Tropical forests and climate change: filling knowledge gaps in a different way

Tropical forests cover around 7% of the Earth's surface but contain 25% of global carbon dioxide reserves, as well as contributing to more than 30% of the world’s primary production. "That is why these forests have such an incredibly important effect on CO2 levels in the atmosphere and are decisive in controlling climate change." Pieter Zuidema said. "And while forests influence climate, changes in the climate also have an effect on forest dynamics and tree growth. It is therefore extremely important to understand these mechanisms. Yet we have an enormous gap in our knowledge. We do not know the precise causes of the changes we observe in forest dynamics, and we understand very little of the responses of trees and forests to climatic changes.”

Change of strategy

Pieter Zuidema and his five colleagues from the Forest Ecology and Forest Management Group plus a colleague from the University of Melbourne therefore advocate another strategy for studying the effects of climate change on these very important forests: "A lot of research has been done to quantify biomass in tropical forests and detect whether this has changed over the last few decades. Recorded changes in biomass have been attributed to the fact that CO2 stimulates photosynthesis, or that global warming increases plant respiration. In fact, we are not able to link changes in forest biomass to such causes as so little is known about the mechanisms involved. This is worrying because it means models that predict the future of tropical forests are based on poor understanding of processes. Our suggestion is therefore to make use of annual ring analyses, isotope research, models and large-scale experiments." 

Rings, isotopes and models

With support from the European Research Council the authors are now applying this approach to projects in Bolivia, Cameroon and Thailand: "In these countries we have performed stable isotope analyses on around 1200 trees in order to detect whether they have benefited from raised CO2 levels in the atmosphere. More CO2 means that trees can utilise water more efficiently and perhaps grow more rapidly as well. Annual rings can tell us whether growth has changed since the start of the Industrial Revolution. And in order to actually understand how trees may respond to climate change we have created models for tree growth.” However, we haven't followed all of our own recommendations, Pieter Zuidema says: “Large-scale experiments are really essential for determining beyond a shadow of doubt the effects of warming and CO2 enrichment, but they're terribly expensive and logistically very demanding. And we already have our hands full with the rings, isotopes and models.”