Mountain Treelines: A Roadmap for Research Orientation

Abstract

For over 100 years, mountain treelines have been the subject of varied research endeavors and remain a strong area of investigation. The purpose of this paper is to examine aspects of the epistemology of mountain treeline research-that is, to investigate how knowledge on treelines has been acquired and the changes in knowledge acquisition over time, through a review of fundamental questions and approaches. The questions treeline researchers have raised and continue to raise have undoubtedly directed the current state of knowledge. A continuing, fundamental emphasis has centered on seeking the general cause of mountain treelines, thus seeking an answer to the question, ''What causes treeline?'' with a primary emphasis on searching for ecophysiological mechanisms of low-temperature limitation for tree growth and regeneration. However, treeline research today also includes a rich literature that seeks local, landscape-scale causes of treelines and reasons why treelines vary so widely in three-dimensional patterns from one location to the next, and this approach and some of its consequences are elaborated here. In recent years, both lines of research have been motivated greatly by global climate change. Given the current state of knowledge, we propose that future research directions focused on a spatial approach should specifically address cross-scale hypotheses using statistics and simulations designed for nested hierarchies; these analyses will benefit from geographic extension of treeline research.

Treelines—Approaches at Different Scales

“…Figure 1) are key factors controlling spatial and temporal treeline structures at the landscape and finer scales, (e.g., [4,5,7,[48][49][50][51][52][53][54][55][56]). Microclimates, distribution of soil temperatures and soil moisture, relocation of snow, depth, and duration of the winter snowpack, avalanches, and runoff all depend on microtopography (length and width ≤ 10 m, area 10-100 m 2 ).…”

Do Himalayan treelines respond to recent climate change? An evaluation of sensitivity indicators

“…Both research lines have contributed significantly in recent years to answering the key question of treeline response to recent climate change. This response will not linearly follow altitudinal shifts of isotherms but rather vary according to the interaction of broad-scale controls (global/regional temperature) and fine-scale modulators of treeline patterns (Holtmeier and Broll, 2005, 2007Holtmeier, 2009;Malanson et al, 2011). It has thus become routine to take the scale dependency of drivers controlling treeline patterns into account (Malanson et al, 2007;Danby, 2011).…”

“…It has also to be emphasized that various treeline-forming species will have different growth and regeneration responses to a changing climate. In the long term, treelines will advance under continued global warming, but not in a closed front parallel to the shift of an isotherm (Holtmeier andBroll, 2005, 2007;Malanson et al, 2011;Körner, 2012a).…”

Warming-induced upslope advance of subalpine forest is severely limited by geomorphic processes

“…Changes in high-elevation tree cover will, thus, result from modifications on any of these controlling processes. Although topography and geomorphology have been identified as important in setting the observed heterogeneity of highelevation mountain tree cover (13)(14)(15)(16)(17)(18)(19), the effect of geomorphology on present and future high-elevation tree cover remains unquantified, and site-based studies overwhelmingly treat terrain physiognomy as a uniform neutral background. To address these questions, we conducted a statistical modeling exercise of tree presence at high spatial resolution (10 m) over a ∼100-km 2 area comprising the geologic and geomorphic diversity found in the Front Ranges of the Canadian Rocky Mountains of Alberta ( Fig.…”