Friedrich-Karl Holtmeier

Treeline advance - driving processes and adverse factors

The general trend of climatically-driven treeline advance is modified by regional, local and temporal variations. Treelines will not advance in a closed front parallel to the shift of any isotherm to higher elevations and more northern latitudes. The effects of varying topography on site conditions and the after-effects of historical disturbances by natural and anthropogenic factors may override the effects of slightly higher average temperatures. Moreover, the varying treeline-forming species respond in different ways to a changing climate. Forest advance upwards and northwards primarily depends on successful regeneration and survival of young growth rather than on increasing growth rates of mature trees. Every assessment of treeline response to future climate change must consider the effects of local site conditions and feedbacks of increasing tree population in modulating the climatically-driven change. Treeline-shift will influence regional and local climates, pedogenesis, plant communities, animal populations and biodiversity as well as having a considerable effect on economic changes in primary production. A better understanding of the functional relationships between the many treeline-relevant factors and treeline dynamics can be achieved only by extensive research at different scales within different climatic regions supported by as many as possible experimental studies in the field together with laboratory and remote sensing techniques.

Wind as an Ecological Agent at Treelines in North America, the Alps, and the European Subarctic

The altitudinal treeline ecotone is a windy environment where wind velocities and directions are controlled by local mountain topography and also by the distribution pattern and structures of tree stands. Wind may override the role of heat deficiency in determining treeline position, spatial pattern, ecological conditions, and tree growth. Regular strong permanent winds restrict tree height and usually cause asymmetric and suppressed growth forms that are common in the treeline ecotone. Apart from direct physiological and mechanical effects on trees and ground vegetation, wind also disperses seeds, relocates snow, and locally erodes soils in the treeline ecotone. Wind effects must be considered an important factor that may delay or even preclude establishment of seedling trees on wind-swept terrain. Discussions of a potential climatically driven upward shift of the treeline at the landscape and smaller scales should give greater attention to the varying wind effects because warming cannot compensate for these other factors. The relative importance of microsite facilitation providing shelter from the wind will increase in parallel with the upslope migration of the tree limit into a much windier environment.

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.

Humus Forms in the Forest-Alpine Tundra Ecotone at Stillberg (Dischmatal, Switzerland): Spatial Heterogeneity and Classification

In the forest-alpine ecotone at Stillberg (Dischmatal/Switzerland) the morphology of humus forms and the spatial variability of organic layer properties were investigated. At northeast-exposed gully sites mulls with high acidity in the Ahorizon occur. They were classified after the Canadian classification of humus forms as Rhizomulls. Mors occur on ridges and on their east- and north-exposed aspects. They can be differentiated by the ratio between the thickness of the Fhorizon and the combined thickness of the F- and H-horizon. The relative thickness of the F-horizon increases significantly in the order: east aspects < ridges < north aspect. The humus forms of the east aspects and the ridges were classified as Humimors and those of the north aspects as Hemimors. The Canadian classification was suitable to describe the properties of the horizons and to classify the humus forms. The results of a grid sampling at the study sites and the computation of nonergodic correlograms show that the spatial variability of organic-layer thickness, bulk density, and moisture is high (CV around 50%), with a pronounced small-scale heterogeneity (range usually below 2 m and more than 50% variability occurs within 0.3 m). Only 33% of the variance of organic-layer thickness were explained by site and vegetation structure, but in spite of the low percentage both proved to be a significant factor. In the forest-alpine tundra ecotone about 30 to 35 soil samples per site are needed for a reliable estimation of the mean of the organic-layer thickness.

Mountain Birch Seedlings in the Treeline Ecotone, Subarctic Finland: Variation in Above- and Below-Ground Growth Depending on Microtopography

The treeline ecotone in northern Finnish Lapland is characterized by a mosaic of sites with highly varying environmental conditions. Density, age structure, growth, and root systems of mountain birch seedlings (Betula pubescens ssp. czerepanovii [Orlova] Hämet-Ahti) were studied in different microsite classes (deflation, lichen heath, dwarf shrub heath, hummock, willow shrub, sedge mire). On wind-exposed convex topography characterized by shortage of moisture and nutrients, seedling establishment is impeded, as is indicated by low seedling densities and lack of very young seedlings as well as by high rooting depths and root/shoot ratios of the few individuals. In sedge mires, birch seedlings occasionally occur in great numbers but die off at an early stage. Extremely shallow root systems point to anoxia as the main reason. Severe shoot damage is common to almost all saplings. This is likely due to grazing by reindeer (Rangifer t. tarandus). As a result, height growth is suppressed even on sites with otherwise relatively suitable conditions (e.g. willow shrub sites). Future effects of climate change which could locally improve conditions for germination and establishment of birch young growth might be overridden by the effects of high reindeer density.

Landform Influences on Treeline Patchiness and Dynamics in a Changing Climate

Local landforms and microtopography control site conditions, spatial patterns, and dynamics in treeline landscapes. Several topography-related treeline types are presented and their responses to a warming climate discussed. On rugged mountain terrain, pronounced changes in vertical range and variety of treeline landscapes will not take place as long as debris slides and avalanches occur regularly. On intensively eroded steep terrain, trees will mainly colonize convexities. On trough shoulders and similar gentle topography with irregular mosaics of convex and concave landforms, tree establishment is most likely to take place on convex topography. On gentle slopes and rolling uplands, where the proportion of wind-swept terrain is comparatively large, microsite facilitation appears to be a precondition for tree establishment. At higher elevations, the relative importance of shelter-providing landforms will increase due to windier conditions. At the beginning of treeline rise, seedling establishment, growth, and survival are closely related to local landforms and microtopography and their effects on site conditions. Later, the feedback from trees on their environment may overrule the effects of landforms.

Snowbeds on Silicate Rocks in the Upper Engadine (Central Alps, Switzerland)—Pedogenesis and Interactions among Soil, Vegetation, and Snow Cover

Abstract The pedogenesis and interactions among soil, vegetation, and snow cover of four alpine snowbeds on silicate rocks in the Upper Engadine (Central Alps, Switzerland) were investigated. The long-lasting snow cover of snowbeds causes differences in pedogenesis and soil properties compared to adjacent alpine sward. Because of the drainage characteristics of the silicate parent material, pedogenesis of snowbeds varies. On well-drained parent material, percolating meltwater favors podzolization during snowmelt. On less permeable parent material, meltwater causes temporarily water-logged conditions. Thus, the snowbed soils show redoximorphic features such as iron concretions. Snowbed soils are classified as Inceptisols or Entisols. A more detailed differentiation is only possible at the soil family level. Outside the snowbeds, moder humus forms (Rhizic Leptomoder, Rhizic Mullmoder) are common, whereas inside the snowbeds, mull humus forms (Rhizomull) as well as moder humus forms (Leptomoder) occur.

Impact of wild herbivorous mammals and birds on the altitudinal and northern treeline ecotones

Wild herbivorous mammals may damage treeline vegetation an cause soil erosion at a local scale. In many high mountain areas of Europe and North America, large numbers of red deer have become a threat to the maintenance of high-elevation forests and attempts to restore the climatic treeline. In northern Fennoscandia, overgrazing by reindeer in combination with mass outbreaks of the autumnal moth are influencing treeline dynamics. Moose are also increasingly involved damaging treeline forest. In the Alps, the re-introduction of ibex is causing local damage to subalpine forests and tree establishment above the forest limit as well as aggravating soil erosion. High-elevation forests and treeline in Europe are susceptible to the deleterious impact of wild ungulate populations because of former extensive pastoral use. Rodents may damage tree seedlings and saplings by girdling, root cutting, bark stripping and burrowing. Hares damage young trees by gnawing. Large numbers of small rodents may occasionally impede tree regeneration by depleting the seed sources. Rodents do not contribute to forest expansion beyond the current treeline. Among birds, nutcrackers are highly effective in influencing tree distribution patterns and treeline dynamics. Without the nutcracker caching of stone pine seeds any upward advance of the trees in response to climatic warming would be impossible. Some bird species such as black grouse, willow grouse and ptarmigan can impair tree growth by feeding on buds, catkins and fresh terminal shoots.

Treelines in a Changing Global Environment

Over the last century the global mean surface temperature has increased by about 0.6 °C and was most pronounced at high elevation and high latitude. Because the elevations and latitudes of treelines are strongly correlated with the occurrence of heat deficiency, climate warming is expected to generate denser forests below the treeline, as well as treeline movement to greater elevations and poleward. Herein, conclusions are presented about the future of treeline movement following a review of mechanisms and limiting factors for tree growth, differences between tall trees and low stature vegetation (including seedlings), and seedling establishment and growth to forest tree stature.

Physiognomic and Ecological Differentiation of Mountain Timberline

A mosaic of tree clumps, scattered groves, isolated, more or less deformed tree individuals and treeless patches covered by low shrubs, herbs, and grasses characterizes the timberline ecotone. It depends on the geographical position of a mountain range whether the treeless communities consist mainly of dwarf shrubs and grasses (e. g., European Alps), dwarf shrub-lichen heath (fjell, Fennoscandia), mountain steppe (arid zone) or tussock grassland (e. g., in the tropics, New Zealand). Which and how many tree species occur at timberline depends on the climate zone and on the history of floral development. If timberline is formed by more than one species, the different ecological properties and requirements of the species (e. g. shade tolerant and intolerant, pioneer or climax species, animal or wind mediated seed dispersal, etc.) may play an important role as to the development of the tree stands, and also in respect of structure, physiognomy and climatically induced shifts of timberline. Competitive species such as beech (Fagus sylvatica), Norway or Engelmann spruce (Picea abies, Picea engelmannii), for example, often form dense stands and abrupt timberlines, while in the case of less competitive species such as larch, most pines or juniper forests are comparatively open giving way gradually to grassland or other alpine vegetation (e. g., Walter, 1968; Armand, 1992; see also Photos 12 and 13). Some authors call such forests “open forests” (e. g., Kessler, 1995; Miehe et al., 1998).