As I’ve been enjoying the tropical weather here in Sudbury over the past week
I’ve been thinking about how ecologists report radial tree growth. And I’m not sure we’re doing it right…
During my PhD, one of the papers I read often was:
- Bee, J. N., Kunstler G. & Coomes, D.A. (2007) Resistance and resilience of New Zealand tree species to browsing. Journal of Ecology 95, 1014–1026.
Table 5 specifically is worth drawing attention to:
It looks like trees in north temperate regions grow really fast – more so even than in the tropics (i.e. Panama). But what happens in winter? In Sudbury, temperatures have been awfully cold in December and they’re likely to hover around similar levels until March.
Surely, there isn’t much growth in trees during this period. So does it make any sense to report diameter growth from January to December? Wouldn’t it be better to report growth relative to some standardized measure of growing season, such as the number of days when air temperatures are >6°C?
In the case of global comparisons, such as in the Bee et al. table, this might be a moot point. North American trees still win, just by more. But standardization is likely to be a real issue for studies that use latitudinal gradients as a space-for-time substitution to test the potential effects of climate warming. A few examples are:
- Silva LCR, Anand M, Leithead MD (2010) Recent widespread tree growth decline despite increasing atmospheric CO2. PLoS ONE 5(7): e11543.
- Huang, J., J. C. Tardif, Y. Bergeron, B. Denneler, F. Berninger, and M. P. Girardin. (2010) Radial growth response of four dominant boreal tree species to climate along a latitudinal gradient in the eastern Canadian boreal forest. Global Change Biology 16:711–731.
- Lloyd AH, Bunn AG and Berner L (2011) A latitudinal gradient in tree growth response to climate warming in the Siberian taiga Global Change Biology 17: 1935–45.
Focusing on Fig. 1 in Silva et al. 2010, red maple (Acer rubrum) at 47°N seems to be growing slower than at 52°N. But temperature, and hence growing season, differs between these two sites. I wonder whether trees would grow at similar rates if annual basal increments were reported relative to the length of the growing season?… Ultimately, what ecologists are regularly reporting is an “absolute” outcome – the product of growth and length of the growing season – rather than the “true” rate of growth.
Leave a comment and let me know what you think!