Seedling diversity and spatially related regenaration dynamics in holly woodlands and surrounding habitats

摘要

Spatialpatterns of seedling distribution and diversity were analysed in smallfragments of holly Ilex aquifolium L. woodlands and in their surrounding areas.Two sampling locations with similar structure were selected for this study:Oncala and Robregordo. They consist of nearly monospecific Ilex standssurrounded by grasslands with high scrub abundance.Theseedling appearance of woody species was quantified from March to November1998. Sampled areas were: 1) closed holly canopy; 2) open holly canopy or smallforest gaps; 3) holly woodland edge; 4) surrounding grassland; 5) underisolated fleshy-fruited shrubs scattered over the grassland; 6) underdry-fruited shrubs and 7) the closest forest to the holly woodland.Additionally, a pine forest at a distance of 20 km from Oncala was sampled. Inevery area ten permanent 50 × 50 cm quadrats were fixed for monthlyseedling control.Thehighest germination density occurs under the holly woodland, especially inclosed canopy areas. Nevertheless, these closed woodlands neither maintain agreat quantity of surviving seedlings nor a high diversity. Seedling density isconsiderable in canopy gaps, shrubs and forest edge, and these habitats havegreater diversity values than understorey habitats. Fleshy-fruited shrubs maintainhigher seedling densities and diversity than dry-fruited shrubs. Woodyseedlings are rare over the grassland. The three non-holly forests studied havevery similar seedling densities and diversity values, higher than those underclosed-canopy holly.Regionaldifferences are important for the numbers of seedlings surviving from previousyears, which are scareer in Robregordo. However, little difference is observedin spatial patterns of seedling diversity between the two locations.Wediscuss a number of processes affecting seed rain density and differentialmortality rates that could account for these spatial patterns, namelycompetition for light inside the woodlands, facilitation under nurse shrubs, orcompetition escape at the edge. Environmental factors such as moisture stressmay play a role in regional differences.