Do Annual Prescribed Fires Enhance or Slow the Loss of Coastal Marsh Habitat at Blackwater National Wildlife Refuge?

The rate of estuarine marsh loss at Blackwater NWR has been high (up to 2,000 ha) during the past 80 years because the vertical buildup of the marsh surface has lagged behind the local rate of relative sea-level rise. In this mineral sediment-poor estuary, marsh vertical development is driven primarily by the accumulation of plant matter in the soil (roots and rhizomes). Thus any activity that affects plant productivity can affect the ability of these marshes to keep pace with sea-level rise. Blackwater NWR has employed an annual prescribed fire regime since the 1970’s to achieve multiple management objectives. However, the influence of annual burning on plant production and marsh elevation dynamics is unknown. The Refuge Manager has stated, “Our most critical science need is to know if an annual return fire interval adversely or positively affects marsh elevation, and whether fire is contributing to or slowing the loss of marsh habitats.” We addressed this concern through manipulative experiments in field plots with varying burn return frequencies (annual, 3-5 years, 7-10 years, and no burn control) established by the refuge in 1998. Key findings: Annual burning significantly increases above- and below-ground plant production over the other three treatment regimes. There was a consistent pattern of other effects of annual burning but they were not statistically significant. The pattern includes annually burned marshes having the lowest surface accretion, root zone subsidence, shallow subsidence, and elevation deficit (i.e., lagged behind the relative sea-level rise rate less than the other treatments). Annual burning has a strong positive influence on plant production, which is a critical component of marsh vertical development in this mineral sediment-poor estuary. Yet, we cannot say unequivocally that annual burning increases the elevation and thereby the sustainability of the marshes at Blackwater NWR. However we can say that annual burning does not pose an additional risk to long-term sustainability of the marsh habitats there, compared to the other treatment regimes