The Brocken tree limit: treeline, summit syndrome or forest advance?

Abstract Climatic treelines mark the low-temperature limit of upright tree growth, yet visible upper tree limits do not necessarily coincide with this thermal boundary. The treeless summit plateau of Mt. Brocken (1,141 m a.s.l.), northern Germany, has repeatedly been interpreted as a climatic treeline. However, visible upper tree limits may also reflect summit syndrome, i.e. exposure-driven suppression of tree stature near mountain summits, as well as historical land use or ongoing management rather than temperature limitation. Here, we reassess the Brocken tree limit using long-term climate records, root-zone temperature measurements, wind data, and recent growth observations of Picea abies . Growing-season temperatures consistently exceeded thermal thresholds commonly associated with global treelines, with mean root-zone temperatures around 8°C in 2025. Growing-season soil temperature well predicted air temperature at the monthly scale, supporting the use of soil temperature measurements as a proxy for thermal conditions at the tree limit: an approach of general utility for logger-based treeline assessment. Long-term air-temperature records likewise remained above commonly reported treeline thresholds and showed significant warming since the 1980s. Spruce individuals surrounding the plateau exhibited vigorous recent height growth, while juvenile trees were observed establishing directly on the plateau. We conclude that the Brocken plateau does not represent a classical temperature-limited treeline but rather an upper tree limit shaped by wind, snow and ice loading and anthropogenic influences (summit syndrome). As such, Mt. Brocken provides a useful model system for distinguishing climatic treelines from exposure- and management-maintained upper tree limits in low-elevation temperate mountains.