Spatial And Temporal Variation Of Soil Moisture Content And Vegetation Productivity At A Small Watershed Of The Loess Plateau

The shortage of water often limits plant growth and hinders the establishment of stable and sustainable plant communities in the hilly region of the Loess Plateau. The objective of my experiment was to learn more about differences in the spatial-temporal variation of soil water content and its effect on vegetation within a gully catchment. The experiment was conducted at Kanggelaogou Catchment in Northern Shaanxi Province. Soil water content was measured at 28 points in the catchment. Biomass and productivity of the plant communities, transpiration rates of dominant plants, and plant community diversity were also measured at various points along the slope.l.In general, it seemed that soil water content increased from the top to the bottom of the catchment., however the soil moisture content was affected by the type of vegetation growing at different parts of the slope. I tried to account for this effect, by choosing sample points with similar types of vegetation. The distribution and type of native vegetation was highly related to soil moisture. On the condition of no effect from vegetation and form of slope, soil moisture increased down to the bottom of the catchment; the vegetation diminished or even reversed this trend. The amount of water in soil profiles under different kinds of native vegetation decreased with depth in the 0-400cm soil profile. In contrast, the water content of soil under orchard and barren slope increased with depth. During the growing season, the soil water content from 0-200 cm of vegetated sites was closely related to precipitation, but the barren slope was different. Soil water content under bare soil remained relatively high (12%) throughout the growing season.2. The biomass and productivity of each plant community were positively correlated. The spatiotemporal distribution of biomass and productivity in the herb communities did not follow an obvious pattern. This was due to human activity and water availability. Human activity disrupted the structure of the herb community.Biomass and productivity decreased as human disruption increased. Water resources in this area were sufficient for the growth of herbs. Unlike the shrub and forest communities, -water was not a limiting factor for the herb community in this area . The biomass and productivity of the shrub community had an obvious pattern in its spatiotemporal distribution. In most cases, biomass and productivity on the north slope was more than on the south slope. Furthermore, the biomass and productivity of the shrub community increased from the top to the bottom of the north slope. This trend seemed to be related to the spatial distribution of soil moisture in the catchment.3. During the year when I conducted this experiment, plant transpiration rates were related to soil moisture and precipitation. Transpiration rates decreased as soil moisture decreased. Diurnal variation in transpiration rates for different plants followed a similar pattern; transpiration rates were low in the morning, increased as temperature and light intensity increased, and then declined again. Transpiration rates for most plants had a single peak each day. Some plants had two peaks per day. Some plants had single peaks each month, but others had two peaks per month. With the exception of Sophora viciifolia, the transpiration rates for all plants were lowest in August. Transpiration rates were positively correlated to soil moisture and precipitation. The long term deficiency of soil water resulted in a decrease in transpiration rates.4. The diversity of the plant communities decreased from the bottom to the top of the slope. The Simpson indicator was highest on the south, intermediate on the southwest slope and lowest on the north slope. In contrast, Shannon-Wiener and Pilow indicators were just the opposite. The steepness of the slope had no apparent effect on plant diversity in this catchment. Differences in plant diversity seemed to be related to human activity and soil moisture conditions.