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1. Site Characteristics Mediate the Relationship Between Forest Productivity and Satellite Measured Solar Induced Fluorescence

   https://doi.org/10.3389/ffgc.2021.695269  

   Yazbeck, Theresia ; Bohrer, Gil ; Gentine, Pierre ; Ye, Luping ; Arriga, Nicola ; Bernhofer, Christian ; Blanken, Peter D. ; Desai, Ankur R. ; Durden, David ; Knohl, Alexander ; et al ( December 2021 , Frontiers in Forests and Global Change)

   Solar-Induced Chlorophyll Fluorescence (SIF) can provide key information about the state of photosynthesis and offers the prospect of defining remote sensing-based estimation of Gross Primary Production (GPP). There is strong theoretical support for the link between SIF and GPP and this relationship has been empirically demonstrated using ground-based, airborne, and satellite-based SIF observations, as well as modeling. However, most evaluations have been based on monthly and annual scales, yet the GPP:SIF relations can be strongly influenced by both vegetation structure and physiology. At the monthly timescales, the structural response often dominates but short-term physiological variations can strongly impact the GPP:SIF relations. Here, we test how well SIF can predict the inter-daily variation of GPP during the growing season and under stress conditions, while taking into account the local effect of sites and abiotic conditions. We compare the accuracy of GPP predictions from SIF at different timescales (half-hourly, daily, and weekly), while evaluating effect of adding environmental variables to the relationship. We utilize observations for years 2018–2019 at 31 mid-latitudes, forested, eddy covariance (EC) flux sites in North America and Europe and use TROPOMI satellite data for SIF. Our results show that SIF is a good predictor of GPP, when accountingmore »for inter-site variation, probably due to differences in canopy structure. Seasonally averaged leaf area index, fraction of absorbed photosynthetically active radiation (fPAR) and canopy conductance provide a predictor to the site-level effect. We show that fPAR is the main factor driving errors in the linear model at high temporal resolution. Adding water stress indicators, namely canopy conductance, to a multi-linear SIF-based GPP model provides the best improvement in the model precision at the three considered timescales, showing the importance of accounting for water stress in GPP predictions, independent of the SIF signal. SIF is a promising predictor for GPP among other remote sensing variables, but more focus should be placed on including canopy structure, and water stress effects in the relationship, especially when considering intra-seasonal, and inter- and intra-daily resolutions.« less
2. Assessing spatial patterns of soil erosion in a high‐latitude rangeland

   https://doi.org/10.1002/ldr.3585  

   Streeter, Richard T. ; Cutler, Nick A. ( March 2020 , Land Degradation & Development)

   High‐latitude areas are experiencing rapid change: we therefore need a better understanding of the processes controlling soil erosion in these environments. We used a spatiotemporal approach to investigate soil erosion in Svalbarðstunga, Iceland (66°N, 15°W), a degraded rangeland. We used three complementary datasets: (a) high‐resolution unmanned‐aerial vehicle imagery collected from 12 sites (total area ~0.75 km²); (b) historical imagery of the same sites; and (c) a simple, spatially‐explicit cellular automata model. Sites were located along a gradient of increasing altitude and distance from the sea, and varied in erosion severity (5–47% eroded). We found that there was no simple relationship between location along the environmental gradient and the spatial characteristics of erosion. Patch‐size frequency distributions lacked a characteristic scale of variation, but followed a power‐law distribution on five of the 12 sites. Present total eroded area is poorly related to current, site‐scale levels of environmental stress, but the number of small erosion patches did reflect site‐level stress. Small (<25 m²) erosion patches clustered near large patches. The model results suggested that the large‐scale patterns observed likely arise from strong, local interactions, which mean that erosion spreads from degraded areas. Our findings suggest that contemporary erosion patterns reflect historical stresses, as wellmore »as current environmental conditions. The importance of abiotic processes to the growth of large erosion patches and their relative insensitivity to current environmental conditions makes it likely that the total eroded area will continue to increase, despite a warming climate and reducing levels of grazing pressure.

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3. Fine-scale variability in coral bleaching and mortality during a marine heatwave

   https://doi.org/10.3389/fmars.2023.1108365  

   Yadav, Shreya ; Roach, Ty N. ; McWilliam, Michael J. ; Caruso, Carlo ; de Souza, Mariana Rocha ; Foley, Catherine ; Allen, Corinne ; Dilworth, Jenna ; Huckeba, Joel ; Santoro, Erika P. ; et al ( June 2023 , Frontiers in Marine Science)

   Coral bleaching and mortality can show significant spatial and taxonomic heterogeneity at local scales, highlighting the need to understand the fine-scale drivers and impacts of thermal stress. In this study, we used structure-from-motion photogrammetry to track coral bleaching, mortality, and changes in community composition during the 2019 marine heatwave in Kāneʻohe Bay, Hawaiʻi. We surveyed 30 shallow reef patches every 3 weeks for the duration of the bleaching event (August-December) and one year after, resulting in a total of 210 large-area, high-resolution photomosaics that enabled us to follow the fate of thousands of coral colonies through time. We also measured environmental variables such as temperature, sedimentation, depth, and wave velocity at each of these sites, and extracted estimates of habitat complexity (rugosity R and fractal dimension D) from digital elevation models to better understand their effects on patterns of bleaching and mortality. We found that up to 80% of corals experienced moderate to severe bleaching in this period, with peak bleaching occurring in October when heat stress (Degree Heating Weeks) reached its maximum. Mortality continued to accumulate as bleaching levels dropped, driving large declines in more heat-susceptible species (77% loss of Pocillopora cover) and moderate declines in heat-tolerant species (19%more »and 23% for Porites compressa and Montipora capitata , respectively). Declines in live coral were accompanied by a rapid increase in algal cover across the survey sites. Spatial differences in bleaching were significantly linked to habitat complexity and coral species composition, with reefs that were dominated by Pocillopora experiencing the most severe bleaching. Mortality was also influenced by species composition, fractal dimension, and site-level differences in thermal stress. Our results show that spatial heterogeneity in the impacts of bleaching are driven by a mix of environmental variation, habitat complexity, and differences in assemblage composition.« less
4. Turbulent Thermal-Wind-Driven Coastal Upwelling: Current Observations and Dynamics

   https://doi.org/10.1175/JPO-D-22-0063.1  

   Lentz, S. J. ( December 2022 , Journal of Physical Oceanography)

   Abstract A remarkably consistent Lagrangian upwelling circulation at monthly and longer time scales is observed in a 17-yr time series of current profiles in 12 m of water on the southern New England inner shelf. The upwelling circulation is strongest in summer, with a current magnitude of ∼1 cm s −1 , which flushes the inner shelf in ∼2.5 days. The average winter upwelling circulation is about one-half of the average summer upwelling circulation, but with larger month-to-month variations driven, in part, by cross-shelf wind stresses. The persistent upwelling circulation is not wind-driven; it is driven by a cross-shelf buoyancy force associated with less-dense water near the coast. The cross-shelf density gradient is primarily due to temperature in summer, when strong surface heating warms shallower nearshore water more than deeper offshore water, and to salinity in winter, caused by fresher water near the coast. In the absence of turbulent stresses, the cross-shelf density gradient would be in a geostrophic, thermal-wind balance with the vertical shear in the along-shelf current. However, turbulent stresses over the inner shelf attributable to strong tidal currents and wind stress cause a partial breakdown of the thermal-wind balance that releases the buoyancy force, which drives themore »observed upwelling circulation. The presence of a cross-shelf density gradient has a profound impact on exchange across this inner shelf. Many inner shelves are characterized by turbulent stresses and cross-shelf density gradients with lighter water near the coast, suggesting turbulent thermal-wind-driven coastal upwelling may be a broadly important cross-shelf exchange mechanism. Significance Statement A remarkably consistent upwelling circulation at monthly time scales is observed in a 17-yr time series of current profiles in shallow water off southern New England. This is not the traditional wind-driven coastal upwelling; instead, it is forced by cross-shelf buoyancy (density) gradients, released by turbulent stresses in shallow water. The persistent upwelling circulation is strongest in summer, when wind and wave forcing are weak, and flushes the inner portion of the continental shelf in a few days. Consequently, this buoyancy-driven coastal upwelling is important for cooling the inner shelf and provides a reliable mechanism for cross-shelf exchange. Many inner shelves are characterized by cross-shelf density gradients and turbulent stresses, suggesting this may be a broadly important cross-shelf exchange mechanism.« less
5. Does plant ecosystem thermoregulation occur? An extratropical assessment at different spatial and temporal scales

   https://doi.org/10.1111/nph.18632  

   Guo, Zhengfei ; Still, Christopher J. ; Lee, Calvin K. F. ; Ryu, Youngryel ; Blonder, Benjamin ; Wang, Jing ; Bonebrake, Timothy C. ; Hughes, Alice ; Li, Yan ; Yeung, Henry C. H. ; et al ( December 2022 , New Phytologist)

   To what degree plant ecosystems thermoregulate their canopy temperature (T_c) is critical to assess ecosystems' metabolisms and resilience with climate change, but remains controversial, with opinions from no to moderate thermoregulation capability.

   With global datasets ofT_c, air temperature (T_a), and other environmental and biotic variables from FLUXNET and satellites, we tested the ‘limited homeothermy’ hypothesis (indicated byT_c&T_aregression slope < 1 orT_c < T_aaround midday) across global extratropics, including temporal and spatial dimensions.

   Across daily to weekly and monthly timescales, over 80% of sites/ecosystems have slopes ≥1 orT_c > T_aaround midday, rejecting the above hypothesis. For those sites unsupporting the hypothesis, theirT_c–T_adifference (ΔT) exhibits considerable seasonality that shows negative, partial correlations with leaf area index, implying a certain degree of thermoregulation capability. Spatially, site‐mean ΔTexhibits larger variations than the slope indicator, suggesting ΔTis a more sensitive indicator for detecting thermoregulatory differences across biomes. Furthermore, this large spatial‐wide ΔTvariation (0–6°C) is primarily explained by environmental variables (38%) and secondarily by biotic factors (15%).

   These results demonstrate diverse thermoregulation patterns across global extratropics, with most ecosystems negating the ‘limited homeothermy’ hypothesis, but their thermoregulation still occurs, implying that slope < 1 orT_c < T_aare not necessary conditions for plant thermoregulation.