Treeline form - a potential key to understanding treeline dynamics

Abstract

Aim Treelines occur globally within a narrow range of mean growing season temperatures, suggesting that low-temperature growth limitation determines the position of the treeline. However, treelines also exhibit features that indicate that other mechanisms, such as biomass loss not resulting in mortality (dieback) and mortality, determine treeline position and dynamics. Debate regarding the mechanisms controlling treeline position and dynamics may be resolved by identifying the mechanisms controlling prominent treeline spatial patterns (or 'form') such as the spatial structure of the transition from closed forest to the tree limit. Recent treeline studies world-wide have confirmed a close link between form and dynamics.Location The concepts presented refer to alpine treelines globally. MethodsIn this review, we describe how varying dominance of three general 'first-level' mechanisms (tree performance: growth limitation, seedling mortality and dieback) result in different treeline forms, what 'second-level' mechanisms (stresses, e.g. freezing damage, photoinhibition) may underlie these general mechanisms, and how they are modulated by interactions with neighbours ('third-level' mechanisms). This hierarchy of mechanisms should facilitate discussions about treeline formation and dynamics. ResultsWe distinguish four primary treeline forms: diffuse, abrupt, island and krummholz. Growth limitation is dominant only at the diffuse treeline, which is the form that has most frequently responded as expected to growing-season warming, whereas the other forms are controlled by dieback and seedling mortality and are relatively unresponsive. Main conclusionsTreeline form provides a means for explaining the current variability in treeline position and dynamics and for exploring the general mechanisms controlling the responses of treelines to climatic change. Form indicates the relative dependence of tree performance on various aspects of the external climate (especially summer warmth versus winter stressors) and on internal feedbacks, thus allowing inferences on the type as well as strength of climate-change responses.

Wood Microsites at Timberline-Alpine Meadow Borders: Implications for Conifer Seedling Regeneration and Alpine Meadow Conifer Invasion

“…Treeline form, considered diffuse, was characterized by gradually declining tree height and density with proximity to the tree limit (Harsch and Bader 2011). All of the sites were managed as wilderness areas.…”

“…Upward advance of the alpine treeline ecotone (ATE), the zone ranging from the upper limit of subalpine forest (forest line) to the treeline, is dependent on successful germination and survival of seedlings. Transitions from forest line to treeline can be abrupt or diffuse and include alpine meadows, krummholz, and tree islands-features associated with regeneration and upward advance potential (Harsch and Bader 2011).…”

“…Globally, soil temperature is associated with treeline dynamics , but lack of moisture, considered the primary cause of seedling mortality (Germino et al 2002;Moles and Westoby 2004), may also restrict seedling regeneration and limit forest upward advance (Gieger and Leuschner 2004;Maher et al 2005;Harsch and Bader 2011;Moyes et al 2015;M€ uller et al 2016). Although microsites, ranging from millimeters to meters in size, are known to have a key role in promoting seedling regeneration (Rochefort et al 1994;Holtmeier and Broll 2012), examinations of functional relationships between seeding survival and Mountain Research and Development (MRD) An international, peer-reviewed open access journal published by the International Mountain Society (IMS) www.mrd-journal.org MountainResearch Systems knowledge topographically controlled ATE site conditions are still rare (Holtmeier and Broll 2007;Harsch and Bader 2011;M€ uller et al 2016). Studies linking abiotic factors (ie temperature, radiation, moisture, and nutrient availability) to physiologic factors (eg stomatal conductance, water potential, and foliar nitrogen content) for young seedlings growing on microsites above the forest line provide a clearer understanding of current climatic factors affecting potential expansion of the ATE (Rochefort et al 1994;Smith et al 2009).…”

The Coupling of Treeline Elevation and Temperature is Mediated by Non-Thermal Factors on the Tibetan Plateau

“…Furthermore, little information is available about the impacts of non-temperature factors on treeline dynamics, even though climate warming and treeline dynamics often appear decoupled. This suggests that treeline shifts are partly determined by non-thermal variables including lagged treeline responses to climate, biotic interactions, or geomorphic constraints (e.g., [10][11][12][13][14]). …”

Recent expansion of Pinus nigra Arn. above the timberline in the central Apennines, Italy

“…In the last few decades, climate and land use changes have triggered treeline ecotone dynamics both at high latitude and altitude in several areas of the planet; however, not all have shown evidence of timberline advancement Broll 2005, 2007;Harsch et al 2009;Harsch and Bader 2011). Where forest tree recruitment has occurred on tundra or alpine grasslands the structure and function of these ecotones are changing.…”