An Emerging Carbon Sink in Headwater Streams and the Role of Large Wood and Riparian Forest Structure

Riparian forests influence stream ecosystems by controlling light availability, nutrient inputs and adding large wood (LW). While many functions of in-stream LW are well studied, there is limited research on their carbon storage potential, especially in eastern North America. Due to forest recovery following historic clearing regionally, riparian forest structure is changing, with implications for LW recruitment, accumulation and carbon dynamics. To better understand the LW carbon pool and relationships with riparian forest structure, we collected data on the forest and in-stream LW in headwater streams in the mature, northern hardwood Hubbard Brook Experimental Forest (HBEF) in New Hampshire, USA. To understand how in-stream carbon storage will change as these second-growth forests develop, we collected comparison data at streams in old-growth forests of the Adirondacks of New York State. Streams at the HBEF contained 7.5 Mg C/ha in LW (SD = 5.8 Mg C/ha), exhibiting substantial variation within and between sites. This variation is linked to heterogeneity in riparian forest structure, especially the large tree basal area. Our data suggest the storage potential of stream LW will increase as riparian forests age, with old-growth stands storing 23.8 Mg C/ha as LW (SD = 9.8). This provides a first assessment of the LW carbon pool in the region and the biotic factors that influence this storage. The positive relationship between LW carbon and large trees, and the increased storage in old-growth forests supports conservation and management that promote large trees and old forests in riparian zones. Such practices may improve the value of in-stream LW carbon as a natural climate solution.