Structural heterogeneity modulates the effects of stem density on standing biomass in mature dry Isoberlinia woodlands

Abstract

Introduction While the impacts of forest structural attributes on above-ground biomass (AGB) has been largely documented in closed-canopy forests, much less is known in dry forests, where water rather than light drives biomass accumulation. In particular, little is known about how AGB of Isoberlinia spp. stands, a dominant ecosystem of dry West Africa, is shaped by structural attributes. Methods To overcome this gap stem density, summed crown projection area (SCPA), and tree diameter at breast height (DBH) were measured in forty 400 m 2 plots established in young and mature Isoberlinia spp. stands in the Sudano-Guinean zone of Benin. AGB was estimated using an allometric model, and a piecewise structural equation model (pSEM) was developed to analyze the direct and indirect effects of stem density, SCPA, and the coefficient of variation of the diameter at breast height (CVDBH) on AGB. Results In young stands (442.50 ± 34.51 stems/ha on average), higher stem density had a significantly negative direct effect on AGB ( β = −0.95, p < 0.001), while SCPA influence remained non-significant. In mature stands (255 ± 32.04 stems/ha on average), increase in stem density also had a significant direct negative effect on AGB. In addition, higher CVDBH significantly reduced AGB ( β = −0.32, p = 0.04), while denser SCPA showed a positive but non-significant trend ( β = 0.19, p = 0.49). Whatever the stand stage, there was no indirect effects of stem density through SCPA and CVDBH. Discussion Our results provide actionable insights for managing Isoberlinia woodlands: density regulation is critical in young stands to optimize individual growth, while maintaining moderate diameter heterogeneity (intermediate CVDBH values at 15%) is essential in mature stands, as excessive structural variability can reduce AGB. These findings contribute to both local resource needs and broader climate mitigation goals by enhancing standing AGB while preserving ecosystem integrity.