Variation the in relationship between urban tree canopy and air temperature reduction under a range of daily weather conditions.

Mitigating heat is a vital ecosystem service of trees, particularly with climate change. Land surface temperature measures captured at a single time of day (in the morning) dominate the urban heat island literature. Less is known about how local tree canopy and impervious surface regulate air temperature throughout the day, and/or across many days with varied weather conditions, including cloud cover. We use bike-mounted air temperature sensors throughout the day in New Haven, Connecticut, USA, from 2019 to 2021 and generalized additive mixed models across 156 rides to estimate the daily variation in cooling benefits associated with tree canopy cover, and warming from impervious surface cover in 90 m buffers surrounding bike observations. Cooling is inferred by subtracting the bicycle-observed temperature from a reference station. The cooling benefits from tree canopy cover were strongest in the midday (11:00-14:00, -1.62 °C), afternoon (14:00-17:00, -1.19 °C), and morning (8:00-11:00, -1.15 °C) on clear days. The cooling effect was comparatively smaller on cloudy mornings -0.92 °C and afternoons -0.51 °C. Warming from impervious surfaces was most pronounced in the evening (17:00-20:00, 1.11 °C) irrespective of clouds, and during cloudy nights (20:00-23:00) and cloudy mornings 1.03 °C 95 % CI [1.03, 1.04]. Among the hottest observed days (top 25th percentile of reference station daily maxima), tree canopy was associated with lower temperatures on clear afternoons -1.78 °C [-1.78, -1.78], cloudy midday -1.17 °C [-1.19, -1.15], clear midday -1.12 °C [-1.12, -1.11]. We add a broader spectrum of weather conditions by explicitly including clouds, and greater temporal resolution by measuring throughout the day to bike-based urban heat research. Future mobile sampling campaigns may broaden the spatial extent with more environmental variation, representing an opportunity for public science and engagement.

Decline in wild bee species richness associated with honey bee (Apis mellifera L.) abundance in an urban ecosystem.

The spatial heterogeneity of urban landscapes, relatively low agrochemical use, and species-rich floral communities often support a surprising diversity of wild pollinators in cities. However, the management of Western honey bees (Apis mellifera L.) in urban areas may represent a new threat to wild bee communities. Urban beekeeping is commonly perceived as an environmentally friendly practice or a way to combat pollinator declines, when high-density beekeeping operations may actually have a negative influence on native and wild bee populations through floral resource competition and pathogen transmission. On the Island of Montréal, Canada there has been a particularly large increase in beekeeping across the city. Over the years following a large bee diversity survey ending in 2013, there was an influx of almost three thousand honey bee colonies to the city. In this study, we examined the wild bee communities and floral resources across a gradient of honey bee abundances in urban greenspaces in 2020, and compared the bee communities at the same sites before and after the large influx of honey bees. Overall, we found a negative relationship between urban beekeeping, pollen availability, and wild bee species richness. We also found that honey bee abundance had the strongest negative effect on small (inter-tegular span <2.25 mm) wild bee species richness. Small bee species may be at higher risk in areas with abundant honey bee populations as their limited foraging range may reduce their access to floral resources in times of increased competition. Further research on the influence of urban beekeeping on native and wild pollinators, coupled with evidence-based beekeeping regulations, is essential to ensure cities contain sufficient resources to support wild bee diversity alongside managed honey bees.

Cryptic eco-evolutionary feedback in the city.

Research Highlight: Brans, K. I., Tüzün, N., Sentis, A., De Meester, L., & Stoks, R. (2021). Cryptic eco-evolutionary feedback in the city: Urban evolution of prey dampens the effect of urban evolution of the predator. Journal of Animal Ecology. https://doi.org/10.1111/1365-2656.13601. Despite the strength and ubiquity of urban stressors on multiple taxa, there have been minimal attempts to determine the ecological consequences of urban evolution on multiple species. Brans & Tüzün et al. use a well-known predator prey system-damselfly nymphs Ischnura elegans and water fleas Daphnia magna-to test whether scenarios in which both species evolve in response to urbanization differ from scenarios in which only the predator or prey evolves. The authors show that urban damselflies showed higher encounter and predation rates when paired with rural prey, but that the advantages conferred by urbanization-driven adaptation disappeared when urban predators encountered urban prey. This represents a cryptic eco-evolutionary feedback, where evolution of both predator and prey concealed the effect of evolution in each partner individually. Results suggest that mismatches in the evolutionary responses of interacting species may have significant ecological consequences, and highlight the importance of a multi-species approach in eco-evolutionary dynamics research.

Scale-dependent interactions between tree canopy cover and impervious surfaces reduce daytime urban heat during summer.

As cities warm and the need for climate adaptation strategies increases, a more detailed understanding of the cooling effects of land cover across a continuum of spatial scales will be necessary to guide management decisions. We asked how tree canopy cover and impervious surface cover interact to influence daytime and nighttime summer air temperature, and how effects vary with the spatial scale at which land-cover data are analyzed (10-, 30-, 60-, and 90-m radii). A bicycle-mounted measurement system was used to sample air temperature every 5 m along 10 transects (∼7 km length, sampled 3-12 times each) spanning a range of impervious and tree canopy cover (0-100%, each) in a midsized city in the Upper Midwest United States. Variability in daytime air temperature within the urban landscape averaged 3.5 °C (range, 1.1-5.7 °C). Temperature decreased nonlinearly with increasing canopy cover, with the greatest cooling when canopy cover exceeded 40%. The magnitude of daytime cooling also increased with spatial scale and was greatest at the size of a typical city block (60-90 m). Daytime air temperature increased linearly with increasing impervious cover, but the magnitude of warming was less than the cooling associated with increased canopy cover. Variation in nighttime air temperature averaged 2.1 °C (range, 1.2-3.0 °C), and temperature increased with impervious surface. Effects of canopy were limited at night; thus, reduction of impervious surfaces remains critical for reducing nighttime urban heat. Results suggest strategies for managing urban land-cover patterns to enhance resilience of cities to climate warming.