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A Review of the Theories to Explain Arctic and Alpine Treelines Around the World†

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A Review of the Theories to Explain Arctic and Alpine Treelines Around the World† Article in press (2007), Journal of Sustainable Forestry [G.P. Berlyn Festschrift Special Issue, Volume 25, Issue 1-2] A review of the theories to explain Arctic and alpine treelines around the world† Andrew D. Richardson1* and Andrew J. Friedland2 ABSTRACT Forest growth is restricted at high latitudes and high elevations, and the limits of tree growth in these environments are dramatically marked by the treeline transition from vertical, erect tree stems to prostrate, stunted shrub forms. However, after four centuries of research, there is still debate over the precise mechanism that causes Arctic and alpine treelines. We review the various theories for treeline, including excessive light, low partial pressure of CO2, snow depth, wind exposure, reproductive failure, frost drought, and temperature. Some of these theories are very old and are no longer held in high esteem; while they may help to explain treeline physiognomy or local variation in treeline position, they generally fail as global explanations. Temperature- based theories appear to be the most reasonable, since cold temperature is really the only trait that is universally characteristic of treelines around the world. Temperature may limit a variety of physiological processes, such as carbon fixation, cuticular ripening, or new tissue development, and theories invoking these mechanisms are discussed. The vertical growth habit of trees is unfavorable to growth in this hostile environment: low- profile vegetation enjoys a far more favorable microenvironment for growth. Recent evidence gives strong support for a theory based on “sink limitation”, i.e., that new tissue development is restricted not by carbon availability but by cold treeline temperatures which limit cell division, and that this situation is exacerbated by arborescent growth (above-ground meristems coupled to cold ambient air temperatures) and self-shading (which keeps soil temperatures cold and restricts below-ground activity). KEYWORDS. Alpine, Arctic, biogeography, climate change, elevation, forest limit, Krummholz, latitude, montane forests, sink limitation, sub-alpine, treeline. 1Andrew Richardson is a Research Scientist at the University of New Hampshire, Complex Systems Research Center, Durham NH 03824. 2Andrew Friedland is a Professor and Chair of the Environmental Studies Program, Dartmouth College, 6182 Steele Hall, Hanover NH 03755. *Corresponding author. Mailing address: USDA Forest Service, 271 Mast Road, Durham NH 03824 USA. Tel: 603 868 7654, Fax: 603 862 0188, e-mail: [email protected] †This is a contribution to the Festschrift in honor of Professor Graeme Berlyn and his long career at the Yale School of Forestry and Environmental Studies. Our choice of topic is a tribute to Graeme’s lifelong interest in Arctic and alpine ecosystems, and his participation in numerous studies of the mountain environment (see Fig. 1). We thank Jim Kellner for helpful comments on the manuscript. THE CAUSES OF ALPINE AND ARCTIC TREELINES INTRODUCTION Treelines are phenomena that occur at the transition from forestland to some other type of vegetation, such as between forest and grassland, or at the margins of a swamp or bog: treelines are therefore a “tension zone” (Griggs, 1934). Causes of treeline can be varied, but include abiotic factors (e.g., temperature, salinity, drought, waterlogging, or soil nutrients), and disturbance, both natural (e.g., fire), and anthropogenic (e.g., timber harvesting, development or agriculture). [FIGURE 1 ABOUT HERE] One of nature’s most dramatic examples of treeline is the border marking the upper limit of forest growth, i.e., the alpine (high elevation) or Arctic (high latitude) treelines that will be the focus of this review paper (Fig. 1). These treelines may be sudden or indistinct (Fig. 2), they may be wavy or straight, and they may advance or recede over time. Here our goal is to review the theories, both historic and current, that explain alpine and Arctic treelines. We will limit our discussion to those treelines that are climate-driven, and ignore anomalies such as the grass balds of the southern Appalachians, for which there are other causes (Mark, 1958). [FIGURE 2 ABOUT HERE] Air cools as it rises and consequently the air temperature decreases at a rate intermediate between 0.50°C/100 m (the saturated adiabatic lapse rate) and 0.98°C/100 m (the dry adiabatic lapse rate). On average, the lapse rate is usually about 0.60°C/100 m increase in elevation (Barry, 1992; Körner, 1999; Richardson et al., 2004). Temperature also generally decreases with increasing latitude. This occurs because at higher latitudes the sun’s radiation is less direct. Richardson et al. (2003a) calculated that mean annual temperature in Alaska decreases by 1.15°C per degree increase in latitude, and based on these data, they concluded that a 1°C increase in latitude was roughly comparable to a 192 m increase in elevation. Humbolt’s Law, which dates to 1817, proposes that in terms of temperature, latitude compensates for altitude, and treeline thus occurs at a lower elevation as one moves from equator to Arctic (Salisbury and Ross, 1992). There are, however, differences between alpine and Arctic (Billings, 1973), and there are similar differences between sub-alpine and sub-Arctic. Generally the sub-alpine zone has greater light intensity and more UV, with less annual day length variation but more diurnal temperature variation, than the sub-Arctic. However, both sub-alpine and sub-Arctic frequently have short growing seasons, low temperatures and high winds (Berlyn, 1993). Low temperature is one of the few factors globally common to both. Many authors differentiate between Arctic and alpine treelines, Arctic treelines being associated with increasing latitude, and alpine treelines being associated with increasing altitude. However, in some of the world’s high-latitude mountain ranges, it is difficult to distinguish between the two. An example of this is the Richardson Mountains (Yukon-NWT border, 67°-70°N in Canada), where the forest limit is determined both by high latitude and altitude. In most cases, the distinction between Arctic and alpine is more easily made. The scientific study of treeline has a long history, as according to Troll (1973), it began in the Swiss Alps during the 16th and 17th centuries. Despite four centuries of research, there is not yet a consensus theory to explain Arctic and alpine treelines universally around the world. However, some explanations are more valid than others at a universal level, whereas some theories seem to provide good explanations for local THE CAUSES OF ALPINE AND ARCTIC TREELINES variation in treeline positions. The purpose of this paper is to assess the arguments both for and against the theories that have been popular in the past and those that are popular now. We will begin by offering some definitions of treeline and describing the physiognomy of different treelines around the world, and then proceed to an assessment of the different theories. In recent years, there has been a renewed interest in treeline research, spurred on by concerns about human-induced climate change and potentially dramatic changes in high-mountain vegetation patterns. A better understanding of the controls over treeline location will enable better predictions to be made about the effects of climate change on these climate-sensitive indicator ecosystems. DEFINITION OF TREELINE Different definitions of tree, forest, timber, and line complicate things somewhat. For example, although most authors agree that trees are erect, woody stems, Troll (1973) insists on a minimum height of 5 m, whereas Körner (1998) specifies 3 m, and Wardle (1974) settles for 2 m. These authors therefore distinguish between “trees” and the prostate, stunted “Krummholz” (from the German for “elfin wood”) found at the extreme limits of growth. Körner defined timberline to be the limit of closed forest, but noted that this transition is rarely abrupt: with increasing elevation or latitude, tree size generally decreases gradually while at the same time the canopy opens up. Wardle (1965) defined timberline to be the upper limit of tall, erect timber-sized trees. Treeline marks the highest patches of forest across slopes of similar aspect according to Körner (1998), but in Daubenmire’s (1954) nomenclature, the forest line is the upper edge of continuous forest and tree line is the elevation of the highest “tree”, whether Krummholz or not. We acknowledge that these terms all have somewhat different connotations. For the purpose of this paper, however, we wish to keep a broad view of treeline, for the delineation of any line or limit is inherently subjective. It is important to focus on the phenomenon of interest (rather than semantics), namely the vegetation change, be it gradual or sudden, from tall trees to prostrate shrubs that occurs at high elevations and high latitudes. We follow Wardle’s (1974) definitions, but will use the term treeline rather than Wardle’s tree limit. Thus: treeline is the extreme limit of trees and tall shrubs (more than 2 m in height). Trees growing closely together make a forest and therefore define the forest limit. In some places, forest limit and treeline are the same (e.g., most Nothofagus in New Zealand). In other systems, there may be a zone of parkland between forest limit and treeline, or the trees between the forest limit and treeline might be stunted and deformed Krummholz: in this case, treeline is the point where Krummholz with flagged stems changes to prostrate Krummholz. We consider vegetation immediately below the treeline to be sub-alpine (or, as the case may be, sub-Arctic) whereas vegetation above the treeline is alpine (or Arctic). GENERAL PATTERNS OF TREELINE Treeline elevations range from near sea level, as in northern Canada and Alaska, up to 4,700 m in Tibet and 5,000 m in the Andes of Bolivia and Chile (Troll, 1973). Treeline elevation generally increases as one moves from the poles to the equator, but there is a wide variation in treeline elevation at a given latitude.
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