COS 61-3 - Soil reduction in degraded tropical timber production forests causes a decline of soil carbon stock at a decadal time scale

Abstract: Unsustainable timber production that has been conducted in the tropics leads to a failure in the recovery of above-ground biomass (AGB) and tree community composition during succession (< 30 years). The degradation of vegetation directly influences water movement and material circulation in the ecosystem, where responses of the below-ground may also be seen. This study investigated how soil carbon content and physicochemical properties varied along a gradient of forest degradation caused by timber production. Field survey was conducted in 36 plots in two timber production forest reserves in Malaysia Borneo, Deramakot and Tangkulap. The two reserves shared the same primary vegetation type (lowland mixed dipterocarp forests), yet after decades of timber production, the current plot-level vegetation ranged from a near-pristine forest to a highly degraded fern thicket. AGB and tree community composition were calculated from tree census data of 2018. Soil samples were collected in each plot from the surface to 30-cm depth at 10-cm depth increment. Soil moisture, total carbon (C), total nitrogen (N), short-range ordered iron minerals (Fe_O) and pH were measured for each soil sample. Soil redox potential was measured at 3 random points in each plot. Taking Normalized Burn Ratio (NBR) as an indicator, occurrence and intensity of historical disturbances for each plot were detected with Landsat image time series dating back to 1988. We compared soil properties against current vegetation status and investigated whether and how the relationship varied among depths.

Logging event(s) in each plot occurred during the last 40 years based on time series analysis. AGB and tree community composition highly varied among 36 plots, both showed a tendency of degradation (i.e., lower AGB and less climax genera) under higher disturbance intensities. Taking decreasing AGB as an indicator of increasing magnitude of forest degradation, soil C and Fe_O concentration decreased in top 30-cm soils. Meanwhile, soil redox potential and pH increased as forest degradation intensified. We observed consistent relationships between soil properties and forest degradation in each 10-cm depth, but a somewhat larger variation was found in the top 10-cm soil. The soil reduction in more degraded forests must be ascribed to reduced transpiration and consequent water-logging resulting from the loss of canopy leaves, which could have promoted the reductive dissolution of reactive soil minerals and loss of soil C. Overall, our data demonstrate that soil degradation can happen under failed succession within a decadal time.