| Simulation models are needed to study primary production of ecosystems with complex biological and environmental interactions at various temporal and spatial scales. Climate and land use change are the main driving forces that influence the net primary production processes. Loess Plateau is well known for its high erosion rate and other special environmental conditions in China and all around world. Reestablishment of appropriate vegetation coverage is recognized as main measures to prevent soil and water loss and restore the degradation ecosystem. The primary production of different vegetation types and plant species under various environments reflect their adaptability to the environment of Loess Plateau. So the study on the primary production is the basis of vegetation reconstruction and ecosystem restoration. Chapter 2 simulated the stomatal conductance, transpiration and photosynthesis characteristic of 14 plant species in forest-steppe ecotone of Loess Plateau. The results indicated that the models were capable of predicting stomatal conductance and net photosynthesis rates of all the 14 plant species under wide ranges of environmental conditions. Major conclusions included that (1) Robinia pseudoacacia L., Molus pumila Mill. and Caroganga korshinskii Kom. were more tolerant for and resistant to soil water stress than the other species due to their lignified guard cell walls and lower osmotic potential. Hippophae rhamnoides L., A. gmelinii Web. Ex Stechm., A. capillaris Thunb., Zea mays L. , Bothriochloa ischaemum L. and Astragalus adsurgens Pall. were superior to other plant species in preserving soil water to resistant long-term aridity for their soft guard cell walls and high osmotic potential. (2) The stomatal conductance of Hippophae rhamnoides L., A. gmelinii Web. Ex Stechm., A. capillaris Thunb., L.davurica Schindl, panicum virgatum and Zea mays L. was slightly affected by photosynthetic active radiation for their osmotic potential insensitive to photosynthetic active radiation, and stomatal conductance only had the peak at early of the day, then decreased during 8:00-16:00 with the increased vapor pressure deficit for their softer guard cell structure than other species,which enhanced the characteristic of preserving soil water. M.sativa L., panicum virgatu., L.davurica Schindland, Hippophae rhamnoides L. had the higher photochemical efficiency, and A. capillaris Thunb., Zea mays L. and Astragalus adsurgens Pall. had the higher carboxylation conductance, and Bothriochloa ischaemum L., Zea mays L. and Setaria italica had lower photorespiration coefficient. So all these species had higher net photosynthesis rate than others. (4) The diurnal variations of net photosynthesis rate had high relation with photosynthetic active radiation. The depression in photosynthesis of Robinia pseudoacacia L., Hippophae rhamnoides L., A. gmelinii Web. Ex Stechm. and A. capillaris Thunb. at afternoon may be explained by decreased stomatal conductance, and that for others was due to the decreased biochemical photosynthetic capacity of mesophyll cells. Chapter 3 scales up the result of ecophysiological models at leaf scale to landscape according to the influence of canopy structure and topography on radiation. The model was tested in Zhifanggou watershed, which belongs to hilly area of Loess Plateau. The model has successfully simulated the variation trends of primary production and soil water dynamics for typical ecosystems of arid and semi-arid under various environments. The main conclusion include (1) Form. R. pseudoacaciat and M. pumila have highest primary production with lower transpiration, and crop, Form. B. ischaemum and L.davurica have higher primary production with highest transpiration, and the primary production and transpiration of Form. A. gmelinii and A. giradii. are all lowest, while Form. H. rhamnoides and C. korshinskii have moderate primary production with higher transpiration. (2) Soil water keep balance at long term, but with fluctuating in short term. Soil moisture of Form. R. pseudoacaciat, M. pumila and H. rhamnoides is deficit higher, soil water is complemented at end of year for Form. A. gmelini and A. giradii, and soil water keep balance for others. Soil moisture was higher in high-flow year than that in low-flow year for all vegetation types, and showed serious deficit in low-flow year and abundant in high-flow year. (3) Primary production in southward aspect was lower than other aspects, and in slopes steeper than 35o was lowest. Form. R. pseudoacaciat and M. pumila showed preferable between slopes of 15-35o, Form. H. rhamnoides, A. gmelinii, A. giradii. and L. davurica showed slightly difference at all aspects and slopes expect southward aspect and steep slope great than 35o. Form. B. ischaemum could grow well at steep slope, and crop would grow preferably in slope less than 15o, but the primary production decreased in slope steeper than 35o. (4) Soil moisture in southward aspect was lower than that in others, and showed serious deficit at the end of year. Southward aspect had lower transpiration and higher evaporation than other aspects and the tendency became significant with slope becoming steep. Chapter 4 analyzed the trends of climate change for 50 years of study area and performed simulations under 4 climate scenarios, which include 20% changes in the amount of daily rainfall, 2℃increase chosen arbitrarily, synthesize 20% increase in rainfall and 2℃increase in temperature during the model simulation period (1990-1999). The simulated results were compared with that of present climate to analyze the impact on the net primary production processes of possible changes in climate variables. (1) Primary production reduced dramatically for 2℃increase in the average air temperature. Among all vegetation types Ass. Artemisia sacrorum and Ass. Artemisia giraldii behaved more sensitive than others, and Black Locust decreased less. Transpiration declined with the increase in temperature. The increase in potential evapotranspiration followed 2℃increase in temperature caused more soil water loss by evaporation, and soil moisture reduced simultaneity. Runoff under various vegetation types declined by more rain infiltrate to soil expects Ass. Artemisia sacrorum and Ass. Artemisia giraldii. (2) A 20% change in rainfall had less impact on primary production than increase in temperaure. Among all vegetation types, primary production of Seabuckthorn changed significantly and Black Locust, Malus pumila and Caragana korshinskii changed slightly. Transpiration of Seabuckthorn changed little with the rain fall, while the change in transpiration of Black Locust and Malus pumila approximately equaled to the extent of rainfall changed, and...
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