SEED ECOLOGY OF SOME SELECTED DRY FOREST SPECIES OF SRI LANKA

Abstract

This study examines seed ecology of ten native dry zone forest species: Manilkarahexandra, Drypetes sepiaria, Salvadora persica, Bauhinia racemosa, Cassia fistula,Cassia siamea, Schlechera oleosa, Oroxylum indicum, Randia uliginosa, Dichrostachyscineria and in one alien invasive species: Prosopis juliflora.

Seed samples were collected from the dry zone of Sri Lanka (in Anuradhapura,Polonnaruwa, Monaragala and Hambanthota districts). Morphology (colour, width,length of seeds) and the weight of 60 seeds per each species were recorded in threereplicates. Viability of seeds was tested using standard TIC test while the percentagegermination of seeds was recorded in moist petridishes laid according to RandomizeComplete Block Design. Rate of water absorption of seeds and seed coat widths were alsomeasured, while the presence of water-soluble growth inhibitors were examined througha bioassay using Brasica juncea seeds. Hot water treatments, Hot air treatments,Gibberalic acid treatments, Manual scarification and combinations of above treatmentswere tested to break the seed dormancy of these species. Seed and plant characteristicswere observed to determine the ecological state (Climax or Pioneer) of study species.Seed predation and dispersal mechanisms were observed during field visits.

P.jutiflora and 0. indicum seeds are recalcitrant, while the remaining species showedseed dormancy. D.cineria Cfistula, Csiamea and B.racemosa seeds appeared to bedormant due to its water impermeable seed coat and the dormancy can be overcome bymanual-scarification. S.persica seeds contain underdeveloped embryo and hot watertreatment (at 45°C for 5 days) was found to be the best treatment to break the seeddormancy. D.sepiaria, S.oleosa and R.uliginosa seeds seem to be dormant due to thepresence Of a growth inhibitor and the hard seed coat. Manual scarification together withGibberalic acid (4000ppm) treatment was found to be suitable to break the seed dormancyof the species. M.hexandra seeds too have an undeveloped embryos and hard seed coats,which may cause seed dormancy, but seed germination can be improved by hot watertreatment (at 85°C for 15minutes) fallowed by manual scarification. o'indicum andR. uliginosa can be described as Pioneer species. Other 8 natural forest species can bedescribed as climax species, which mainly dispersed by animals. High seed predationcan be observed in these species. Especially insects including termites and several larvaeof insects attack the seeds of tbese species.