Influences of fern and vine coverage on the above-ground biomass recovery in a Bornean logged-over degraded secondary forest

Tropical secondary forests are widespread in the anthropogenically modified landscapes. Because tropical secondary forests were known to show a steady rate of above-ground biomass (AGB) recovery, they were expected to play a crucial role in carbon sequestration. However, in a preliminary survey of logged-over forests in Borneo, some patches covered with ferns and vines do not seem to recover steadily despite the fact that several decades have passed since the last logging. The presence and abundance of fern/vine thickets may affect the AGB recovery, but few studies have investigated the effects of the thickets. We established a total of seventeen 20-m radius circular plots in logged-over forests, Sabah, Malaysia, with a varying degree of fern/vine coverage, and tested the hypothesis that the greater fern/vine coverage would retard the AGB recovery. The net AGB accumulation rate from 2014 to 2019 was lower in the forests with a higher fern/vine coverage. Our lower bound of the accumulation rate was much lower than the previously reported rates elsewhere. The number of newly recruited and small-diameter trees was lower, and the mortality of remnant trees was higher with increasing fern/vine coverage. The growth rate of the extant pioneer trees, which are expected to significantly contribute to the initial build-up phase of the secondary succession, was inhibited when ferns and vines covered the canopy. Our study suggests that the secondary succession is arrested and the recoverability of forest stands is lower than formerly predicted when they are covered by thick ferns and vines.