Watershed-scale forest biomass distribution in a perhumid temperate rainforest as driven by topographic, soil, and disturbance variables

Abstract

Temperate rainforests are the most carbon dense forest ecosystem on the planet, with C stocks several times higher than most other forested biomes. While climatic and disturbance drivers of these C stocks are relatively well explored, the spatial distribution of those stocks at the scale of entire watersheds is less well known, particularly in perhumid rainforests where research has been minimal. This study explored biomass distributions across an entire watershed simultaneously, from ocean to glacial icefields, in Southeast Alaska. Utilizing LiDAR and ground surveys, biomass was modelled throughout the landscape and distributions are described statistically. The dominant driver of biomass distributions at this scale (controlling for elevation) was the flow of water through the landscape: areas of higher water accumulation typically had low biomass (often <10 Mg·ha–1), whereas well-drained areas supported biomass approaching 950 Mg·ha–1. This relationship was strong at all elevations; only riparian locations (typically well-drained soils) maintained high biomass at low slopes. Exposure to stand-replacing disturbances, often a dominant driver, was only a minor factor. This work emphasizes the importance of water in temperate rainforests and the potentially significant impacts of changes to biomass given changes in precipitation (both increasing and decreasing) due to global climate change.