Spatial pattern of subalpine forest-alpine grassland ecotones in the Spanish Central Pyrenees

We describe the spatial structure of two contrasting subalpine Pinus uncinata forest-alpine grassland ecotones located in the Central Pyrenees (Ordesa and Tesso sites) as a preliminary step to infer the processes that produced their spatial patterns. All trees were mapped and measured within 4200 m(2) rectangular plots parallel to the maximum slope and encompassing timberline and treeline. The spatial description of the ecotones was accomplished using several methodologies. Point pattern analysis (Ripley's K) was first used to quantify the spatial pattern of trees using each stem x-y coordinates. Then, surface pattern analyses (Moran and Mantel spatial correlograms) were used to quantify the spatial pattern of tree characteristics across the ecotone (size, growth-form, estimated age). In the Ordesa site, krummholz individuals showed significant and positive spatial interaction with seedlings. In this site, P. unicinata individuals evolved from shrubby to vertical. growth-forms abruptly producing a steep spatial gradient. In the Tesso site, regeneration was concentrated near the treeline and the spatial gradient was gradual. Both ecotones formed similar to 45 m long zones of influence along the slope based on different variables. Wind and snow avalanches seem to be the main controlling factors of the spatial pattern of trees in the sites Ordesa and Tesso, respectively. Our results point out potential different responses of treeline populations to environmental changes according to the spatial pattern. (C) 2000 Elsevier Science B.V. All rights reserved.