Beta-diversity partitioning reveals high floristic complementarity among tropical dry forest remnants in the Cauca River Geographic Valley, Colombia

Background and aims – Tropical dry forests are among the most threatened ecosystems on Earth, and in the Cauca River Geographic Valley of Colombia only about 500 ha remain. Where two sites share few species, either each holds a different set of taxa (turnover) or the poorer is a reduced version of the richer (nestedness), and these two components call for opposite conservation responses. We asked which one governs this flora, whether the answer depends on the method, and which fragment attributes predict each component. Material and methods – We analysed the presence and absence of 915 vascular plant taxa across 35 fragments. Compositional difference was separated into replacement and richness-difference contributions under two frameworks. Each component was related to the fragment’s position, elevation and area by constrained ordination, and tested for an increase with the geographic distance. Fragments were also ranked by the taxa they add to a growing selection. Key results – Compositional difference was very high (multi-site Jaccard = 0.977). Replacement dominated under both frameworks, though its estimated share differed (98% and 60%). Constrained ordination showed position and elevation to structure replacement, whereas area was unrelated to it and predicted only the richness-difference component. Dissimilarity increased with distance (Mantel r = 0.238, p < 0.001), but no component carried that signal under both frameworks. Two fragments held 54% of the flora, 11 reached 90% and 31 out of 35 carried a complete representation of all taxa. Conclusion – These forest fragments are not impoverished copies of one another, but distinct assemblages, each holding taxa the others lack. The area governs how many taxa a fragment holds, not which ones, so protecting the largest fragment would capture only part of the flora. That the spatial signal could not be attributed to one component under both frameworks is a caution for a literature in which such attributions rest on a single partition.