Warmer Forests Could Accumulate More Dead Wood by 2099

Warmer forests have long been expected to rot more rapidly. That has been the prevailing assumption among scientists.

When dead trees fall onto a warming forest floor, they are expected to decompose faster, matching the processes that killed them. Rising temperatures support the fungi and beetles that consume dead timber.

Scientists put this assumption to the test by producing a comprehensive projection for the world’s forests up to 2099.

Their assessment found that dead wood is accumulating more quickly than it can decay. The difference was greatest across the cold northern regions, where tree mortality is already increasing.

A concealed carbon reservoir

All this fallen timber represents a largely overlooked giant within the climate system. Globally, dead wood stores approximately 80 billion tons of carbon (73 billion tonnes).

That represents around eight percent of the carbon held in all forests. This reserve changes according to two competing processes.

Fresh dead wood is created whenever trees die, while older material disappears as organisms return its carbon to the atmosphere. Climate change influences both processes.

Pascal Edelmann, an ecologist at the Technical University of Munich (TUM), set out to establish which process would prevail. Would forests build up dead wood or lose it? The issue had not previously been resolved.

New understanding of dead wood

Edelmann’s team developed its answer using three sources of evidence. First, they used a map showing dead wood stocks across the world’s forests in 2010.

They then added forecasts of tree growth and mortality from five vegetation models.

To assess decomposition, the researchers used a global experiment monitoring decaying logs at 55 locations across six continents. At every site, researchers recorded how quickly wood broke down under local temperatures and rainfall.

They projected this complete picture to 2099, comparing carbon entering dead wood with carbon leaving it.

A value above one indicated an accumulation of dead wood, whereas a score below one showed that stocks were declining.

Two competing processes

Warming increases both processes simultaneously. As more trees die and forests grow more quickly, additional wood collects on the forest floor.

A recent study showed that drought, fires and insect outbreaks are already increasing tree deaths around the world. More deaths mean a greater supply of dead wood.

Heat also affects the other side of the equation. Warmer conditions increase decomposition rates, as measurements from forest sites worldwide have shown.

This indicates that fungi, beetles and other organisms that consume dead wood become more active as temperatures rise.

The two effects might offset one another, keeping the global stockpile broadly stable. Alternatively, one could exceed the other.

Even a slight difference between them determines whether forests worldwide gain or lose dead wood during the century.

Findings from the models

Before this research, no study had established which process would come out ahead. Edelmann’s findings pointed to a single outcome.

Across every model and climate scenario examined, dead wood production exceeded decomposition by roughly five percent. Although modest, this advantage was consistent.

Both processes intensified over the century, but formation increased faster than decomposition.

The annual amount of newly created dead wood rose by an average of about 15 percent, while decomposition increased by around eight percent.

By the late 2080s, the yearly volume of new dead wood entering forests rises substantially.

Important regional differences

The global figure conceals striking contrasts between regions. The strongest build-up occurs in the cold north, across boreal forests and the Arctic margins of Canada and Russia.

Temperate areas follow closely behind, whereas the tropical pattern is markedly different.

There, formation only slightly exceeded decomposition because the heat and moisture that enable rapid tree growth also make wood rot quickly.

In time, this contrast could alter the global distribution of dead wood. At present, tropical forests contain about two-thirds of the world’s dead wood.

Yet, as northern forests accumulate it most rapidly, dead wood could become more evenly distributed across the world in the coming decades.

Uncertain outcomes

These results are not guaranteed. By the end of the century, roughly one in five simulations showed dead wood decreasing rather than increasing. Decomposition is particularly difficult to predict.

One study found that termites greatly accelerate wood decay as temperatures increase, and warming may allow these decomposers to move into places where they were previously uncommon.

The researchers also excluded two major uncertainties. Wildfires can completely burn away dead wood, while human decisions could overwhelm the climate effect in either direction.

Why dead wood matters

Rotting timber is anything but waste. It provides habitat and food for about a quarter of all forest species and supports biodiversity throughout the world’s forests.

“Climate change will profoundly accelerate deadwood dynamics in the forests of the world,” Edelmann and his colleagues write.

Many insects reliant on dead wood are now at risk of extinction. Warming is likely to leave forests with more rather than less dead wood, with the greatest increases occurring in the cold north.

For foresters and policymakers, this makes dead wood something to retain instead of remove.