The Dynamics of Dry Matter and Chemical Substances in a Juncus effusus Population

Abstract

Seasonal changes in standing crops of living and dead dry matter and tissue levels of chemical substances were determined for a Juncus effusus population. These data were used to determine rates of production, pigment synthesis and nutrient absorption during different parts of the annual growth cycle. The annual cycle began with the appearance of young shoots in late summer. Net productivity was greatest during early autumn and midspring. Little growth occurred during the winter and there was a rapid dieback of shoots in the summer. Tissue concentrations of most chemical constituents declined during spring growth and summer dieback. Uptake rates of mineral nutrients other than nitrogen were large during autumn growth, but slight during spring growth. The same was true of pigment synthesis. Energy accrual was generally proportional to dry-matter increases during both phases of growth. Results of this investigation and previous studies are discussed to show that rates of net nutrient uptake and synthesis of pigments are not merely functions of net dry-matter production, but physiological characteristics of the species. Quantities of chemical substances per unit area of stand did not necessarily follow the same patterns of change as did dry-matter standing crops. Dead Juncus effusus shoots decomposed rapidly, about 50% of the dry matter disappearing within 4 months. Mineral nutrients were lost more quickly than the degradation of dry matter. INTRODUCTION Herbaceous vegetation of fresh-water marshes often produces large quantities of biomass (Penfound, 1956; Westlake, 1963). Chemical analyses have been made on several marsh species (Straskraba, 1966; Lawrence, 1968; Boyd, 1968, 1970a). These data were used to estimate amounts of chemical constituents per unit area of plant stand and to compare the nutritive value of various species. In addition, a few studies have dealt with the dynamics of mineral nutrient uptake and biochemical synthesis during growth and nutrient losses during decomposition. Uptake rates and tissue concentrations of most minerals declined as the growing season progressed, but maximum mineral accrual per unit area of stand was obtained at or near the time of peak standing crop (Stake, 1967, 1968; Boyd, 1969, 1970b). Pigment concentrations followed a similar trend of change (Boyd, 1970b). Drymatter standing crops decreased during late summer and total quantities of nutrients per m2 declined even faster than dry matter (Boyd, 1969, 1970b). There is very rapid release of most minerals, particularly cations, from emergent macrophytes once the shoots fall into the water (Boyd, 1970c). These processes are obviously very important aspects of the physiological ecology of macrophytes and additional research is needed. The present study was undertaken to describe the patterns of change in tissue concentrations and quantities per unit area of minerals, pigments and energy in a Juncus effusus L. stand during a yearly 1 This research was supported by contract AT (38-1) -310 between the U.S. Atomic Energy Commission and the University of Georgia.