Reproductive output and biomass allocation in Sesbania emerus in a tropical swamp.

This study examined the correlation of moisture, reproductive phenology, density of mature plants, and herbivory of apical meristems with the morphology and reproductive output of Sesbania emerus, an annual legume growing along a moisture gradient in a swamp in Guanacaste Province, Costa Rica. It also determined how biomass allocation varied and how it was related to reproductive output of plants growing along the moisture gradient within the swamp. Morphological changes included production of more stems and branches in response to herbivory, more and higher prop roots and more aerenchyma as water depth increased, and greater stem diameter in lower densities. Plant height varied greatly within a site, but not among habitats. Plants began to produce flowers and fruits later in wet sites than in dry sites. Reproductive output was generally more sensitive to environmental variables than was plant size. Fruit number and plant height were positively correlated for almost every treatment. Greater fruit and seed production were correlated with drier sites, earlier phenology, and lower density, but not with herbivory. Total biomass accumulation did not vary among moisture sites, but root production appeared to occur at the expense of reproductive output in the wetter sites. Plants in the wetter sites had both a greater percent and a greater absolute amount of biomass in roots, and a lower percent and a lower absolute amount of biomass in fruits and seeds than plants in drier sites. The root: shoot ratio was nearly five times higher in the wet than the dry site. Seed number per plant ranged from a mean of 6,800 at the wet site to a mean of 16,878 at the dry site. If this striking phenotypic variation in reproductive output and biomass accumulation has a genetic basis, the possibility of ecotypic differentiation exists in S. emerus. ANNUAL PLANTS often exhibit striking size variation within single populations and habitats (Obeid, Machin and Harper, 1967; Ogden, 1970). This variation arises in even-aged cohorts of seedlings, and can result in a few large individuals dominating the population (Stern, 1965). Furthermore, because reproductive output and seed production in plants are generally positively correlated with plant size (Harper, 1977; Kawano and Miyaki, 1983), a few dominant individuals make a disproportionate contribution to the seed pool. The establishment of dominance in a population may be due to initial diffierences among microsites of seedlings (Harper, Williams and Sager, 1965; Hartgerink and Bazzaz, 1984), diffierences in time of seedling emergence (Ross and Harper, 1 Reeeived for publieation 16 July 1984; revision aecepted 7 February 1985. This researeh was eompleted while partieipating in the Tropieal Eeology eourse 84-1 ofthe Organization for Tropieal Studies in Costa Riea. We thank D. Futuyma for his effiorts in coordinating the eourse, G. Miller, C. Nalepa, L. Prevetti, H. Tiebout, and D. Townsend for field assistance, and B. Benner, D. H. Janzen, and two anonymous reviewers for eonstructive advice on the manuscript. 1972), or to genetic differences among individuals in relative growth rates (Burdon and Harper, 1980) or in biomass allocation patterns (Meagher and Antonovics, 1982). Over time, the size and reproductive output of annual plants also can be affected by numerous environmental factors such as water, light, and nutrient availability, plant density and competition, and herbivory (Watkinson and Harper, 1978; Waller, 1980). In order to assess the impact of these environmental factors on plant fitness it is useful to know first the range of response of a plant within a relatively uniform environment. One approach that can be used is to determine the allometric relationship of plant size to reproductive output within one site, and then to compare the allometries obtained across environments. In this manner, one can deduce the response of a plant to changing environments in terms of seed production and patterns of resource allocation. We utilized this approach in a study of the environmental correlates of plant size, morphology, and reproductive output in Sesbania emerus Urb. (Fabaceae), an annual legume