Controls on carbon cycling in terrestrial ecosystems continue to be a central theme in ecosystem ecology, in part due to the increasing concerns regarding human disruption of the carbon cycle and the consequences for the global climate system. There is a growing urgency to better understand the terrestrial ecosystems ability to absorb and store carbon as a way of mitigating emissions from the burning of fossil fuels, and part of that potential lies in the reservoirs of organic carbon in forest ecosystems.
Downed logs and trunks, and woody roots (collectively known as woody debris) play an important role in the carbon cycle in many forest ecosystems, particularly in boreal and temperate zones, where woody debris persists for long periods of time on the forest floor. Downed wood represents a large fraction of the above-ground organic carbon in the ecosystem (Harmon et al. 1986), and are an important source for habitat for many organisms. In spite of their recognized importance to temperate forest biogeochemistry (Carmona et al. 2002; Janisch & Harmon 2002), it has been difficult to assess their rates of turnover and the impact of species composition and state of degradation. This is due to the fact that the decomposition process is generally so slow that it exceeds the time-frame of most experiments, which complicates model validation.
Understanding the ways that wood rots
In this issue's Editor's Choice paper, Interspecific differences in wood decay rates: insights from a new short-term method to study long-term wood decomposition by Grégoire T. Freschet, James T. Weedon, Rien Aerts, Jurgen R. van Hal and Johannes H. C. Cornelissen, the authors use a combination of empirical short-term measurements in a range of species in a chronosequence of decay classes to develop an new methodology for estimating long-term woody decay rates for boreal forest ecosystems. The authors use a three-pronged approach, combining short-term estimations of woody decay with a chronosequence of decay classes and a use model to integrate this information, so generating a new tool for the assessment of woody decomposition.
Six dominant tree species, representing the main functional groups at the Swedish study site, were collected in different states of decay and left to decompose (or ´further decompose´) for two years in a common incubation site using litterbags in a classic design. The data for mass loss of the different decay stages and species, along with initial nutrient and lignin concentrations, was incorporated into an iterative model describing a range of different decay functions (e.g. linear, exponential or sigmoidal).
One of the most surprising results was that there was that no single model accurately described decay in all species. Only Alnus and Salix woody debris followed a classic negative exponential decay model, while the other species demonstrated a more complex picture depending on the decay class, species identity and organ (stem or root). Furthermore, only lignin content and pH were good predictors of rate of decay, which contrasts with some recent studies of woody decomposition (Cornwell et al., 2009). On the other hand, nutrient concentrations and carbon:nutrient ratios were not well correlated with woody decomposition.
Traits are important, even for wood
The combined approach of direct short-term observation and the application of an iterative model demonstrates two advances for our understanding of carbon turnover of woody debris. One is that all wood is not created equally and that it appears that models of long-term decomposition must incorporate the complexity of responses for both decay class and physical wood characteristics. Second, while traits associated with species or functional groups have been shown to be important determinants of the early stages of woody decomposition (Cornwell et al., 2009; Weedon et al., 2009), the importance of these traits also extends into later stages of woody decay. While care should be taken to apply this approach in other environments (i.e. tropical forests), this study serves as a useful jumping off point for a new integrated methodology to incorporate more accurate estimates of woody debris turnover in boreal forest ecosystems, a key variable in our understanding of the global terrestrial carbon cycle.
Associate Editor, Journal of Ecology
- Carmona, M.R., Armesto, J.J., Aravena, J.C., & Pérez, C.A. (2002) Coarse woody debris biomass in successional and primary temperate forests in Chiloé Island, Chile. Forest Ecology and Management, 164, 265-275.
- Cornwell, W.K., Cornelissen, J.H.C., Allison, S.D., Bauhus, J., Eggleton, P., Preston, C.M., Scarff, F., Weedon, J.T., Wirth, C., & Zanne, A.E. (2009) Plant traits and wood fates across the globe: Rotted, burned, or consumed? Global Change Biology, 15, 2431-2449.
- Freschet, G.T., Aerts, R., Cornelissen, J.H.C., van Hal, J.R., Weedon, J.T. (2011) Interspecific differences in wood decay rates: insights from a new short-term method to study long-term wood decomposition. Journal of Ecology, Vol. 100:1, 161-170
- Harmon, M.E., Franklin, J.F., Swanson, F.J., Sollins, P., Gregory, S.V., Lattin, J.D., Anderson, N.H., Cline, S.P., Aumen, N.G., Sedell, J.R., Lienkaemper, G.W., Cromack Jr, K., & Cummins, K.W. (1986) Ecology of coarse woody debris in temperate ecosystems. In Advances in Ecological Research, Vol. 15, 133-302.
- Janisch, J.E. & Harmon, M.E. (2002) Successional changes in live and dead wood carbon stores: Implications for net ecosystem productivity. Tree Physiology, 22, 77-89.
- Weedon, J.T., Cornwell, W.K., Cornelissen, J.H.C., Zanne, A.E., Wirth, C., & Coomes, D.A. (2009) Global meta-analysis of wood decomposition rates: a role for trait variation among tree species? Ecology Letters, 12, 45-56.
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