Woody-tissue respiration


The complete accounting of carbon entering and exiting forest ecosystems is a vital task, and Damesin et al. have examined the contribution of woody-tissue respiration to this, as reported on pp. 159–172 in this issue. However, scaling up from tree measurements to estimations at the stand level is fraught with difficulties. The sizeable effort put into determining volume and surface area of branches was an important component of the study.

What was noteworthy? First, it was found that branch respiration rate differs from stem respiration rate at all times of the year, with the direction and magnitude of the difference depending on the base for expressing the fluxes. Sprugel (1990) and Maier et al. (1998) reported similar findings, but this is the first such report for a deciduous species. Damesin et al. show that as a consequence of these differences between stems and branches, the common practice of using measurements of stem respiration to estimate branch respiration leads to underestimates when sapwood volume is the basis for scaling, or overestimates when surface area is the basis.

The second point of note was that branch respiration appears to make about the same contribution to ecosystem respiration as does stem respiration in the beech ecosystem, because of both higher specific rates and the large amount of respiring branch matter. Partly because of the relatively high flux from branches, the above-ground, woody-tissue respiration contributed a large fraction of total ecosystem respiration in the beech ecosystem in comparison to previous reports for coniferous forests (e.g. Ryan et al. (1996), Lavigne et al. (1997)). The authors found that one third of ecosystem respiration is derived from woody-tissues in their beech forest whereas values of 15% and less are typical in coniferous ecosystems.

Assuming that the present results are typical of deciduous forests, then measurements of woody-tissue respiration are essential in studies having as an objective the complete accounting of C entering and exiting the ecosystem. Increasingly chamber measurements are taken at sites where the eddy covariance method is used to estimate net ecosystem exchange, for the purpose of explaining the observed exchange between ecosystem and atmosphere. Damesin et al. have shown that woody-tissue respiration deserves as much attention as other fluxes, such as soil respiration, that branch respiration should be measured in addition to stem respiration, and that allometric relationships established by harvesting a sample of trees at the site should facilitate the scaling of woody tissue respiration.