|Description||Solar panel farms (SPFs) may be a part of a sustainable solution to pollinator decline and the loss of ecosystem services they provide. Some SPFs are seeking to landscape with diverse native seed mixes, which has the potential to benefit pollinators, the solar industry, as well as surrounding ecosystems and communities. Native plants selected for restoration should be diverse and offer ample resources for pollinator biodiversity. Yet, we lack important knowledge of native plant performance on SPFs and pollinator attractiveness for specific regions, including the Coastal Plain of eastern North Carolina.
To better inform native plant selection for SPFs, I investigated the microclimate under a simulated solar panel, evaluated pollinator visitation to native plant species used in landscaping, and asked how 13 plant species might respond under solar panels. I used Apogee® light meters (Apogee Instruments, Logan, UT) to record photosynthetically active radiation (PAR), both in the shade and unshaded (control) plots, for 2-wk periods in both winter and again in summer 2018. I also recorded temperature during these same times using iButton® data loggers (Maxim Integrated, San Jose, CA). I found PAR at ground level was reduced on average by 82.6% in winter and 79.8% in summer, during peak daylight hours. Temperature also varied under the simulated panel, becoming cooler during the afternoon hours and warmer in the evening and morning, in both winter and summer, possibly enhancing winter germination rates and lowering winter frost mortality under PV panels.
I selected 14 shade-tolerant perennial native plants to evaluate for possible use promoting pollinators on SPFs. Selection was based on known pollinator use, maximum height (to minimize contact with panels), and availability. Select native plant species (10 of the original 14) were also evaluated for pollinator attractiveness in shaded pollinator beds. Correspondence analyses (one based on 23 insect genera and one based on morpho-group, e.g. small native bee, large native bee, etc.) suggested that insects were not randomly visiting these plant species. In both correspondence analyses, Echinacea purpurea and Chrysogonum virginianum were outliers, suggesting a difference in their insect visitation, driven primarily by visits from Bombus sp. to Echinacea and Toxomerus sp. to Chrysogonum. The remaining eight native plant taxa were clustered, suggesting similarity in their insect visitation patterns. According to the visitation index (visits per floral unit V/F), Stokesia laevis was the most, followed by Coreopsis lanceolata, Gaillardia pulchella, Asclepias tuberosa, and Rudbeckia fulgida. Least attractive by this measure was Conoclinium coelestinum. Shannon’s Diversity Index (H’) based on genera of visiting insects was calculated using visitation rates for each plant species. Chrysogonum virginianum had the greatest diversity index among these ten selected natives, followed by R. hirta, G. pulchella, and Echinacea purpurea. Least attractive by this metric was R. fulgida. I then ranked the overall attractiveness of each plant species using a simple combination of their rankings based on the combined Shannon’s Diversity index and visitation index. The appeal to insect visitors as shown by the combined ranking suggests that the floral resources of G. pulchella, C. virginianum, C. lanceolata, A. tuberosa, R. hirta, S. laevis, and E. purpurea are most useful to pollinators and could provide necessary resources to eastern North Carolina’s pollinators. Rudbeckia fulgida, C. coelestinum, and Marshallia obovata did not rank as highly, however, did attract insects.
I also compared germination performance for 13 of the original 14 native species under simulated shading by solar panels in a greenhouse experiment. A. tuberosa, C. lanceolata, E. purpurea, C. coelestinum, G. pulchella, M. obovata, R. fulgida, R. hirta, and S. laevis germinated successfully and show promise for landscaping on SPFs. I then conducted a field germination experiment using eight native plant species that showed high germinability in the greenhouse to compare germination rates under and outside simulated solar panel arrays. In the field, G. pulchella (Indian blanket), R. hirta (black-eyed Susan), E. purpurea (eastern purple coneflower), C. lanceolata (lanceleaf coreopsis), and S. laevis (Stokes’ aster) had the highest and most rapid germination and show the most promise for use on SPFs. However, field germination was reduced when compared to greenhouse results for A. tuberosa, C. coelestinum, C. lanceolata, E. purpurea, R. fulgida, R. hirta, and S. laevis, suggesting microclimate and weedy competitor effects.
Seed mixes with high proportions of fast establishing natives such as G. pulchella and R. hirta could be useful in establishing vegetation on SPFs. Additionally, land preparation techniques and sowing a cover crop with these native plant species may increase successful germination and establishment of pollinator habitat on solar farms of eastern North Carolina. More in-situ studies of native plant performance are needed to provide more information to landscape managers and reduce pressures on native seed stocks. Use of native plants for landscaping on SPFs remains a promising option for promotion and maintenance of both plant and insect biodiversity on these human-modified landscapes.||