Background: The alpine treeline ecotone is regarded as a sensor of the effects of global changes on alpine plant communities. However, little is known about how treeline dynamics influence the diversity and composition of alpine plant communities. Such information is necessary to forecast how ascending montane forests may affect the composition of alpine flora. Aims: We analysed the temporal variations in tree cover, plant diversity and composition, and the effect of tree cover dynamics on field layer vegetation over a period of 11 years, in two alpine treeline ecotones in the central Pyrenees, Spain. Methods: Tree and field layer vegetation was sampled in permanent transects in 1998 and 2009, using the point-intercept method. Temporal changes in tree cover, plant species richness and abundance were characterised along the ecotone by using a randomisation approach, rarefaction curves, and a non-parametric multivariate test, respectively. Results: Tree cover increased significantly at one of the sites, whereas plant species richness only increased at the other site where tree cover had not changed. Vegetation composition changed significantly at both sites, but it was not spatially coupled with changes in tree cover along the ecotone. Conclusions: A change of tree cover does not necessarily trigger changes in the ground flora at the treeline over relatively short periods (decade scale). The results challenge our ability to infer short-term biodiversity impacts from upslope advance of forests. Integrated tree and field layer monitoring approaches are necessary to produce a better understanding of the impact of ongoing global change on treeline ecotones.

We thank G. Sangüesa and J. Villellas for field assistance in 2009; J. Zabalza and P. Errea for technical assistance with Figure 1; L. Cayuela for comments on an earlier draft; J. Grace for suggestions and kindly improving the English; A. Hofgaard, L. Nagy and four anonymous referees for their useful comments and suggestions. We are also grateful to the staff of the Ordesa-Monte Perdido and Aigüestortes-Estany de Sant Maurici National Parks for permissions. Funding was provided by the Spanish Ministry of Science and Technology through a doctoral grant to IP (AP2008-01449), and projects (018/2008 of National Parks and CGL2010-21642 to MBG, and AMB95-0160 to EG). JJC thanks the support of the ARAID Foundation.

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22 páginas