Spatial patterns of tree recruitment in a relict population of Pinus uncinata:: forest expansion through stratified diffusion

Aim To infer future changes in the distribution of isolated relict tree populations at the limit of a species' geographical range, a deep understanding of the regeneration niche and the spatial pattern of tree recruitment is needed. Location A relict Pinus uncinata population located at the south-western limit of distribution of the species in the Iberian System of north-eastern Spain. Methods Pinus uncinata individuals were mapped within a 50 x 40-m plot, and their size, age and reproductive status were estimated. Data on seed dispersal were obtained from a seed-release experiment. The regeneration niche of the species was assessed based on the associations of seedling density with substrate and understorey cover. The spatial pattern of seedlings was described using point-pattern (Ripley's K) and surface-pattern (correlograms, Moran's I) analyses. Statistical and inverse modelling were used to characterize seedling clustering. Results Pine seedlings appeared aggregated in 6-m patches. Inverse modelling estimated a longer mean dispersal distance (27 m), which corresponded to the size of a large cluster along the north to north-eastward direction paralleled by an eastward trend of increasing seedling age. The two spatial scales of recruitment were related to two dispersal processes. The small-scale clustering of seedlings was due to local seed dispersal in open areas near the edge of Calluna vulgaris mats: the regeneration niche. The long-range expansion might be caused by less frequent medium-distance dispersal events due to the dominant north-westerly winds. Main conclusions To understand future range shifts of marginal tree populations, data on seed dispersal, regeneration niche and spatial pattern of recruitment at local scales should be obtained. The monitoring of understorey communities should be a priority in order to predict correctly shifts in tree species range in response to global warming.