Abstract
Photosynthetic carbon assimilation is dependent on the canopy-air CO 2 concentration ( Ca), which is an essential driving parameter of mechanistic models built to predict plant responses to changing environmental conditions. Short-term studies have shown that crop-canopy Ca undergoes substantial diurnal and seasonal variations as compared to Ca values measured several meters above plant canopies in well mixed conditions. Whether these canopy Ca patterns measured in crops also apply to wetland plant canopies and over long periods of time remains largely unknown. The first objective of this study was to analyze the consistency of short- and long-term Ca patterns in salt marsh canopies over a 10-year period. The second objective was to assess the impact of these canopy Ca patterns on simulated ecosystem productivity. In this study, we used Ca data collected from 1990 to 2000 in salt marsh canopies of the Chesapeake Bay, Maryland. The possible effects of short- and long-term canopy Ca patterns on plant productivity were simulated with a mechanistic model specifically adapted to the Scirpus olneyi vegetation of the salt marsh ecosystem. The annual average of daytime canopy Ca of a brackish marsh community rose by a significant ( P<0.01) 1.55 μmol CO 2 mol −1 year −1 between 1990 and 2000, which parallels the atmospheric records of Mauna Loa over the same period of time. Annual Ca averages displayed some variability around this trend, which were highly correlated with the year-to-year variation in aboveground plant biomass ( r 2=0.83, P<0.001). Therefore, our study demonstrated that aboveground plant biomass is the main driver for inter-annual canopy Ca fluctuations. Daytime canopy Ca displayed maximum values in May and September and minimum values in July, with an amplitude of about 20 μmol CO 2 mol −1. The amplitude of the diurnal Ca cycle measured at the wetland appeared similar to that of North American agricultural ecosystems, exceeding 100 μmol CO 2 mol −1 in July and August and characterized by a sharp drop in the CO 2 concentration shortly after dawn. Model simulations suggested that the increase in canopy Ca from 1990 to 2000 resulted in a substantial increase in Scirpus gross plant productivity (GPP) of 0.31% per year, which confirms that plant ecosystem simulations need to consider the actual increase in canopy Ca when the simulated time period exceeds a few years. Forcing the model with the measured diurnal Ca pattern increased the simulated net plant productivity (NPP) immediately after dawn and decreased it for the rest of the daytime period, which resulted in a simulated net NPP decrease of about 5% over the growing season.
Published Version
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