Biomass and diversity in Africa’s dry forests vastly underestimated

The Miombo ecoregion in eastern and southern Africa shows diverse plant communities influenced by precipitation. However, research has mainly focused on large trees, neglecting the understorey's small trees and shrubs that are key to the ecosystem's biomass and diversity.

In a new study conducted by an international team of researchers, small trees and shrubs in forest understoreys – the layer of vegetation beneath the forest canopy – were found to play a much bigger role in carbon storage and biodiversity than we often realize.

“In drier regions like Namibia, small trees and shrubs contribute almost one-third of the total woody biomass. Yet, they are rarely accounted for in biomass estimates at a national or global scale.” shares lead author Hermane Diesse, a doctoral student from the Namibia University of Science and Technology. “We also found that the species richness of the shrubs in the understorey decreases as the overstorey biomass increases, while the species richness of the young trees in the understorey increases.
 

This confirms the important role of large trees in shaping the understorey layer; open canopies allow for denser, shrub-dominated understoreys, whereas denser overstoreys promote more understorey tree species.

Published in the Forest Ecosystem Journal, the research looked at woody biomass and species diversity across five sites, covering a rainfall gradient from 500 mm to over 1200 mm mean annual precipitation.

“Most carbon assessment studies and programs focus only on large overstorey trees, using a minimum stem diameter of 5 cm or 10 cm. This means that small trees and shrubs are often ignored, even though they can make up a substantial part of the total biomass,” adds Diesse.

Interestingly, the team also found that understorey biomass does not necessarily decrease along the gradient from dry to wet Miombo woodlands. Instead, the overstorey trees grow bigger, which reduces the proportion of total biomass held by the understorey layer, as shown in the graph.

“However, most carbon assessment theories and methods, especially those based on trees above a certain stem diameter size, come from studies in wet Miombo, meaning they don’t fully account for the structure of drier woodlands,” says Diesse.

These findings can reignite the debate on the potential of carbon sequestration of dry woodlands.

“If we want to accurately estimate forest carbon stocks and carbon fluxes, we must include small trees and shrubs, particularly in dry ecosystems where their biomass contribution is significant and often changes more rapidly,” notes Diesse. “Recognizing the complete structure of these woodlands will lead to better carbon assessments and more effective conservation strategies.”

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Contact the author: Hermane Diesse, Namibia University of Science and Technology, SEOSAW Network, diessehermane@gmail.com

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