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of Influences Geomorphology and Geology on Alpine Treeline in the American West More Important Than Climatic Influences?
by David R. Butler* Department of Geography Texas State University-San Marcos San Marcos, TX 78666-4616 * corresponding author e-mail: email@example.com phone: 512-245-7977 fax : 512-245-8353
George P. Malanson Department of Geography University of Iowa Iowa City, IA 52242
Stephen J. Walsh Department of Geography University of North Carolina Chapel Hill, NC 27599-3220
Daniel B. Fagre U.S. Geological Survey Glacier Field Station Glacier National Park West Glacier, MT 58836
The spatial distribution and pattern of alpine treeline in the American West reflect the overarching influences of geological history, lithology and structure, and geomorphic processes and landforms. These influences occur at spatial scales ranging from the continental scale to fine scale processes and landforms at the slope scale. Past geomorphic influences, particularly Pleistocene glaciation, have also left their impact on treeline, and treelines across the west are still adjusting to post-Pleistocene conditions. Digging by animals illustrates a slope-scale set of processes that impact treeline and that should be examined in detail in future studies to facilitate a better understanding of where individual tree seedlings become established.
Key words: alpine treeline, geomorphic facilitation, geologic control, biogeomorphology, ecotone, Rocky Mountains, American West
Alpine treeline in the American West is the ecotone between coniferous forest at lower elevations and alpine tundra or bare bedrock above. Conventional wisdom in studies of alpine treeline in the west and elsewhere states that alpine treeline is primarily a result of some aspect of climatic or edaphic control (e.g., temperature, length of growing season, length of snow cover, soil temperature, or soil moisture, to mention some of the most commonly cited variables [see Holtmeier, 2003, for a magisterial coverage of climatic factors affecting treeline]). We assert here, however, that the impact of geology and geomorphology must first be examined as primary determinants of the location and pattern of alpine treeline, rather than climate. This assertion may be expressed most clearly as follows: climate is subservient to the role of geology and geomorphology across both spatial and temporal scales as a determinant of alpine treeline in the American West. This perspective on the importance of geomorphology and geology has also been mentioned by Holtmeier (2003) and Holtmeier and Broll (2005), although not to the extent that we assert here. They recognize three fundamental types of treeline: climatic, orographic, and anthropogenic (which we explicitly do not address here). Orographic treeline (Holtmeier  prefers the term timberline) is always located more or less far below the elevation to which the forest would advance at the given climatic conditions (Holtmeier (2003), p. 25), and is induced by steep rocky trough walls, talus cones, slope debris and avalanche chutes (p. 25). This perspective on the role of geomorphic conditions and disturbances goes partway to our perspective, but does not go
sufficiently far, because even climatic treelines illustrate, through the fine-scale patterns of seedling distribution, the role of geomorphic processes and landform patterns. Inherent geological conditions also strongly influence treeline locations and patterns, and also frequently trump climate as the primary causal control on those locations and patterns. Kruckeberg (2002) asserts in effect that all plant distributions are the result of geological control, and although we do not go that far in our perspective on the role of geomorphology and geology as treeline determinants, our position is closer to his than that of Holtmeier. As Wiegand et al. (2006, 880) noted, treeline features are not arbitrary but . there is a clear signal in the pattern which allows for inference of the underlying processes. We assert that those underlying processes are geomorphic and geologic.
Spatial and Temporal Scales at Treeline A consideration of scale is critical to our position on geomorphology and geology at treeline. Most studies of treeline have focused on the plants themselves rather than on the broader pattern. Even global scale analyses have been about global patterns of temperature influences on individual plant physiology (e.g., ). Malanson et al. (here) have emphasized multiple scale interactions, but primarily examine how affects at finer scales influence patterns at coarser scales, not vice versa. While we also consider finer scales, we take a somewhat inverted approach and consider the scale question from the perspective of what constraints coarser scale phenomena impose on finer scale process and pattern relations. The importance of coarse scale constraints can affect what happens to individual plants if one follows the constraints down the hierarchy.
In their examination of the Western Cordillera of North America, Malanson and Butler (2002) identified four spatial scales of significance for the study of mountains there the continental scale, the mountain range scale, a within-range scale, and a withinvalley scale. Here, we compress and slightly revise these into three scales of note: the continental-to-range scale, the within-range scale, and the slope scale, the latter term better encompassing the finer processes and landforms associated with local issues of alpine treeline. We examine each of these in sequence below, with specific examples of where treeline conditions are at least moderated, if not outright controlled, by geology and geomorphology. Issues of temporal rates of change at treeline are also addressed where appropriate. Our examples are taken from mountain ranges throughout the American West (Table 1), many of which serve as primary and secondary sites for the U.S. Geological Surveys Western Mountain Initiative program (Figure 1).
Continental-to-Range Scale At the geographic scale of an individual mountain range or larger, geology trumps climate as a primary control of treeline, because plate tectonics are responsible for the very presence of the mountains (Malanson and Butler, 2002). Kruckeberg (2002, p. 87) conjectures that (m)ajor weather systems are products of the mountain systems they encounter. From that perspective, all mountain weather and climate is a function of plate tectonics, and thus all treelines are controlled by geological histories associated with each respective mountain range. This perspective does, of course, ignore the postorogenic reality of varying climates within a mountain range; yet, even with varying climates at the within-range (or finer) scale, geology and geomorphology must be
considered as primary treeline determinants, as described below. The overall pattern of north-south trending ranges in North America, in line with the global latitudinal energy gradient and perpendicular to the west-east precipitation gradient, determines a continental gradient in treeline elevation but also in variation in glaciation coincident or not with current treeline, and so affects the degree of variation in landforms and geomorphic processes within ranges.
Within-Range Scale Within an individual range, the geologic structure and glacial history determines the spatial arrangement of major ridges and valleys (Malanson and Butler, 2002), and thus the spatial pattern of treeline. For example, on the West Spanish Peak in the Sangre de Cristo Range of Colorado (Figure 2), treeline is a function of the structure of the peak (an intrusive stock with radiating dikes) which, upon exhumation, has induced the development of a radial drainage pattern that profoundly affects the upper limits of treeline. The stream channels radiating from the rounded peak create geomorphically active sites where trees are unable to survive at elevations where adjacent ridgecrests are tree-covered. The isolated Cascade Range volcanoes are another example where the geologic structure of a stratovolcano is the primary determinant of subsequent erosion patterns, and subsequently of treeline. Lithology also serves as the primary determinant of treeline within some ranges. In the White Mountains on the Nevada-California border, the location and pattern of upper treeline is a function of the distribution of sandstone versus dolomitic limestone (Wright and Mooney, 1965; Kruckeberg, 2002) (Figure 3). On the sandstones of the
White Mountains, treeline is well below the climatic limits where trees may survive, but on the dolomitic sites a zone of bristlecone pine (Pinus longaeva) extend up to the climatic limits of tree growth (Figure 4). The dolomitic sites are coarse and nutrient poor, but bristlecone pine is able to survive in such harsh conditions, whereas other trees and sagebrush cannot. On adjacent areas of sandstone, bristlecone pine could survive but is out-competed there by other tree species and particularly by sagebrush (Wright and Mooney, 1965). Although not exclusive to treeline sites, Parkers work (1991, 1995) also illustrated the significant role of lithology in determining tree species distribution and density in the Sierra Nevada and southern Cascades of California. Disturbance treelines (sensu the orographic treelines of Holtmeier, 2003) at the within-range scale are also typically a function of underlying geologic control. In Glacier National Park, Montana, for example, the presence of the Lewis Overthrust fault along the eastern edge of the Park powerfully influences the location of the disturbance/orographic treeline produced by large-scale bergsturz (Figure 5) (Oelfke and Butler, 1985; Malanson and Butler, 2002). In the same area, Butler and Walsh (1990) and Butler and Malanson (1992) illustrated that lithology and geologic structure exerted primary control over the spatial distribution of snow-avalanche paths, themselves a spatially widespread form of orographic treeline in the Park. Butler and Walsh (1994) showed that debris flows in eastern Glacier National Park severely depress alpine treeline below a hypothetical climatic optimum, and Walsh et al. (1994) did the same for snowavalanche paths. Both forms of mass movement are widespread throughout the West, and act throughout the region as a geomorphic inhibiter of upward advance of treeline.
Within most mountain ranges in the American West, one must also note that current treeline, geomorphic processes, and soils are still adjusting to climatic change covering a variety of time scales, i.e., many ranges are in disequilibrium with current climatic conditions. The effects of the Little Ice Age are probably most prevalent at the slope scale, and are discussed below; however, at the within-range scale, many areas in the West are still adjusting to the geomorphic effects of the Pleistocene. In the Grand Tetons, for example (Figure 6), deep scouring by alpine glaciers descending to the adjacent valley floor removed surface soils and exposed bedrock that has yet to recover with sufficient soils for trees to ascend to elevations where nearby trees exist in areas of deeper soil. In such a situation, conditions are slowly improving and will allow eventual tree establishment and upward movement, but at a rate slower than at sites with better soils/edaphic conditions (Figure 7). Elsewhere, such as in parts of the Sierra Nevada and Glacier National Park, Montana, the intensity of Pleistocene glaciation produced glacial landforms such as cirque headwalls (Figure 8) that are simply not going to be invaded by trees within the foreseeable future regardless of whether the current climate would allow it (Figure 9). The current-day disturbance treeline (Butler and Walsh, 1994; Walsh et al., 1994; Butler et al., 2003) also acts to preclude upward advance of treeline under currently prevailing climatic conditions. Overall, landforms also differentiate the patterns that plants can develop within the ecotone because of differences in slope extents and angles at the right elevation range. Broad gentle slopes exist just above treeline in Rocky Mountain National Park, CO, where treeline is compressed on steeper slopes, in contrast to the east slopes of the Snowy Range in Wyoming where treeline is spread across a broad gentle slope below the main
peaks. The coincidence of current treeline elevation with landforms determined by past glaciation varies across all of western North America.
Slope Scale At first glance, the slope scale seems to best represent the influence of climate on treeline. Locations where the upper limit of tree growth approximates a straight line (Figure 10) suggest an upper altitudinal limit to tree growth imposed by climate (although even in these cases, geomorphic processes can impose a disturbance treeline upon a climatic treeline; Figure 11). In such cases, it is posited that seedling establishment above the general horizontal treeline represents a response to changing climatic conditions (e.g., Bekker, 2005; and Zeng et al., 2007, and references therein). In some areas in the West, however, the landscape is still adjusting to the end of the Little Ice Age, and upward advance of treeline has not occurred; instead, patches of krummholz have become denser and/or taller without upward movement (Butler et al., 1994; Klasner and Fagre, 2002). Such a situation certainly could be a function of a time lag, as suggested by Malanson (1999); however, it is also feasible that the geomorphic setting in the immediate surroundings of the patches is simply inimical for survival. Conditions immediately adjacent to the established patches were not colonized for a reason, whereas the patch was. We suggest that the conditions that were not appropriate for initial establishment were associated with geomorphic and geologic conditions deleterious to seedling establishment and survival. The issue of tree seedling establishment and survival at the slope scale has been addressed in previous research of the Mountain GeoDynamics Research Group (Allen
and Walsh, 1996; Malanson et al., 2002, 2007; Walsh et al., 2003; Butler et al., 2003, 2004; Resler et al., 2005; Resler, 2006; Zeng and Malanson, 2006; Zeng et al., 2007). This body of work has focused particularly on the role of fine-scale geomorphic processes and landform features as facilitators of tree seedling establishment and survival. This work has illustrated that, even in locations where climate at first glance seems to be the controlling factor at treeline, fine-scale geomorphic processes and landforms ultimately dictate where a tree seedling may become successfully established; or conversely, where a seedling will find geomorphically unfavorable conditions inimical to survival. Solifluction terraces are a relatively common landform found adjacent to treeline in many ranges in the American West (Figure 12) (e.g., Hansen-Bristow and Price, 1985; Caine, 2001; Walsh et al., 2003). Solifluction treads and risers are very difficult environments on which a tree seedling attempts to become established and survive; risers are comprised of dense tundra vegetation that is difficult if not impossible for seedlings to penetrate, and treads are rocky, barren, and highly susceptible to frost heaving (Butler et al., 2004). Soil conditions and quality vary little on the solifluction treads (Malanson et al., 2002), but in general are better with higher nutrient status than in adjacent krummholz (Malanson and Butler, 1994). On the solifluction surfaces, turf exfoliation (also known by the German term Rasenabschlung) facilitates tree seedling establishment by creating fine-textured, penetrable surfaces at the base of risers (Butler et al., 2004). These sites also offer topographic protection from wind and wind-driven ice, as do randomly distributed boulders across the tundra surface, where seedling survival is enhanced (Resler et al., 2005; Resler, 2006). At this fine scale, the geomorphic
conditions are what may serve as an accelerator that allows upward establishment and migration of the treeline (Figure 13). It is also interesting to note that the same geomorphic process (e.g., solifluction) may operate at a different scale in ranges across the West (Figure 12), but with the same general effect on treeline. Another slope-scale group of processes that may have profound localized effects on treeline alpine location and pattern is the work of digging by animals. Grizzly bears excavate widespread surfaces on slopes in the treeline ecotone (Figure 14) (Butler, 1992; Hall and Lamont, 2003), and these excavations produce higher concentrations of forms of nitrate than in adjacent, intact meadows (Tardiff and Stanford, 1998) that may benefit tree seedlings. Clarks nutcrackers (Nucifraga columbiana) bury whitebark pine (Pinus albicaulis) seeds throughout the West from the Sierra Nevada (Tomback, 1982) to northwest Montana (Resler and Tomback, in press), and their choice of burial locations determines where whitebark pine seeds sprout. Those choices may, in turn, benefit from localized protection (facilitation) offered by slope-scale geomorphic processes and geologic settings such as at the base of solifluction terraces, rock outcrops, or boulders (Figure 12). Numerous mammals also create widespread excavations at alpine treeline through the creation of a variety of burrow structures (Figure 15) (Butler, 1995; Hall and Lamont, 2003), and their digging activities may have pronounced effects on nitrogen and carbon in soils there (Aho et al., 1998). Virtually no research has been done at present, however, concerning whether or not these excavations aid or hinder tree seedling establishment and survival. Pocket gophers (Geomyidae) in particular have been examined for the role they play in affecting soils characteristics in alpine tundra adjacent
to treeline (Seastedt, 2001; Sherrod and Seastedt, 2001; Forbis et al., 2004; Sherrod et al., 2005). No work exists on the roles of gopher digging, gopher mounds, or gopherinduced changes in soil characteristics as facilitators of tree seedling success at alpine treeline. Gopher mounds in Georgia (Simkin and Michener, 2004) illustrated greater longevity of longleaf pine seedlings than in the surrounding matrix, although the difference was not statistically significant. Reichman and Seabloom (2002, 45) noted that mortality of seedlings tends to be very high on gopher mounds, because of exposure to herbivores and dry soil conditions, but they were not specifically addressing tree seedlings. They also pointed out that individual seedlings that survive on mounds are larger and produce more seeds than similar plants embedded in the surrounding plant matrix. Forbis et al. (2004) illustrated that recently created gopher mounds on Niwot Ridge, CO, had lower seedling emergence and survival rates than undisturbed areas in the first five years after mound creation, but also that the mounds had higher seedling emergence density from five years to at least 20 years after disturbance. The study by Forbis et al. (2004) was carried out in alpine tundra, however, and no tree seedlings were found in their seedlings that emerged on gopher mounds. Would mounds in the immediate treeline ecotone (Figure 15) show tree seedling emergence and survival? Schtz (2005) examined the question of whether gopher digging damaged young trees on Niwot Ridge, and found no evidence that pocket gophers are responsible for visible damage on juvenile conifers there. Given the potential significance of gopher mounds as a geomorphic facilitator or inhibitor at alpine treeline, we suggest that they merit greater examination.
Concluding Statements We are not dismissing the concept of climatic treeline here. Indeed, horizontal, roughly straight-line, treelines such as those illustrated in Figures 10 and 11 reflect a primary climatic control. Treelines such as these typically occur in areas of relatively uniform geologic conditions, particularly in areas of homogeneous granitic lithology such as typify parts of the Sierra Nevada and Colorado Rockies where Pleistocene glacial scouring was limited in extent and severity. Such treelines are diagrammatically represented by Figure 10c, where upward advancement of treeline is neither hindered nor facilitated by geomorphic or geologic conditions. We are, however, emphasizing that treelines throughout the American West are, in many more cases than traditionally recognized, strongly controlled by geological history, geologic structure, lithology, geomorphic processes, and landforms. Furthermore, the distribution of many of the landforms that influence the location and pattern of alpine treeline are themselves directly controlled by geologic structure and lithology. Disequilibrium conditions associated with the profound impacts of Pleistocene glaciation further illustrate the significance of geomorphic processes in controlling present-day treeline. Figure 9 summarizes the situation where geologic and/or geomorphic conditions utterly preclude upward migration of treeline, and Figure 7 does so for situations where upward migration is slow, inhibited by current or past (disequilibrium) conditions, and likely not to be geographically widespread. Even at the slope scale, where a first glance may reveal a relatively uniform, horizontal treeline, we urge a closer inspection. At this scale, the specific location of present-day seedlings is likely the result of fine-scale geomorphic facilitation (sensu Butler et al., 2004, and
Resler et al., 2005) that allows upward migration at an accelerated rate compared to the surrounding landscape (Figure 13). The fine-scale geomorphic effects of digging by animals may serve to facilitate or inhibit seedling establishment on treeline, and the jury is still out on which situation is more likely (or if both may occur under varying, sitespecific conditions). Future research is needed to expand our knowledge of the significance of animal excavations as facilitators or inhibitors of seedling success in the alpine treeline ecotone. The relations between geology and geomorphology and the ecological processes and patterns of the treeline ecotone will also change with changing climate in any one place. If a treeline elevation shifts, it may move into an area of different landforms at the within-range scale. Of equal interest will be how changes in climate can affect slopescale processes such as solifluction and animal activity, which would then change how plants respond to the climate change. In any case, interpretation of the scale-dependent influence of geology and geomorphology is necessary to understanding treeline pattern and process.
This paper is a Contribution of the Mountain GeoDynamics Research Group. Funding for fieldwork came from the USGS Biological Resources Division, Global Change Programs Western Mountain Initiative, through the office of Dan Fagre and the Glacier Field Station. The National Park Service provided research permits and access to Glacier, Rocky Mountain, Sequoia/Kings Canyon, Olympic, and North Cascades National Parks. Tim Seastedt organized and hosted an excellent tour of the INSTAAR facilities and landscape of Niwot Ridge, Colorado, and Nel Caine graciously shared his extensive geomorphological experience on Niwot Ridge with us. The field assistance of Dan (Cookie) Weiss, Zahao (The Wanderer) Shen, Darren (Darius, King of Persia) Grafius, and Will (Farm Boy Fred) Butler is gratefully acknowledged. Dan Weiss also produced Figure 1.
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Figure 7 (below)
E L E V A T I O N Elevation of treeline over time with climate warming
Geomorphic/geologic conditions encountered that decelerate upward migration of treeline Elevation threshold above which treeline would not likely advance without geomorphic/ geologic facilitation
Figure 9 (below)
E L E V A T I O N Elevation of treeline over time with climate warming
Geomorphic/geologic condition encountered that essentially precludes any upward advance of treeline, regardless of climate change
Figure 10 a (above), b (middle), and c (bottom)
E L E V A T I O N Elevation of treeline over time with climate warming
Continuing upward advance of treeline facilitated by geomorphic/geologic conditions Elevation threshold above which treeline would not likely advance without geomorphic/geologic facilitation
Figure 12 a (above) and b (below)
Geomorphic/geologic conditions encountered that accelerate upward migration of treeline E L E V A T I O N Elevation of treeline over time with climate warming Elevation threshold above which treeline would not likely advance without geomorphic/geologic facilitation
Figure 14 (a, above ; b, below)