Are old forests underestimated as global carbon sinks?

Abstract

Summary Old forests are important carbon pools, but are thought to be insignificant as current atmospheric carbon sinks. This perception is based on the assumption that changes in productivity with age in complex, multiaged, multispecies natural forests can be modelled simply as scaled‐up versions of individual trees or even‐aged stands. This assumption was tested by measuring the net primary productivity (NPP) of natural subalpine forests in the Northern Rocky Mountains, where NPP is from 50% to 100% higher than predicted by a model of an even‐age forest composed of a single species. If process‐based terrestrial carbon models underestimate NPP by 50% in just one quarter of the temperate coniferous forests throughout the world, then global NPP is being underestimated by 145 Tg of carbon annually. This is equivalent to 4.3–7.6% of the missing atmospheric carbon sink. These results emphasize the need to account for multiple‐aged, species‐diverse, mature forests in models of terrestrial carbon dynamics to approximate the global carbon budget.

A collection of functions to determine annual tree carbon increment via stem-analysis

“…The estimation of forest aboveground carbon (C) pools and increments is essential to address the issue of the role of forest ecosystems on the global C balance [6,15].…”

Rate of tree carbon accumulation increases continuously with tree size

“…Our results resolve conflicting assumptions about the nature of tree growth, inform efforts to undertand and model forest carbon dynamics, and have additional implications for theories of resource allocation 11 and plant senescence 12 . A widely held assumption is that after an initial period of increasing growth, the mass growth rate of individual trees declines with increasing tree size 4,5,[13][14][15][16] . Although the results of a few single-species studies have been consistent with this assumption 15 , the bulk of evidence cited in support of declining growth is not based on measurements of individual tree mass growth.…”

Using the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) to examine the impact of harvest and fire on carbon dynamics in selected forest types of the Canadian Boreal shield

“…However, this explanation, which is based on hypotheses outlined by the authors mentioned above, must be used with caution. As pointed out by Carey et al (2001), other studies have indicated that stem respiration rate decreases with increase in tree size because the amount of CO 2 respired per unit sapwood apparently decreases with age. If the observation of Carey et al (2001) is correct, it is possible that CBM overestimates the amount of CO 2 emitted for long rotations.…”

“…Process-based models can be programmed to include effects of silviculture and/or climate change in growth predictions. Several authors have highlighted the importance of stand structure (e.g., even-or uneven-aged, Carey et al 2001) and successional stage on C cycling at the landscape or country level (Kurz et al 1998, Schulze et al 2000, Law et al 2000, Bottcher et al 2008.…”