The effects of warming on wood decomposition and decomposer communities differ between two sites

Background/Question/Methods

Faster decomposition of leaf litter and soil organic matter with warming has been well studied. However, few studies have tested how wood decomposition will respond to warming although woody debris comprises about 20% of carbon in forests. The response of decomposer communities, especially microbial communities, is often decoupled from the response of decomposition rates and we don't know whether changes in the microbial community structure will accentuate or mitigate the acceleration of enzyme activity under warming.

We used a large-scale field manipulation with a novel design to test how wood decomposition and microbial communities in wood respond to warming across a range of temperatures at two sites. We placed wood in two size classes (fine- woody debris and coarse- woody debris) in 12 chambers at each of two sites representing the northern and southern range limits of eastern hardwood forests. The chambers were warmed in a regression design with nine treatment chambers (1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 and 5.5° C above ambient) and three control chambers at ambient temperature. We removed samples annually for four years and assessed rates of mass loss and changes in fungal community structure, arthropod community structure, and fungal and bacterial abundance.

Results/Conclusions

Warming altered wood decomposition but the effects varied depending on the size of the wood and the site. Smaller diameter wood (fine woody debris) decomposed faster at the cooler, northern site whereas larger diameter wood (coarse woody debris) decomposed faster at the warmer, southern site.  Warming did not alter rates of coarse woody debris decomposition at either site, although the fungal communities in coarse woody debris were more different among warming treatments at the warmer, southern site where temperatures were closer to maximum temperatures for the fungi. Fine woody debris decomposition rates increased with warming at the cooler northern site, but decreased with warming at the warmer southern site.  Moisture limitation or the shifts in wood decomposer community structure may explain slower fine woody debris decomposition rates at warmer temperatures at the southern site. Our results indicate that the effects of warming on decomposition rates can vary based on substrate size, and site.