Where temperate meets tropical: multi‐factorial effects of elevated CO₂, nitrogen enrichment, and competition on a mangrove‐salt marsh community

Abstract

Our understanding of how elevated CO 2 and interactions with other factors will affect coastal plant communities is limited. Such information is particularly needed for transitional communities where major vegetation types converge. Tropical mangroves (Avicennia germinans) intergrade with temperate salt marshes (Spartina alterniflora) in the northern Gulf of Mexico, and this transitional community represents an important experimental system to test hypotheses about global change impacts on critical ecosystems. We examined the responses of A. germinans (C 3 ) and S. alterniflora (C 4 ), grown in monoculture and mixture in mesocosms for 18 months, to interactive effects of atmospheric CO 2 and pore water nitrogen (N) concentrations typical of these marshes. A. germinans, grown without competition from S. alterniflora, increased final biomass (35%) under elevated CO 2 treatment and higher N availability. Growth of A. germinans was severely curtailed, however, when grown in mixture with S. alterniflora, and enrichment with CO 2 and N could not reverse this growth suppression. A field experiment using mangrove seedlings produced by CO 2 -and N-enriched trees confirmed that competition from S. alterniflora suppressed growth under natural conditions and further showed that herbivory greatly reduced survival of all seedlings. Thus, mangroves will not supplant marsh vegetation due to elevated CO 2 alone, but instead will require changes in climate, environmental stress, or disturbance to alter the competitive balance between these species. However, where competition and herbivory are low, elevated CO 2 may accelerate mangrove transition from the seedling to sapling stage and also increase above-and belowground production of existing mangrove stands, particularly in combination with higher soil N.

Coastal Habitats of the Gulf of Mexico

“…Propagules also tend to strand at higher elevations on creekbanks in Louisiana marshes and sustain less damage by predators compared to those stranding at lower elevations. Those seedlings that do become established in salt marshes may be suppressed by competition from grasses such as S. alterniflora (McKee and Rooth 2008). Only when S. alterniflora is disturbed, reducing competition or creating bare patches of ground, can A. germinans recruit to the sapling stage .…”

“…For example, McKee and Rooth (2008) examined effects of nitrogen addition on the competition between the mangrove, Avicennia germinans, and the salt marsh dominant, Spartina alterniflora, in coastal Louisiana. When grown in mixture in greenhouse mesocosms, nitrogen addition greatly favored S. alterniflora, which is a stronger competitor for nitrogen.…”

Can elevated CO2 modify regeneration from seed banks of floating freshwater marshes subjected to rising sea-level?

“…In one experiment, growth of the mangrove Avicennia germinans (C 3 ) was higher in elevated CO 2 when grown in monoculture, but when grown in mixed cultures with Spartina alterniflora (C 4 ), the growth was suppressed (McKee & Rooth, 2008). While both C 3 and C 4 species may have enhanced growth in elevated CO 2 , the specific response of species may depend on a large array of biotic and abiotic factors.…”

“…Two greenhouses were randomly assigned to elevated CO 2 (720 ll l -1 = elevated) and the other two to ambient CO 2 (385 ll l -1 ) treatments. CO 2 concentrations were continuously monitored and kept within ±5% of the set-point through automated feedback regulation of flow meters supplying CO 2 gas to the greenhouses (see McKee & Rooth, 2008). One model of the IPCC (2007) projects a doubling of CO 2 by 2050, which was the basis for selecting the elevated level of CO 2 in the experiment.…”

“…Careful assessment of any interactive effects of these environmental factors is important in projecting the future responses of freshwater wetland species to climate change. Of these factors, CO 2 has received much emphasis in ecosystem modeling (Mayle et al, 2004;Hickler et al, 2009) and empirical field studies in wetlands (Rasse et al, 2005), but only rarely studied in tandem with multifactor interactions (McKee & Rooth, 2008;Cherry et al, 2009;Langley et al, 2009).…”