EFFECTS OF NUTRIENT PULSES ON THE GROWTH AND REPRODUCTION OF THE ANNUALS ABUTILON THEOPHRASTI AND DATURA STRAMONIUM (RESOURCE ALLOCATION, PHENOLOGY, PLANT COMPETITION)

Availability of plant resources varies temporally as well as spatially. The timing of resource pulses in relation to plants' growth and developmental schedules can affect plants' use of those resources. Because plants can differ, within and among species, in relative capacities to use resources at different times, pulse timing may affect population and community dynamics.; Plant response to timing of nutrient pulses was studied in individuals and in two-species competition, using the co-occurring annuals Abutilon theophrasti Medic. and Datura stramonium L. A. theophrasti plants received pulses that varied in size and frequency among treatments but contained the same total nutrient input. Plants receiving a single large pulse that was delayed until fruiting had lower seed production than plants in all other treatments.; In the first of two experiments involving conspecific and heterospecific pairs. A. theophrasti grown with conspecifics had lower seed production under a late pulse than an early pulse, but D. stramonium seed production did not vary. In a second experiment, the species' responses were reversed. The second experiment also included variation in emergence time between members of competing pairs. Under heterospecific competition, both species had greater seed production under the late pulse than the early pulse. A late pulse tended to give less advantage to the earlier emerging member of a pair.; The two species were also grown individually in an experiment using a broad range of single-pulse times, and a subset of plants was harvested at intervals for measurement of biomass and nutrient levels. D. stramonium again had lower seed production under the late pulses, while A. theophrasti was minimally affected by pulse timing. Differences in seed production among the pulse treatments resulted from differences in total biomass and nutrient accumulation, rather than differences in proportional allocations to reproduction. The ability of A. theophrasti to respond to nutrient pulses at a later age than D. stramonium may be related to its later initiation of reproductive growth and/or its greater tendency for growth of lower lateral branches late in the season.