Neighbourhood density and genetic relatedness interact to determine fruit set and abortion rates in a continuous tropical tree population

Abstract

Tropical trees may show positive density dependence in fruit set and maturation due to pollen limitation in low-density populations. However, pollen from closely related individuals in the local neighbourhood might reduce fruit set or increase fruit abortion in self-incompatible tree species. We investigated the role of neighbourhood density and genetic relatedness on individual fruit set and abortion in the neotropical tree Jacaranda copaia in a large forest plot in central Panama. Using nested neighbourhood models, we found a strong positive effect of increased conspecific density on fruit set and maturation. However, high neighbourhood genetic relatedness interacted with density to reduce total fruit set and increase the proportion of aborted fruit. Our results imply a fitness advantage for individuals growing in high densities as measured by fruit set, but realized fruit set is lowered by increased neighbourhood relatedness. We hypothesize that the mechanism involved is increased visitation by density-dependent invertebrate pollinators in high-density populations, which increases pollen quantity and carry-over and increases fruit set and maturation, coupled with self-incompatibility at early and late stages due to biparental inbreeding that lowers fruit set and increases fruit abortion. Implications for the reproductive ecology and conservation of tropical tree communities in continuous and fragmented habitats are discussed.

Pollen flow and effects of population structure on selfing rates and female and male reproductive success in fragmented Magnolia stellata populations

“…Second, local competition for pollinators among neighboring plants could explain the negative effect of local density on seed production. Although it is unusual for local plant density to be negatively correlated with female reproductive success [61,62], there are a few known examples of large plant aggregations increasing competition for pollinators [15,63]. In spite of the occurrence of pollen dispersal over several hundred meters, the leptokurtic dispersal curve indicates that most pollination events occurred within about 50 m, corresponding to the scale of the local genet density (Figure 2).…”

Genetic consequences of subtropical rainforest fragmentation onMacadamia tetraphylla(Proteaceae)

“…When flowering trees are clumped, pollinators tend to follow nearneighbour behaviour, whereas when trees are broadly spaced the insect is more likely to miss the next closest tree in a fragment and simply fly to the next fragment (STACY et al, 1996). Such processes may influence fruit set (JONES and COMITA, 2008) and ultimately affect tree population dynamics.…”

Unravelling conflicting density‐ and distance‐dependent effects on plant reproduction using a spatially explicit approach

“…Ishihama et al . ; Jones & Comita ). In entomophilous species, pollen deposition from genetically related donor conspecifics can strongly contribute to qualitative pollen limitation given that insect pollinators typically disperse pollen locally (Harder & Barrett ; Aizen & Harder ).…”