Dispersal of Amazonian Trees: Hydrochory in Swartzia polyphylla1

Hydrochory was investigated in the seeds of the Amazonian floodplain tree, Swartzia polyphylla, in which pods open on the tree to release one large seed. Seeds collected from beach drift along the Rio Negro showed a high percentage of floaters (82%). Yet most seeds sank following collection from: adult trees (89%), unflooded ground under adults (96%), and flooded ground under adults (86%). The specific gravity of the seeds was near that of water, 1.04 ± 0.03 for sinkers and 0.98 ± 0.02 for floaters. The ability to float was correlated directly with the volume of the air pocket between the two cotyledons, which varied from 5.6 to 20.5 percent of the total seed volume. In a long-term floatation test lasting 81 days, 45 percent of the seeds never floated, 33 percent always floated, and 22 percent first sank for one week and then floated for at least one month. Seeds that never floated eventually rotted, but not until days 63-73. Seeds that were floating at day 81, regardless of how long they had been floating, were placed on moistened filter paper for 18 days during which time 36 percent germinated, 45 percent rotted, and 19 percent did neither but remained viable. These results suggest that S. polyphylla achieves dimorphism in flotation of its seeds, some sinking and some floating, by producing seeds of continuous variation in specific gravity around a mean close to 1.00. Seeds that float can be dispersed long distances along river margins, while those that sink may be moved only marginally from the parent tree. RESUMO Investigou-se a hidrocoria em sementes de Swartzia polyphylla, uma especie arborea das florestas inundadas da Amazonia. A vagem se abre na arvore soltando normalmente uma grande semente. Sementes coletadas nas linhas de deposiCaao da praia do Rio Negro mostraram alta porcentagem de flutuaCao (82%), enquanto a maioria das sementes coletadas diretamente da copa das arvores (89%), no solo sob as arvores (96%) e em solo alagado sob as arvores (86%) afundou. A gravidade especifica destas sementes foi bem proxima a da agua, 1.04 ± 0.03 para as que afundaram e 0.98 ± 0.02 para as que flutuaram. A habilidade de flutuaCao esteve diretamente relacionada ao volume relativo da bolsa de ar que existe entre os cotiledones, variando de 5.6 a 20.5 porcento do volume da semente. Em um teste de flutuaCao de longa duraCao (81 dias), 45 porcento das sementes nunca flutuararm, 33 porcento flutuaram durante todo o teste, 22 porcento inicialmente afundaram por uma semana e depois flutuaram ao menos por um mes. Algumas das sementes afundadas apodreceram, mas nao antes de 63 dias de incubaCao. As sementes que estavam flutuando no 81° dia foram colocadas em papel de filtro umido, destas 36 porcento germinaram, 45 porcento apodreceram, e 19 porcento ficaram viaveis (nao germinaram, nem apodreceram). Estes resultados sugerem que as sementes de S.polyphylla apresentam dimorfismo para flutuafao, algumas flutuaCao e outras afundando, sendo isto consequencia da produCao de sementes com uma variaCao continua de gravidade especifica em torno de uma media proxima a 1.0. As sementes que flutuam podem set dispersas a longas distancias ao longo das margens dos rios, enquanto aquelas que afundam podem ser apenas marginalmente afastadas marginalmente da arvore-mae.