Tree giants are carbon giants – above and below ground

Typically, measurements of soil organic carbon do not take into account the higher soil carbon concentration adjacent to, within, and beneath the trunks of large trees. Root volume, which displaces soil and thereby reduces the spatial density of soil organic carbon, is also not included in the usual calculations.

Accounting for these two variables could have a significant impact on carbon balances when converting a primary forest with large trees to a secondary forest with much smaller trees or to a treeless area. The authors measured soil bulk density and soil organic carbon directly beneath and within large tree trunks and in and beneath humus mounds near board roots. The data were recorded in tropical mixed eucalypti forests in Australia.

There is more to it than meets the eye

Here, the scientists determined soil carbon as a function of soil depth. Immediately adjacent to the trees studied, they estimated that approximately 90% of the cumulative soil carbon content in the mineral soil occurs to a depth of 2.60 m. This value was compared with the amount of soil carbon between the trees – that is, in the area between the board roots – from previous measurements. The result was a four times higher concentration of soil organic carbon under the trunk than between the trees. When the soil carbon under the log is added to the amounts directly adjacent to it (in and under the humus mounds between the board roots), the amount of carbon there per unit area is 7% higher than between the trees.

The results suggest that more CO2 may have been produced by land-use changes in the past when trees with trunk diameters greater than 1 m were felled than previously thought. The scientists were able to identify an additional 50 tree species worldwide that are likely to also have higher soil organic carbon levels, like the eucalyptus trees examined in this study. These include mixed oak forests, such as those found in Europe. The additional soil carbon per hectare correlated positively with the basal area of the trees, which increases with the number of large trees in the stand. It follows that preserving large trees can help store higher amounts of carbon in the forest. Therefore, it is important to protect especially medium-sized trees in order to be able to ensure the existence of large trees in the future.

Comment

The results of Dean et al. (2020) highlight the importance of old, large-diameter trees and the complexity of the forest carbon cycle:

1. The importance of large old trees is very significant not only for biodiversity and carbon storage of aboveground biomass, but also for soil carbon. Consequently, in order to increase the carbon storage in forests, it is essential to allow trees to grow old and large, not to completely harvest existing old and thick trees, and to make sure that all forest development stages, including the decay stage, are present. Carbon stored under old trees cannot be recaptured by new young trees as quickly as it is released when a single tree is cut. Timber harvesting should therefore be done carefully and with caution, allowing as many trees as possible to reach old age.
2. The study also shows how complex soil carbon accumulation is and how heterogeneously it is distributed over a forest area. It becomes clear that the simplified calculation methods do not do justice to the complexity of the carbon cycle and that the carbon storage potential is often estimated in an oversimplified way.
3. The need for further research in the field of soil organic carbon becomes clear and it is necessary to continuously adapt existing models to new findings.