What do nonstructural carbohydrate concentrations of trees indicate about plant carbon balance?

Background/Question/Methods

Non-structural carbohydrates have long been assumed to indicate the internal carbon balance of trees. Hundreds of studies have assumed that ‘low’ carbohydrate levels indicate that the tree’s demand for carbohydrates for growth, maintenance and transport exceeds the supply from photosynthesis, and that ‘high’ levels indicate that supply exceeds demand. Measurement of non-structural carbohydrates coupled with this assumption has been used to make many inferences about the presence or absence of a carbon limitation (carbohydrate demand > supply). For example, this approach has been used to infer that tropical trees under drought stress are not carbon limited, that carbon limitation was not the cause of widespread aspen mortality, that trees at altitudinal tree line are not carbon limited, and that older, taller trees are not carbon limited.

The assumption linking carbohydrates and carbon balance results from assumption about the priority for storage in carbon partitioning: that carbohydrate storage has the lowest priority to sinks of respiration; wood, foliage, root, and reproductive growth; defensive compounds; and storage. If carbohydrate storage pools were the last to fill after stressful periods and the first to empty with the onset of stress, then carbohydrate storage would likely be a low priority (if simple priorities really exist for partitioning among pools). Carbohydrate status would also indicate internal carbon balance if storage is simply a passive process that absorbs excess and contributes during times of need. However, if carbohydrate storage is a high priority and an active sink then the assumption linking carbohydrates and carbon balance is incorrect, and no inference can be drawn about the balance of carbohydrate supply and demand from carbohydrate concentrations. We examine the literature for evidence of the priority for carbohydrate storage.

Results/Conclusions

The literature overwhelmingly shows that carbohydrate storage is an active sink with a high priority. Several defoliation studies did not deplete carbohydrate levels, even though stem growth declined, indicating that storage had a higher priority than wood growth. Harvesting latex from rubber trees (a large sink for carbohydrate) actually increased carbohydrate concentrations. And, carbohydrates increase in droughted trees when growth has ceased. Inferences about the internal carbon balance of trees made from carbohydrate concentrations are likely incorrect. Inferences about tree carbon balance need to be made by measuring all of the fluxes that comprise tree carbon balance, including flux to carbohydrate storage.