Standardizing radial growth

As I’ve been enjoying the tropical weather here in Sudbury over the past week

Everyone’s gone home for winter

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:

Picture1

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.

Sudbury minimum air temperatures (brrrrr)

Minimum air temperatures in Sudbury, ON — brrrrr……

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!

AJT

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2 comments

  1. Thanks for the interesting post; a great excuse to procrastinate this afternoon!

    First things first, it’s nice to know that somewhere in the northern hemisphere winter is actually taking place – Cambridge is still unseasonably warm and I bet the trees in my garden wish they were evergreen right about now!

    Regarding your suggestion that we need to rethink how we measure tree growth, you make a good point: are two trees increase in diameter by 1 cm during the course of the year – but one does it in 6 months while the other takes 8 – really growing at the same rate?

    I guess my feeling is that it depends on what the aim is. If the objective is to quantify the asymptotic growth rate of a species and determine at which temperature it is maximal, then averaging growth across an entire year is of little help. If, instead, we want to know how much more/less C a tree is going to sequester if the temperature increases by 1°C, then I think the “absolute” growth is what we are after. Using Fig. 1 of the Silva et al. paper you mention as an example, it’s true that the difference in instantaneous growth rate between red maple trees growing at 42°N vs 47°N might be close to zero, but at the end of the day (of the year?) trees further south sequestered twice as much C than those at 47°N (side note: Fig. 1 of the Silva paper isn’t the easiest to interpret, especially since BAI axes are reported with different ranges!).

    One issue that should be considered if you were to standardize growth according to some measure of growing season length (e.g., days when temperature is >5°C; GDD5), is that (a) growing season length depends on a combination of factors aside from just temperature and (b) that different species have different climatic cues based on which they initiate and end the growing season. So while GDD5 might be a good predictor of growing season length for one species, it might tell you a lot less about another. Unrelated, but one thing I was wondering about when I read your post is how liable growth cues are as a trait. Can trees easily adapt when they begin/stop growing, or is this trait phylogenetically conserved?

    Another point (and with this I’m finished, promise!) is what measure we should use to express tree growth. Climate (as well as other abiotic factors) is known to influence how trees allocate C reserves. For example, trees in warmer climates tend to invest more in height compared to diameter growth. It is therefore conceivable that two trees (of the same species) that sequester the same amount of C each year could have different diameter increments.

    Tommaso

    1. As always, it depends on the question. 🙂

      I agree with your interpretation that it makes little sense to average across a year in order to ask at which latitude is growth rate fastest. But asking how much C might be stored in a tree at different latitudes is definitely sensible with current approaches, though you probably want to be mindful about issues like height!

      Your point about plasticity in growth cues is worth thinking about more.

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