Adaptive Strategy Of Plant Species To Grazing/Mowing And Community Assembly Mechanisms Along The Resource Availability Gradient

Plant responses to disturbance and community assembly mechanisms have been central to the study of ecology for decades. The trait-based approaches is growing in recent ecology studies. Functional traits are defined as ’morpho-physio-phenological’ traits that impact fitness indirectly via their effects on growth, reproduction and survival of individual plant performance. Trait-based approaches have been advocated as a useful tool to explain species adaptive strategies and explore the different processes suggested to affect community assembly. This dissertation uses trait-based approaches to explore plant species adaptive strategies under different landuse types of arid-and semi-arid grassland, additionally, we examine the community assembly mechanisms along a resource availability gradient mainly mediated by elevation. Specifically, chapter2and chapter3examine the plasticity and reproductive strategy of two constructive species Stipa grandis and S. krylovii in response to grazing and mowing in Inner Mongolia steppe; Chapter4examines the relationship between herbivore diet selectivity and species response to a manipulated grazing gradient in California Valley grassland; Chapter5examines the pattern of functional traits diversity along a resource availability gradient and relates these results to the different processes suggested to affect species coexistence within plant communities. We get several useful results:1. The dominant species S. krylovii had higher leaf plasticity while the other dominant species S. grandis had higher whole plant traits plasticity in grazing treatment. Specifically, S. krylovii had higher SLA and leaf blade/sheath ratio than that of S. grandis under grazing, it indicated that S. krylovii had stronger regeneration rate and photosynthetic capacity than that of S. grandis in grazing.’The variability of basal area and root/shoot ratio of S. grandis were increased under grazing treatment, it suggested that grazing induced tussock fragmentation and the tiller increment of S. grandis. Thus, S. krylovii was more tolerant to grazing than S. grandis. The plasticity of both species under grazing was larger than under mowing condition. 2. Grazing had different effects on reproductive processes of S. grandis and S. krylovii. Grazing significantly increased vegetative tiller to flowering tiller biomass ratio of S. krylovii, while significantly decreased vegetative tiller to flowering tiller biomass ratio of S. grandis. It suggested that S. krylovii investigated more resources into vegetative reproduction, while S. grandis investigated more resources into sexual reproduction in grazing. The reverse reproductive strategies of both species in grazing would be one of important mechanisms of species replacement.3. Plant height and RGR were good predictor traits of species in response to grazing; different species took different adaptive strategies in response to grazing, not all increasers were tolerant species, although some of them had high foliage quality (high SLA, low LDMC), they used avoidance strategies through short stature, thorn and high silica content to increase their abundance in grazing.4. The results of functional trait patterns showed that less productive communities were structured by environmental filters, belowground competition, while more productive communities were structured by aboveground competition and species abundance.5. In northern California Valley grassland, the long history grazing management had been playing an important role in evolutionary process of plant communities; especially it had stronger selective pressure on plant height and seed size.