Grasslands in the Tibetan Plateau are claimed to be sensitive and vulnerable to climate change and anthropogenic activities. Quantifying the impacts of climate change and anthropogenic activities on grassland growth is an essential step for developing sustainable grassland ecosystem management strategies under the background of climate change and increasing anthropogenic activities occurring in the plateau. Net primary productivity (NPP) is one of the key components in the carbon cycle of terrestrial ecosystems, and can serve an important role in the assessment of vegetation growth. In this study, a modified Carnegie–Ames–Stanford Approach (CASA) model, which considers remote sensing information for the estimation of the water stress coefficient and time-lag effects of climatic factors on NPP simulation, was applied to simulate NPP in the Tibetan Plateau from 2001 to 2015. Then, the spatiotemporal variations of NPP and its correlation with climatic factors and anthropogenic activities were analyzed. The results showed that the mean values of NPP were 0.18 kg∙C∙m⁻²∙a⁻¹ and 0.16 kg∙C∙m⁻²∙a⁻¹ for the original CASA model and modified CASA model, respectively. The modified CASA model performed well in estimating NPP compared with field-observed data, with root mean square error (RMSE) and mean absolute error (MAE) of 0.13 kg∙C∙m⁻²∙a⁻¹ and 0.10 kg∙C∙m⁻²∙a⁻¹, respectively. Relative RMSE and MAE decreased by 45.8% and 44.4%, respectively, compared to the original CASA model. The variation of NPP showed gradients decreasing from southeast to northwest spatially, and displayed an overall decreasing trend for the study area temporally, with a mean value of −0.02 × 10⁻² kg∙C∙m⁻²∙a⁻¹ due to climate change and increasing anthropogenic activities (i.e., land use and land cover change). Generally, 54% and 89% of the total pixels displayed a negative relationship between NPP and mean annual temperature, as well as annual cumulative precipitation, respectively, with average values of –0.0003 (kg∙C∙m⁻² a⁻¹)/°C and −0.254 (g∙C∙m⁻²∙a⁻¹)/mm for mean annual temperature and annual cumulative precipitation, respectively. Additionally, about 68% of the total pixels displayed a positive relationship between annual cumulative solar radiation and NPP, with a mean value of 0.038 (g∙C∙m⁻²·a⁻¹)/(MJ m⁻²). Anthropogenic activities had a negative effect on NPP variation, and it was larger than that of climate change, implying that human intervention plays a critical role in mitigating the degenerating ecosystem. In terms of human intervention, ecological destruction has a significantly negative effect on the NPP trend, and the absolute value was larger than that of ecological restoration, which has a significantly positive effect on NPP the trend. Our results indicate that ecological destruction should be paid more attention, and ecological restoration should be conducted to mitigate the overall decreasing trend of NPP in the plateau.

Climate change can significantly affect carbon cycling of forest ecosystems. The diurnal and seasonal dynamics of soil respiration (R _s) in Cinnamomum camphora and Liquidambar formosana forests were investigated by using infrared gas exchange analyzer of Li-Cor 6400-09 each month in 2006. Soil temperature and moisture were also measured. Diurnal variations in R _s varied with daily soil temperature in the two forests. Across the growing season, soil respiration peaked on July 28 due to higher soil temperature and moisture conditions. Seasonal R _s variations were predominantly influenced by soil temperature. Seasonal soil respirations in C. camphora and L. formosana forests were strongly related to soil temperature (x) at 5-cm depth (R _s = 0.1598e^0.1377x (R ² = 0.9289, P = 0.001) and R _s = 0.2177e^0.0962x (R ² = 0.927, P = 0.000)), while annual mean R _s rate was 2.614 and 1.397 μmol m⁻² s⁻¹ and Q ₁₀ values was 3.96 and 2.62 for the two forests, respectively. However, R _s was not influenced by seasonal changes in soil moisture (w) in the two forests in 2006. We developed two equations (R _s = −0.020w ²+0.497w+0.562 (R ² = 0.109, P > 0.05), R _s = −0.001w ²+0.072w+0.731(R ² = 0.053, P > 0.05)) to describe the relationship between R _s and soil moisture. A positive relationship between R _s and soil moisture were observed when soil moisture was below 12.43% and 18.00%, but when soil moisture exceeded threshold values of 12.43% and 18.00%, soil moisture became the restraint factor for R _s. Long-term observations of soil respiration and moisture are required to understand the temporal dynamics of soil respiration.

The planting of trees on mine wastelands is an effective, long-term technique for phytoremediation of heavy metal-contaminated wastes. In this study, a pot experiment with seedlings of Koelreuteria paniculata under six treatments of local mine wastes was designed to determine the major constraints on tree establishment and to evaluate the feasibility of planting K. paniculata on manganese mine wastelands. Results showed that K. paniculata grew well in mine tailings, and also under a regime of equal amounts of mine tailings and soil provided in adjacent halves of pots. In contrast, mine sludge did not favor survival and growth because its clay texture limited fine root development. The bio-concentration factor and the translocation factor were mostly less than 1, indicating a low phytoextraction potential for K. paniculata. K. paniculata is suited to restore manganese mine sludge by mixing the mine sludge with local mine tailings or soil.