Over the next century rising sea level will move the land/sea interface inland, exposing coastal ecosystems to increasing tidal influence including increased flooding frequency and salinity. We want to understand how these environmental changes will modify coastal forest structure, productivity and carbon cycling. Many southeastern US coastal plain rivers have already experienced shifts in extent of tidal influence due to human hydrologic modifications. Employing this gradient of tidal influence as a proxy for rising sea level, we investigate these impacts in cypress swamps located along the intensively modified lower Savannah River. Utilizing monthly censuses of fine root populations growing adjacent to minirhizotron tubes, we employ a demographic approach to quantify patterns of root distribution, morphology and production. We combine these with measures of aboveground production and monthly surface flux assays of soil respiration to compose a carbon cycle for each tidal forest site.
We observed shifts in fine root form, distribution in the soil profile and lifespan that influence root turnover and rhizosphere carbon cycling along this gradient of tidal influence. Rhizome biomass increases and fine root and total belowground plant biomass shift to deeper positions in the soil profile with increasing site pore water salinity. We found a greater proportion of fine root length in smaller diameter classes with increasing site salinity. Roots at the soil surface are of smaller diameter than those deeper in the soil profile, and this pattern strengthens as one moves downriver and site porewater salinity increases. Fine root specific gravity decreases with increasing site salinity. Like aboveground woody increment and litterfall production, which decrease as site pore water salinity increases, total fine root turnover is lowest at the site with highest pore water salinity. However, this is not a monotonic pattern belowground: fine root standing stocks, turnover and soil respiration peak at the midpoint of this gradient, where herbaceous vegetation forms a vigorous understory. We explore our ability to distinguish cypress roots from the entire root community and to describe shifts in root distribution and turnover, and in carbon cycling, along this gradient of cypress swamp disintegration.
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