Spatiotemporal variation and drivers of cool microclimates in high-latitude ecosystems

Microclimates have recently been emphasized for their key role in shaping local ecosystems. Their profound impact on species distributions and ecosystem dynamics are increasingly recognized. The variations in temperature, humidity, and other climatic factors within microenvironments may be important for the adaptability and resilience of diverse species. For example, in warming climate, cold-adapted species can occur and persist in cool microclimate pockets beyond their macro-climatic ranges. In this study, we explore the spatial and temporal variations of cool microclimates at the forest-tundra ecotone, situated at 68° N latitude in the Pallas region, northern Finland. This study seeks to identify the locations and factors influencing the persistence and spatial structuring of cool microclimatic pockets. We build our study on continuous in-situ near-surface temperature measurements during peak growing season from 193 study sites, representing diverse topographical settings, soil conditions, vegetation types and land covers such as peatlands. We generated microclimate surfaces at a spatial resolution of 10m using statistical multivariate modelling. This process allowed for the mapping of cool microclimates throughout the entire study area. The results showed the versatility of temperature variations in the landscape. The average daily minimum temperatures in July ranged between 3.7° C and 8.8° C, whereas the average daily maximum temperatures ranged between 15.5° C and 24.5° C, respectively. During the coldest period of the day, typically nighttime, the coolest areas were open wetlands in the bottoms of valleys, whereas during the warmest period of the day, typically afternoon, the coolest areas were the shaded hill slopes with poleward aspects and characterized by coniferous forests. The cool microclimates during nighttime were strongly driven by the cold air pooling into low-elevation areas and the intense long-wave radiative cooling in treeless environments. In contrast, the daytime cool microclimates were determined by the interactive effect of low short wave-solar radiation conditions on north-facing slopes and the dense canopies of mature spruce forests. Cool microclimates exhibited connected spatial patterns both during the nighttime and daytime, but due to different driving factors, these patterns rarely overlapped. Our results offer novel understanding on the spatiotemporal variation and drivers of cool microclimates in topographically versatile boreal-tundra ecotone. We expect this information to be important in highlighting the significance of cool microclimates for the persistence of cold-adapted species under climate change across high-latitude ecosystems.