Does forest structure hold the key to understanding forest carbon uptake?

Will increased atmospheric CO2 result in increased biomass in forest ecosystems? At the leaf scale, the evidence that elevated CO2 increases photosynthetic rates is overwhelming1. However, evidence of increased biomass is equivocal, particularly in mature stands2 For example, many previous FACE experiments were located in regrowing or disturbed stands. As a result, observed responses may simply have represented accelerated growth, rather than an increase in the long-term carbon store in forest ecosystems.

We know that not all trees benefit to the same extent from additional carbon dioxide2. Where within the forest do the largest growth enhancements due to carbon dioxide occur? Do younger trees benefit more than older ones? What does this mean for the carbon balance across whole ecosystems and even the globe? The work in this project will help to address a big uncertainty regarding the response of mature forest stands to elevated CO2.

This is crucial knowledge, because terrestrial ecosystems currently take up about 30% of all the carbon emitted by human activities each year3, reducing the rate of climate change. In order to understand if this uptake will continue, we have to understand the processes that lead to it.

This PhD project will seek to answer these questions using and developing a state-of-the-art model of terrestrial ecosystems, and combining it with observations from a mature experimental forest in the United Kingdom and other sites worldwide.

The Free Air CO2 Experiments being conducted at the Birmingham Institute of Forest Research is the only research facility of its kind in the world, focusing on how mature temperate forests are affected by human emissions of carbon4. The LPJ-GUESS ecosystem model scales plant growth and function from the leaf, to the forest stand, to the globe. It is widely used in assessments of the role of terrestrial ecosystems in the global carbon cycle5, one of the largest uncertainties in future projections of climate change. Observations from these forests and other sources will be combined with the LPJ-GUESS model to test hypotheses regarding the role of nutrients, plant allocation decisions and competition between trees in enabling or limiting the growth response of forests to elevated CO2examine the sensitivity of carbon uptake to different forest structures. Ultimately the modelling system will be applied globally, assessing the contribution of different forest states to the global carbon cycle.