Wetenschap - 8 december 2016

Large trees grow more slowly than was thought

Roelof Kleis

Carbon capture volumes are fairly well known for ordinary trees. Mathematical formulas describe the relationship between tree size and biomass growth. Forest ecologists can use this to get a reasonable estimate of the contribution that trees and forests make to the carbon balance, which is important for climate change. But this is trickier for large trees, as Pieter Zuidema and international colleagues explained in Functional Ecology in November.

Photo: Peter Groenendijk

The group used annual rings to track the growth history of more than 700 large tropical trees in Thailand, Cameroon and Bolivia from a database containing 1400 trees and 14 different species. The life histories show that the biomass growth of individual large trees levels off. Zuidema and colleagues are therefore disputing the conclusions of a Nature article in 2014 that argued that the biomass growth of trees continues to increase up to the end. But that was a population-level study. Zuidema: ‘You can’t draw conclusions about individual trees based on observations of a population as a whole. If the largest trees in the forest grow fastest, that still doesn’t mean that trees grow at a faster rate towards the end of their life.’

The study by Zuidema originated in the fortnightly meetings in which researchers in the Forest Ecology & Management chair group discuss a recent scientific paper. This Nature article not only raised questions, the researchers also realized that they were able to answer those questions with their own data. 

According to Zuidema, the findings do not suddenly mean forests are storing less carbon. ‘But we do need to be incredibly careful with assertions about the growth of large trees and the contribution that makes to carbon sequestration in forests, and we need to carry out more measurements of large trees.’