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Fungi and wood decomposition


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Fungi are astonishing, even beautiful, with their wide range of sizes, shapes and colors. They are also seemingly everywhere and, together with bacteria, nematodes and other microbes, they play a crucial role at the global scale: breaking down organic matter and releasing carbon, oxygen, nitrogen, and phosphorus into the soil and the atmosphere. In other words, they are a key player in carbon and nutrient cycling.
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While carbon cycling models are more frequently accounting for microbial biomass, they do not as often incorporate the differences among the kind of microorganisms or their many traits. Different fungi have different abilities to decompose wood, so accounting for these differences could result in a potentially massive variation in carbon dynamics.

So what are the critical traits of fungi that may affect carbon and nutrient cycling? To name just a few: certain species grow quickly, others slowly. Some are tolerant to stress such as drought or extreme temperatures. Others not. Some grow densely, while others grow long filaments (hyphae). While these traits are being studied, there’s still much we don’t yet know about the wonderful world of fungi.

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Given the tremendous functional diversity of fungi, scientists have hypothesized that there may be a relationship between their traits and the rate at which they decompose wood. Some studies considered slow-growing fungi with dense filaments would decompose wood faster than fast-growing fungi with long filaments. Other studies assumed the opposite. Still, others considered performance traits such as moisture stress-tolerance, believing more tolerant fungi would decompose wood slower than less tolerant ones.

This study tests the relationship between fungal characteristics and wood decomposition rates. Understanding how decay rates vary between fungi could ultimately enhance the accuracy of terrestrial carbon dynamic forecasts.

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To explore the relationship between fungal traits and wood decomposition rates we combined two approaches: lab experiments and a long-term field study. In the lab, we studied fungal wood decomposition rates under controlled conditions. We observed how quickly 34 fungi from North America decomposed the same type of wood. We then cross referenced the decomposition rates with a database of 22 fungal traits previously measured on the same samples in order to identify which fungal traits most affect wood decomposition. Over a 5-year field study, we measured the decay of over 70 logs from 20 woody species. We isolated 1,582 fungi from those logs and measured their growth rates to evaluate the robustness of our laboratory observations under complex natural conditions. This dual approach provides greater confidence in the strength of the highlighted relation between fungal traits and wood decomposition rates.

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The type of wood and climate conditions account for roughly half of the wood decomposition rate. Fungi account for around a quarter - making it a very important factor in decomposition. Based on observations in the lab (22 traits across 34 different fungi), as well as in the field (1,582 isolated fungi in total), we determined that fungal growth rate (measured by how quickly fungi extend their hyphae) is the single strongest trait predictor of fungal wood decomposition. In fact, fungal decomposition ability appears to be closely related to a well-known trade off among fungi between competitive dominance and stress tolerance. We observed that fungal decomposition ability exists along the same spectrum from slow-growing, stress-tolerant fungi that are poor decomposers, to fast-growing, highly competitive fungi with fast decomposition rates, regardless of the local microclimate.
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In natural environments, fungi compete for resources. Some fungi are more dominant within their community while others are less competitive but capable of withstanding stressful environments. On one end of the spectrum, competitive fungi grow fast and decompose wood quickly; on the other end,dense fungi tolerate a wide range of moisture but grow slowly and decompose wood slowly.

In the end winners and losers are a matter of perspective. While slow growers may not be dominant, they can survive in harsher conditions, ensuring that key nutrients such as nitrogen and phosphorus are recycled into the soils. And although faster decomposition means carbon is released into the atmosphere quicker, it can also increase plant productivity and tree growth by breaking down more quickly dead matter for the production of soil.

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Our study shows that fungal decomposition is negatively associated with moisture and temperature stress tolerance and production of enzymes, while positively associated with combative ability and growth rate. While our study focused on North America, we hope that it will inspire global studies. A better understanding of the biogeographic distribution of fungal communities and their functional traits would help scientists translate these traits into predicted decomposition rates. Such an approach would enhance the accuracy of wood decay predictions and ultimately global carbon cycling models.
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