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Las actividades antrópicas han modificado la vegetación en los cinturones subalpinos durante mucho tiempo, disminuyendo el ecotono del límite del bosque e influyendo en el paisaje principalmente a través del pastoreo y el fuego. El abandono de estas prácticas tradicionales de uso de la tierra ocurrido durante las últimas décadas y el calentamiento global están contribuyendo en gran medida a la colonización de especies leñosas en pastos subalpinos, causando cambios irreversibles en el funcionamiento de los ecosistemas. Actualmente se están llevando a cabo diferentes estrategias de gestión que requieren el uso de fuego y / o desbroce para prevenir la expansión de arbustos altamente invasores y mejorar el estado de conservación de los ecosistemas de pastizales subhúmedos de alta productividad. Sin embargo, todavía desconocemos cómo las diferentes estrategias de gestión afectan la recuperación de los pastos subalpinos, la cual depende a su vez de la comunidad vegetal involucrada. En este estudio, utilizamos experimentos de campo para evaluar el impacto de los tratamientos de manejo sobre las propiedades del suelo y las respuestas de la vegetación en las comunidades de pastos subalpinos de Bromion erecti colonizados por el arbusto pirófito Echinospartum horridum (Vahl) Rothm. en el sur de los Pirineos. La vegetación fue monitoreada por 5 años en parcelas de E. horridum (dos parcelas por tratamiento) donde la vegetación fue eliminada por fuego prescrito (tratamiento de Quemado) o por desbroce mecánico (tratamiento de Desbroce). Se utilizaron parcelas no perturbadas de E. horridum como control (C-Erizón) para los tratamientos de eliminación de arbustos y se usó una comunidad de pastos Bromion erecti regularmente pastada (C-Grass) como control de pastos subalpinos. El tratamiento de Desbroce de la comunidad de E. horridum fue más eficaz para controlar la colonización de E. horridum que el tratamiento de Quemado y contribuyó en mayor medida a recuperar la vegetación original de pastizales, porque las plántulas de E. horridum tuvieron un peor rendimiento (menores tasas de germinación) y las especies de plantas fueron más similar al pasto original (leguminosas, hierbas no leguminosas y pastos). La cobertura de arbustos y sub-arbustos en las áreas quemadas aumentó más rápido que en las áreas desbrozadas durante los 5 años posteriores al tratamiento, aunque no alcanzó el nivel de C-Erizón. La riqueza de especies y la diversidad aumentaron en comparación con C-Erizón en ambos tratamientos, pero fueron significativamente más bajos que los de C-Grass. La conectividad de la red fue mayor en los pastos bien conservados, es decir, C-Grass, que en cualquier otro tratamiento, principalmente dominado por la asociación negativa planta-planta. Los nutrientes del suelo disminuyeron en sitios quemados 4 años después del tratamiento con fuego y no se observaron diferencias entre los sitios desbrozados y C-Erizón, aunque no alcanzaron los valores de fertilidad del suelo de C-Grass en términos de materia orgánica y relación C/N. Este estudio mostró que el pastoreo favorece la diversidad de plantas y la complejidad de la comunidad en los pastos subalpinos. También demostró que el desbroce es mejor estrategia que la quema para restaurar los pastos tras la invasión de los arbustos porque la quema implica una degradación más profunda del suelo y una recuperación más rápida del arbusto pirófito, E. horridum. Anthropogenic activities have modified vegetation on subalpine belts for a long time, lowering the treeline ecotone and influencing the landscape mainly through grazing and fire. The abandonment of these traditional land use practices during the last decades and global warming are contributing largely to the colonization of woody species in subalpine grasslands, causing irreversible changes in ecosystem functioning. A variety of management strategies requiring the use of fire and/or clear-cutting are carried out to prevent the expansion of highly encroaching shrubs and improve the conservation status of subhumid high-productive grasslands ecosystems. However, it is still poorly understood how different management strategies affect the recovery of subalpine grasslands, which is influenced concurrently by the vegetation community involved. In this study we used field experiments to test the impact of management treatments on soil properties and vegetation responses in subalpine Bromion erecti grassland communities colonized by the pyrophyte shrub Echinospartum horridum (Vahl) Rothm. on the southern Pyrenees. Vegetation was monitored for 5 years in E. horridum plots (two plots per treatment) where the vegetation was removed by prescribed fire (Burnt treatment) or by mechanical removal (Clear-cut treatment). Undisturbed E. horridum plots were used as a control (C-Erizón) for shrub removal treatments and a Bromion erecti grassland community regularly grazed (C-Grass) was used as a control for subalpine grassland. Clear-cut treatment of E. horridum community was more effective to control E. horridum colonization than Burnt treatment and contributed to a higher extent to recover original grassland vegetation, because E. horridum seedling performed worse (lower germination rates) and plant species were more similar to the original grassland (legumes, non-legume forbs, and grasses). Shrubs and sub-shrubs cover in Burnt areas increased faster than in Clear-cut areas during the 5 years following the treatment, although it did not reach the level of C-Erizón. Species richness and diversity increased in comparison with C-Erizón in both treatments, but they were significantly lower than those in the C-Grass. Network connectivity was larger in well preserved grasslands, i.e, C-Grass, than in any other treatments, mainly dominated by negative plant-plant association. Soils nutrients declined in Burnt sites 4 years after the fire treatment and no difference was observed between Clear-cut and C-Erizón sites, although they did not reach the soil fertility values of C-Grass in terms of organic matter and C/N ratio. This study showed that grazing favors plant diversity and community complexity in subalpine grasslands. It also demonstrated that clearing is a better strategy than burning to restore grasslands after shrub encroachment because burning entails deeper soil degradation and faster recovering of the pyrophyte shrub, E. horridum.

Many studies have reported the phytotoxicity of allelopathic compounds under controlled conditions. However, more field studies are required to provide realistic evidences for the significance of allelopathic interference in natural communities. We conducted a 2-years field experiment in a semiarid plant community (NE Spain). Specifically, we planted juvenile individuals and sowed seeds of Salsola vermiculata L., Lygeum spartum L. and Artemisia herba-alba Asso. (three co-dominant species in the community) beneath adult individuals of the allelopathic shrub A. herba-alba, and assessed the growth, vitality, seed germination and seedling survival of those target species with and without the presence of chemical interference by the incorporation of activated carbon (AC) to the soil. In addition, juveniles and seeds of the same three target species were planted and sown beneath the canopy of adults of S. vermiculata (a shrub similar to A. herba-alba, but non-allelopathic) and in open bare soil to evaluate whether the allelopathic activity of A. herba-alba modulates the net outcome of its interactions with neighboring plants under contrasting abiotic stress conditions. We found that vitality of A. herba-alba juveniles was enhanced beneath A. herba-alba individuals when AC was present. Furthermore, we found that the interaction outcome in A. herba-alba microsite was neutral, whereas a positive outcome was found for S. vermiculata microsite, suggesting that allelopathy may limit the potential facilitative effects of the enhanced microclimatic conditions in A. herba-alba microsite. Yet, L. spartum juveniles were facilitated in A. herba-alba microsite. The interaction outcome in A. herba-alba microsite was positive under conditions of very high abiotic stress, indicating that facilitative interactions predominated over the interference of allelopathic plants under those conditions. These results highlight that laboratory studies can overestimate the significance of allelopathy in nature, and consequently, results obtained under controlled conditions should be interpreted carefully.

Vegetation spatial patterns emerge in response to feedback interactions between organisms and their environment, because of the redistribution of water and nutrients around the plant canopy or as a consequence of facilitation/competition interactions at the plant level, even in the absence of pre-existing substratum heterogeneities. It has been suggested that changes in vegetation spatial patterns are a signal of transition shift in ecosystems. Understanding the factors that lead to aggregated spatial patterns and control the transition to random distributions requires that environmental and species information is taken into account. In this study, we investigated the relative contributions of aridity (a long-term process), to which vegetation is adapted, and the area covered by bare soil (short-term process) to plant-plant associations and their contribution to aggregated spatial patterns. The study was conducted in a gradient of aridity ranging from that in subalpine grassland habitats in the Pyrenees and Sierra Nevada mountains to that in the semiarid steppes of Cabo de Gata and the middle Ebro Valley in Spain. We compared sites that differed in aridity and a geophysical feature (north- vs. south-facing slope). We observed that the relative contribution of aridity and bare soil to plant-plant facilitation and vegetation aggregation differed in subalpine and semiarid areas. Facilitation in subalpine habitats had a marked effect on aggregated spatial patterns, while aridity contributed to disruption of these patterns. Conversely, in semiarid habitats, the disruption of aggregated patterns was mainly promoted by an increase in bare soil area rather than in aridity. In semiarid habitats, the higher level of stress on south-facing slopes increased facilitation interactions relative to north-facing slopes, although this did not enhance the persistence of aggregated spatial patterns. We conclude that the use of aggregated spatial patterns as an indicator of ecosystem shift must distinguish and separately take account of long-term processes to which vegetation adapt, and short-term process.

In December 2012, the first Jornadas IPErinas took place at the Pyrenean Institute of Ecology (CSIC). The aim of this meeting was to show many of the new research topics that nowadays are being developed in the Institute. This workshop grew out of the need to disseminate the main new research lines developed at the center, as a fundamental part of scientific communication and knowledge transfer to society. In this paper, we briefly review the topics presented in the first meeting Jornadas IPErinas, summarizing the oral contributions., En diciembre de 2012, se celebraron las primeras Jornadas IPErinas, reunión que permitió compartir al personal del Instituto Pirenaico de Ecología (CSIC), muchos de los trabajos de investigación que actualmente se están llevando a cabo por los distintos grupos del centro. Estas Jornadas surgieron de la necesidad de compartir y difundir los principales objetivos y resultados más relevantes de nuevas líneas de investigación desarrolladas actualmente en el centro, como parte fundamental de la comunicación científica y transferencia de conocimiento a la sociedad. Bajo estas líneas, se presenta una reseña de la las primeras Jornadas IPErinas, a través de un breve resumen de las charlas presentadas.

A 109/084(39)

Carta remitida por D. José Zapater y Ugueda a D. Vicente Pueyo y Aviñó, comunicándole el envío de cierta comunicación.

Carta remitida por D. Vicente Pueyo y Aviñó a D. José Zapater y Ugeda, comunicándole el envío de un oficio del Asilo de Párvulos de San Jerónimo.