Substantial spatial heterogeneity in growing season dynamics across northern environments

Arctic tundra and boreal forest landscapes exhibit substantial microclimatic temperature variability, driven by strong topographic gradients, heterogeneous snowmelt patterns, and contrasting vegetation structures. However, this fine-scale variability is often poorly represented in reanalyses, climate models, and conventional weather station networks. In this study, we use a total of 347 microclimate loggers deployed across three regions in northern Fennoscandia during 2020–2024 to examine how the onset, end, duration, and degree days of the growing season vary across the landscapes. Our results show pronounced spatial variation in the onset of the growing season, particularly in the tundra environments, where the earliest and latest onsets differed by up to 40 days within the same season. In contrast, the end of the growing season often occurred abruptly at the three regions, likely driven by changes in large-scale weather conditions. Consequently, the spatial variability in growing season length was primarily driven by differences in its onset. Growing degree sums also varied markedly within areas, with differences of about 400°C days between the warmest and coldest places. The exceptionally warm summer of 2024 corresponded to conditions normally experienced several hundred kilometres further south, highlighting the unprecedented nature of this anomalous summer in northern ecosystems. The findings of this study demonstrate the value of dense microclimate observation networks that advance the understanding of microclimatic processes at ecological scales in cold ecosystems.