Afforestation or abandonment of agricultural fields to forest regeneration is a method of sequestering carbon to offset the increasing atmospheric concentration of CO2. We selected 11 sites with altitudes ranging from 14 to 2056 m and with paired forest regenerated and adjacent agricultural fields. Our objectives were to (1) examine the changes in soil organic carbon (SOC) concentration and stock after forest regeneration of agricultural fields and (2) identify the factors related to elevation and adjacent agricultural practices that affect the SOC accumulation rate. Our results demonstrated overall increases in both SOC concentrations and stocks after forest regeneration of the abandoned agricultural fields. The average increase rates of SOC concentrations in the forest regenerated soil samples were 1.65 and 0.95 g C kg−1 at 0–10 and 10–20 cm depths, respectively, representing 101% and 65% increases relative to those in the soil samples from agricultural fields. The average accumulation rates of SOC stocks in the regenerated forests were 13.0 and 6.7 ton C ha−1 at the 0–10 and 10–20 cm depths, respectively, representing 96% and 62% increases relative to those in the agricultural soil samples. The average annual sequestration rate was 1.03 Mg C ha−1 year−1 for the top 0–20 cm soils, which is greater than that observed by previous reviews and meta-analyses. The tropical/subtropical climate, sampling soil depth, forest regeneration period, and tree species in this study are likely to have contributed to the high average SOC accumulation levels. In addition, the SOC stock accumulation rates were higher at low-elevation sites than at middle-elevation sites, which could also be attributed to the favorable climatic conditions at the low-elevation sites. Along with the build-up of carbon sequestration in the forest floor and tree biomass, the afforestation/abandonment of agricultural fields to forest regeneration appears to be a promising carbon offset mechanism.

<p>The cropland afforestation policy was initiated in 2002 in Taiwan and had been approaching the 20-year term. From the scientific perspective, it is a critical issue to understand the public welfare role and ecosystem services provided by the cropland afforestation. In this study, we investigated the changes of soil organic carbon (SOC) on plantations after 14 years conversion from the sugarcane fields. Soil samples were collected at 0-10 and 10-20 cm depth. Soil organic C concentration, bulk density, soil aggregation, and the stable isotopic <sup>13</sup>C of the SOC and aggregates were determined. The results indicated the SOC stocks on the afforested plots were between 1000 and 1500 g m<sup>-2 </sup>significantly higher than those under the sugarcane plots (p < 0.05). The analyses of stable <sup>13</sup>C indicated that the net increases in SOC stocks on the afforested plots were mainly attributed to the inputs of the forest-derived SOC that outweighed the loss of sugarcane-derived SOC. The afforestation also enhanced the aggregation with higher stability and SOC concentration. The comparatively depleted <sup>13</sup>C values in the stable macroaggregates further suggested the ecological function from this new SOC source. Combining with the stand development and aboveground biomass accumulation, we expected the cropland afforestation would provide ecosystem services and functions.</p>

Dendrometer bands have been proposed as an accurate method for measuring tree growth. However, the constrained observation window and the material used in them hamper long-term tree growth monitoring. This study devised a dendrometer band made from stainless steel and primarily extended the extension length of the band spring to yield ample space to monitor diameter increments long-term. A total of more than 500 individual trees, including both coniferous and broadleaf trees, were examined. We compared the dendrometer band’s long-term performance with diameter tape for 5- and 10-year measurements. The results showed that the measurements of the two methods were highly correlated (R > 0.89) in both measuring periods. Differences between the two measurements for individual trees were typically less than 5 mm, and the mean differences at a stand level were less than 2 mm. These consistent observations suggested that the dendrometer band measurements were reliable for long-term measurement. Using the dendrometer bands, we further demonstrated the annual tree growths of diameter at breast height (DBH) and basal area (BA) for ten years of measurements. The size-dependent relationships between DBH/BA growth and initial DBH were also presented. Owing to their simple installation, low cost, and reliable measurement, these dendrometer bands would be helpful in forestry and forest ecology research.