Vegetation Ecological Features And Net Primary Productivity Simulation In Yangou Watershed In The Loess Hill-gully Areas Of China

The Loess Plateau in China is one of the key regions of soil erosion control and ecological environment construction. Vegetation net primary productivity (NPP) is an important indicator in evaluating the effectiveness of vegetation recovery and ecosystem management. Climate and land use change are the major components of global change, and also apparently affect the NPP. Currently, global climate change has become an indisputable fact, and the land use pattern in the Loess Plateau also has changed greatly since the implementation of Grain for Green Project. Therefore, studying the formation of vegetation productivity and its response to climate and land use change has great significance in clarifying the response of NPP to global change, and in guiding the vegetation construction in this region.In the study, we take Yangou watershed in loess hill-gully region as a case. Base on the continuous monitoring on soil physical and chemical properties, community features and plant ecophysiological characteristics of the eleven typical plants, the soil physical and chemical characteristics of different vegetation types, the seasonal variation of plant photosynthesis and leaf structure traits and C, N, P stoichiometry were discussed. Moreover, in considering the effects of soil moisture content and terrain factors on vegetation carbon assimilation process and basing on the DEM of the study area, a NPP model applied in complex terrain was constructed (CT-VPP). And then, the water balance and NPP of different vegetation ecosystem in the watershed were simulated and the responses of NPP of different vegetation types to climate and land use change were analysized.The improvement on soil physical and chemical properties of different vegetation type was different, and the improving effect of natural vegetation was higher than artificial vegetation. Compared with farmland, soil bulk and soil moisture decreased while soil nutrient increased under other four vegetation types. Soil organic matter and total nitrogen in natural shrubland and grassland were significantly higher than that in artificial woodland and shrubland, and the lowest soil organic matter and total nitrogen content were in farmland and orchard. With soil depth, the difference of soil physical and chemical properties among different vegetation types gradually diminished. While the difference of soil total phosphorus were not significant among different vegetation types and different soil depth.The photosynthetic characteristics, leaf physical traits, and stoichiometric parameters of eleven typical plants varied greatly with species and seasonal change. Overall, the plant photosynthetic rate, nitrogen and water use efficiency of different plants were higher in the middle growth stage (July) than that in the early (May) and late (September) growth stage, and the transpiration rate was higher in the early growth stage than that in the middle and late growth stages, while specific leaf weight were higher in the late growth stage than that in the early and middle growth stages. Leaf C content was lower in the early growth stage than that in the middle and late growth stage, while the leaf N and P content had a contrast seasonal variation. Leaf C: N, C: P had similar seasonal variation trend with leaf C content, while the seasonal variation of leaf N: P was relatively complex. These results indicated that plants invested more energy on productive organ in the early growth periods while invested more energy on defensive organ in the late growth periods during their developing process.A NPP model applied in complex terrain was constructed and validated. The model consisted of three modules which were solar radiation sub-model, soil water balance sub-model and plant carbon assimilation sub-model. The model was drove by weather, soil, vegetation, and spatial data, and the output of the model included solar radiation, evapotranspiration, soil moisture, and NPP, etc. at daily, monthly and annual step. The model was tested with the measured data, and also with simulated results of other similar models. The verification results showed that the model had good prediction in the study area. The simulated results showed that the NPP of the watershed in 2007 was about 7681.5 t·C with the order of woodland> orchard> shrubland> farmland> grassland, shady and semi-shady slope> sunny and semi-sunny slope, and 5°-15°slope > other slope. NPP in the watershed had responded differently to the occurring time of climate change. NPP increased significantly with increasing precipitation, and decreased with increasing temperature where climate change occurred through the whole year and in the summer half-year. Where climate change occurred in the winter half-year, increased precipitation had little effect on NPP and increased temperature significantly reduced NPP. The effects of precipitation changes on NPP was greater than the temperature changes in the whole year or in the summer half-year, while it reversed in the winter half-year. There were clear differences between the response sensitivities of different vegetation types. Trees and shrubs were more sensitive to changes in temperature and precipitation than crops and grasses.There were obviously effects of land use change on NPP. The areas of farmland and grassland in 2007 declined by 20.4% and 10.5% comparing to 1997, while woodland, shrubland and orchard increased by 13.9%, 3.1% and 8.7%. NPP in the watershed increased by 10% from 1997 to 2007. To take the land use pattern in 2007 as a base, NPP increased by 11.1% and 15.5% when sloping land steeper than 25°replaced by woodland and orchard, while NPP decreased by 7.8% and 18.9% when replaced by farmland and grassland, while NPP not changed obviously when replaced by shrubland.In conclusion, a NPP model applied in complex terrain was constructed on the basis of field test and laboratory data collected, and the responses of NPP of the Yangou watershed in the Loess Plateau to climate and land use change were explored in this study. The results are helpful to understand the mechanisms of vegetation carbon assimilation process and to clarify the response and adaptive difference of different vegetation to global change. However, this paper only took a small watershed in the Loess Plateau as a case to study the vegetation primary productive process, regional scale NPP simulation studies need to do to reveal the ecosystem carbon cycle process and its distribution pattern changes in the Loess Plateau.