Carbon in the Pacific Coastal Temperate Rainforest: New Research
Beneath the canopy of North America’s largest remaining old-growth forest, globally-significant amounts of carbon are captured from the atmosphere by plant life cycled through the soil, rivers, and water bodies of coastal Southeast Alaska and British Columbia. The Pacific Coastal Temperate Rainforest (PCTR) is a key region for research into carbon stocks and movement between forested and glacierized areas and the highly productive coastal waters.
Four recent publications from ACRC researchers and affiliates explore carbon movement in the PCTR in relation to climate change, storm events, glacial retreat, and carbon speciation.
Storms drive carbon export from the Tongass
Rainstorms change the concentration and type of carbon transported by streams in the PCTR by reaching more carbon pools and creating hydrologic pathways to the coast.
Studies have found that rainfall and stream runoff are key drivers of the movement of carbon from land into coastal waters, and the majority of this transportation happens during storms. The Juneau area, in the northern PCTR, is one of the wettest locations in North America with an average of 200 centimeters of annual precipitation that generally falls during large storms moving across the Pacific Ocean. Climate change is projected to bring warmer, wetter conditions to the region in the coming decades, suggesting that there will be larger storm events with more precipitation falling as rain rather than snow. This flow of carbon from land to sea is an often underestimated part of the carbon cycle, and in the case of storm-driven carbon flows, little is known about the sources and types of carbon that is transported in these events.
Arecent publicationinJGR Biogeosciences, led by Jason Fellman and Eran Hood, explores the concentration, composition, and source of carbon over a six-day storm event at three sites of varying land cover in Juneau. Because storm events can mobilize different carbon pools by flowing through different soil layers, the form and source of carbon can change dramatically over the course of a storm. Using specialized equipment, the researchers were able to identify a precise molecular fingerprint that shows where and when the carbon originated from, and track carbon from its source out into marine environments. During the storm event, the dissolved organic carbon concentration in streams increased, which suggests that in the absence of storms, the lack of hydrologic connectivity to move carbon limits the flow of carbon to the marine environment, rather than the lack of carbon sources. The high flow event also transported a higher proportion of an energy‐rich form of carbon that is highly edible to microbial communities to surface and coastal waters, providing an energy subsidy to coastal food webs that might otherwise be consumed by soil and stream microbial communities.
Glacier retreat impacting carbon transport to the ocean
Glacial retreat is occurring rapidly in Southeast Alaska, and as a result, changing how carbon is produced, stored, and transported across the landscape.
Globally, glaciers are expected to lose roughly 30-40% of their volume between 2006 to 2100, and global glacier runoff is projected to decrease by about 20% over the same time period. As glacier runoff is a significant source of carbon flowing to coastal waters in Southeast Alaska, this shift will have an impact on carbon cycles in the region. Carbon that originates from glacial sources is largely rock-derived, or petrogenic carbon, while carbon moved by rainfall and snowmelt streams in forested ecosystems is mostly biogenic, derived from plants and organic material. Understanding carbon movement to the ocean in the PCTR is important because fjords in the region are global hotspots of carbon burial, a process that impacts the atmospheric reservoir of CO2.
In arecent studypublished by Hood and Fellman inGeophysical Research Letters, the scientists looked at three watersheds in coastal Southeast Alaska that vary in glacier cover to understand how glacier shrinkage will impact the amount and type of organic carbon in rivers. They found that changes in glacier coverage will increase the transport of dissolved organic carbon in rivers and decrease the transport of particulate organic carbon. Sources of particulate organic carbon in rivers may also change as glaciers shrink and have less erosive power.
Carbon and Coastal Environments
The quality and amount of carbon flowing into marine environments in the northern Pacific Ocean is controlled by watershed location and characteristics within the southeastern coast of Alaska.
Thousands of streams and rivers drain from Southeast Alaska into coastal waters, acting as hotspots for carbon transport to the marine environment of the Alexander Archipelago and northern Pacific Ocean. The importance of this organic matter contribution to marine ecosystems is not well understood but the concentration and biological availability suggest it could influence marine productivity.
A paper led by Rick Edwards at the US Forest Service Pacific Northwest Research Center estimates that 1.17 million metric tons of carbon pours into estuarine and marine waters in the Gulf of Alaska each year. That’s roughly the equivalent weight of 10,000 blue whales, and augments the organic matter brought in by marine primary production. Of this dissolved organic carbon, the researchers estimate that 23 to 66% is bioavailable, or usable to marine organisms. This means it can be taken up by bacteria and other microbes, becoming carbon dioxide released back into the atmosphere or food that supports the marine food web. To understand the importance of this large carbon source to marine ecosystems, further research is needed into where and when freshwater and carbon enter the ocean and what its ultimate fate is.
Tracking Carbon Composition Changes due to Glacial Retreat
Climate-driven glacier loss is contributing to changes in the export and composition of carbon traveling from land to coastal waters, with potential consequences for downstream aquatic ecosystems.
Glacial retreat due to climate change has the potential to impact dissolved organic matter in streams by shifting the contributions of carbon from outside the aquatic ecosystem (such as plant and soil material), and carbon from within the aquatic system.
Research publishedby ACRC-supported PhD candidate Megan Behnke (Florida State University) assessed the molecular composition and input of dissolved organic matter coming from glacial and terrestrial sources in watersheds that are seeing rapid glacial retreat and forest expansion into newly deglaciated areas. Behnke and co-authors collected streamwater over the summer glacial runoff period from streams in watersheds with no glacial influence toa subglacial outflow with ~70% glacial coverage. Behnke found that streams with more glacial meltwater input had higher amounts of old dissolved organic carbon (up to ∼ 3200 yr old radiocarbon age), suggesting that glacial retreat will decrease the proportion of old, protein-rich dissolved organic carbon delivered to aquatic ecosystems.She also demonstrated that molecular composition could be accurately modeled based on how much the glacial and terrestrial sources contributed, showing that the carbon sources in Southeast Alaska produce consistent types of compounds.As glaciers disappear from watersheds and are replaced by forests, the carbon compounds unique to glacial runoff are likely to be replaced by terrestrial organic matter compounds. In the future, carbon entering Southeast Alaska’s coastal waters may be less diverse in age, molecular composition, and source materials. This could have unknown consequences for downstream freshwater and marine food webs.