Another perspective on altitudinal limits of alpine timberlines

Tree Physiology 23, 1101–1112 © 2003 Heron Publishing—Victoria, Canada Another perspective on altitudinal limits of alpine timberlines† WILLIAM K. SMITH,1,2 MATTHEW J. GERMINO,3 THOMAS E. HANCOCK1 and DANIEL M. JOHNSON1 1 Department of Biology, Wake Forest University, Winston-Salem, NC 27109-7325, USA 2 Author to whom correspondence should be addressed () 3 Department of Biology, Idaho State University, Pocatello, ID 83209-8007, USA Received October 17, 2002; accepted March 8, 2003; published online October 1, 2003 Summary 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. Keywords: altitude, ecological facilitation, flagged trees, growth limitation, krummholz, photosynthesis, sapling growth, seedling establishment, treeline, water relations. Introduction For more than a century, the question of why forest trees are not found above certain altitudinal limits has remained unanswered. The question is particularly relevant today because of the potential sensitivity of the limit to climate change (Moir et al. 1999, Jobbagy and Jackson 2000, Paulsen et al. 2000, Rupp et al. 2001, Grace et al. 2002). In particular, the possible loss of alpine species as a result of the upward encroachment of subalpine forest under a global warming regime is an important concern (e.g., Messerli and Ives 1997, Luckman and Kavanagh 2000). In the broadest sense, an upper “timberline” is the highest elevation at which trees occur with a stature characteristic of trees found within the contiguous subalpine forest (Figure 1). In comparison, trees growing above this elevational limit are usually distorted structurally (e.g., krummholz mats, flagged, and stunted tree forms) and occur in diminishing numbers across a transitional region (ecotone) extending from the forest edge into the alpine tundra (Tranquillini 1979, Arno and Hammerly 1990, Stevens and Fox 1991, Körner 1999, Sveinbjornsson 2000). Likewise, the upper altitude limit above which trees no longer occur, even in severely stunted forms, is considered here to be the “treeline,” although a definition of treeline based on minimum tree height has also been used (e.g., Körner 1998). Between the timberline and treeline, the transitional ecotone may vary in width from tens to hundreds of meters, depending on the steepness of the gradient in biophysical and physicochemical factors. These altitudinal limits may also vary locally depending on wind exposure and the altitude of the perennial snowpack. Although seedling age and size vary greatly at timberline, the term seedling is used for trees 0–10 years old and the term sapling for trees older than about 10 years, but not sexually mature (e.g., cone forming). To understand the upper altitudinal limits of a particular timberline, the interactions of both abiotic and biotic factors must be considered. Abiotically, microsite and growth form differences that modify microclimate could raise the maximum altitude at which a particular tree might survive, and advance the † This paper was among those presented at the 17th North American Forest Biology Workshop “Rocky Mountain ecosystems: Diversity, complexity and interactions,” sponsored by the Tree Physiology and Forest Genetics working groups of the Society of American Foresters and held at Washington State University, Pullman, WA. 1102 SMITH, GERMINO, HANCOCK AND JOHNSON tion, life history, and physiological tolerance probably dictate establishment success in the treeline ecotone, as well as other boundary systems. Only abiotic factors influencing the upward migration of a timberline will be considered here. In addition, timberlines limited to a specific upper altitude primarily because of the steepness of the slope and absence of soil are excluded from this discussion. Review of recent hypotheses of timberline causation Figure 1. Photographs of the basic facilitation processes involved in sapling growth at the alpine treeline ecotone in the Snowy Range of southeastern Wyoming, USA. (Top panel) Needle mortality caused by snow abrasion, and desiccation of vertical leaders that extend above the snow surface (living shoots beneath snow cover have green needles). (Middle panel) Dead (brown) needles on vertical leaders shown after snow melt in early summer. (Bottom panel) Survival of vertical leaders from the leeward edge of krummholz mats that will become flagged trees (apical buds may survive on leaders with dead needles at leeward edge of mat). well-documented benefits of ecological facilitation over the negative impacts of competition (e.g., Egerton et al. 2000, Choler et al. 2001). A species microsite preference during the germination phase, structural facilitation from growth form modification, intraspecific and interspecific microsite facilita- Numerous studies have focused on correlative relationships between timberline altitudes and specific environmental parameters such as mean annual minimum temperatures. Körner (19 (...truncated)