| This research explores the little studied effects of adjacency arrangements on species composition in the understory of aspen woodlands. I define an adjacency arrangement as a focal patch (in this study an aspen woodland) with its adjacent elements (grassland and shrubland) and characterize the adjacency arrangement with the length of the common border with each adjacent element. Thirty aspen woodlands and 457 2-m2 plots within 11 aspen woodlands have been sampled for vegetational, environmental, and spatial data. The field data show that the number of shrubland species increases (and the number of grassland species decreases) in a woodland as the percent shrubland on the woodland border increases. The field data also show that percent shrubland, together with traditional resource and disturbance related factors (elevation, slope, and sheep and cattle grazing), best explains the understory composition. For measuring the adjacency effects on specific locations within an aspen woodland, I find the following four spatial variables crucial: distance to shrubland, distance to grassland, distance to the nearest edge, and angle to grassland. The field data show that at least one of the four spatial variables is significant for describing the distribution patterns of virtually all 41 tested species. Predictive vegetation models with these spatial variables perform significantly better than do models with only environmental variables for species richness and for nine out of 10 modeled species. Overall, the distribution patterns of the understory vegetation in aspen woodlands is strongly controlled by the distance to shrubland. Seed dispersal and vegetative reproduction may be the most important mechanisms for the majority of the observed adjacency effects. I conclude that adjacency effects emerge as a leading factor explaining species composition in patches with one or two adjacent elements. I argue that spatial relationships, and specifically adjacency effects, should become an integral part of vegetation models, together with factors of resource availability and disturbance history. Even before these models are widely available, landscape architects can gain important information, in addition to the information represented by the vegetation-units themselves, from adjacency arrangements of vegetation units mapped in vegetation maps. |
