Seedling recruitment factors in low-diversity Hawaiian wet forest: towards global comparisons among tropical forests

Recruitment limitations determine forest community regeneration patterns. Source limitation and dispersal limitation contribute to overall seed limitation, while environmental conditions and habitat associations influence establishment limitation. Several hypotheses have made contradictory predictions for how the relative importance of these limitations should vary with diversity. However, comparative data have not been available for low‐diversity tropical forests. We quantified recruitment limitations using 2.5 yrs of seed rain and seedling distribution data collected within a 4 ha forest dynamics plot in low‐diversity native‐dominated Hawaiian wet forest. We further quantified seedling irradiance and substrate habitat associations and niche overlap (using Pianka's niche overlap index). Additionally, we compared recruitment limitations and the frequency of seedling habitat associations across forests using the few available published data from sites employing similar field and analytical methods. In Hawaiian wet forest, seed dispersal more strongly limited recruitment than did establishment limitation across species, with 11 of 18 species completely seed limited (i.e., no seeds found). However, the relative importance of limitations varied greatly among species. For the three most abundant species, habitat conditions more strongly limited regeneration than did seed arrival, especially for the dominant canopy species, Metrosideros polymorpha, which was not seed limited. Most species were significantly associated with specific ranges of irradiance and/or substrates. Although habitat associations may indicate niche differentiation, Hawaiian species also showed significant niche overlap. Across the three forests compared, community‐wide mean seed and establishment limitation values were similar, despite wide variation in diversity. However, recruitment limitations differed strongly among species within forests due to species' life‐history differences. While seed limitation in Hawaiian forest was as high as in high‐diversity forests, mechanisms may differ; seed limitation in Hawaii may arise from loss of pollinators and dispersers rather than from a high proportion of rare species as occurs in high‐diversity forests. The strong habitat associations in Hawaiian forest relative to high‐diversity forests supported theoretical expectations that lower species diversity should increase the importance of habitat associations. However, these habitat associations were not linked to niche differentiation in Hawaii. Our findings suggest that high recruitment limitation may facilitate coexistence despite niche overlap in low‐diversity Hawaiian forest.

Inman-Narahari, Faith Ostertag, Rebecca Cordell, Susan Giardina, Christian P. Nelson-Kaula, Kehauwealani Sack, Lawren