Integrating forest biomass and distance from channel to develop an indicator of riparian condition

Abstract

Living and detrital biomass in a riparian buffer zones ameliorate diffuse-source pollution originating from adjacent landscapes, with higher nutrient removal potential, particularly with respect to nitrogen and sediments, associated with more biomass (i.e., older forests). Pollution removal is mediated by sediment trapping and uptake of nitrogen by plants and by denitrification by microbial communities in root zones using organic matter and root exudates as energy sources. However, as a consequence of various land management activities, the amount of biomass of most forest riparian zones is much lower than its potential, which is mature forest. This makes restoration of riparian buffer zones to mature forest an ideal way to both improve water quality and increase carbon sequestration. This study measures the amount of biomass aboveground and in soils for a variety of common, age-related condition-types associated with riparian buffer zones along low order (headwater) streams in agricultural landscapes of Coastal Plain North Carolina. The data are used to determine pollution removal potential in relation to stand age and distance from channel and a basis for developing an indicator of riparian condition. Mean biomass for common riparian cover types were Mature Forest (>50 y old): 483Mg/ha; Young Forest (25–50 y old): 257Mg/ha; Regenerating Forest (5–25 y old): 205Mg/ha; recently Harvested Forest (0–5 y old): 165Mg/ha; Perennial Herb: 67Mg/ha; Shrub/Sapling: 63Mg/ha; and Annual Rowcrop: 36Mg/ha. Trees contained >96% of all aboveground biomass and >58% of total biomass present in the forested conditions. Biomass in recently harvested forests were >97% detrital, mostly due to the large amount of slash left after harvesting and the mostly intact soil organic matter. Most (>80%) of the biomass in non-forest cover types was stored in the detrital pool. By partitioning the riparian zone into inverse distance-weighted subzones based on distance from stream channel, we developed an indicator for determining riparian condition based on both biomass and proximity of biomass to channel.