Rainfall is the most important factor controlling how much carbon is stored in the soil of coastal temperate rainforests. That’s according to researchers who used machine-learning to characterize soils across British Columbia and Alaska. The results will inform models of how carbon stocks respond to climate change.
Temperate rainforests like those of Scandinavia and the Pacific Northwest contain the densest accumulation of above-ground carbon in the world, with up to 1500 tonnes per hectare. Storage beneath the forest floor is harder to estimate, but in places could exceed 500 tonnes per hectare in the top metre of soil alone. This would place coastal temperate rainforests above grassland, cropland and all other types of forest for its below-ground carbon concentration, and leave it second only to Arctic permafrost.
To pin down the figure, Gavin McNicol of University of Alaska Southeast, US, and colleagues compiled a database of more than a thousand soil profiles taken across the North Pacific coastal temperate rainforest. From each profile, the researchers determined the mass of soil organic carbon down to a depth of one metre. Then they associated each data point with geographical attributes such as annual precipitation, topography and underlying rock type, and used the combined data to train a machine-learning algorithm.
As the greatest concentrations of soil organic carbon occurred where the ground was wettest, annual precipitation was the most important attribute, followed by topographical factors like slope and elevation. Informed by these empirical relationships, the algorithm was set to work on a digital map of the study region, predicting soil organic carbon density based on the geographic characterization of each pixel.
The total mass of soil organic carbon calculated for North Pacific coastal temperate rainforest was 4.5 gigatonnes of carbon, which the researchers believe is probably an underestimate. The limited spatial resolution of the landscape data means that small, wet depressions were overlooked, yet these are places where soil organic carbon density tends to be highest. Another reason to suspect that the total figure is conservative is that carbon stores in many places probably extend much deeper than the one metre used in the study.
“By limiting our estimates to the top metre, we permitted our work to be readily compared to estimates for other systems globally,” says McNicol. “But while researching the literature for this project I came across some old work from the 1950s and 60s showing that the peatlands in southeast Alaska can be more than 5–6 m deep.”
For now, soil organic carbon is accumulating in the North Pacific coastal temperate rainforest. But that situation is not necessarily permanent. Soil organic carbon appears to decompose more quickly at higher temperatures, so some of this carbon could return to circulation as the climate warms.
“Some studies have estimated that 1 °C of warming could release over 50 GtC from the top 30 cm of soils globally by 2050,” says McNicol. For perspective, 50 GtC is about five times the amount of carbon released by human activity in 2018.
Conversely, if climate change brings more annual precipitation, and causes less of it to fall as snow, an increased area of water-saturated soil could bring even larger accumulations of soil organic carbon. Either way, by providing a robust estimate of how much carbon is in the soil, and an understanding of what influences its distribution, the research contributes to models of carbon-biosphere interactions.
Gavin McNicol and colleagues reported their findings in Environmental Research Letters (ERL).