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

We describe the spatial structure of two contrasting subalpine Pinus uncina ta forest-alpine grassland ecotones located in the Central Pyrenees (Ordesa and Tesso sites) as a preliminary step to infer the processes that produce d their spatial patterns. All trees were mapped and measured within 4200 m( 2) rectangular plots parallel to the maximum slope and encompassing timberl ine 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 coordinat es. Then, surface pattern analyses (Moran and Mantel spatial correlograms) were used to quantify the spatial pattern of tree characteristics across th e ecotone (size, growth-form, estimated age). In the Ordesa site, krummholz individuals showed significant and positive spatial interaction with seedl ings. In this site, P. unicinata individuals evolved from shrubby to vertic al. growth-forms abruptly producing a steep spatial gradient. In the Tesso site, regeneration was concentrated near the treeline and the spatial gradi ent was gradual. Both ecotones formed similar to 45 m long zones of influen ce along the slope based on different variables. Wind and snow avalanches s eem to be the main controlling factors of the spatial pattern of trees in t he sites Ordesa and Tesso, respectively. Our results point out potential di fferent responses of treeline populations to environmental changes accordin g to the spatial pattern. (C) 2000 Elsevier Science B.V. All rights reserve d.