Ecological determinants of life history evolution of two Indian bamboo species

Abstract

Almost all (70 out of 72) of the Indian bamboo species possess a long period of vegetative growth followed by a single suicidal bout of reproduction. Only about eight of these 70 monocarpic species exhibit synchronized seeding over an area of several hundred square kilometers. Of the two commonest species, Bambusa arundinacea, which forms dense stands along water courses in areas of less variable rainfall, belongs to this minority of synchronized seeders; Dendrocalamus strictus, which occurs in scattered numbers in areas of more variable rainfall, exhibits sporadic seeding behavior. Individuals of both species grow exponentially during the vegetative phase. Observations on the synchronized seeding of B. arundinacea show it to be spread over 5-6 years, with flower and seed production by any individual clump being completed over the dry season from December to April. An average clump produced 100 kg of seed, corresponding to 30 percent of its above-ground biomass. One major cohort of B. arundinacea widely distributed over peninsular India seeded in 1868-72, 1912-16, and 1958-62; while a number of minor cohorts have seeded at other times. There is no defined direction over which seeding progresses in time. The bamboo seeds are subject to high levels of predation, and we propose that the exponential nature of clump growth in conjunction with predator swamping has led to the evolution of a long pre-reproductive period and monocarpy, which characterize most Indian bamboo species. Synchronized mast seeding is a further independent adaptation favored in a few species occurring in dense stands in more constant environments.