Gross primary productivity of Phragmites australis under elevated CO2 and N enrichment
The primary productivity of coastal wetlands is changing dramatically in response to rising atmospheric CO2, nitrogen proliferation, and invasions by novel species. Changes in productivity will likely alter coastal wetlands' resilience to sea level rise and the ecosystem services they provide. We sought to determine how the invasion of Phragmites australis (common reed) will alter the gross primary productivity (GPP) of coastal wetlands in North America under conditions of elevated CO2 and N enrichment. Our approach was to combine empirical data on canopy structure and photosynthesis in a simulation model, thereby quantifying Phragmites carbon assimilation at the stand level. Empirical data came from plants grown in an open-top chamber study in which CO2 and N have been manipulated since 2011 (ambient vs. +320 ppm CO2 and/or fertilization with 0 vs. 25 g N m-2 yr-1). We modeled the four environmental scenarios at the site separately, and calculated the total amount of carbon assimilated by monotypic stands at daily intervals and through a full growing season.
Our simulations indicated that monotypic stands of Phragmites growing under near-future conditions of CO2 and N will fix 3.65 kg C m-2 yr-1 at typical stand densities (100 culms m-2). Compared to Phragmites GPP under ambient CO2 (1.69 kg C m-2 yr-1), the combined CO2+N treatment experienced a 116% stimulation effect. This represented an approximately additive combination of the stimulation effects for single additions of CO2 or N (49% and 62%, respectively). Our results suggest that eutrophied wetlands will be particularly susceptible to invasion by Phragmites as CO2 rises. However, if higher GPP translates into increased soil organic matter accumulation under Phragmites stands, its invasion may prevent some tidal wetland ecosystems from succumbing to accelerating rates of sea level rise.