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1. The extent of vegetation-driven panel cooling and consequent increase in electricity generation from solar PV sites depend on climate and soil properties

   Bertel, R. ; Choi, C. S. ; Macknick, J. ; McCall, J. ; Ravi, S ( December 2021 , AGU 2021 Fall Meeting Meeting New Orleans, LA)

   Co-locating solar photovoltaics (PV) with agriculture or natural vegetation could provide a sustainable solution to meeting growing food and energy demands, particularly considering the recent concerns of solar PV encroaching on agricultural and natural areas. However, the identification and quantification of the mutual interactions between the solar panels and the underlying soil-vegetation system are scarce. This is a critical research gap, as understanding these feedbacks are important for minimizing environmental impacts and for designing resource conserving and climate-resilient food-energy production systems. We monitored the microclimate, soil moisture distribution, and soil properties at three utility-scale solar facilities (MN, USA) with different site management practices, with an emphasis on verifying previously hypothesized vegetation-driven cooling of solar panels. The microclimatic variables (air and soil temperature, relative humidity, wind speed and direction) and soil moisture were significantly different between the PV site with bare soil (bare-PV) and vegetated PV (veg.-PV) site. Compared to the bare-PV site, the veg.-PV site also had significantly higher levels of total soil carbon and total soil nitrogen, as well as higher humidity and lower air and soil temperatures. Further, soil moisture heterogeneity created by the solar panels was homogenized by vegetation at the veg.-PV sites. However, we found no significant panel cooling or increase in electricity output that could be linked to co-location of the panels with vegetation in these facilities. We link these outcomes to the background climatic conditions (not water limited system) and soil moisture conditions. In regions with persistent high soil moisture (more frequent rainfall events) soil evaporation from wet bare soil may be comparable or even higher than from a vegetated surface. Thus, the cooling effects of vegetation on solar panels are not universal but rather site-specific depending on the background climate and soil properties. Regardless, the other co-benefits of maintaining vegetation at solar PV sites including the impacts on microclimate, soil moisture distribution, and soil quality support the case for solar PV–vegetation co-located systems. 

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2. Managed sheep grazing can improve soil quality and carbon sequestration at solar photovoltaic sites

   https://doi.org/10.1002/essoar.10510141.1  

   Elizabeth Towner, Tom Karas ( January 2022 , AGU 2021 Fall Meeting)

   Solar energy development is land intensive and recent studies have demonstrated the negative impacts of large-scale solar deployment on vegetation and soil. Co-locating vegetation with managed grazing on utility scale solar PV sites could provide a sustainable solution to meeting the growing food and energy demands, along with providing several co-benefits. However, the impacts of introducing grazing on soil properties at vegetated solar PV sites are not well understood. To address this knowledge gap, we investigated the impacts of episodic sheep grazing on soil properties (micro and macro nutrients, carbon storage, soil grain size distribution) at six commercial solar PV sites (MN, USA) and compared that to undisturbed control sites. Results indicate that implementing managed sheep grazing significantly increased total carbon storage (10-80%) and available nutrients, and the magnitude of change correlated with the grazing frequency (1-5 years) at the study sites. Furthermore, it was found that sites that experienced consecutive annual grazing treatments benefitted more than intermittently grazed sites. The findings will help in designing resource conserving integrated solar energy and food/fodder systems, along with increasing soil quality and carbon sequestration. 

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3. China’s terrestrial ecosystem carbon balance during the 20th century: an analysis with a process-based biogeochemistry model

   https://doi.org/10.1186/s13021-022-00215-9  

   Lu, Yanyu ; Huang, Yao ; Zhuang, Qianlai ; Sun, Wei ; Chen, Shutao ; Lu, Jun ( October 2022 , Carbon Balance and Management)

   China’s terrestrial ecosystems play a pronounced role in the global carbon cycle. Here we combine spatially-explicit information on vegetation, soil, topography, climate and land use change with a process-based biogeochemistry model to quantify the responses of terrestrial carbon cycle in China during the 20th century.

   At a century scale, China’s terrestrial ecosystems have acted as a carbon sink averaging at 96 Tg C yr^− 1, with large inter-annual and decadal variabilities. The regional sink has been enhanced due to the rising temperature and CO₂concentration, with a slight increase trend in carbon sink strength along with the enhanced net primary production in the century. The areas characterized by C source are simulated to extend in the west and north of the Hu Huanyong line, while the eastern and southern regions increase their area and intensity of C sink, particularly in the late 20th century. Forest ecosystems dominate the C sink in China and are responsible for about 64% of the total sink. On the century scale, the increase in carbon sinks in China’s terrestrial ecosystems is mainly contributed by rising CO₂. Afforestation and reforestation promote an increase in terrestrial carbon uptake in China from 1950s. Although climate change has generally contributed to the increase of carbon sinks in terrestrial ecosystems in China, the positive effect of climate change has been diminishing in the last decades of the 20th century.

   This study focuses on the impacts of climate, CO₂and land use change on the carbon cycle, and presents the potential trends of terrestrial ecosystem carbon balance in China at a century scale. While a slight increase in carbon sink strength benefits from the enhanced vegetation carbon uptake in China’s terrestrial ecosystems during the 20th century, the increase trend may diminish or even change to a decrease trend under future climate change.

    

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4. The response of boreal peatland community composition and NDVI to hydrologic change, warming, and elevated carbon dioxide

   https://doi.org/10.1111/gcb.14465  

   McPartland, Mara Y. ; Kane, Evan S. ; Falkowski, Michael J. ; Kolka, Randy ; Turetsky, Merritt R. ; Palik, Brian ; Montgomery, Rebecca A. ( October 2018 , Global Change Biology)

   Widespread changes in arctic and boreal Normalized Difference Vegetation Index (NDVI) values captured by satellite platforms indicate that northern ecosystems are experiencing rapid ecological change in response to climate warming. Increasing temperatures and altered hydrology are driving shifts in ecosystem biophysical properties that, observed by satellites, manifest as long‐term changes in regionalNDVI. In an effort to examine the underlying ecological drivers of these changes, we used field‐scale remote sensing ofNDVIto track peatland vegetation in experiments that manipulated hydrology, temperature, and carbon dioxide (CO₂) levels. In addition toNDVI, we measured percent cover by species and leaf area index (LAI). We monitored two peatland types broadly representative of the boreal region. One site was a rich fen located near Fairbanks, Alaska, at the Alaska Peatland Experiment (APEX), and the second site was a nutrient‐poor bog located in Northern Minnesota within the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment. We found thatNDVIdecreased with long‐term reductions in soil moisture at theAPEXsite, coincident with a decrease in photosynthetic leaf area and the relative abundance of sedges. We observed increasingNDVIwith elevated temperature at theSPRUCEsite, associated with an increase in the relative abundance of shrubs and a decrease in forb cover. Warming treatments at theSPRUCEsite also led to increases in theLAIof the shrub layer. We found no strong effects of elevatedCO₂on community composition. Our findings support recent studies suggesting that changes inNDVIobserved from satellite platforms may be the result of changes in community composition and ecosystem structure in response to climate warming.

    

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5. Arctic shrub growth trajectories differ across soil moisture levels

   https://doi.org/10.1111/gcb.13677  

   Ackerman, Daniel ; Griffin, Daniel ; Hobbie, Sarah E. ; Finlay, Jacques C. ( March 2017 , Global Change Biology)

   The circumpolar expansion of woody deciduous shrubs in arctic tundra alters key ecosystem properties including carbon balance and hydrology. However, landscape‐scale patterns and drivers of shrub expansion remain poorly understood, inhibiting accurate incorporation of shrub effects into climate models. Here, we use dendroecology to elucidate the role of soil moisture in modifying the relationship between climate and growth for a dominant deciduous shrub,Salix pulchra, on the North Slope of Alaska,USA. We improve upon previous modeling approaches by using ecological theory to guide model selection for the relationship between climate and shrub growth. Finally, we present novel dendroecology‐based estimates of shrub biomass change under a future climate regime, made possible by recently developed shrub allometry models. We find thatS. pulchragrowth has responded positively to mean June temperature over the past 2.5 decades at both a dry upland tundra site and an adjacent mesic riparian site. For the upland site, including a negative second‐order term in the climate–growth model significantly improved explanatory power, matching theoretical predictions of diminishing growth returns to increasing temperature. A first‐order linear model fit best at the riparian site, indicating consistent growth increases in response to sustained warming, possibly due to lack of temperature‐induced moisture limitation in mesic habitats. These contrasting results indicate thatS. pulchrain mesic habitats may respond positively to a wider range of temperature increase thanS. pulchrain dry habitats. Lastly, we estimate that a 2°C increase in current mean June temperature will yield a 19% increase in abovegroundS. pulchrabiomass at the upland site and a 36% increase at the riparian site. Our method of biomass estimation provides an important link toward incorporating dendroecology data into coupled vegetation and climate models.

    

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