Summary 1We mapped and identified all trees ≥ 10 mm in diameter in 25 ha of lowland wet forest in Amazonian Ecuador, and found 1104 morphospecies among 152 353 individuals. The largest number of species was mid-sized canopy trees with maximum height 10-20 m and understorey treelets with maximum height of 5-10 m. 2 Several species of understorey treelets in the genera Matisia and Rinorea dominated the forest numerically, while important canopy species were Iriartea deltoidea and Eschweilera coriacea . 3 We examined how species partition local topographic variation into niches, and how much this partitioning contributes to forest diversity. Evidence in favour of topographic niche-partitioning was found: similarity in species composition between ridge and valley quadrats was lower than similarity between two valley (or two ridge) quadrats, and 25% of the species had large abundance differences between valley and ridge-top. On the other hand, 25% of the species were generalists, with similar abundance on both valley and ridges, and half the species had only moderate abundance differences between valley and ridge. 4 Topographic niche-partitioning was not finely grained. There were no more than three distinct vegetation zones: valley, mid-slope, and upper-ridge, and the latter two differed only slightly in species composition. 5 Similarity in species composition declined with distance even within a topographic habitat, to about the same degree as it declined between habitats. This suggests patchiness not related to topographic variation, and possibly due to dispersal limitation. 6 We conclude that partitioning of topographic niches does make a contribution to the α -diversity of Amazonian trees, but only a minor one. It provides no explanation for the co-occurrence of hundreds of topographic generalists, nor for the hundreds of species with similar life-form appearing on a single ridge-top.

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Aim  To assess the effects of altitude and historic and recent forest fragmentation on the genetic diversity and structure of the wind‐pollinated tropical tree line species Polylepis incana. Location  One of the highest mountain forest regions of the world, located in the Eastern Cordillera of the Ecuadorian Andes. Methods  We compared genetic diversity and structure of adult trees with those of seedlings (n= 118 in both cases) in nine forest stands spanning an altitudinal gradient from 3500 to 4100 m a.s.l. using amplified fragment length polymorphisms (AFLPs). Genetic diversity was calculated as percentage of polymorphic bands (P) and Nei's expected heterozygosity (He); genetic differentiation was assessed using analysis of molecular variance, ΦST statistics and Bayesian cluster analysis. Results  Estimates of genetic diversity at the population level were significantly lower in seedlings than in adults. Genetic diversity (He‐value) was, in both cases, negatively correlated to altitude and positively correlated to population size in the seedlings. Genetic differentiation of the seedlings was approximately as high (φST= 0.298) as that of the adults (φST= 0.307), and geographical differentiation was clearly reflected in both AFLP profiles, with mountain ridges acting as barriers to gene flow. Main conclusions  Our study provides evidence of a historic upslope migration of P. incana in central Ecuador. In addition, it highlights the detrimental effects of unexpectedly strong genetic isolation, both recent and historical, particularly for our wind‐pollinated species where the distance between forest stands was less than 25 km. We therefore additionally propose that in habitats with pronounced high‐mountain landscape structures, gene flow may be hampered to such an extent that species have a more pronounced sensitivity to habitat fragmentation, even among populations of wind‐pollinated trees.

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