by Stephen Johnson
Though it has been assumed that the rate of carbon accumulation declines with the age of an individual tree, little empirical evidence has been produced to support this assumption. Understanding how carbon storage capacity changes throughout the life of the tree is important in modeling carbon dynamics in forests, which can be used to determine how forests will contribute to climate change mitigation plans. Net primary productivity is well known to decline in even-aged forests, as does mass gain per unit leaf area. However, few forests are completely even-aged, and many are subjected to selective logging that removes the largest trees. Proper modeling of the amount of carbon lost through this logging can be used to more accurately price carbon credits for the preservation of natural forests, aiding efforts to keep them intact. In order to determine how carbon storage rates change with tree age, Stephenson et al. (2014) collected data from long-term monitoring plots in tropical and temperate areas across the globe. By measuring the diameter of each tree and using allometric equations, the researchers determined how much carbon was being stored over time. They found that while stand productivity declined with age, individual tree carbon gain rate increased, with no signs of declines at any age.
Stephenson et al. used data from long-term ecological monitoring plots in forests on every continent. Forests in each plot are surveyed approximately every five to ten years, with measurements re-taken on each individual tree. The diameter of each tree was measured, and combined with allometric equations in the literature to determine the amount of carbon present in each tree. Measurements were compared for individual trees through time to generate the change in growth rate with age. Models were constructed to determine if the shape of the relationship was increasing, decreasing, or parabolic, and the Akaike Information Criterion was used to determine which model matched the data most effectively. They also tested for bias caused by cross-plot measurements or differing allometry, but found no systematic errors. The data included measurements on 403 species in both tropical and temperate areas, and found that rate of mass gain increased with tree mass in 87% of the species examined. The authors found no evidence of a decline even in the largest trees, with the largest trees potentially adding more than half a metric ton of biomass per year.
There is no evidence that large trees decline in their ability to add mass and sequester carbon. Thus, older trees do not act merely as passive reservoirs of carbon, but actively sequester significant levels of carbon. This contrasts with previously noted declines in stand productivity, which can be explained by reductions in tree density. This research also indicates the value of large trees in carbon capture schemes. Large trees are often removed during the selective logging process, potentially severely reducing the amount of carbon storage in the forest. This can help inform carbon credit values, which are proposed to reward farmers who incorporate trees into agroforestry systems. Larger, older trees are more valuable, and consequently carbon credits should increase on older farms. The results also indicate that reductions in growth rate may not be associated with senescence, as previously assumed.
Stephenson, N.L., Das, A.J., Condit, R., Russo, S.E., Baker, P.J., Beckman, N.G., Coomes, D.A., Lines, E.R., Morris, W.K., Rueger, N., Alvarez, E., Blundo, C., Bunyavejchewin, S., Chuyong, G., Davies, S.J., Duque, A., Ewango, C.N., Flores, O., Franklin, J.F., Grau, H.R., Hao, Z., Harmon, M.E., Hubbell, S.P., Kenfack, D., Lin, Y., Makana, J.-R., Malizia, A., Malizia, L.R., Pabst, R.J., Pongpattananurak, N., Su, S.-H., Sun, I.-F., Tan, S., Thomas, D., van Mantgem, P.J., Wang, X., Wiser, S.K., Zavala, M.A., 2014. Rate of tree carbon accumulation increases continuously with tree size. Nature 507, 90–+. doi:10.1038/nature12914