From forested coastal wetlands to northern boreal forests, forests are considered essential cogs in the global carbon cycle. Generally, forests act as “sinks” by absorbing more carbon from the atmosphere through photosynthesis than they release, but they still do emit a considerable amount of carbon dioxide. Much of this release happens through respiration by soil microbes.
However, not all carbon lost by forests escapes to the atmosphere: A small but significant amount is exported into aquatic systems via dissolved organic carbon, which derives from soil material like plant litter and peat. Compared to atmospheric carbon export, aquatic export is small, but it is still considered a critical carbon flux.
In a new study, Senar et al. investigated how hydrologic connectivity controls carbon transport in the ecosystem and how carbon is partitioned between the atmosphere and waterways in Canada’s northern hardwood forests. The researchers hypothesized that hydrologic connectivity—the water-mediated transfer of matter and energy between landscape positions—determines carbon’s fate in the ecosystem. The flow of water between habitat types is closely tied to soil moisture, water table depth, and stream discharge.
The research took place in the Turkey Lakes Watershed, an experimental watershed located approximately 60 kilometers north of Sault Ste. Marie, Ontario, Canada. The researchers collected stream samples for 5 years to monitor dissolved organic carbon, and they monitored carbon dioxide emissions using flux chambers placed across habitat types. Other measurements included soil temperature, moisture, and organic carbon.
The results indicated that hydrologic connectivity between uplands, ecotones (regions of transition), and wetland habitats does indeed control the fate of carbon—both atmospheric and aquatic—in the northern hardwood forest; however, the study unexpectedly found that hydrological connectivity also dictates the magnitude of carbon exported from the ecosystem. In water-limited habitats, like uplands, the increase in soil water stimulated microbial activity and, subsequently, carbon dioxide released from respiration. In contrast, as wetlands and other water-saturated areas became hydrologically linked to the surrounding uplands, the increase in soil water tamped down the soil bacteria liveliness in the resulting anaerobic soils.
The study also found that hydrologically connected habitats resulted in an increase in aquatic transport of carbon. In other words, as more water entered the ecosystem, more carbon washed downstream into streams and lakes. The findings showed a distinct seasonal pattern, with increased aquatic transport during periods of high hydrologic connectivity, namely, spring snowmelts and fall storms.
Future climate predictions project a trend toward higher temperatures and prolonged periods of disconnected hydrology. The authors suggest that under those circumstances, northern hardwood forests will initially increase atmospheric carbon emissions from upland and ecotone habitats, with an eventual decrease as water becomes limited. The reduction in hydrologic connectivity will also result in less aquatic carbon transport downstream. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2018JG004468, 2018)
—Aaron Sidder, Freelance Writer