Not every high‐latitude or high‐elevation forest edge is a treeline

Abstract

Attempts at identifying climate warming effects on mountain and arctic vegetation caused a recent hype in treeline studies. In this perspectives article, we recall the need of clear‐cut definitions, a consistent terminology and a theoretical framework that permits hypothesis testing. Founded in the ecological niche concept, the application of the fundamental niche edge to treeline permits defining the potential climatic limit of tree growth, while the realized niche edge captures all deviations for reasons related to other, more local, abiotic factors, biotic interactions, disturbances and human interventions. An important point is that a globally common phenomenon calls for a common abiotic driver which is the temperature at the low temperature edge of the niche of the life form tree. We explain why other abiotic factors that may affect the local range limits, such as microclimate, moisture and wind do not devaluate the classical isotherm concept. Our key message is that applying a clearly defined concept of potential treeline, also allows defining deviations from it and explaining the deviations within a reproducible theoretical framework.

Four ways to define the growing season

“…The very same 'stress' that reduces crop yield, determines the composition of a plant community, with the relief from such 'limitation' causing the community's extinction and subsequent replacement by another type of community. The century-long debate about 'treeline' was fueled by ill-founded definitions of the life-form 'tree', the notion of a 'line', and the confusion of the realized versus fundamental niche concept (Körner, 1998(Körner, , 2021bKörner & Hoch, 2023). Similarly, the term growing season covers a suite of potential definitions with very different meanings and functional consequences.…”

Introducing the concepts of range‐pinning and Allee effects to explain reduced temperature sensitivity of global treeline dynamics

“…theoretical) treeline positions (Figure 1a). While realized treelines refer to all elevational and latitudinal ecotones that can be found around the world, potential treelines describe the hypothetical upper or poleward niche edge of upright tree growth that closely follows a highly variable isotherm (Körner & Hoch, 2023). Potential and realized treeline positions are therefore almost never in equilibrium, and their current offset under rapid global warming can reach several hundreds of meters in elevation (Büntgen et al., 2022), with the realized treeline lagging the potential one.…”

“…Though helpful, this simple distinction between realized and potential treelines is not sufficient and should further separate realized treelines into undisturbed natural and suppressed artificial ecotones (Figure 1a). Since undisturbed natural and suppressed artificial treelines are usually characterized by different levels of species diversity, stand density and age structure, they are likely to exhibit slower and faster responses to recent warming, respectively. Despite strong evidence for a potential global treeline isotherm that reaches highest elevations near the equator and approaches sea‐level at high latitudes (Körner & Hoch, 2023), the mechanistic determinants of alpine and arctic distribution limits of upright tree growth remain puzzling. Although the interplay of biotic and abiotic factors and processes that may contribute to natural treeline positions around the world have been studied intensively, the various growth‐ and stress‐related hypotheses focus on ring formation in trees under relatively cold conditions.…”

Inclusions and exclusions in treeline definitions

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Körner and Hoch's (2023) definition of "treeline" to include only forest edges that are arctic or alpine, globally distributed and thermally limited is reconsidered for the alpine, with attention to the alpine treeline ecotone. They B I OS K E TCH George P. Malanson has pursued research on mountain vegetation since 1983 with a focus on treelines since 1990 but with major excursions into fire ecology, riparian systems and land use science.

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“…Recognizing other fundamental niche limited edges as treelines, if not alpine, and defining "alpine treeline" as a zone or line within the broader "alpine treeline ecotone", are proposed to promote synergies in research in these related systems. Körner and Hoch (2023) (hereafter, K&H) have provided a definition of "treeline" that can sharpen research questions by tightly circumscribing this biogeographic phenomenon. However, questions remain about the terminology of what they have excluded, and their fusion of "treeline ecotone" with "treeline" is problematic.…”