydrique de phytocénoses halophiles sous climat méditerranéen

Des recherches sur le régime hydrique de la couverture végé tale (potentiel hydrique et transpiration), les variations de la bio masse végétale (matière sèche, composition chimique et valeur énergétique) et la production primaire ont été effectuées parallè lement à une étude de la composition floristique, de la salinité et de la profondeur de la nappe phréatique le long d’un transect implanté dans une végétation halophile dominée par Salicornia fruticosa et Arthrocnemum glaucum. Les biomasses végétales se sont avérées relativement élevées dans le Salicornietum fruticosae : de l’ordre de 3 kg de matière sèche par m2, alors qu’elles n’atteignent que 0,4 kg m-2 dans Y Arthrocnemetum. L’analyse des variations saisonnières de cette biomasse a permis de situer la production primaire annuelle du Salicornietum fruticosae entre 0,5 et 1 kg de matière sèche par m2, correspondant à un équivalent énergétique de 8-16 MJ m— 2, pour un rayonnement global incident de l’ordre de 6000 MJ par an. Sans atteindre les valeurs élevées qui caractérisent les marais d’eau douce ou les cultures, elle est du même ordre de grandeur que celle d’un peuplement de Quercus ilex étudié sous le même climat. La zone étudiée est caractérisée par la présence d’une nappe phréatique salée (10-45 g 1— 1) présentant des oscillations saison nières de niveau. A une période de submersion hivernale, au cours de laquelle il y a des risques d’asphyxie des racines, succède un assèchement, puis un abaissement de la nappe qui descend jus qu’à environ 70 cm dans le courant de l’été. Cet abaissement se manifeste par une chute du potentiel hydrique dans la vasculari sation et par une régulation de la transpiration ; parallèlement on observe un ralentissement très net de la croissance, qui fait que la production primaire est réalisée, pour l’essentiel, en trois mois (mai à juillet). La comparaison de deux zones se distinguant par la salinité de la nappe et le mode d’enracinement a montré la complexité et la nature de certaines relations entre les fluctuations de salinité et de profondeur de la nappe, l’évolution du régime hydrique de la plante et les variations de la biomasse. La répartition des raci nes en plusieurs strates joue un rôle important dans le maintien du potentiel hydrique de la plante. La présence d’une strate pro fonde lui permet, en été, de s’affranchir des horizons superficiels, où viennent s’accumuler les sels, et de garder le contact avec la frange capillaire. La présence d’une strate superficielle lui permet, au début de son cycle végétatif, d’éviter l’asphyxie lorsque la nappe est encore proche de la surface.Research on the water relations of the vegetation (water potential and transpiration), on seasonal changes in plant biomass (dry matter, chemical composition and caloric value), and on primary production have been carried out concurrently with records of species distribution, and depth and salinity of the water table along a transect in a halophytic vegetation dominated by Salicornia fruticosa and Arthrocnemum glaucum. Values of above-ground plant biomass are rather high in Salicornia fruticosa stands : about 3 kg m— 2 (dry matter), compa red to only 0,4 kg m— 2 in Arthrocnemum stands. Shoot net pro ductivity of Salicornia fruticosa stands was estimated to be between 0.5 and 1.0 kg m— 2 yr—1, which is equivalent to a caloric content of 8-16 MJ m— 2, for an incident global radiation of about 6000 MJ m— 2 yr- 1. This value is lower than those recorded for freshwater marshes or cultivated land, but of the same order of magnitude as in a Quercus ilex stand under the same climatic conditions. The area studied is characterized by the presence of saline underground water (10-45 g 1— 1), presenting seasonal variations in level. After a period of winter submersion, with risks of root asphyxia, there is a period of drying up and a lowering of the water table to a level of about — 70 cm in midsummer. This lowering is reflected by a drop of the water potential in the vas cularization and by a stomatal regulation leading to a decrease in transpiration ; concurrently a sharp reduction in growth is observed, with the result that most of the primary production is carried out in three months (May to July). Comparison between two areas differing in salinity of ground water and in root distribution showed the complexity and the nature of certain relationships between fluctuations of the water level and salinity of the ground water on the one hand, and evo lution of the water regime of the plant and biomass changes on the other hand. Distribution of roots in several layers plays an important role in the maintenance of the plant water potential. The presence of a deep root layer is useful to the plants in sum mer : it allows them to become independent from the upper horizons, in which salts accumulate, and to remain in contact with the capillary fringe. The presence of a superficial layer enables the plant to avoid asphyxia at the beginning of the gro wing season, when the water is still near the surface

The Central Amazon Biomass Sink Under Current and Future Atmospheric CO2: Predictions From Big-Leaf and Demographic Vegetation Models

There is large uncertainty whether Amazon forests will remain a carbon sink as atmospheric CO2 increases. Hence, we simulated an old-growth tropical forest using six versions of four terrestrial models differing in scale of vegetation structure and representation of biogeochemical (BGC) cycling, all driven with CO2 forcing from the preindustrial period to 2100. The models were benchmarked against tree inventory and eddy covariance data from a Brazilian site for present-day predictions. All models predicted positive vegetation growth that outpaced mortality, leading to continual increases in present-day biomass accumulation. Notably, the two vegetation demographic models (VDMs) (ED2 and ELM-FATES) always predicted positive stem diameter growth in all size classes. The field data, however, indicated that a quarter of canopy trees didn't grow over the 15-year period, and while high interannual variation existed, biomass change was near neutral. With a doubling of CO2, three of the four models predicted an appreciable biomass sink (0.77 to 1.24 Mg ha−1 year−1). ELMv1-ECA, the only model used here that includes phosphorus constraints, predicted the lowest biomass sink relative to initial biomass stocks (+21%), lower than the other BGC model, CLM5 (+48%). Models projections differed primarily through variations in nutrient constraints, then carbon allocation, initial biomass, and density-dependent mortality. The VDM's performance was similar or better than the BGC models run in carbon-only mode, suggesting that nutrient competition in VDMs will improve predictions. We demonstrate that VDMs are comparable to nondemographic (i.e., “big-leaf”) models but also include finer scale demography and competition that can be evaluated against field observations. ©2020. The Authors

Horticultural weed control 1988 report

Crops were grown at the experimental farms using accepted cultural practices within the limits of experimentation trials were conducted on growers' fields. Most experiments were designed as randomized complete blocks with two to five replications. Herbicide treatments were applied uniformly with precision plot sprayers or granular formulations were distributed from quart jar shakers. Unless otherwise indicated, preplant herbicide applications were incorporated with a PTO horizontal rotary tiller operated at a depth of approximately three inches. After critical application timings, crops were irrigated with overhead sprinklers at weekly intervals or as needed. Crop and weed responses are primarily visual evaluations of stand reduction (SR) and growth reduction (GR), ranging from 0-100 with 100 as the maximum response for each rating, or an over-all rating of 0-10 for crop response or control of specific weed species with 10 being complete control of the weed or good crop vigor (no injury). Additional data such as crop yields are reported for certain studies and may be reported in either English or metric systems