Chih-Hsin Cheng

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.

Amending soils with biochar, a pyrolyzed organic material, is an emerging practice to potentially increase plant available water and reduce the risks associated with climatic variability in traditionally‐rainfed tropical agricultural systems. To investigate the impacts of biochar amendment on soil water storage relative to non‐amended soils, we performed an upland rice field experiment in a tropical seasonally dry region of Costa Rica consisting of plots with two different biochar amendments and a control plot. Across all plots, we collected hydrometric and isotopic data (δ¹⁸O and δ²H of rain, mobile soil, ground and rice xylem water). We observed that the soil water retention curves for biochar treated soils shifted, indicating that rice plants had 2% to 7% more water available throughout the growing season relative to the control plots and thus could withstand dry spells up to seven extra days. Furthermore, the isotopic composition of plant water in biochar and control treatments were rather similar, indicating that rice plants in different treatments likely consumed similar water. Hence, we observed that biochar amendments can stabilize water supplies for the rice plants; however, still supplemental irrigation was required to facilitate plant growth during extended dry periods. Ultimately, our findings indicate, that biochar amendments can complement, but not necessarily replace, other water management strategies to help reduce the threat of rainfall variability to rainfed agriculture in tropical regions.

Biochar has been put forward as a potential technology that could help achieve sustainable water management in agriculture through its ability to increase water holding capacity in soils. Despite this opportunity, there are still a limited number of studies, especially in vulnerable regions like the tropics, quantifying the impacts of biochar on soil water storage and characterizing the impacts of biochar additions on plant water composition. To address this critical gap, we present a case study using stable water isotopes and hydrometric data from melon production in tropical agriculture to explore the hydrological impacts of biochar as a soil amendment. Results from our 10-week growing season experiment in Costa Rica under drip irrigation demonstrated an average increase in volumetric soil moisture content of about 10% with an average moisture content of 25.4 cm³ cm⁻³ versus 23.1 cm³ cm⁻³, respectively, for biochar amended plots compared with control plots. Further, there was a reduction in the variability of soil matric potential for biochar amended plots compared with control plots. Our isotopic investigation demonstrated that for both biochar and control plots, there was a consistent increase (or enrichment) in isotopic composition for plant materials moving from the roots, where the average δ¹⁸O was −8.1‰ and the average δ²H was −58.5‰ across all plots and samples, up through the leaves, where the average δ¹⁸O was 4.3‰ and the average δ²H was 0.1‰ across all plots and samples. However, as there was no discernible difference in isotopic composition for plant water samples when comparing across biochar and control plots, we find that biochar did not alter the composition of water found in the melon plant material, indicating that biochar and plants are not competing for the same water sources. In addition, and through the holistic lens of sustainability, biochar additions allowed locally sourced feedstock carbon to be directly sequestered into the soil while improving soil water availability without jeopardizing production for the melon crop. Given that most of the expansion and intensification of global agricultural production over the next several decades will take place in the tropics and that the variability of tropical water cycling is expected to increase due to climate change, biochar amendments could offer a pathway forward towards sustainable tropical agricultural water management.

Forest stand density has been shown to have different albeit small effects on soil carbon. We hypothesized that the absence of density effect on soil carbon (C) storage can be explained by a loss of old soil C. This replacement of old by fresh C results in zero net C sequestration by soils but could alter the quality of soil organic matter.   We used one afforestation experiment in Siberia, in which three tree species (spruce, larch and Scots pine) were grown for the last 30 years at 18 levels of stand density, ranging originally from 500 to 125,000 stems per ha. We selected five density levels and studied C and nitrogen (N) contents in mineral soils at 0-5 cm depth. The age of soil C was measured under larch and spruce for three levels of density by radiocarbon (14C) dating. In all soil samples we determined stability of soil organic matter (SOM) to mineralization (decomposability) and to elevated input of readily decomposable C – glucose (primability).  Stand density affected soil C and N contents differently depending on the tree species. Only under spruce both C and N contents were increasing with density, under larch and pine the covariation was insignificant, while N tended even to decline with density increase.  With the 14C data we were able to show strong dilution of old SOM by fresh C derived from the trees, the effect was stronger with higher density. This provides first evidence that density increase increases the fractions of new C versus old C and this can happen without altering the total C contents like under larch. While stand density altered soil C and N contents only under spruce, it altered C decomposability under all tree species: with density increase the C decomposability (per unit of C) declined under spruce but increased under larch and pine. This is relevant to predicting C losses from forest soils with different tree species and densities.  Higher losses would occur under larch and pine with higher densities, but increase of density under spruce would reduce the C losses from soil. Furthermore, while no significant covariation of stand density with C primability was detected, we first observed strong tree species effects on C primability. Twice as much C is lost from soil under larch than under spruce and pine by equal addition of C-glucose.  This indicates that elevated C deposition from roots and exudates to soil as predicted due to elevated CO2 concentration would most strongly accelerate soil C turnover and C losses under larch than under spruce and pine. Overall, tree species altered the susceptibility of soil C to elevated C input and stand density had strong effect on the decomposability of SOM, which is important parameter of C stability. The effect of stand density is thus important to consider even if stand density does not affect total soil C content. 

Forest stand density has been shown to have different, albeit small, effects on soil carbon. We hypothesized that the absence of a density effect on soil carbon (C) storage could be explained by a loss of old soil C. This replacement of old by fresh C could result in zero net C sequestration by soils but could also alter the quality of the soil organic matter. We used one afforestation experiment in Siberia, in which three tree species (spruce, larch and Scots pine) have been grown for the last 30 years at 18 levels of stand density, ranging originally from 500 to 125,000 stems per ha. We selected five density levels and studied the C and nitrogen (N) contents in mineral soils at 0–5 cm depth. The age of the soil C was measured under larch and spruce for three levels of density by radiocarbon (¹⁴C) dating. In all soil samples, we determined the stability of the soil organic matter (SOM) by assessing two indices: C decomposability (mineralization of C per unit of soil C) and primability (susceptibility of the SOM to microbial priming). The stand density affected the soil C and N contents differently depending on the tree species. Only under spruce did both the C and N contents increase with density; under larch and pine, the covariation was insignificant and N even tended to decline with a density increase. With the ¹⁴C data, we were able to show the strong dilution of old SOM by fresh C derived from the trees; the effect was stronger with a higher density. This provides the first evidence that a density increase increases the fractions of new C versus old C and this can happen without altering the total C contents such as under larch. Although the stand density altered the soil C and N contents only under spruce, it altered C decomposability under all tree species; with a density increase, the C decomposability declined under spruce but increased under larch and pine. This is relevant to predicting C losses from forest soils with different tree species and densities. Higher C losses would occur under larch and pine with higher densities but under spruce, a density increase would reduce the losses of C from the soil. Furthermore, although no significant covariation of stand density with C primability was detected, we first observed strong tree species effects on C primability. Twice as much C was lost from the soil under larch than under spruce or pine by an equal addition of C-glucose. This indicated that elevated C deposition from roots and exudates to the soil as predicted due to an elevated CO₂ concentration would most strongly accelerate the soil C turnover and C losses under larch than under spruce and Scots pine. Overall, the tree species altered the susceptibility of the soil C to an elevated C input and the stand density had a strong effect on the decomposability of the SOM, which is an important parameter of C stability. The effect of stand density is, therefore, important to consider even if the stand density does not affect the total soil C.

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.

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 13C of the SOC and aggregates were determined. The results indicated the SOC stocks on the afforested plots were between 1000 and 1500 g m-2 significantly higher than those under the sugarcane plots (p < 0.05). The analyses of stable 13C 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 13C 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.

Amending soils with biochar, a pyrolyzed organic material, is an emerging practice to potentially increase plant available water. However, it is not clear (1) to what extent biochar amendments increase soil water storage relative to non-amended soils and (2) whether plants grown in biochar amended soils access different pools of water compared to those grown in non-amended soils. To investigate these questions, we set up an upland rice field experiment in a tropical seasonally dry region in Costa Rica, with plots treated with two different biochar amendments and control plots, from where we collected hydrometric and isotopic data (δ¹⁸O and δ²H from rain, soil, groundwater and rice plants). Our results show that the soil water retention curves for biochar treated soils shifted, indicating that rice plants had 2 % to 7 % more water available throughout the growing season relative to the control plots. In addition, we observed a within treatment variability in the soil water retention curves which was in the same order of magnitude as one would expect from responses due to differences in biochar application rates or due to differences in biochar typologies. The stable water isotope composition of plant water showed that the rice plants across all plots preferentially utilized the more variable soil water from the top 20 cm of the soil instead of using the deeper and less variable sources of water. Our results indicated that rice plants in biochar amended soils could access larger stores of water more consistently and thus could withstand dry spells of seven extra days relative to rice grown in non-treated soils. Though supplemental irrigation was required to facilitate plant growth during extended dry periods. Therefore, biochar amendments can complement, but not necessarily replace, other water management strategies.

Many environmental and climatic disturbances can significantly change the magnitude and pattern of litterfall. This study investigated the effects of typhoon disturbances on the seasonal and interannual patterns of litterfall on coniferous and broadleaf plantation stands in Xitou, central Taiwan. Throughout the study period from 2012 to 2018, typhoon disturbances were recorded in 4 of these 7 years, whereas only minor or even no typhoon disturbances occurred in the other 3 years. Our results demonstrated that the pattern of monthly litterfall varied substantially between the coniferous and broadleaf stands. The coniferous stands exhibited a substantial litterfall pulse due to typhoon disturbances. By contrast, typhoon disturbances exerted a minor impact on the broadleaf stands. The litterfall seasonality of the coniferous stands was higher than that of the broadleaf stands, especially in the years with typhoon disturbances. Furthermore, the yearly variation caused by typhoon disturbances was distinct at the coniferous stands; the annual litterfall mass at the coniferous stands in the years with typhoon disturbances was more than twice as high as that in the years without typhoon disturbances, namely 6,000 kg ha⁻¹ yr⁻¹ versus 2,500 kg ha⁻¹ yr⁻¹. By contrast, the annual litterfall mass at the broadleaf stands did not differ significantly among the study years. Because of high interannual variation, long-term sampling is essential for accurate estimation of annual litterfall at coniferous stands. For broadleaf plantations, interannual litterfall variation seems less critical, and spatial variability should be considered because of higher variability in terms of site conditions.

Unfortunately, the original article (Huang et al. 2017) contained some errors. The Fig. 4 displayed incorrectly. The correct figure can be found below: