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Seedling establishment is a critical step for the regeneration of plant populations, both because of the commonly high mortality rates during this stage and because of the potential to alter and fix the spatial and temporal patterns of recruitment (Harper 1977; Silvertown & Lovett-Doust 1993; Clark et al. 1999). Seeds are dispersed to a variety of microhabitats that constitute the starting environment for all subsequent recruitment stages (Harper 1977; Schupp 1995). This provides a range of differences in biotic and abiotic conditions affecting seedling emergence, survival or growth, hence influencing establishment success (Collins 1990; Schupp 1995; Castro et al. 2002a).
In plant species with a wide distribution, different populations are usually subjected to different biotic and abiotic conditions, and therefore to environments with contrasting limitations on seedling establishment. This is the case of boreo-alpine species in the northern hemisphere. Their range has undergone continuous latitudinal and altitudinal changes during glacial-interglacial cycles of the Pleistocene. They had a wider distribution more southerly at the peak of the last glacial period than they do in the current interglacial (Peñalba 1994; Bennet 1997; Taberlet et al. 1998). Their main area is now in the northern part of the continents, persisting in the south only in microclimatic islands within a region whose climate is generally adverse, often in refuges at high altitude. Given the impact of climate on establishment success (e.g. Milton 1995; Marone et al. 2000), factors controlling seedling recruitment of these species may differ sharply in contrasting areas of their geographical range.
In Europe, the Scots pine, Pinus sylvestris L., is distributed mainly in central and northern parts, and, in the south, is restricted to the high mountains of the Mediterranean basin (Ceballos & Ruiz de la Torre 1971; Boratynski 1991). Here, it persists as isolated nuclei facing ecological conditions very different from those of the main distribution area and from those considered to be optimum (Ceballos & Ruiz de la Torre 1971; Catalán 1991; Hódar et al. 2003). Most information concerning the ecology of Scots pine seedling establishment comes from central and northern Europe, where the main abiotic constraints for establishment are low temperatures (James et al. 1994; Ryyppöet al. 1998; Domisch et al. 2002), and the major biotic factors are invertebrate herbivores (Vaartaja 1950; Hagner & Jonsson 1995; Nystrand & Granström 1997, 2000), pathogens (Burdon et al. 1994), or interference with existing vegetation (e.g. Kuuluvainen & Juntunen 1998; Nilsson et al. 2000; Valkonen et al. 2002). The interactions between these factors exerts a powerful impact on the spatial pattern of recruitment, concentrating regeneration in some microhabitats and preventing seedling establishment in others (Burdon et al. 1994; Steijlen et al. 1995; Zackrisson et al. 1995, 1997). Factors controlling seedling establishment of Scots pine in Mediterranean areas, however, are largely unknown, despite the relevance that this may have for forest regeneration.
Contrary to the situation in central and northern Europe, the main abiotic factor constraining establishment of woody species in the Mediterranean region, as well as in Mediterranean-type ecosystems in general, is usually summer drought (Dunne & Parker 1999; Rey & Alcántara 2000; Castro et al. 2002a,b). Similarly, biotic factors influencing seedling establishment in the Mediterranean region may also differ from those of more mesic, northern areas. For instance, in temperate environments, the typically dense vegetation cover often competes with establishing seedlings (e.g. Lorimer et al. 1994; Kolb & Robberecht 1996; Buckley et al. 1998). However, in drought-stressed Mediterranean ecosystems, the presence of vegetation may protect establishing seedlings against high radiation, high temperatures and losses of soil moisture, thereby increasing survival (Callaway 1995; Castro et al. 2002a; Gómez et al. 2004). Knowledge of the factors determining seedling establishment of Scots pine in southern areas could thus aid the understanding of the patterns and processes involved in the recruitment of plant populations growing in contrasting ecological habitats.
This study explores the factors controlling establishment of Scots pine in Sierra Nevada and Sierra de Baza (south-east Spain). These two mountain ranges, some 80 km apart, constitute the southernmost geographical limit of Scots pine, and are separated from the nearest population (in central Spain) by c. 500 km (Ceballos & Ruiz de la Torre 1971; Boratynski 1991). The relict, isolated populations have also experienced decline in recent centuries due to human activity (Willkomm 1882; Voigt 1889; Blanca et al. 1998; Hódar et al. 2003), resulting in a mosaic of woodland interspersed with areas of successional shrublands (Catalán 1991; Castro 2000). Our aim is to determine the spatial pattern of seedling recruitment and its consequences for forest regeneration. Our study was performed in woodland stands as well as in adjacent shrublands into which the forest could expand. We monitored seedling survival, growth and cause of mortality in the field, and performed sowing experiments. Given the impact of habitat structure on seedling recruitment, we also considered the performance of seeds and seedlings in the most common microhabitats where seeds are found after dispersal, i.e. areas of bare soil, under the canopy of abundant shrubby species and under the canopy of adult pines. The differences in environmental conditions suggest that factors controlling seedling establishment in southern areas will differ from those in northern and Fennoscandian areas, and that, as a result, the mechanisms underlying the spatial pattern of recruitment will also differ. We predicted that summer drought would be the main abiotic factor constraining seedling survival in the Mediterranean mountain and that existing vegetation would protect seedlings against such stress, resulting in differences in microhabitat suitability for establishment. Juveniles were therefore expected to be associated with microhabitats providing this nurse effect. We asked:
What is the magnitude of population losses during seedling emergence and establishment in the different microhabitats?
What are the biotic and abiotic agents that cause seedling mortality of Scots pine at its southern geographical limit?
What is the effect of the stand type (woodland vs. successional shrublands) and microhabitat type on seedling recruitment and performance?
Are the patterns of seedling survival and performance coupled?
What is the resulting spatial pattern of seedling recruitment and how does this affect forest regeneration under Mediterranean drought-stressed ecosystems compared with more mesic northern and Fennoscandian areas?
Does the pattern of seedling establishment translate to the pattern of juvenile distribution?