Current ecological theory hypothesizes that boundaries between adjacent ecosystem units are important in determining ecosystem structure and function across heterogeneous landscapes, and that such boundaries are potentially important sites for early detection of global climate change effects. Hence, there is an increasing research effort to elucidate the structure and function of ecological boundaries. Yet traditional data analysis methods focus primarily on homogeneous units rather than on the boundaries between them; thus, new methods are being developed for detecting, characterizing and classifying boundaries, e.g., split moving—window boundary analysis (SMW). SMW is a simple yet sensitive method for locating discontinuities that may exist within multivariate, serial data (ordered in one dimension) at various scales relative to the length of the data series. However, SMW is subjective and relative, and therefore locates apparent discontinuities even within random, serial data. In this paper we present two nonparametric methods for determining the statistical significance of discontinuities detected by SMW. First, we describe a Monte Carlo method for determining the statistical significance of scale—dependent discontinuities (i.e., discontinuities that are significant relative to only one scale). Second, we propose a nonparametric, scale—independent method (it also is dependent upon scale size, but to a much lesser degree than the Monte Carlo method) that is more appropriate for locating statistically significant discontinuities that separate different, relatively homogeneous groups of varying size along a series. We examine the robustness of these two methods using computer—generated data having varying intensities of imposed discontinuities, and illustrate their application to locating boundaries between vegetation samples collected at systematic intervals across a desert landscape in southern New Mexico, USA.

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Abstract. We studied distribution patterns of vascular plant species and environmental variables for three years along a permanent transect traversing a closed‐drainage watershed in the northern Chihuahuan Desert of south‐central New Mexico, USA. The transect extended for 2.7 km from a basin floor playa (1310 m elevation, fine‐textured soil), across a piedmont slope, and onto the base of a granitic mountain (1410 m elevation, coarse‐textured soil). The gradients in elevation and soils across our transect, along with variable seasonal rainfall, downslope redistribution of water and organic matter, and soil texture‐related variation in infiltration, water holding capacity, and moisture release characteristics, interact to generate a complex spatial and temporal gradient of available soil water and nitrogen. We grouped plant species into guilds according to growth form and photo synthetic pathway type. These guilds are spatially and temporally differentiated along the transect such that particular groups utilize particular seasonal phases or spatial regions of the gradient. We identified six distinct plant communities along the transect. C4 perennial grasses dominated the mesic/high nitrogen portion of the gradient, which occurred at the highest (upper piedmont grassland, dominated by Bouteloua eriopoda) and lowest (playa, dominated by Panicum obtusum) elevations along the transect. C3 shrubs were dominant in the xeric/low nitrogen portion of the gradient located near the middle of the transect (bajada shrubland, dominated by Larrea tridentata). C3 shrubs also dominated a narrow zone of vegetation located adjacent to the playa (playa fringe, dominated by Prosopisglandulosa). C4 perennial grasses, C3 subshrubs, and C3 and C4 perennial forbs and annuals were co‐dominant in the intermediate locations along the gradient, which occurred below (mixed basin slopes) and above (lower piedmont grassland) the bajada shrubland. Life‐form distribution patterns at the small scale of our study reflect some of the patterning that occurs at larger scales in response to climate gradients. The distributions of some species and guilds along the transect are apparently modified by competitive interactions.