Vegetation patterns at the alpine treeline ecotone: the influence of tree cover on abrupt change in species composition of alpine communities Batllori, E. 1Ã ; Blanco-Moreno, J.M. 2,3 ; Ninot, J.M. 2,4 ; Gutie´rrez, E. 1,5 & Carrillo, E. 2,6 1 Departament d’Ecologia, Universitat de Barcelona, Avinguda diagonal 645, 08028 Barcelona, Spain; 2 Departament de Biologia Vegetal, Unitat de Bota `nica, Universitat de Barcelona, Avinguda Diagonal 645, 08028 Barcelona, Spain; 3 E-mail jmblanco@ub.edu; 4 E-mail jninot@ub.edu; 5 E-mail emgutierrez@ub.edu; 6 E-mail mcarrillo@ub.edu; Ã Corresponding author; E-mail enric.batllori@ub.edu Abstract Aims: The upper elevation limit of forest vegetation in mountain ranges (the alpine treeline ecotone) is expected to be highly sensitive to global change. Treeline shifts and/ or ecotone afforestation could cause fragmentation and loss of alpine habitat, and are expected to trigger con- siderable alterations in alpine vegetation. We performed an analysis of vegetation structure at the treeline ecotone to evaluate whether distribution of the tree population determines the spatial pattern of vegetation (species com- position and diversity) across the transition from subalpine forest to alpine vegetation. Location: Iberian eastern range of the Pyrenees. Methods: We studied 12 alpine Pinus uncinata treeline ecotones. Rectangular plots ranging from 940 to 1900 m 2 were placed along the forest-alpine vegetation transition, from closed forest to the treeless alpine area. To determine community structure and species distribution in the treeline ecotone, species variation along the forest-alpine vegetation transition was sampled using releve´ s of 0.5m 2 set every 2 m along the length of each plot. Fuzzy C-means clustering was performed to assess the transitional status of the releve´ s in terms of species composition. The relation of P. uncinata canopy cover to spatial pattern of vegetation was evaluated using continuous wavelet transform analysis. Results: Vegetation analyses revealed a large degree of uniformity of the subalpine forest between all treeline ecotone areas studied. In contrast, the vegetation mosaic found upslope displayed great variation between sites and was characterized by abrupt changes in plant community across the treeline ecotone. Plant richness and diversity significantly increased across the ecotone, but tree cover and diversity boundaries were not spatially coincident. Conclusions: Our results revealed that no intermediate communities, in terms of species composition, are present in the treeline ecotone. Ecotone vegetation reflected both bedrock type and fine-scale heterogeneity at ground level, thereby reinforcing the importance of microenvironmental conditions for alpine community composition. Tree cover did not appear to be the principal driver of alpine community changes across the treeline ecotone. Micro- environmental heterogeneity, together with effects of past climatic and land-use changes on ecotone vegetation, may weaken the expected correlation between species distribu- tion and vegetation structure. Keywords: Community structure; Ecotone; Pinus uncinata; Plant diversity; Pyrenees; Vegetation analysis; Wavelets. Plant nomenclature: Bolo` s et al. (2005). Introduction The Pyrenees, like other mountain systems, are characterized by altitudinal zonation formed by contrasting vegetation belts. In the upper parts of the Pyrenees, a subalpine (or highest forest) belt and an alpine (supraforest) belt have been traditionally distinguished (Braun-Blanquet 1948; Ninot et al. 2007). At higher altitudes, the potential upper limit of the forest – treeline ecotone – coincides, by defi- nition, with the boundary between the subalpine and alpine belts. Ecotones are defined as transition zones between adjacent ecological systems (Hansen et al. 1988; van der Maarel 1990), and two types have been differentiated, (1) ecotones in the strict sense, where strong fluctuations create contrasting environments that are individually relatively homo- geneous, and (2) ecoclines, where gradual differences in at least one major environmental factor allow transitional states. In this study, we use the term treeline ecotone (or treeline) in a broad sense to de- scribe the transition between the subalpine forest and alpine vegetation (Ko¨rner 1998; Holtmeier & Broll 2005). This transition occurs through an ele- vational gradient of increasingly adverse conditions Journal of Vegetation Science 20: 814–825, 2009 & 2009 International Association for Vegetation Science