As global observations of solar-induced chlorophyll fluorescence (SIF) have become available from multiple satellite platforms, SIF is increasingly used as a proxy for photosynthetic activity and ecosystem productivity. Because the relationship between SIF and gross primary productivity (GPP) depends on a variety of factors including ecosystem type and environmental conditions, it is necessary to study SIF observations across various spatiotemporal scales and ecosystems. To explore how SIF signals relate to productivity over a temperate deciduous forest, we deployed a PhotoSpec spectrometer system at the University of Michigan Biological Station AmeriFlux site (US-UMB) in the northern Lower Peninsula of Michigan during the 2018 and 2019 growing seasons. The PhotoSpec system consisted of two narrowband spectrometers, for the retrieval of SIF in the red (680-686 nm) and far-red (745-758 nm) regions of the electromagnetic spectrum, and a broadband spectrometer for the assessment of vegetation indices. We found that SIF correlated with GPP across diurnal and seasonal cycles, but that SIF irradiances were more strongly related to downwelling radiation than GPP. However, while this dependence of SIF on radiation obscured drought signals in SIF itself, we demonstrate that a SIF response to severe drought was apparent as a decrease in relative SIF. These results highlight the potential of SIF for detecting stress-induced losses in forest productivity. Additionally, we found that the red:far-red SIF ratio did not exhibit a response to drought stress, but was largely driven by seasonal and interannual changes in canopy structure, as well as by synoptic changes in downwelling radiation.

Drought and pluvial extremes are defined as deviations from typical climatology; however, the background climatology can shift over time in a non-stationary climate, impacting interpretations of extremes. This study evaluated changes in meteorological drought and pluvial extremes by merging tree-ring reconstructions, observations, and climate-model simulations spanning 850 – 2100 CE across North America to determine whether the Industrial era and projected future lie outside the range of natural climate variability. Our results found widespread and spatially consistent exacerbation of both extremes, especially summer drought and winter pluvials, with west and south drying, the northeast wetting trends, and increased interannual variability across the east and north. Our study underscores climate change has already shifted precipitation climatology beyond pre-Industrial climatology and is projected to further intensify ongoing shifts.

Improving the representation of plant hydraulic behavior in vegetation and land-surface models is critical for improving our predictions of the impacts of drought stress on ecosystem carbon and water fluxes. Species-specific hydraulic traits play an important role in determining the response of ecosystem carbon and water fluxes to water stress. Here, we present plans for the development of the Finite-difference Ecosystem-scale Tree Crown Hydrodynamics model version 3 (FETCH3), a tree hydrodynamic model which builds upon its predecessors FETCH and FETCH2. FETCH3 simulates water transport through the soil, roots, and xylem as flow through porous media. The model resolves water potentials along the vertical dimension, and stomatal response is linked to xylem water potential. The tree-level model is scaled to the plot scale based on the species composition and canopy structure of the plot, allowing the model to be validated using both tree-level observations (sap flux) and plot-level observations (eddy covariance). We will collect data from multiple sites that have both sap flux and eddy covariance measurements for analysis. The Predictive Ecosystem Analyzer (PEcAn) will be used for optimization of the hydraulic parameters in FETCH3 for different plant types in multiple sites. We plan to use this new modeling framework to examine the interactions among water stress, species-specific hydraulic strategies, and stomatal regulation across different species and ecosystem types.

Accounting for temporal changes in carbon dioxide (CO2) emissions from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of eddy covariance flux measurements of CO2 from 13 lakes and reservoirs in the Northern Hemisphere (NH) and quantify the magnitude and dynamics at multiple temporal scales. We found pronounced diel and sub-monthly oscillatory variations in CO2 flux at all sites. Diel variation converted sites to daily net sinks of CO2 in only 11% of site-months. Upscaled annual emissions had an average of 25% (range 3-58%) interannual variation. Given temporal variation remains under-represented in inventories of CO2 emissions from lakes and reservoirs, revisions in CO2 flux are needed using a better representation of sub-daily to interannual variability. Constraining short- and long-term variability is necessary to improve detection of temporal changes of CO2 fluxes in response to natural and anthropogenic drivers.

Accounting for temporal changes in carbon dioxide (CO2) emissions from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of eddy covariance flux measurements of CO2 from 13 lakes and reservoirs in the Northern Hemisphere (NH) and quantify dynamics at multiple temporal scales. We found pronounced sub-annual variability in CO2 flux at all sites. Accounting for diel variation, only 11% of site-months were net daily sinks of CO2. Annual CO2 emissions had an average of 25% (range 3-58%) interannual variation. Nighttime emissions regularly exceeded daytime emissions. Sources of CO2 flux variability were delineated through mutual information analysis. Sample analysis of CO2 fluxes indicate importance of continuous sampling. Constraining short- and long-term variability is necessary to improve detection of temporal changes of CO2 fluxes in response to natural and anthropogenic drivers.

Representation of canopy and tree function in earth system models (ESMs) is going through rapid advancements in the last decade. The simplistic big-leaf representation of canopy in model is being expanded and replaced by more complex canopy representations that include multiple functional types, species and age/size stages, multiple patches with different canopy height and leaf area characteristics, and vertically detailed representations with between 2 (light and shade) to n canopy leaf layers. Recently, the hydrodynamic approach to modeling stomata conductance has advanced, and many ESMs include a hydrodynamic version. Under the hydrodynamic approach, stomata respond to water availability in the xylem, and not directly in the soil. The result is a model that include the stem, and xylem dimensions as canopy characteristics for determining the flow and storage of water in the xylem. However, pipe diameter, area and volume do not scale linearly, and thus, the simulations cannot resolve canopy-scale hydrodynamics by pulling all the stem conductive area and volume to a single virtual patch scale, but instead, need to consider the hydrodynamics of a single virtual tree and scale its resulting fluxes to the entire canopy. While that is trivial in a homogeneous canopy, the difference between the tree-level description of the hydrodynamic canopy and the horizontally pulled description of the canopy in the surface-flux and radiation-exchange modules leads to a conundrum. Imagine a mixed canopy: 50% oaks and 50% maples. Assume oaks have 20% more LAI and are 1 m taller than the maples. As far as hydrodynamics are concerned, the evaporative demands of maple follow from light attenuation and leaf area profiles of a maple-like canopy. However the roughness length and aerodynamic resistance, and the resulting wind profile inside the canopy is not characteristic of maple or oak but is a result of the mixed canopy. I will present formulation for a consistent scaling of tree-level canopies to patch-level for mixed forests with multiple species of different functional types. The formulation scales canopy characteristics and the resulting canopy fluxes from tree to forest in an energy and mass-conservative way, and allow a smooth and consistent multi-species canopy description for hydrodynamic models in mixed-forests.

Methane emissions from freshwater, mineral-soil wetlands represent an important portion of the global greenhouse gas budgets. We use long-term observations in Old Woman Creek (OWC), an estuarine wetland at the coast of Lake Erie. OWC is characterized by a fluctuating water level controlled by a natural sand barrier. OWC water levels are high when the barrier is closed. When it breaks, OWC is directly connected to the lake. Long term water level rise of Lake Erie provides a trend of water level at OWC. These changes to hydrology drive changes to the ecology. In OWC the dominant eco-hydrological patch types are mudflats, cattails (Typha), floating-leaf vegetation (Lotus, water lily), and open water. The seasonal and long term changes to water level lead to dynamic changes in the patch type composition, and as OWC gets deeper, mudflats and cattails give way to open water and floating-leaf vegetation. We developed an approach to classify the eco-hydrological patch type from remote sensing images. We used seasonal time series of NDVI from HLS (a composite dataset of Sentinel and Landsat). These time series were classified to patch types according to their similarity to the seasonal NDVI profiles of pixels identified and ground-truthed as pure pixels of a specific patch type. We then used the DG-SWEM high resolution hydrodynamic model to simulate the flow velocity throughout the wetland. Combining the eco-hydrological patch locations and the high-resolution flow simulation allowed for the calculation of an effective patch-type-level residence time. We found differences in residence times between the different patch types. We measured the relations between methane flux and CO2 uptake at the whole wetland scale, the vegetation patch scale, and directly from leaves. We found different methane-CO2 relations among the floating leaved and emergent species. Phenological transitions throughout the growing season continued to make an important effect only in Typha. Our observations represent a valuable foundation towards a more robust models of methane fluxes in wetlands at the resolution of within-wetland vegetation patch type, and resolving the effects of seasonal and within-season vegetation phenology in ecosystem-scale models.

The plant hydrodynamic approach represents a recent advancement to land surface modeling, in which stomatal conductance responds to water availability in the xylem rather than in the soil. To provide a realistic representation of tree hydrodynamics, hydrodynamic models must resolve processes at the level of a single modelled tree, and then scale the resulting fluxes to the canopy and land surface. While this tree-to-canopy scaling is trivial in a homogeneous canopy, mixed-species canopies require careful representation of the species properties and a scaling approach that results in a realistic description of both the canopy and individual-tree hydrodynamics, as well as leaf-level fluxes from the canopy and their forcing. Here, we outline advantages and pitfalls of three commonly used approaches for representing mixed-species forests in land surface models, and present a new framework for scaling vegetation characteristics and fluxes in mixed-species forests. The new formulation scales fluxes from the tree- to canopy-level in an energy- and mass-conservative way and allows for a consistent multi-species canopy description for hydrodynamic models.