Vertical Canopy Gap Profiles Reveal Structural Divergence Among Central African Old-Growth Forest Types

Forest canopy gaps are key indicators of forest dynamics, yet their vertical organization remains poorly quantified across forest types. While canopy gaps are commonly interpreted as signatures of disturbance regimes, their size distribution and vertical organization may also reflect intrinsic differences in stand structure, crown architecture, and density-dependent interactions. We hypothesize that the vertical distribution of canopy gap size differs among forest types, reflecting contrasting structural organization and developmental trajectories. Using airborne LiDAR, we quantified vertical gap size distributions in two Central African old-growth forest types: monodominant Gilbertiodendron dewevrei (GIL) and MIXED forests, across two sites in the Democratic Republic of the Congo (Lileko and Yangambi). We also derived structural ratios normalizing gap area by basal area and canopy height. Results show clear divergence in vertical gap profiles. GIL forests consistently exhibit lower canopy openness than MIXED forests. Between heights of 10 and 20 m, total gap area per hectare ranged from 39.11 (95% CI: 19.84–83.5) to 567.56 (95% CI: 229.44–707.48) m2 ha−1 in GIL forests, compared with 69.10 (95% CI: 55.28–123.51) to 1244.97 (95% CI: 784.73–1360.45) m2 ha−1 in MIXED forests across both sites. Structural ratios further confirm that GIL forests maintain smaller gap areas relative to basal area and canopy height. Overall, the results support the three study hypotheses, demonstrating that vertical gap size distributions and their normalization by stand structural attributes provide robust and transferable indicators for discriminating among Central African old-growth forest types and developmental stages, highlighting the importance of structural context for interpreting tropical forest dynamics.