Understanding how forests grow and how they metabolize carbon is crucial to ecosystem carbon flux, feedbacks between vegetation and climate, and the forestry. However, a key question remains unanswered: how is carbon metabolism converted into growth in trees?
Respiration (the breakdown of carbon compounds for energy) provides the resources for a plant to drive growth in its organs. At the ecosystem level, whole-tree respiration can be estimated, however parsing out the contributions of each organ is difficult and confounds studies trying to link respiration to growth. We may expect that higher rates of respiration within an organ contribute to greater growth. So how does respiration in a tree organ affect the growth of said organ?
In a recent article in Tree Physiology, Tommy Chan and colleagues study the seasonal relationship between stem growth and stem respiration in Scots pine (Pinus sylvestris). To measure stem growth, they used a dendrometer, which measures variation in stem diameter that can be related to growth. Stem respiration was measured using gas exchange chambers. So how was stem respiration linked to growth?
They found that next-day stem respiration was best correlated with daily stem growth, and these correlations were strongest during summer when stem growth was greatest. Why would respiration peak after the growth? Mature trees are large enough that diffusion of CO2 from stem respiration can take a very long time. However, given that this is one of the first studies to investigate a link between stem respiration and growth, there could be other undiscovered processes contributing to time lags, such as source-sink relationships with carbohydrate production and demand.