” If a tree falls in a forest and no one is around to hear it, does it make a sound?” – George Berkeley

A question to get many young minds thinking. But here’s another:

If a tree falls in a river, where does it go?

Perhaps not an immediate issue for those of us in Orkney, but it is one which will affect us, eventually.

Scientists from Colorado State University have been examining the fallen trees which form log jams in the Mackenzie River. Its delta is the third largest in the world by land area and drains about 20% of Canada. The team studied about 13,000 square kilometres (5,000 square miles) of delta in the biggest attempt to map woody deposits so far.

Why?

Throughout the Arctic, fallen trees make their way from forests to the ocean by way of rivers. Those logs can stack up as the river twists and turns, resulting in long-term carbon storage.

Researcher  Alicia Sendrowski explained:

 “We have great knowledge about carbon in other forms, like dissolved or particulate organic carbon, but not what we call ‘large carbon’ — large wood.” 

The team spent three weeks in the field measuring river driftwood, sampling the wood to date using radiocarbon dating and then used remote imagery to identify wood at the river’s surface and estimate the areal extent of the logjam.

The deposit, comprising more than 400,000 miniature caches of wood, is storing about 3.4 million tons (3.1 million metric tons) of carbon. The largest single deposit, which covers around 20 American football fields, stores 7,385 tons (6,700 metric tons) of carbon alone. But because there are even more logs buried in soil, submerged underwater and hidden from aerial imagery under vegetation, the total amount of carbon stored in the delta’s wood could be about twice as large.

The Arctic’s cold, often dry or icy conditions mean trees can be preserved for tens of thousands of years. With climate change and warming conditions we risk losing this carbon storage.

The Mackenzie River Delta is a “hotspot” of carbon storage thanks to incredibly carbon-rich soils so the logs’ carbon storage makes up a relatively small fraction of the delta’s total carbon storage, which is around 3 quadrillion grams of carbon.

Alicia Sendrowski said:

“But we think it’s still important because as changes in the basin occur, like logging or damming, and as climate change alters precipitation patterns and warming, wood preservation will decrease. It’s a significant amount of carbon, so there’s a potentially significant loss of carbon storage.”.

The Mackenzie logjam also reflects only one basin in the Arctic; at least a dozen deltas larger than 500 square kilometers dot the north, so all together, large woody deposits throughout the Arctic could add up to be a significant carbon storage pool, and one we know little about.

The researchers were also interested in how long a tree can last in the Arctic, which is important when modeling how “active” a carbon pool is — that is, how rapidly material is moved around. Carbon dating revealed that while many of the trees they sampled began growing around or after 1950, some were much older, reaching back to around 700 CE. (A study in the 1960s carbon-dated wood from a tree preserved in an icy mound to about 33,000 years ago.)

Click on this link to access, Wood-based carbon storage in the Mackenzie River Delta: The world’s largest mapped riverine wood deposit, published in Geophysical Research Letters.