Floral biology of two woody species of Dalbergia at high risk of timber extraction

Dalbergia granadillo Pittier and D. stevensonii Standl. are two important native timber-yielding forest tree legume species, the first distributed along the Pacific coast of Mexico and southwestern Mexico, and the other in southwestern Mexico. Both are threatened by illegal trade, deforestation, and habitat fragmentation. To generate information to help improve their conservation status, we studied several aspects of their reproductive ecology: reproductive phenology, floral morphology, floral rewards, and pollinators. Reproductive phenology was studied by monitoring trees from flowering to fruiting. Floral morphometrics was used to describe floral morphology and infer the mating strategy using a morphological approach (pollen-ovule ratio, herkogamy, flower size). Pollinators were determined by capturing insects and observing insect behavior on flowers. Results showed that flowering in both species was synchronous and massive. A high rate of early flower abortion was documented in both species. Both species have small, numerous, whitish, and fragrant flowers, typical of Dalbergia. Pollen-ovule ratios and floral traits suggest that outcrossing is favored. Floral visitors are insects, but only bees manipulate flowers when looking for rewards, a behavior that permits pollination. The two Dalbergia species apparently depend on bees for successful reproduction. The most critical reproductive bottleneck was a meager fruiting success probably due to either the lack of compatible individuals for outcrossing or high rates of fruit abortion. The regeneration of populations of these slow-growing Dalbergia species depends on a suitable quantity of seeds, which is severely limited by the overharvesting of mature, reproductive individuals for the timber trade.