Tales from the past: remapping dynamic tree- and forest lines in response to changing climate and current land use

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Abstract
Average temperatures are rising more rapidly in high-latitude and alpine regions than elsewhere, leading to a gradual compression of the alpine bioclimatic zone due to the upward shift of the tree- and forest lines (TFLs). While most studies on TFL dynamics indicate advance, the rate at which regional and local treelines respond to climate warming remains uncertain. Furthermore, not every empirical TFL is determined by climate alone; edaphic conditions, species traits, and, in particular, land use, affect tree growth and distribution. In many regions, domestic grazing and other forms of traditional mountain summer farming have historically depressed forest lines. Previous research has been limited by the comparison of data sampled with mixed methods and poor temporal data coverage. Additionally, there is still a lack of studies accounting for time-lags, thus including data spanning a long time. In this study, we used consistently remapped in situ measurements of mountain birch (Betula pubescens ssp. czerepanovii) in Norway, dating from 50 to 130 years back, to: (1) document the rate of TFL change; (2) understand the impact of land use and climate change on regional TFL dynamics; and (3) discuss regional aspects and quality components of the data. We find that Norwegian TFLs are advancing at rates exceeding 0.5 m yr−1, primarily driven by climate change for TLs and land use for FLs. Still, the rates of TFL change vary considerably between regions, likely due to stochastic disturbances (e.g. snow avalanches, insect outbreaks, pests, landslides, rockfalls). We highlight the need for better quantification of time lags in treeline responses and for consistent definitions and methodologies when assessing long-term TFL dynamics in boreal–alpine ecotones.