High production in a herbaceous perennial plant achieved by continuous growth and synchronized population dynamics

Continuous changes in the population dynamics, above- and belowground biomass, growth rates, and production of the emergent rush Juncus effusus were evaluated over an annual period in a sub-temperate riparian wetland. Extant and emerging individual photosynthetic culms (shoots) were labeled individually and their growth dynamics quantified in replicated plots. Lengths of living (chlorophyllous) portions and basal diameters of several thousand individual culms were determined at weekly intervals and converted to biomass by regression of surface areas to mass. Shoots emerged continuously at all times of the year; the number of ramets increased four times from summer to winter (12,000 - >30,000 culms m(-2)). Individual culms in November-December, however, were generally smaller (41-52%) than in summer. Maximum growth of individual new culms ranged from a maximum of 6.4 mg per day in June (40-50 days to maximum biomass) to 1.1 mg per day in November (70-110 days to reach maximum biomass in winter). Due to continuous recruitment and gradual culm senescence, numerous multiple cohorts occurred on an annual basis. Root production was also evaluated from changes in seasonal biomass and from experimental studies of changes in aboveground biomass in relation to those of rooting tissues. Annual root production was approximately 42% of shoot production. Average annual production estimates of combined above- and belowground components was 9832 g AFDM m(-2) per year. The extremely high production rates of this Juncus species result from the continuous growth while simultaneously shoot densities changed as the inverse of biomass seasonally. These processes maximized photosynthetic carbon fixation under mild climatic conditions of the winter and spring seasons. This dynamic growth strategy is likely to be common among herbaceous perennial emergent aquatic plants in mild climatic regions. (C) 1999 Elsevier Science B.V. All rights reserved.