Another perspective on altitudinal limits of alpine timberlines

Abstract

Recent hypotheses of timberline causation include the possibility that limitations to growth processes may be more limiting than restrictions on photosynthetic carbon gain, and that cold soil is a primary limiting factor at high altitude. However, almost all of the supporting data for timberline causation have come from studies on older trees, with little focus on the mechanisms of seedling establishment and the growth of saplings away from the forest edge into the treeline ecotone. We describe a conceptual model of timberline migration that invokes a strong dependence on ecological facilitation, beginning with seed germination and continuing through seedling establishment and sapling growth to the stage where trees with forest-like stature form new subalpine forest at a higher altitude. In addition to protection from severe mechanical damage, facilitation of photosynthetic carbon gain and carbon processing is enhanced by plasticity in plant form and microsite preference, enabling seedling survival and sapling growth inside and through the often severe boundary layer just above the ground cover. Several forms of facilitation (inanimate, interspecific, intraspecific and structural) result in substantial increases in photosynthetic carbon gain throughout the summer growth period, leading to enhanced root growth, subsequent amelioration of drought stress, and increased seedling survival. Avoidance of low temperatures and low-temperature photoinhibition of photosynthesis may be major benefits of the facilitation, enhancing photosynthetic carbon gain and respiratory-driven growth processes. We propose that the growth of vertical stems (flagged tree forms) from krummholz mats is analogous functionally to the facilitated growth of a seedling/sapling in and away from ground cover. Increasing abundance and growth of newly established trees in the treeline ecotone generates a structural and microsite facilitation characteristic of the subalpine forest below. This is followed by the formation of new subalpine forest with forest-like trees, and a new timberline at higher altitude.

Feedback effects of clonal groups and tree clusters on site conditions at the treeline: implications for treeline dynamics

“…Germino et al 2002, Brodersen et al 2006, Maher & Germino 2006). In addition, out going long-wave radiation and nocturnal summer frosts are reduced (Germino & Smith 1999, Smith et al 2003, Johnson et al 2004. Winter snowpack on the ground, however, often lingers into summer, even on southern aspects (see also Elliott & Kipfmueller 2010, Elliott 2012, Elliott & Cowell 2015.…”

High solar radiation hinders tree regeneration above the alpine treeline in northern Ecuador

“…The topsoil of cleared plots warmed up more than that of other plots, but because of the good water retention capacity the pá ramo soil dried out only superficially. Still, because of the high water demand for transpirational cooling of exposed leaves, and because of the dryer air in the exposed plots, water stress and subsequent overheating may have negatively affected seedling performance (Smith et al 2003;Valladares and Pearcy 1997). Although our results clearly demonstrate that radiation causes damage and hinders tree regeneration, the exact mechanisms of radiation damage in the studied species definitely need further investigation.…”

“…At least at some temperate treelines, seedlings seem to benefit rather than suffer from surrounding alpine vegetation (Germino et al 2002;Maher et al 2005;Smith et al 2003), but for tropical treelines this balance is yet unknown. In the pá ramo, the alpine vegetation of the northern Andes, plant cover is often very dense and over half a meter high, and may therefore compete heavily with tree seedlings, although it could also offer protection.…”

Complex climate constraints of upper treeline formation in the Pyrenees

“…why they are located at certain elevations and which climatic factors determine the uppermost limit of tree existence (Smith et al 2003;Körner 1998). Nowadays, treeline formation is attributed to the influence of a minimum threshold of soil temperatures on root growth (Körner 2012).…”