Ramet Population Dynamics and Net Aerial Primary Productivity of Spartina Alterniflora

Abstract

Ramet dynamics and net aerial primary productivity (NAPP) were studied in samples of Spartina alterniflora (smooth cordgrass) at Sapelo Island, Georgia. Three populations were compared: tall, short, and short with nitrogen fertilization in spring (short/N). Tall and short S. alterniflora populations had different demographic characteristics. The short population had a shorter leaf longevity (49 vs. 72 d) and a higher leaf turnover than the tall population, which may be due to high salinity and nitrogen limitation in the high marsh. Although the average ramet longevities of tall and short populations were similar (231 and 204 d, respectively), cohorts of the tall population that emerged early in the growing season had a significantly longer average life—span than those of the short population, probably because they had more support from belowground reserves that led to a higher initial survival rate. Leaf number, leaf area, shoot density, and biomass production of the short population were greatly increased by spring nitrogen fertilization, but the longevity of leaves and ramets was little affected. Using the demographic data and phytometric equations (nondestructive method), new growth was found throughout the year in S. alterniflora populations at Sapelo Island. The average dry mass NAPP was estimated to be 1105, 2244, and 1520 g.\mathrm{m}{—2}.\mathrm{yr}{—1}$ for the short, short/N, and tall populations, respectively. Because of its higher leaf turnover, the short population had a higher leaf to stem production ratio than the tall population. NAPP estimates obtained by nondestructive methods usually lie between overestimates and underestimates from harvest methods, indicating that nondestructive methods give accurate estimates of NAPP for salt marshes. Using the highest spring aerial production rates, the upper limits of annual dry mass total production of S. alterniflora at Sapelo Island were calculated as 2555 g/\mathrm{m}2$ for the short population and 4526 g/\mathrm{m}2$ for the tall population. These limits are lower than many previous estimates of annual total primary production for S. alterniflora.