Wind-Conditioned 20th Century Decline of Birch Treeline Vegetation in the Swedish Scandes

Abstract

ABSTRACT. The study focused on a frequent, although not dominant, mode of treeline change in the Swedish Scandes over the past century. Monitoring of stand density decline in a wind-exposed subalpine birch (Betula pubescens ssp. tortuosa) population was carried out over the past 30 years. The overall result included substantial and unbalanced individual mortality and a drastic reduction in stature of surviving birches. Radiocarbon dating of in situ subfossil birch-tree remains in an adjoining but windier site revealed complete stand demise over the past century or so. It is inferred that the extant and extinct birch populations represent different phases in an unbroken process of stand-level demise, hypothetically initiated in response to reindeer disturbance during the final phase of the Little Ice Age. This process was subsequently communicated more or less autogenically in a leeward direction as a consequence of the exposed and drought-prone nature of the site. Increasing westerly wind circulation and decreasing soil moisture (earlier snowmelt) during the past century may have contributed in this respect, although these aspects require further study. Within strongly wind-exposed terrain, a century of substantial warming has not sufficed to offset the birch decline. The results highlight the conclusion that under certain circumstances, wind and associated ecological and physiological effects are the overriding determinants of treeline position and structure. These are fundamental insights for generating realistic landscape-scale models of treeline change in a hypothetically warmer future.Key words: Betula pubescens ssp. tortuosa, deflation, drought, models, reindeer, retrogression, Swedish Scandes, treeline, warming, wind RÉSUMÉ. Cette étude porte sur un mode fréquent, bien que non dominant, de changement de la limite des arbres dans les Scandes suédoises au cours du siècle précédent. Durant les 30 dernières années, on a suivi le déclin dans la densité d'un peuplement de bouleaux pubescents subalpins (Betula pubescens ssp. tortuosa) qui étaient exposés au vent. Le résultat global comprenait une importante mortalité individuelle non compensée ainsi qu'une baisse radicale dans la taille des bouleaux survivants. La datation au radiocarbone de restes subfossiles de bouleaux trouvés in situ dans un lieu voisin mais plus venteux a révélé qu'un peuplement s'était complètement effondré au cours du siècle dernier ou à peu près. On en déduit que les peuplements de bouleaux existants et disparus représentent différentes phases d'un processus ininterrompu d'effondrement du niveau de peuplement, qui aurait son origine dans la perturbation causée par le renne durant l'ultime phase du petit âge glaciaire. Ce processus s'est ensuite poursuivi de façon plus ou moins autogène dans la direction du vent en raison de la nature du site qui était exposé au vent et à la sécheresse. Un accroissement de la circulation du vent d'ouest et une diminution de l'humidité du sol (fonte des neiges précoce) au cours du siècle dernier pourraient...

Selected Landscapes Under the Influence of Wild Herbivorous Mammals

“…Similar observations have been made in the southern Swedish Scandes (Kullman 2005 ) and in northern Norway (Evans 1995 ). There, large-scale landscape degradation combined with overgrazing by reindeer has caused a depression of the altitudinal belts and birch forest limit together with soil erosion (Löffl er 2005 ).…”

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 ).…”

“…Trees that can establish themselves beyond the current treeline are usually exposed to a much windier climate than at lower elevation (e.g., [3,49]), regardless of whether the treeline is natural or anthropogenic. Thus, the anthropogenic treeline is often climatically shaped like a natural climatic treeline [27,128].…”

Dynamics of alpine treelines: positive feedbacks and global, regional and local controls

“…Long-term developmental and structural changes occur in canopy architecture, leaf, stem and root morphology, and modifications of cell structure and biomechanical properties of the xylem. Especially, wind appears to play a role in many northern hemisphere treelines because of common desiccation of high-elevation trees (Scott et al 1993, Hätten-schwiler and Smith 1999, Sveinbjörnsson 2000, Kullman 2005, Richardson and Friedland 2009. Wind also plays a role in determining the tree heights and treeline positions at most of the world's treelines (Grace 1977, Sveinbjörns-son 2000.…”