Abstract
In several tropical and subtropical forests, plants of the understorey act as an ecological filter that differentially affects woody species regeneration. In convex sectors of the Schinopsis balansae (Anacardiaceae) forests of the Southeastern Chaco there are dense colonies of terrestrial bromeliads. These may influence forest regeneration by intercepting rain water and propagules in their tanks. Within colonies, the spatial distribution of bromeliads is clumped because their clonal growth leaves numerous internal gaps. In this study we describe the internal heterogeneity of three bromeliad colonies (plots) and analyze how this heterogeneity affects Acacia praecox regeneration (i.e. seedling recruitment and survival). In January 1996, we randomly placed three transects with 150 contiguous quadrats of 100 cm(2) in each plot. For each quadrat we recorded the type of floor cover (i.e. bromeliads, herbs, litter, or bare soil) and the presence of A. praecox seeds or seedlings. In July 1996 we relocated the transects and recorded seedling survival. Bromeliad colonies showed a high internal heterogeneity. Almost half of the 450 quadrats were covered by two terrestrial bromeliads. Aechmea distichantha was recorded in 81% of all quadrats with bromeliads, and Bromelia serra in the others. All quadrats with bromeliads were covered by litter. Half of them were occupied by the bases of bromeliads and the others were covered by their leaves. In contrast, where bromeliads were not present, soil surface was covered by litter in 83% and by herbaceous vegetation in 11% of the quadrats; very few quadrats were covered by bare soil. In January 1996, we recorded 127 seeds and 176 seedlings of A. praecox. Seed and seedling densities of A. praecox were similar in quadrats with and without bromeliads, but variability in seedling density of A. praecox was higher within than among plots. Seed density was higher in quadrats covered by bromeliad leaves than inside the tanks. Seedling survival of A. praecox was slightly higher in quadrats with bromeliads in only one of the three plots. No seedling survived inside the bromeliad tanks. Apparently. bromeliad colonies have no effect on seedling regeneration of A. praecox.
Highlights
Spatial patterns of woody species regeneration are often associated with environmental heterogeneity (Streng et al 1989, Svenning 1999, 2001)
As all newly emerged seedlings were Acacia praecox Gris. and the number of old seedlings was very low (i.e. 17 individuals belonging to 6 species), we only considered A. praecox for the analysis
About half of them were covered with bromeliad leaves (54%) and the remainders were occupied by their bases (46%; Fig. 1a)
Summary
Spatial patterns of woody species regeneration are often associated with environmental heterogeneity (Streng et al 1989, Svenning 1999, 2001). In several forests, dense plant colonies in the understorey affect woody species regeneration through competition for light, water, and soil nutrients, reduction of the physical space where to establish, as well as providing shelter for seed and seedling predators (Denslow et al 1991, Wada 1993, George and Bazzaz 1999a, b, Abe et al 2001). The spatial distribution of bromeliads is clumped due to their clonal growth leaving numerous internal gaps This finer heterogeneity may affect forest regeneration by allowing woody seedling establishment in these empty microsites between bromeliads. That study did not analyze whether the internal heterogeneity of the bromeliad colonies affected in different ways particular regeneration processes (e.g. seedling recruitment or survival). The objects of this paper are (1) to describe the internal heterogeneity of the bromeliad colonies of the ‘quebrachal’, and (2) to analyze how this heterogeneity may affect woody species regeneration (i.e. seedling recruitment and survival)
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