Rate of tree carbon accumulation increases continuously with tree size

Forests are major components of the global carbon cycle, providing substantial feedback to atmospheric greenhouse gas concentrations¹. Our ability to understand and predict changes in the forest carbon cycle—particularly net primary productivity and carbon storage—increasingly relies on models that represent biological processes across several scales of biological organization, from tree leaves to forest stands^{2, 3}. Yet, despite advances in our understanding of productivity at the scales of leaves and stands, no consensus exists about the nature of productivity at the scale of the individual tree^{4, 5, 6, 7,} in part because we lack a broad empirical assessment of whether rates of absolute tree mass growth (and thus carbon accumulation) decrease, remain constant, or increase as trees increase in size and age. Here we present a global analysis of 403 tropical and temperate tree species, showing that for most species mass growth rate increases continuously with tree size. Thus, large, old trees do not act simply as senescent carbon reservoirs but actively fix large amounts of carbon compared to smaller trees; at the extreme, a single big tree can add the same amount of carbon to the forest within a year as is contained in an entire mid-sized tree. The apparent paradoxes of individual tree growth increasing with tree size despite declining leaf-level,sup>8, 9, 10 and stand-level¹⁰ productivity can be explained, respectively, by increases in a tree’s total leaf area that outpace declines in productivity per unit of leaf area and, among other factors, age-related reductions in population density. 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 allocation¹¹ and plant senescence12.

1. Pan, Y. et al. A large and persistent carbon sink in the world’s forests. Science 333, 988–993 (2011)

2. Medvigy, D., Wofsy, S. C., Munger, J. W., Hollinger, D. Y. & Moorcroft, P. R. Mechanistic scaling of ecosystem function and dynamics in space and time: Ecosystem Demography model version 2. J. Geophys. Res. 114, G01002 (2009)

3. Caspersen, J. P., Vanderwel, M. C., Cole, W. G. & Purves, D. W. How stand productivity results from size- and competition-dependent growth and mortality. PLoS ONE 6, e28660 (2011)

4. Kutsch, W. L. et al. in Old-Growth Forests: Function, Fate and Value (eds Wirth, C., Gleixner, G. & Heimann, M.) 57–79 (Springer, 2009)

5. Meinzer F. C., Lachenbruch B., Dawson T. E., eds. Size- and Age-Related Changes in Tree Structure and Function (Springer, 2011)

6. Enquist, B. J., West, G. B., Charnov, E. L. & Brown, J. H. Allometric scaling of production and life-history variation in vascular plants. Nature 401, 907–911 (1999)

7. Sillett, S. C. et al. Increasing wood production through old age in tall trees. For. Ecol. Manage. 259, 976–994 (2010)

8. Mencuccini, M. et al. Size-mediated ageing reduces vigour in trees. Ecol. Lett. 8, 1183–1190 (2005)

9. Drake, J. E., Raetz, L. M., Davis, S. C. & DeLucia, E. H. Hydraulic limitation not declining nitrogen availability causes the age-related photosynthetic decline in loblolly pine (Pinus taeda L.). Plant Cell Environ. 33, 1756–1766 (2010)

10. Ryan, M. G., Binkley, D. & Fownes, J. H. Age-related decline in forest productivity: pattern and process. Adv. Ecol. Res. 27, 213–262 (1997)

11. Thomas, S. C. in Size- and Age-Related Changes in Tree Structure and Function (eds Meinzer, F. C., Lachenbruch, B. & Dawson, T. E.) 33–64 (Springer, 2011)

12. Thomas, H. Senescence, ageing and death of the whole plant. New Phytol. 197, 696–711 (2013)

Originally published at www.nature.com/nature/journal/v507/n7490/full/nature12914.html

• Category News, Science Reports
• Topics Forests and Climate Connection, Old Growth