Another perspective on altitudinal limits of alpine timberlines

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.