“Follow the carbon” is a growing pastime of ecologists and environmental researchers worldwide. In the process of cellular respiration, organisms use carbon compounds to fuel their metabolic pathways, so having carbon around makes life possible.  Within ecosystems, following the carbon is equivalent to following how energy flows among the producers, consumers, detritivores and decomposers. In soils, decomposers play a central role in energy flow, but we might not appreciate their importance because many decomposers are tiny, and decomposition is very slow.  We are thrilled by a hawk subduing a rodent, but are less appreciative of a bacterium breaking down a lignin molecule, even though at their molecular heart, both processes are the same, in that complex carbon enters the organism and fuels cellular respiration.  However. from a global perspective, cellular respiration produces carbon dioxide as a waste product, which if allowed to escape the ecosystem, will increase the pool of atmospheric carbon dioxide thereby increasing the rate of global warming. So following the carbon is an ecological imperative.

As the world warms, trees and shrubs are colonizing regions that previously were inaccessible to them. In northern Sweden, mountain birch forests (Betula pubescens) and birch shrubs (Betula nana) are advancing into the tundra, replacing the heath that is dominated by the crowberry, Empetrum nigrum. As he began his PhD studies, Thomas Parker became interested in the general question of how decomposition changes as trees and shrubs expand further north in the Arctic. On his first trip to a field site in northern Sweden he noticed that the areas of forest and shrubs produced a lot of leaf litter in autumn yet there was no significant accumulation of this litter the following year. He wondered how the litter decomposed, and how this process might change as birch overtook the crowberry.

One of the study sides in autumn: mountain birch forest (yellow) in the background, dwarf birch (red) on the left and crowberry on the right. Credit: Tom Parker.

Several factors can affect leaf litter decomposition in northern climes.  First, depending on what they are made of, different species of leaves will decompose at different rates.  Second, different types of microorganisms present will target different types of leaves with varying degrees of efficiency.  Lastly, the abiotic environment may play a role; for example, due to shade and creation of discrete microenvironments, forests have deeper snowpack, keeping soils warmer in winter and potentially elevating decomposer cellular respiration rates. Working with several other researchers, Parker tested the following three hypotheses: (1) litter from the more productive vegetation types will decompose more quickly, (2) all types of litter decompose more quickly in forest and shrub environments, and (3) deep winter snow (in forest and shrub environments) increase litter decomposition compared to heath environments.

To test these hypotheses, Parker and his colleagues established 12 transects that transitioned from forest to shrub to heath. Along each transect, they set up three 2 m² plots – one each in the forest, shrub, and heath – 36 plots in all. In September of 2012, the researchers collected fresh leaf littler from mountain birch, shrub birch and crowberry, which they sorted, dried and placed into 7X7 cm. polyester mesh bags.  They placed six litter bags of each species at each of the 36 plots, and then harvested these bags periodically over the next three years. Bags were securely attached to the ground so that small decomposers could get in, but the researchers had to choose a relatively small mesh diameter to make sure they successfully enclosed the tiny crowberry leaves. This restricted access to some of the larger decomposers.

Some litter bags attached to the soil surface at the beginning of the experiment. Credit: Tom Parker.

To test for the effect of snow depth, the researchers also set up snow fences on nearby heath sites.  These fences accumulated blowing and drifting snow, creating a snowpack comparable to that in nearby forest and shrub plots.

Parker and his colleagues found that B. pubescens leaves decomposed most rapidly and E. nigrum leases decomposed most slowly.  In addition, leaf litter decomposed fastest in the forest and most slowly in the heath.  Lastly, snow depth did not  influence decomposition rate.

(Left graph) Decomposition rates of E. nigrum, B. nana and B. pubescens in heath, shrub and forest. (Right graph) Decomposition rates of E. nigrum, B. nana and B. pubescens in heath under three different snow depths simulating snow accumulation at different vegetation types: Heath (control), + Snow (Shrub) and ++ Snow (Forest) . Error bars are 1 SE.

B. pubescens in forest and shrub lost the greatest amount (almost 50%) of mass over the three years of the study, while E. nigrum in heath lost the least (less than 30%).  However, B. pubescens decomposed much more rapidly in the forest than in the shrub between days 365 and 641. The bottom graphs below show that snow fences had no significant effect on decomposition.

Percentage of litter mass remaining (a, d) E. nigrum, (b, e) B. nana, (c, f) B. pubescens in heath, shrub, or forest. Top graphs (a, b, c) are natural transects, while the bottom graphs (d, e, f) represent heath tundra under three different snow depths simulating snow accumulation at different vegetation types: Heath (control), + Snow (Shrub) and ++ Snow (Forest) . Error bars represent are 1SE. Shaded areas on the x-axis indicate the snow covered season in the first two years of the study.

Why do mountain birch leaves decompose so much more than do crowberry leaves?  The researchers chemically analyzed both species and discovered that birch leaves had 1.7 times more carbohydrate than did crowberry, while crowberry had 4.9 times more lipids than did birch. Their chemical analysis showed much of birch’s rapid early decomposition was a result of rapid carbohydrate breakdown. In contrast, crowberry’s slow decomposition resulted from its high lipid content being relatively resistant to the actions of decomposers.

Researchers (Parker right, Subke left) harvesting soils and litter in the tundra. Credit: Jens-Arne Subke.

Parker and his colleagues did discover that decomposition was fastest in the forest independent of litter type. Forest soils are rich in brown-rot fungi, which are known to target the carbohydrates (primarily cellulose) that are so abundant in mountain birch leaves.  The researchers propose that a history of high cellulose litter content has selected for a biochemical environment that efficiently breaks down cellulose-rich leaves. Once the brown-rot fungi and their allies have done much of the initial breakdown, another class of fungi (ectomycorrhizal fungi) kicks into action and metabolizes (and decomposes) the more complex organic molecules.

The result of all this decomposition in the forest, but not the heath, is that tundra heath stores much more organic compounds than does the adjacent forest (which loses stored organic compounds to decomposers).  As forests continue their relentless march northward replacing the heath, it is very likely that they will introduce their efficient army of decomposers to the former heathlands.  These decomposers will feast on the vast supply of stored organic carbon compounds, release large quantities of carbon dioxide into the atmosphere, which will further exacerbate global warming. This is one of several positive feedbacks loops expected to destabilize global climate systems in the coming years.

note: the paper that describes this research is from the journal Ecology. The reference is Parker, T. C., Sanderman, J., Holden, R. D., Blume‐Werry, G., Sjögersten, S., Large, D., Castro‐Díaz, M., Street, L. E., Subke, J. and Wookey, P. A. (2018), Exploring drivers of litter decomposition in a greening Arctic: results from a transplant experiment across a treeline. Ecology, 99: 2284-2294. doi:10.1002/ecy.2442. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.