Dynamics of aboveground net primary production of boreal steppe of northern Mongolia.

摘要

Understanding the spatial and temporal variability in aboveground net primary production (ANPP) of grassland ecosystems has remained a challenge because of complexity of ecological processes and patterns relating with temporal and spatial scales. In this dissertation, I studied the landscape scale (spatial) and temporal (annual) variability in ANPP while taking account of year-to-year variation of climatic factors, livestock grazing and vegetation structure as well as the gradient from wet and nutrient-rich sites (north-facing steppe) to dry and nutrient-poor site (south-facing steppe) in the cold and dry boreal steppe of Northern Mongolia, which is one of the hot spots of climate change because of its location at high latitude (51°N).;Livestock grazing increased species richness of wet and nutrient-rich sites as grazing tolerant species locally colonized and the species in the wet and nutrient-rich sites were driven to local extinction. In dry and nutrient-poor sites, no significant change was observed. However, plant communities in the dry and nutrient-poor south-facing steppe under grazing shifted to dominance by short and prostrate forb species.;For temporal variability in ANPP of the boreal steppe, the ungrazed north-facing lower had highest mean annual plant biomass while the lowest was observed in the grazed north-facing lower. For the other three landscape positions, mean annual plant biomass at the peak biomass period was not substantially different between our grazing levels. We found more temporal variability in total plant biomass in the grazed landscape than the ungrazed landscape. Climatic variables included in our analyses had more explanatory power for the temporal variability in total plant biomass in the grazed landscape. This might indicate a shift from dominance of effects of internal vegetation constraints on total plant biomass in an ungrazed landscape to dominance of effects of climate factors in a grazed landscape.;Nitrogen (N) is one of the main factors controlling ANPP in grassland ecosystem. We carried out a water and N manipulation experiment along a topographical gradient (south-facing upper and lower slopes) of the boreal steppe. We found results supporting co-limitation of water and N along the topographical gradient. We found strong positive effect of the interaction of water and N (H20 x N treatment) on ANPP at the both slopes; 3.5 times higher at the upper and 2.5 times at the lower slope.;Without a grazing effect, litter (plant dead material) is accumulated in the steppe. There is almost 4-fold high litter mass in ungrazed areas compared with grazed areas of mountain steppe of Northern Mongolia. Litter influences many important functions in grasslands including the water cycle, vegetation structure and ANPP. Using a litter and rainfall manipulation field experiment and water flux simulations, we found that the litter effect on soil water content was inconsistent among years; in a dry year with frequent small rainfall events, litter significantly affected the soil water while this effect was not detected in a wet year with frequent large storms. In the boreal steppe, the mean litter mass, 64 g m-2, did not significantly affect the amount of infiltration.;Traditional seasonal movement of Mongolian nomads can be an important part of the sustainability of boreal steppe. The traditional movement can lessen loss of species diversity at either local or regional scale. Especially in the permafrost area, livestock grazing resulted in the disappearance of moss cover, and the species adapted to the wet and nutrient rich site. Moss cover is the main insulator of permafrost. The loss of moss cover could potentially accelerate a thawing of permafrost and warming of this region. Also nomadic movement might lessen the high N limitation in the Mongolian grasslands through redistribution of soil nutrients by livestock. In order to prevent loss of litter biomass, which has a significant effect on the ANPP of the boreal steppe, I suggest that encouraging seasonal movement of Mongolian herders will make it possible to maintain the sufficient litter mass to support plant production for livestock. (Abstract shortened by UMI.).