Large-diameter trees buffer monsoonal changes to tree biomass over two decades

Forest carbon storage inherently depends on the frequency and severity of characteristic disturbances and long-term changes in climate. The tropical forest of Lanjenchi, Taiwan is affected by the northeast monsoon wind, resulting in a varying vegetation structure depending on wind exposure. However, the northeast monsoon winds have been decreasing due to the climate change. We used four censuses over 22 years (1997, 2005, 2013, and 2019) to examine how tree density and aboveground biomass change under different levels of wind stress. We assessed tree density, aboveground biomass, aboveground woody productivity, and aboveground woody mortality from trees with diameter at breast height (DBH) ≥ 1 cm across 5.28 ha subdivided into 10 × 10 m quadrats. We tested for differences in tree density and aboveground biomass among three habitat types (windward, intermediate, and leeward), among small-diameter (1 cm ≤ DBH < 10 cm), medium-diameter (10 cm ≤ DBH < 17.2 cm) and large-diameter trees (DBH ≥ 17.2 cm). The 49,481 trees (density 9,272 ± 3612 trees ha⁻¹) of 140 species held 129.37 ± 51.95 Mg ha⁻¹ of aboveground biomass. From 1997 to 2019, tree density decreased and aboveground biomass was stable in the whole forest. However, changes in density and aboveground biomass were apparent among different habitat types and among different diameter classes. Specifically, in the windward habitat, aboveground biomass of small-diameter trees declined over time (from 35.34 Mg ha⁻¹ to 26.29 Mg ha⁻¹), and that of large-diameter trees increased (from 48.62 Mg ha⁻¹ to 57.78 Mg ha⁻¹). In the leeward habitat, large-diameter trees exhibited both high biomass productivity (1.04 Mg ha⁻¹ year⁻¹) and mortality (1.43 Mg ha⁻¹ year⁻¹). Although the overall state of the forest appears to possess multi-decadal stability, differences in dynamics among diameter classes and habitats may lead to forest changes if trees in those habitats continue to respond differentially to shifting magnitudes of monsoon wind speed.