Quantifying unseen woody biomass and diversity in understorey trees and shrubs at the extremes of water availability in the Miombo ecoregion

The Miombo ecoregion covers eastern and southern Africa, with variations in plant species composition, structure, and biomass across a broad precipitation gradient. Most studies of woody plant communities focus exclusively on larger overstorey trees (≥5 or ≥10 ​cm stem diameter), overlooking the contribution of small trees and shrubs in the understorey, which can comprise a significant portion of total biomass and diversity. Here, we evaluate the contribution of both large overstorey and small understorey woody plants to species diversity and above-ground biomass (AGB), with 17 plots (0.5–1 ​ha) across five sites representing both extremes of rainfall gradient spanning the Miombo ecoregion, in northeast Namibia (500–700 ​mm mean annual precipitation, MAP) and southern Democratic Republic of Congo (DRC) (>1,200 ​mm MAP).

Mean AGB per site ranged from 21 to 119 ​Mg·ha^-1, increasing with rainfall, while the proportional AGB contribution of small trees, saplings, and shrubs decreased. In dry Namibia, small trees, saplings, and shrubs (<5 ​cm DBH) contributed up to 28.2% of total AGB (mean ​± ​standard deviation: 18.3% ​± ​3.4%), whereas in wet DRC, they contributed only up to 2.5% (2.3% ​± ​1.4%).

Namibian sites, on average, contained a large proportion of woody species diversity exclusively in small trees and shrubs (<5 ​cm DBH), with 55 species representing 59.4% of the total diversity. In contrast, DRC sites had higher overall small woody plant diversity (66 species) but fewer species found exclusively as small individuals (25.2%), with many saplings that grow to larger trees. Understorey composition also differed, with saplings of overstorey trees dominating in DRC, while shrubs dominated in Namibia.

Our findings show that woody biomass and diversity in dry woodlands are substantially underestimated when studies focus only on larger trees. This highlights the need to consider all woody vegetation to better understand woody plant diversity and biomass variation.