Microclimate temperature variations from boreal forests to the tundra

Microclimate varies greatly over short horizontal and vertical distances, and timescales. This multi-level heterogeneity influences terrestrial biodiversity and ecosystem functions by determining the ambient environment where organisms live in. Fine-scale heterogeneity in microclimate temperatures is driven by local topography, land and water cover, snow, and soil characteristics. However, their relative influence over boreal and tundra biomes and in different seasons, has not been comprehensively quantified. Here, we aim to 1) quantify temperature variations measured at three heights: soil (-6 cm), near-surface (15 cm) and air (150 cm), and 2) determine the relative influence of the environmental variables in driving thermal variability. We measured temperature at 446 sites within seven focus areas covering large macroclimatic, topographic, and ecosystem gradients (tundra, mires, forests) of northern Europe. Our data, consisting of over 60 million temperature readings during the study period of 2019/11-2020/10, reveal substantial thermal variability within and across the focus areas. Near-surface temperatures in the tundra showed the greatest instantaneous differences within a given focus area (32.3°C) while the corresponding differences for soil temperatures ranged from 10.0°C (middle boreal forest) to 27.1°C (tundra). Instantaneous differences in wintertime air temperatures were the largest in the tundra (up to 25.6°C, median 4.2°C), while in summer the differences were largest in the southern boreal forest (13.1°C, median 4.8°C). Statistical analyses indicate that monthly-aggregated temperature variations in boreal forests are closely linked to water bodies, wetlands, and canopy cover, whereas in the tundra, variation was linked to elevation, topographic solar radiation, and snow cover. The results provide new understanding on the magnitude of microclimate temperature variability and its seasonal drivers and will help to project local impacts of climate change on boreal forest and tundra ecosystems.JA and HG acknowledges Academy of Finland Flagship funding (grant no. 337552), and JA and VT acknowledge the funding by the Faculty of Science, University of Helsinki (project MICROCLIM, grant no. 7510145). JK was funded by the Arctic Interactions at the University of Oulu and Academy of Finland (grant no. 318930, Profi 4). TR was funded by the Doctoral programme in Geosciences at the University of Helsinki. PN was funded by the Nessling Foundation and the Kone foundation. We want to thank all field work assistants and the staff of Värriö, Hyytiälä and Kilpisjärvi research stations. We acknowledge funding from Nordenskiöld samfundet, Tiina and Antti Herlin foundation, Oskar Öflunds stiftelse, and Maa- ja vesitekniikan tuki ry