Migration, invasion and decline: changes in recruitment and forest structure in a warming-linked shift of European beech forest in Catalonia (NE Spain) Josep Pen ˜ uelas, Roma ` Ogaya, Martı ´ Boada and Alistair S. Jump J. Pen ˜ uelas (josep.penuelas@uab.cat), R. Ogaya and A. S. Jump, Unitat Ecofisiologia CSIC-CREAF, CREAF (Center for Ecological Research and Forestry Applications), Edifici C, Univ. Auto`noma de Barcelona, ES-08193 Bellaterra, Catalonia, Spain. (Present address of A.S.J.: Environment Dept, Univ. of York, Heslington, York, YO10 5DD, UK.)  M. Boada, Dept de Geografia, Univ. Auto`noma de Barcelona, ES-08193 Bellaterra, Catalonia, Spain. Altitudinal upward shifts of species’ ranges have occurred across a wide range of taxonomic groups and geographical locations during the twentieth century in response to current climate warming. However, actual data of plant species’ altitudinal shifts are still scarce and not always clear. Here we provide a more detailed investigation of a previously reported European beech Fagus sylvatica forest altitudinal shift in the Montseny Mountains (Catalonia, NE Spain) now based on field photographic survey and on the population age structure and the recruitment patterns in the high Fagus limit (HFL), the central forest area (CFA) and the low Fagus limit (LFL). Monitoring of the lowest altitudinal range shows that beech forest is being progressively replaced by Mediterranean holm oak forest. Holm oaks are characterized by recruitment rates more than three times higher than those of beech in the LFL in the last decades. The percentage of young individuals in the LFL is only half that in the HFL and CFA. In the highest altitudinal range, present day and early 20th century photographs show that the HFL has gained density and has shifted altitudinally upwards, advancing with establishment of new, vigorous outpost trees (13 individuals per each 100 m of tree-line). They are mostly (89%) younger than 35 yr old and mostly (97%) located up to 70 m (with a few up to 105 m) ground surface distance above the current tree line (3651 m altitude) at the highest altitudes (16001700 m). The beech forest upward shift is a likely consequence of warming, but land-use practice changes (cessation of burning by shepherds) have made it possible. These changes in vegetation distribution and population structure constitute a new indication of the complex global change effects on life in mountain ecosystems. As a result of past climate warming events, plant species and biomes have shifted towards the poles or higher altitudes (Gates 1993). It is increasingly accepted that, across a wide range of taxonomic groups and geogra- phical locations, species’ distributions have changed during the twentieth century in response to current climate warming (IPCC 2001a, Walther et al. 2002). Numerous studies have reported shifts of species altitudinal ranges in response to recent warming (Wardle and Coleman 1992, Grabherr et al. 1994, Meshinev et al. 2000, Kullman 2001, 2002, 2003, Sturm et al. 2001, Lloyd and Fastie 2003, Pen ˜uelas and Boada 2003, Sanz-Elorza et al. 2003) although some studies failed to find such changes (Camarero and Gutie ´rrez 2004, Wang et al. 2006). Exact and detailed measurements to document altitudinal shifts in woody species, however, are rare. We recently reported an upward altitudinal shift of the temperate beech forest during the 20th century in response to climate and land use changes in the Montseny Mountains in Catalonia (NE Iberian Penin- sula) (Pen ˜uelas and Boada 2003). The European beech forests at medium and highest altitudes (8001700 m a.s.l.) of the Montseny Mountains constitute one of the ecotonic southernmost distribution areas of Fagus sylvatica (beech) forest in western Europe (Bolo `s Ecography 30: 829837, 2007 doi: 10.1111/j.2007.0906-7590.05247.x # 2007 The Authors. Journal compilation # 2007 Ecography Subject Editor: Francisco Pugnaire. Accepted 24 October 2007 829