Cambial activity in the understory of the Mayombe forest, DR Congo

Key message Understanding species-specific response as well as wedging and zero xylem growth is vital for tree-ring analysis of tropical understory trees. Knowledge on intra-annual xylem growth remains understudied in tropical regions, especially for understory species. However, it is important to disentangle seasonal tree response in this complex environment. The aim is to assess intra-annual wood formation and its variability in selected understory tree species of a semi-deciduous tropical forest. The cambium of four species from the Luki reserve of the Mayombe (DR Congo) was monthly marked at the stem base via the pinning method. To assess ring anomalies on the stem disks, digitization of the last 5–10 rings was performed along the circumference. Relative growth was determined based on X-ray CT volumes of the pinning zone, as well as on sanded surfaces and microsections. Stem disks allowed to visualize ring anomalies and growth variations. Intra-annual growth was successfully derived via X-ray CT and could be fitted with a Gompertz function. A species-specific response is observed, although there is circumferential variability. However, the most remarkable result is that many of the trees in the data set had no xylem formation at the stem base, throughout the entire season, thus forming missing rings. Intra-annual variability in growth illustrates the different responses of species and individual trees to environmental drivers. Phenology might explain the differences, although site and competition should be considered as well. A large number of trees show no xylem growth at all, apart from wound-induced local growth, causing missing rings which have important implications for the tree-ring analysis in tropical regions.