PEAT COLLAPSE, PENDING AND WETLAND LOSS IN A RAPIDLY SUBMERGING COASTAL MARSH

Conversion of coastal marshes to inland open water is often associated with plant stresses such as saltwater intrusion into non-saline marsh es and soil waterlogging, but the physical processes that initiate pon d formation are not clear. We hypothesized that marsh elevation decrea sed rapidly following plant mortality because of structural collapse o f the living root network. We monitored the elevation of 20 marsh humm ocks between April 1990 and April 1992. Near total plant mortality occ urred within 1 year and was attributed to excessive flooding. Hummock elevation decreased almost 15 cm within 2 years but elevation of adjac ent ponds showed no trend. Plant stubble was still rooted in place on the submerged hummocks, and even slight evidence of surface erosion wa s not noted until the end of the study. The Cs-137 inventory in soil c ollected before and after the study also indicated that pest collapse rather than erosion caused the elevation decrease. Thus, pest collapse may initiate interior marsh ponds that subsequently spread via erosio n and may partly explain why some marshes experiencing plant mortality convert to open water rather than re-vegetate. Feat collapse appeared to be the primary mechanism of marsh loss in this Louisiana hotspot.