[en] We describe the spatial pattern of a subalpine forest-alpine pasture ecotone in the Central Pyrenees, that includes altitudinal timberline and treeline, and it is dominated by Pinus uncinata Ram. A rectangular (30 x 140 m) plot was located crossing the ecotone with its longest side parallel to the slope. We measured for each P. uncinata individual inside the plot: location (coordinates x, y), and structure (e. g. height) and growth form variables (number and type —living or dead, vertical or shrubby— of stems per individual). P. uncinata individuals were classified according to their size (adults, poles, saplings and seedlings) and growth form (krummholz —shrubby and multistemmed individuals— and krummholz with vertical stems). We described quantitatively the type of substrate (bare soil, organic matter, gravel and rock) and cover of herbs, shrubs and P. uncinata using transects parallel to the slope. The ecotone structure was described through: (1) point pattern (Ripley's K) and (2) surface pattern analyses (spatial correlograms of height); (3) the detection and description of boundaries using density, size or growth form variables; (4) the synthesis of variations of presence and diversity of substrates and herbs and shrubs; and (5) the ordination of quadrats (the plot was previously subdivided into 115 6 x 6 m quadrats), according to their spatial position in the ecotone, the type of substrate, the cover of herbs and shrubs and the number, size and growth form of P. uncinata individuals. Most P. uncinata living individuals were krummholz, located above the timberline. Krummholz individuals showed significant and positive spatial interaction with seedlings. Bigger, vertical and unistemmed individuals predominated in the lower area of the ecotone, in the forest. The change of height with increasing elevation was abrupt and masked an underlying pattern of patches of trees with similar height in the forest. The structure variables were more sensitive because they produced a greater number of boundaries. These boundaries were arranged forming a "diagonal" (in the lower and upper areas of the ecotone for big and small individuals, respectively) because of the sequential location of progressively bigger unistemmed individuals descending across the ecotone. The shrubby individuals were associated with rocky substrates, that decreased in the forest, where organic matter predominated. The snow-wind interaction can explain the location of the studied timberline that could be considered a local phenomenon. Krummholz can buffer seedlings against the harsh climatic conditions of this ecotone (strong wind, reduced snowpack, low temperature). The spatial location of the different classes of individuals, the spatial interaction between seedlings and krummholz individuals, and changes of growth form (from shrubby to vertical growth form or vice versa) can cause some inertia in the response of ecotone P. uncinata populations to environmental changes.

[es] Describimos el patr&oacute;n espacial de un ecotono bosque subalpino-pastos alpinos, que incluye los l&iacute;mites altitudinales del bosque y del &aacute;rbol, dominado por Pinus uncinata Ram. y localizado en los Pirineos Centrales. Situamos una parcela rectangular (30 x 140 m) cruzando el ecotono con su lado mayor paralelo a la pendiente. Para cada individuo de P. uncinata dentro de la parcela medimos: su localizaci&oacute;n (coordenadas x, y), y variables estructurales (p. ej. altura) y de forma de crecimiento (n&uacute;mero y tipo &mdash;vivo o muerto, vertical o arbustivo&mdash; de pies por individuo). Los individuos vivos de P. uncinata se clasificaron seg&uacute;n su tama&ntilde;o (adultos, j&oacute;venes, v&aacute;stagos y pl&aacute;ntulas) y forma de crecimiento (krummholz &mdash;individuos arbustivos polic&oacute;rmicos&mdash;, krummholz con pies verticales). Describimos cuantitativamente el tipo de sustrato (suelo, materia org&aacute;nica, grava, roca) y la cobertura de herb&aacute;ceas, arbustos y P. uncinata usando transectos paralelos a la pendiente. La estructura del ecotono se describi&oacute; mediante: (a) los an&aacute;lisis del patr&oacute;n de puntos (K de Ripley) y (b) del patr&oacute;n de superficies (correlogramas espaciales de la altura); (c) la detecci&oacute;n y descripci&oacute;n de fronteras usando la densidad o variables de tama&ntilde;o y forma de crecimiento de los indidividos; (d) la s&iacute;ntesis de las variaciones en la presencia y diversidad de sustratos y de herb&aacute;ceas y arbustos; y (e) la ordenaci&oacute;n de los cuadrados en que subdividimos la parcela (115 cuadrados de 6 x 6 m), de acuerdo a su posici&oacute;n espacial en el ecotono, al tipo de sustrato, a la cobertura de herb&aacute;ceas y arbustos y al n&uacute;mero, tama&ntilde;o y forma de crecimiento de los individuos de P. uncinata. La mayor&iacute;a de los individuos vivos de P. uncinata eran krummholz, situ&aacute;ndose por encima del l&iacute;mite del bosque. Los individuos krummholz mostraron interacci&oacute;n espacial positiva con las pl&aacute;ntulas. En el &aacute;rea inferior del ecotono, los individuos grandes, verticales y unic&oacute;rmicos predominaban en el bosque. El cambio de altura al ascender era brusco y enmascaraba un patr&oacute;n subyacente de manchas de &aacute;rboles de altura similar dentro del bosque. Las variables estructurales fueron las m&aacute;s sensibles porque produjeron el mayor n&uacute;mero de fronteras. Estas fronteras se dispusieron en &ldquo;diagonal&rdquo; (en las partes inferior y superior del ecotono para los individuos grandes y peque&ntilde;os, respectivamente) debido a la situaci&oacute;n secuencial de individuos progresivamente mayores y unic&oacute;rmicos al descender por el ecotono. Los individuos arbustivos estaban asociados a sustratos rocosos, que disminu&iacute;an al entrar en el bosque, donde la materia org&aacute;nica predominaba. La interacci&oacute;n nieve-viento permite explicar la localizaci&oacute;n de este l&iacute;mite del bosque que puede considerarse como un fen&oacute;meno local. Los individuos krummholz pueden proteger a las pl&aacute;ntulas frente a las duras condiciones clim&aacute;ticas del ecotono (vientos fuertes, escaso espesor de nieve, bajas temperaturas). La situaci&oacute;n espacial de las distintas clases de individuos, la interacci&oacute;n espacial entre pl&aacute;ntulas y krummholz, y los cambios de forma de crecimiento (de forma arbustiva a vertical o viceversa) pueden provocar cierta inercia en la respuesta de las poblaciones de P. uncinata del ecotono frente a cambios ambientales.