Neighborhood-scale phylogenetic relatedness affects sapling survival in the dominant tree family of a Neotropical lowland wet forest
Factors affecting seedling/sapling survival have profound consequences for the future composition of tree communities. One explanation for the maintenance of diversity in tropical forests is the Janzen-Connell effect, in which seedling mortality mediated by host-specific pests and pathogens inhibits seedling establishment under conspecific adults, thus preventing monospecific stands and maintaining community-wide diversity. Recent studies have suggested that the effect extends beyond conspecific-heterospecific; the presence of close relatives in the local neighborhood can also reduce seedling survival. We tested whether survival of saplings (50 cm – 5 m height) is also impacted by neighborhood-scale phylogenetic relatedness, and how strongly this effect compares to other forces influencing sapling survival (light level, stem damage, and basal area of surrounding trees). In a Costa Rican wet forest, Fabaceae saplings were marked, assessed for stem damage and light level, and mapped in 2013. In 2014 we measured and identified all trees ≥ 5 cm diameter in a 10 m radius surrounding each seedling (or its tag, if it had died). We calculated the average phylogenetic branchlength between each focal seedling and its neighbors and the basal area of trees in the 10 m radius. We used logistic regression to assess the strongest predictors of sapling survival.
We assessed 169 sapling neighborhoods, for a total of 4303 stems of 247 species. Between 2013-14, 29 focal saplings died. Light level (P = 0.12), stem damage (P = 0.45), and basal area of trees in the 10 m radius (P = 0.15) were not significant predictors of sapling death. Among the factors tested, the only significant predictor was phylogenetic relatedness of neighbors: saplings that survived were in neighborhoods with higher average branchlength (i.e., among less closely related neighbors; P = 0.013). Our results suggest that closely related neighbors can be detrimental to tree survival even beyond the seedling stage, and that negative biotic interactions with closely related neighbors can be a strong filter affecting local community composition. It is particularly noteworthy that we observed these negative impacts among Fabaceae, the dominant plant family in this forest, since most of the Fabaceae trees in this forest are N-fixers and could presumably provide benefits to neighbors through soil enrichment. Extending the study of the Janzen-Connell effect beyond conspecific-heterospecific interactions will provide greater insight into the mechanisms that maintain tropical tree species diversity.