Does habitat specialization by seedlings contribute to the high diversity of a lowland rain forest?


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1. One of the earliest hypotheses to explain high tropical forest diversity proposes that species are differentially specialized in their germination or growth and survival to particular habitats.

2. I examined evidence for habitat associations in seedling density and demography in 9 years of seedling dynamics data from Yasuní National Park, Ecuador, a lowland rain forest where previous studies have demonstrated habitat preferences among adult trees. I included 136 species or morphospecies from multiple annual seedling cohorts with known age of recruitment.

3. Approximately 90% of the species examined demonstrated negative or positive associations with one or more topographic habitats in their recruitment, growth and/or mortality at some point in the study, and approximately 60% of species had significant associations in at least half of the census periods studied. The survival of newly recruited seedlings varied among seedlings in response to topographic gradients, indicating the potential for species to partition habitat niches at a young stage.

4. There was significant inter-annual variation in seedling habitat associations, indicating the characteristics of the topographic niche important to seedling performance change through time. This variability alone can contribute to the maintenance of species diversity through storage effects. While associations may also be weak or ephemeral, the seedling dynamics for many species supported the possibility that associations seen in adult populations develop through differential mortality across habitats as seedlings.

5.Synthesis. That species’ seedlings perform differently among topographic habitats and that these differences are detectable very early on in a plant’s life indicate the potential for the abiotic environment to mediate or exaggerate the roles of other mechanisms in influencing the composition of the understorey seedling assemblage.