How Diverse Canopies Absorb More Carbon

As the climate crisis becomes more dire, planting trees and turning green space into forest is no longer enough to balance carbon emissions. Negative emissions technologies need to be implemented, but also, existing forests and other ecosystems need to be optimized for maximum productivity. Managing forests and other ecosystems will allow them to perform photosynthesis at the highest rates and remove as much carbon from the air as possible.

“Carbon removal efforts by companies and governments have largely relied on trees and soil. But even under a best-case scenario, these can only provide around half of the removal needed. We only have so much available arable land in which to plant the number of trees we need to store enough carbon.”

Let’s Not Pretend Planting Trees Is a Permanent Climate Solution

Scientists from the University of Wisconsin-Madison took a closer look at forest canopies to determine what physical characteristics cause different forests to consume more carbon than others. They used laser scanning of nine forests in northern Wisconsin to measure the complexity of the forest canopy, then compared that information to how much carbon and water each site absorbed during the summer and fall of 2019. 3D scanning revealed distinct double-layered canopies in some forests, while others had more a consistent single-layered canopy, as shown in the figure below.

LiDAR forest transects adapted from Murphy et al (2022).

Results showed that more complex forests typically featured higher productivity, or more carbon uptake, because they made the best uses of light and water. This comes as no surprise, because other recent studies have reached similar conclusions. However, forest carbon uptake also depends on the kinds of plants within each forest, as well as the history of events like burning, clearing, or planting.

The vertical complexity index, or average VCI, calculated in the study was one of the variables with the strongest control over forest carbon uptake and resource (light and water) use efficiency. It is an ecological metric originating from information theory which represents the evenness of the vertical distribution of trees, shrubs, and other plants. It has to do with how close together a canopy is, as well as tree size and distribution throughout a forest. This means it varies a lot with forest structure and age.

Key takeaways

Timber harvesting reduces canopy complexity far into the future, not just at the time of harvest. This can have an impact on a forest canopy’s ability to effectively use light and water, which impacts its carbon sequestration abilities. Vertically complex or “heterogeneous” forests are generally better at resource use efficiency and photosynthetic productivity.

Careful thinning, selective harvesting, and manual planting could potentially be helping forests become more complex, by allowing trees of different ages and species to form diverse forests. Further studies of existing ecosystems, as well as computer modeling of forests with different management techniques, can help scientists understand the long-term impacts of manipulating forests to increase complexity. Certain relationships such as that of canopy structural complexity and productivity are not always straightforward and can be influenced by other factors as well.

Using 3D imaging on forest canopies can help us understand how forests work, and can tell us how to better manage these ecosystems if we want them to reach their full potential. To read the open access study, recently published in the Journal of Geophysical Research: Biogeosciences, click here.

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