Tropical forest restoration: Fast resilience of plant biomass contrasts with slow recovery of stable soil C stocks

Abstract

Due to intensifying human disturbance, over half of the world's tropical forests are reforested or afforested secondary forests or plantations. Understanding the resilience of carbon (C) stocks in these forests, and estimating the extent to which they can provide equivalent carbon (C) sequestration and stabilization to the old growth forest they replace, is critical for the global C balance. In this study, we combined estimates of biomass C stocks with a detailed assessment of soil C pools in bare land, Eucalyptus plantation, secondary forest and natural old‐growth forest after over 50 years of forest restoration in a degraded tropical region of South China. We used isotope studies, density fractionation and physical fractionation to determine the age and stability of soil C pools at different soil depths. After 52 years, the secondary forests had equivalent biomass C stocks to natural forest, whereas soil C stocks were still much higher in natural forest (97.42 t/ha) than in secondary forest (58.75 t/ha) or Eucalyptus plantation (38.99 t/ha) and lowest in bare land (19.9 t/ha). Analysis of δ13C values revealed that most of the C in the soil surface horizons in the secondary forest was new C, with a limited increase of more recalcitrant old C, and limited accumulation of C in deeper soil horizons. However, occlusion of C in microaggregates in the surface soil layer was similar across forested sites, which suggests that there is great potential for additional soil C sequestration and stabilization in the secondary forest and Eucalyptus plantation. Collectively, our results demonstrate that reforestation on degraded tropical land can restore biomass C and surface soil C stocks within a few decades, but much longer recovery times are needed to restore recalcitrant C pools and C stocks at depth. Repeated harvesting and disturbance in rotation plantations had a substantial negative impact on the recovery of soil C stocks. We suggest that current calculations of soil C in secondary tropical forests (e.g. IPCC Guidelines for National Greenhouse Gas Inventories) could overestimate soil C sequestration and stabilization levels in secondary forests and plantations. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12925/suppinfo is available for this article.

Changes in Soil Microbial Biomass, Community Composition, and Enzyme Activities After Half-Century Forest Restoration in Degraded Tropical Lands

“…To restore the degraded land, scientists launched an afforestation campaign in 1959 and established two types of artificial forests and one bare land (BL) as a control in three geographically similar catchments [9,43]. The two forests include a managed Eucalyptus (Eucalyptus exserta) plantation (MEP) and a restored secondary forest (RSF).…”

“…We also selected a nearby undisturbed forest (UF, a zone forest type that has been preserved by residents for about 200 years) as a reference site. Some more detailed description of the species richness and diversity can be found in previous studies [9,47,48].…”

“…Many countries, including China, have established national programs to increase plantation areas and have successfully increased the areas of vegetation [5][6][7]. Numerous studies have demonstrated that C stocks in plant biomass and soil can rapidly increase with afforestation [8,9] and recover to an equivalent level to the nearby undisturbed forests within a few decades [10]-particularly in tropical regions, where the consistent warm temperature and ample rainfall favor rapid plant growth [7]. However, there are also many restoration projects that achieved very limited success or failed completely [11,12], mostly due to the nutrient limitation, poor tree species selection, and a lack of understanding of the plant-soil-microbe interaction [13][14][15][16].…”

Changes in seasonal precipitation distribution but not annual amount affect litter decomposition in a secondary tropical forest

“…The soil is lateritic and developed from deeply weathered granite (Wang, Ding, et al, 2017). The climate here is a tropical climate with a distinct wet (from April to September) and dry season (from October to March).…”

“…The mean annual temperature is 23°C, and the mean annual precipitation is 1,400-1,700 mm, respectively. The soil is lateritic and developed from deeply weathered granite (Wang, Ding, et al, 2017). Our experimental site was located in a secondary tropical forest.…”