Freeze-thaw process induced by increased precipitation affects root growth of alpine steppe on the Tibetan Plateau

The response of vegetation productivity to precipitation is becoming a worldwide concern.Most reports on responses of vegetation to precipitation trends are based on the growth season.In the soil freeze/thaw process,the soil water phase and heat transport change can affect root growth,especially during the thawing process in early spring.A field experiment with increased precipitation(control,increased 25%and increased 50%)was conducted to measure the effects of soil water in early spring on above-and below-ground productivity in an alpine steppe over two growing seasons from June 2017 to September 2018.The increased 50%treatment significantly increased the soil moisture at the 10 cm depth,there was no difference in soil moisture between the increased 25%treatment and the control in the growing season,which was not consistent in the freeze/thaw process.Increased soil moisture during the non-growing season retarded root growth.Increased precipitation in the freezing-thawing period can partially offset the difference between the control and increased precipitation plots in both above-and below-ground biomass.

Soil moisture response to land use and topography across a semi-arid watershed: Implications for vegetation restoration on the Chinese Loess Plateau

Soil moisture is a limiting factor of ecosystem development in the semi-arid Loess Plateau. Characterizing the soil moisture response to its dominant controlling factors, such as land use and topography, and quantifying the soil-water carrying capacity for revegetation is of great significance for vegetation restoration in this region. In this study, soil moisture was monitored to a depth of 2 m in three land use types(native grassland, introduced grassland,and forestland), two landforms(hillslope and gully),and two slope aspects(sunny and shady) in the Nanxiaohegou watershed of the Loess Plateau,Northwest China. The MIKE SHE model was then applied to simulate the soil moisture dynamics under different conditions and determine the optimal plant coverage. Results showed that the average soil moisture was higher in native grassland than in introduced grassland and Platycladus orientalis forestland for a given topographic condition;however,a high soil moisture content was found in Robinia pseudoacacia forestland, with a value that was even higher than the native grassland of a sunny slope. The divergent results in the two forestlands were likely attributed to the differences in plant coverage. Gully regions and shady slopes usually had higher soil moisture, while lower soil moisture was usually distributed on the hillslope and sunny slope.Furthermore, the mean absolute relative error and Nash-Sutcliffe efficiency coefficient of the MIKE SHE model ranged between 2.8%–7.8% and 0.550–0.902,respectively, indicating that the model could effectively simulate the soil moisture dynamics. The optimal plant coverage was thus determined for hillslope P. orientalis by the model, corresponding to a leaf area index(LAI) value of 1.92. Therefore, for sustainable revegetation on the Loess Plateau,selecting suitable land use types(natural vegetation),controlling the planting density/LAI, and selecting proper planting locations(gully and shady slope regions) should be considered by local policy makers to avoid the over-consumption of soil water resources.

The variation in soil moisture and the appropriate groundwater table for desert riparian forest along the Lower Tarim River

Based on data collected over five years of monitoring the Lower Tarim River,we analyzed the variability of soil moisture content （SMC） and the relationship between SMC,groundwater table depth （GWD） and vegetation by using the methods of coefficient of variation （Cv）,Pearson correlation and regression. The results of the variability of SMC indicate that it rose with increase in depth of soil layer -SMC in the soil layer of 0-60 cm was relatively small compared to SMC in the soil layer of 100-260 cm which showed a significant increase in variability. SMC and GWD before and after ecological water diversions exhibited significant differences at the site of the Yingsu transect and its vicinity of the watercourse,especially SMC in the soil layer of 100-260 cm increased significantly with a significant rise of GWD and reached maximum values at a GWD of about 4 m. Plant coverage and species diversity significantly improved with increases in SMC in the soil layer of 100-260 cm,both of them approached the maximum values and 92.3% of major plant species were able to grow when SMC was 〉 10%. To restore the ecosystem of desert riparian forest along the Lower Tarim River,the GWD must be maintained at 〈 4 m in the vicinity of the watercourse and at about 4 m for the rest of this arid region.

Generalized and species-specific prediction models for aboveground biomass in semi-steppe rangelands

Aims The accurate estimation of aboveground biomass in vegetation is critical for global carbon accounting.Regression models provide an easy estimation of aboveground biomass at large spatial and temporal scales.Yet,only few prediction models are available for aboveground biomass in rangelands,as compared with forests.In addition to the development of prediction models,we tested whether such prediction models vary with plant growth forms and life spans,and with the inclusion of site and/or quadrat-specific factors.Methods We collected dataset of aboveground biomass from destructive harvesting of 8088 individual plants belonging to 79 species in 735 quadrats across 35 sites in semi-steppe rangelands in Iran.A logarithmic transformation of the power-law model was used to develop simple prediction models for the easy estimation of above-ground biomass using plant coverage and vegetation density as predictors for the species-specific model,multispecies and plants of different growth forms and life spans.In addition,additive and multiplicative linear regression models were developed by using plant coverage and one categorical variable from the site and/or quadrat-specific factors.Important Findings The log-transformed power-law model based on plant coverage pre-cisely predicted aboveground biomass across the whole dataset for ei-ther most of the species-specific model,multispecies or plants of the same growth forms(shrubs,forbs or graminoids)and life spans(annuals,biennials or perennials).The addition of vegetation density as a single or in a compound predictor variable had relatively poor performance com-pared with the model having plant coverage only.Although generalizing at the levels of plant group forms and/or life spans did not substantially enhance the model-fit and validation of the plant coverage-based mul-tispecies model,the inclusion of plant growth forms or life spans as a categorical predictor variable had performed well.Generalized models in this study will greatly contribute to the accurate and easy predic-tion of aboveground biomass in the studied rangelands and will be also useful to rangeland practitioners and ecological modellers interested in the global relationship between biodiversity and aboveground biomass productivity across space and time in natural rangelands.