Wenfa Xiao

Rates of litter decomposition and soil respiration in relation to soil temperature and water in different-aged Pinus massoniana forests in the Three Gorges Reservoir Area, China.

To better understand the soil carbon dynamics and cycling in terrestrial ecosystems in response to environmental changes, we studied soil respiration, litter decomposition, and their relations to soil temperature and soil water content for 18-months (Aug. 2010–Jan. 2012) in three different-aged Pinus massoniana forests in the Three Gorges Reservoir Area, China. Across the experimental period, the mean total soil respiration and litter respiration were 1.94 and 0.81, 2.00 and 0.60, 2.19 and 0.71 µmol CO2 m−2 s−1, and the litter dry mass remaining was 57.6%, 56.2% and 61.3% in the 20-, 30-, and 46-year-old forests, respectively. We found that the temporal variations of soil respiration and litter decomposition rates can be well explained by soil temperature at 5 cm depth. Both the total soil respiration and litter respiration were significantly positively correlated with the litter decomposition rates. The mean contribution of the litter respiration to the total soil respiration was 31.0%–45.9% for the three different-aged forests. The present study found that the total soil respiration was not significantly affected by forest age when P. masonniana stands exceed a certain age (e.g. >20 years old), but it increased significantly with increased soil temperature. Hence, forest management strategies need to protect the understory vegetation to limit soil warming, in order to reduce the CO2 emission under the currently rapid global warming. The contribution of litter decomposition to the total soil respiration varies across spatial and temporal scales. This indicates the need for separate consideration of soil and litter respiration when assessing the climate impacts on forest carbon cycling.

Responses of nutrients and mobile carbohydrates in Quercus variabilis seedlings to environmental variations using in situ and ex situ experiments.

Forest tree species distributed across a wide range of geographical areas are subjected to differential climatic and edaphic conditions and long-term selection, leading to genotypes with morphological and physiological adaptation to the local environment. To test the ability of species to cope with changing environmental conditions, we studied the ecophysiological features of Quercus variabilis using seedlings grown in geographically widely isolated populations (Exp. I, in situ) and in a common garden (Exp. II, ex situ) using seedlings originating from those populations. We found that Q. variabilis plants grown in different locations along a south-north gradient had different levels of nutrients (N, P, K) and carbon-physiological performance (photosynthesis, non-structural carbohydrates, such as soluble sugars and starch), and that these physiological differences were not correlated with local soil properties. These geographic variations of plant physiology disappeared when plants from different locations were grown in the same environment. Our results indicate that the physiological performance of Q. variabilis plants is mainly determined by the climatic variations across latitude rather than by their soils or by genetic differentiation. The adaptive ability of Q. variabilis found in the present study suggests that this species has the potential to cope, at least to some extent, with changing environmental conditions.

Spatial variability of soil organic carbon in Three Gorges Reservoir area, China

Soil organic carbon (SOC) is an important component of the global carbon pool. It is a critical indicator of soil quality. We studied SOC content (SOCC) and SOC density (SOCD) of the Three Gorges Reservoir (TGR) area in China. Soil samples from 306 sites across the study area were assessed for SOCC, SOCD and bulk density. Total SOC stocks in the TGR area were estimated at 5.82×10-1Pg. We examined relationships between SOCC and SOCD, and the environmental and land-use/land-cover (LULC) variables. The plow layer (0-0.3m) had a significantly higher mean SOCC (20.6gkg-1) than the subsoil layer (16.5gkg-1); elevation, LULC, soil type and soil thickness were the most influential factors affecting SOCC in the plow layer. In the subsoil layer, elevation and soil thickness were dominant in determining SOCC and SOCD. To study the spatial variability of SOC, we used statistical modeling and GIS-based techniques to map the distribution of SOCC and SOCD of the study area. Both SOCC and SOCD in the plow layer showed patchy distribution and were positively correlated with elevation and vegetation coverage. Spatial variability of SOCD in the subsoil layer showed a gradual transition between LULC categories. The lower SOCC of farmland appeared to be related to the repeated removal of agricultural produce from the land. Preservation of permanent vegetation cover and changing of the traditional farming practices will help to improve SOC stock and increase soil productivity in the TGR area.

The Link Between Litterfall, Substrate Quality, Decomposition Rate, and Soil Nutrient Supply in 30-Year-Old Pinus massoniana Forests in the Three Gorges Reservoir Area, China

Abstract Little is known about whether soil nutrient supply is determined by decomposition rate or litterfall in Pinus massoniana forests. The present study quantified the relationships between litterfall, litter substrate quality, decomposition rate, and soil nutrients to elucidate the contribution of litter to soil nutrient supply in these forests. Twelve 1 × 1–m litterfall collectors (collected monthly) and 108 decomposition bags, 20 × 20 cm (collected every 3 months) in size, were placed in three 30-year-old P. massoniana stands in the Three Gorges Reservoir Area, from August 2010 to January 2011. The accumulation of soil nutrients depended on the relationship of litterfall, substrate quality, and decomposition rate. Soil organic matter content was significantly positively correlated with litterfall and litter substrate quality dynamic (both P < 0.01) (N, P, and C/N ratio). Litter decomposition rate was positively correlated with litter initial N concentration (R2 = 0.82, P = 0.01) but negatively correlated with initial P (R2 = 0.67, P = 0.045) and C/N ratio (R2 = 0.90, P = 0.00). Litter decomposition rate was negatively correlated with soil organic matter content (P = 0.02), total soil N (P < 0.001), and soil available P (P < 0.001), indicating that litter decomposition was faster in nutrient-poor than in nutrient-rich soils, which may function as a mechanism for increasing the nutrient use efficiency in P. massoniana forest ecosystems. Litter nutrient return through decomposition plays a more important role than the other litter parameters studied in determining soil nutrient supply in the P. massoniana forest ecosystem. Forest management practices need to create conditions that favor the litter decomposition to improve the site conditions and growth rate of the P. massoniana trees.

Thinning but not understory removal increased heterotrophic respiration and total soil respiration in Pinus massoniana stands

Quantifying soil respiration (Rs) and its components [autotrophic respiration (Ra) and heterotrophic respiration (Rh)] in relation to forest management is vital to accurately evaluate forest carbon balance. Thus, Rs, Ra, and Rh were continuously monitored from November 2013 to November 2016 in Pinus massoniana forests subjected to four different management practices in China. We hypothesized that understory removal and thinning decrease Ra and Rh and thus Rs, and these decreases will change with time following UR and thinning. Mean values of Rs, Ra, and Rh in light thinned plots (LT=15% of tree basal area thinned) and heavily thinned plots (HT=70% of tree basal area thinned) were significantly higher than in control (CK) and understory removal plots (UR). The annual Rh/Rs ratio ranged from 58% to 70% across all treatments, and this ratio was significantly higher in HT and LT than in UR and CK. Only HT significantly increased soil temperature. Soil temperature could better explain Rh (R2=0.69-0.96) than Ra (R2=0.51-0.86). HT and LT increased Q10 for both Ra and Rh, except for Rh in UR. Soil moisture content (W; %) was significantly higher in HT than in other treatments, but W had limited effects on soil respiration in that rain-rich subtropical China. This result suggests that global warming alone, or in combination with clear-cutting or canopy tree thinning will markedly increase soil heterotrophic respiration and thus the total soil CO2 emission. To get firewood for local people and to reduce soil CO2 emissions under global warming, canopy trees are needed to be protected and understory shrubs may be allowed to be used in the subtropical China.

Responses of landscape metrics to altering grain size in the Three Gorges Reservoir landscape in China

Quantifying the response of landscape metrics to an altering observation scale is crucial to understanding environmental changes and managing ecosystem services. Whereas the scaling behaviors of landscape metrics in spatial heterogeneity analysis have been well identified by previous research, there remains a need to examine these effects in areas undergoing rapid change. Here, we aim to reveal the landscape scale effect in the Three Gorges Reservoir (TGR) area, China, using a case study on Zigui County. We applied a suite of common landscape metrics (12 indices at the class level and 17 indices at the landscape level) to characterize the landscape pattern and examine the response of the metrics to altering grain size using a series of land-use/land-cover data with gradient resolutions. The results reveal that significant scale effects exist in most pattern metrics in the TGR landscape. In addition, the different responses to the altering grain size occurred with different landscape metrics and various land-use/land-cover types. With respect to changing grain size, all of the selected pattern metrics at the landscape level displayed high or medium sensitivity in response to changing grain size except the Fractal Dimension Index and the landscape-diversity indices. The behavior of the metrics in response to altering grain size can be grouped into four types (Type 1, Type 2, Type 3, and Type 4). The class-level metrics with high sensitivity were Mean Patch Size, the Contiguity Index, the Euclidean Nearest-Neighbor Distance, the Perimeter-Area Ratio, and Patch Density for all land-use/land-cover types, whereas low sensitivities were detected in the response of the Fractal Dimension Index and the Largest Patch Index. Based on the response to the altering resolution of input data, the class-level metrics could be grouped into three types (Type a, Type b, and Type c). Considering the scaling behavior of landscape metrics, we suggest using a set of suitable remote-sensing images to quantify the landscape pattern in the TGR landscape and similar areas.

Effects of mixed leaf litter from predominant afforestation tree species on decomposition rates in the Three Gorges Reservoir, China

To determine whether mixed plantations can improve nutrient cycling and to elucidate the mechanisms of such effects, a field litterbag experiment with seven treatments involving Pinus massoniana (P.), Cupressus funebris (C.) and Quercus variabilis (Q.) litter in equal mass proportions (pure litter; pairwise combinations; and the combination of all three species) was conducted in a Pinus massoniana plantation in the region of the Three Gorges Reservoir, China. We measured mass loss and the release of C, N and P from the litter treatments and assessed the effects of mixing litter in each sampling phase and for various decomposition periods. At the end of the study, the mass loss and release of C, N and P among the treatments relative to their initial contents ranged from 47.6% to 62.8%, 59.5% to 75.2%, 63.5% to 78.2% and 58.9% to 72.6%, respectively. Primary mass loss and nutrient release occurred during a phase with high temperatures and precipitation, and decomposition was closely correlated with the initial lignin/N ratio and N concentration. Compared with the decay values of Quercus litter, mixing litter increased N release by 1.2% for the P. + Q. and C. + Q. combinations and increased P release by 3.0-6.3% for the three litter mixture combinations. Additionally, the P. + Q. and C. + Q. two-species mixtures exhibited greater decay than the three-species mixture. Mixing the two coniferous species (P. + C.) also increased decomposition. Furthermore, positive nonadditive mass loss occurred after incubation for 240 d, and mixing effects on the nonadditive release of C, N and P occurred immediately in 60 d incubations in all treatments. In conclusion, mixing these three species or two of species can improve material cycling in plantations, and Quercus appears to be a priority candidate for mixed planting with Pinus and/or Cupressus.

The Concentration of Non-structural Carbohydrates, N, and P in Quercus variabilis Does Not Decline Toward Its Northernmost Distribution Range Along a 1500 km Transect in China

Understanding the mechanisms that determine plant distribution range is crucial for predicting climate-driven range shifts. Compared to altitudinal gradients, less attention has been paid to the mechanisms that determine latitudinal range limit. To test whether intrinsic resource limitation contributes to latitudinal range limits of woody species, we investigated the latitudinal variation in non-structural carbohydrates (NSC; i.e., total soluble sugar plus starch) and nutrients (nitrogen and phosphorus) in mature and juvenile Chinese cork oak (Quercus variabilis Blume) along a 1500 km north-south transect in China. During the growing season and dormant season, leaves, branches, and fine roots were collected from both mature and juvenile oaks in seven sites along the transect. Tissue concentration of NSCs, N, and P did not decrease with increasing latitude irrespective of sampling season and ontogenetic stage. Furthermore, higher levels of NSCs and N in tissues of juveniles relative to mature trees were found during the dormant season. Partial correlation analysis also revealed that during the dormant season, soluble sugar, NSC, the ratio of soluble sugar to starch, and tissue nitrogen concentration were correlated positively with latitude but negatively with precipitation and mean temperature of dormant season. Our results suggest that carbon or nutrient availability may not be the driving factors of the latitudinal range limit of the studied species. Further studies should be carried out at the community or ecosystem level with multiple species to additionally test the roles of factors such as regeneration, competition, and disturbance in determining a species’ northern distribution limit.

Geotechnology-Based Modeling to Optimize Conservation of Forest Network in Urban Area

Forest network development in urban areas faces the challenge from forest fragmentation, human-induced disturbances, and scarce land resources. Here, we proposed a geotechnology-based modeling to optimize conservation of forest network by a case study of Wuhan, China. The potential forest network and their priorities were assessed using an improved least-cost path model and potential utilization efficiency estimation. The modeling process consists of four steps: (i) developing species assemblages, (ii) identifying core forest patches, (iii) identifying potential linkages among core forest patches, and (iv) demarcating forest networks. As a result, three species assemblages, including mammals, pheasants, and other birds, were identified as the conservation targets of urban forest network (UFN) in Wuhan, China. Based on the geotechnology-based model, a forest network proposal was proposed to fulfill the connectivity requirements of selected species assemblages. The proposal consists of seven forest networks at three levels of connectivity, named ideal networks, backbone networks, and comprehensive network. The action priorities of UFN plans were suggested to optimize forest network in the study area. Additionally, a total of 45 forest patches with important conservation significance were identified as prioritized stepping-stone patches in the forest network development. Urban forest conserve was also suggested for preserving woodlands with priority conservation significance. The presented geotechnology-based modeling is fit for planning and optimizing UFNs, because of the inclusion of the stepping-stone effects, human-induced pressures, and priorities. The framework can also be applied to other areas after a sensitivity test of the model and the modification of the parameters to fit the local environment.