Spatial and Compositional Pattern of Alpine Treeline, Glacier National Park, Montana
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Spatial and Compositional Pattern of Alpine Treeline, Glacier National Park, Montana Thomas R. Allen and Stephen J. Walsh Abstract effects of climate change and indirect effects of changing dis- This study sought to quantify the complex patterns of alpine turbance regimes (Beaudoin, 1989; Kullman, 1990). Land- treeline across an extensive area of Glacier National Park, scape disturbances and their regime changes have been Montana. Satellite image classification, digital terrain model- shown to be evident and quantifiable in broader landscapes ing, and geographic information system (GIS) measurements (Rex and Malanson, 1990; Spies et al., 1994). Baker et al. of landscape structure provided important tools for the anal- (1995) applied aerial photography and GIs techniques to map ysis. The study area was topographically partitioned into wa- alpine treeline in the Colorado Front Range. Brown's (1992; tersheds and hillslope units in which to measure treeline 1994) predictive modeling of covertypes in Glacier National patterns. Cluster analysis of selected spatial and composi- Park found that climatic gradients alone could not accurately tional pattern metrics was used to infer major alpine treeline predict cover type distributions. Using techniques similar to forms. Six significant treeline types were differentiated using those reported here, Baker and Weisberg (1995) found and patch richness, contagion, contrast, number of patches, frac- classified spatial patterns of landscape structure in Rocky tal dimension, relative edge density, and forest-tundra juxta- Mountain National Park, Colorado. Their approach used aer- position. Clusters were validated using split-sample replica- ial photography and transect measures of treeline structure tion and discriminant analysis. Patterns were found to differ (Baker et al., 1995; Baker and Weisberg, 1995.) This study among types of terrain, affirming hypothesized sensitivities to presents a similar approach based on digital image process- topoclimatic gradients, natural disturbances, and geologic ing of Landsat Thematic Mapper (TM), terrain modeling, and substrate. landscape analysis in hillslopes. Comparative analysis of treelines in regional contexts should improve our under- standing of ecotone responses to global climatic change. Con- Introduction troversy over patterns that may be diagnostic of climatic The dynamics of ecotones, or boundaries between ecosys- control (Wardle, 1993) may be addressed with the use of tems and landscapes, are of particular interest to studies of quantitative measures of spatial pattern. Quantification of al- landscape ecology (e.g., Johnston and Bonde, 1989; Hansen pine treeline structure may generate data on the form and lo- and dicastri, 1992; Swanson et al., 1992). Alpine treeline, cation of monitoring sites of treeline response to climatic the ecotone separating subalpine forest and alpine tundra changes. Further, understanding the structure of alpine tree- ecosystems, has emerged as a highly studied landscape line will provide more information on the study of mountain boundary because of its potential sensitivity to climate habitats, biodiversity, and aesthetics. change (Hansen-Bristow and Ives, 1984; Hansen-Bristow et The basic intent of this research was to examine the spa- al., 1988; Armand, 1992; Walsh et al., 1994). Spatial metrics tial structure of the alpine treeline ecotone relative to possi- commonly applied in landscape ecology provide important ble biophysical controls. Specific research objectives were to tools to quantify the structure of the treeline ecotone and possibly separate climatic and other influences on its form Map the alpine treeline ecotone utilizing satellite image clas- sification of multidate Landsat Thematic Mapper data, (e.g., Baker et al., 1995). Treeline pattern must be examined Identify common forms of alpine treeline ecotone patterns in the context of potentially complex spatial variation in mi- through GIS measured ecotone patterns and statistical analy- croclimate, disturbances, resource availability, and biotic sis, and processes (Stevens and Fox, 1991; Slatyer and Noble, 1992; Integrate digital elevation models (DEMS) for landscape strati- Malanson and Butler, 1994), factors whose relative influences fication and develop environmental variables to explain tree- on treeline patterns have not been fully synthesized. There is line patterns. also an emerging need to critically evaluate measures of The central hypothesis holds that alpine treeline exhibits landscape structure (Hess, 1994), particularly in light of ef- a small number of distinguishable forms reflecting the influ- forts to understand the dynamics of landscapes and their ence of topographic gradients, disturbances, and geologic fac- boundaries. This study applies digital image processing and tors in the alpine environment, as has been observed in geographic information system (GIS) techniques to examine broader landscapes (Wickham and Norton, 1994). The alpine the landscape structure of the alpine treeline ecotone in Gla- treeline ecotone (ATE) refers to the boundary between sub- cier National Park (GNP), Montana. alpine and alpine ecosystems. This boundary may be ex- The alpine treeline ecotone provides a suitable land- pressed by patterns along a continuum from broad transition scape boundary for analysis of vegetation response to direct zone to a narrow, compressed boundary. "Patches" in this T.R. Allen was with the Department of Geography, Univer- sity of Vermont, Box 54170, Burlington, VT 05405-4170. He Photogrammetric Engineering & Remote Sensing, is presently with the Department of Political Science and Ge- Vol. 62, No. 11, November 1996, pp. 1261-1268. ography, Old Dominion University, Norfolk, VA 23529-0088, 0099-1112/96/6211-1261$3.00/0 S.J. Walsh is with the Department of Geography, University O 1996 American Society for Photogrammetry of North Carolina, Chapel Hill, NC 27599-3220. and Remote Sensing Continental Divide in the northern Rocky Mountains of northwest Montana at approximately 49"N latitude and 113"W longitude. The rugged, glaciated topography; situation between maritime and continental climates; relatively pris- tine environment; and floristic affinities to the middle and northern Rocky Mountains make GNP a highly suitable loca- tion for study of the ATE. The study area encompasses water- sheds along the eastern flank of the northern Rocky Moun- tains in eastern GNP (Figure 1).GNP exhibits steep topography associated with Pleistocene and limited Holocene valley glaciation. The climate is dominated by cold, snowy winters and brief, mild summers (Finklin, 1986). Elevations within the study area range from approximately 1,400 m to 3,000 m. The elevation zone between 1700 m and 2400 m, which captures climatic and elevationally depressed portions of alpine treeline, is the focus of the analysis. The entire study area is located east of the Continental Divide where the climate is harsher, having more severe lower tempera- tures, stronger winds, and drier conditions as compared to locations west of the Divide (Finklin, 1986). The ATE is dom- inated by subalpine fir (Abies lasiocarpa), whitebark pine (Pinus albicaulis) and/or limber pine (Pinus flexilis), and patches of Engelmann spruce (Picea engelmannil] (Arno and Hammerly, 1984). Treeline within GNP has not shown straightforward re- sponses to climate change since the Little Ice Age (Butler et al., 1994). Repeat terrestrial photography has found little re- cent change in treeline patterns, including snow avalanche- impacted ecotones (Butler et al., 1994). Debris flows (Gard- ner, 1980; Butler and Walsh, 1994), snow avalanches (Butler, 1979; Walsh et al., 1994), fire (Kessell, 1979; Johnson and Larsen, 1991), and animals (Butler, 1995) are documented disturbance agents impacting the ATE. GNP is representative of much of the northern Rocky Mountains and provides a suitable study area for modeling ATE sensitivities to climate, disturbance, and stresses. Heretofore, no straightforward pat- Figure 1. Study area location in eastern Glacier National tern or location for treeline advance in GNP has been dis- Park, Montana, including areas above 1,700 metres. Greys- cerned, a likely result of extreme topographic complexity cale display of Landsat TM band 4, 3 September 1990. and recurrent disturbances (Arno and Hammerly, 1984). Identification of ATE patterns could, thus, guide activities to monitor changes in response to climate. study refer to adjacent pixels of similar land-cover types, as opposed to individual patches of trees. Spatial patterns of Methods the ATE include varying degrees of complexity of patch shapes and sizes, the altitudinal breadth of the ecotone, and Image and Land-Cover Data the presence of interdigitating patches such as in ribbon for- Multidate Landsat TM digital data were used to map alpine land cover in the GNP study area. Images sensed on 28 Au- ests and snow glades. The ATE also exhibits compositional gust 1988 and 3 September 1990 were atmospherically and variation through the number of different cover types present (diversity) and the distribution of area among cover types geometrically corrected for integration into the GIS.Both im- ages showed clear conditions and apparently high radiomet- (evenness). Northern Rocky Mountain treeline patterns are ric quality. Atmospheric corrections were applied using the likely influenced by topographic complexity (Arno and Ham- histogram offset method. The 1988 TM image was georefer- merly, 1984), climatic gradients (Wardle,