Effects Of Nitrogen Addition On Flowering Phenology And Community Strucure Of Alpine Meadow In The Eastern Qinghai-Tibet Plateau

The impcat of global climate and environment change on terrestrial plant ecosystems have been increasingly concerned by scientists. In particular, the response of plant phenological events to changes in global climate has been a critical concern in ecology. Different phenological time among plants have an important role in community assembly. Plant phenological events change is the result of the combined effects of multiple environmental factors. Nitrogen has traditionally been considered the primary limiting nutrient for plant growth in terrestrial ecosystems, but in the context of global climate change the rate of nutrient inputs in natural systems has been changed, which resulted in nitrogen (N) enrichment in terrestrial ecosystems. Grassland ecosystem is an important component of terrestrial ecosystems, the study on the impact of nitrogen nutrition on grassland communities are mostly concentrated in the community productivity, species diversity and community structure changes for a long time. Recent studies show that nitrogen addition could affect plant phenology, especially affect flowering phenology. Environmental change of many ecosystems throughout the globe has important ramifications for grassland communities, and the research on grassland plant phenology have received widespread attention. But the majority of plant phenology research has been conducted in lowland areas, alpine meadow ecosystems may be particularly sensitive bellwethers of environmental change. Alpine plants are limited by a short growing season and extreme environmental conditions, which making them highly vulnerable to modification of limiting factors. This suty was conducted in an alpine meadow at The Research Station of Alpine Meadow and Wetland Ecosystems of Lanzhou University, in Maqu (azi)(N33°40’E101°52’, altitude3550m). We set up three levels of nitrogen addition (control, low nitrogen addition, high nitrogen addition), from2009to2011, plant phenology was observed in fixed plots and fixed time (from early May to late-September in each year), and measured plant functional traits (plant height, SLA, individual plant biomass allocation)for most common species in nitrogen addition and control communities, to study the effects of nitrogen addition on flowering phenology, functional traits and community structure of alpine meadow in the eastern Qinghai-Tibet Plateau.We gained following results1. In natural community, we observed58,51,55, species flowered in2009,2010,2011respectively. The flowering sequence distribution of species in the community show that, species flowering phenology started from the late April to the end of September. The frist flowering date (FFD), peak flowering date (PFD) and flowering duration day (FDD) are different among species.2. In alpine meadow community, the first flowering date is negatively correlated with the flowering duration day in different nitrogen addition treatments.3. We tested the effect of nitrogen addition on first flowering date for38common species in2010,34common species in2011. Nitrogen addition significantly delayed the first flowering date for all graminoid species, but accelerated first flowering date for most forb species. In response to nitrogen addition, species of Gramineae, Sedge and Compositae delayed first flowering date; species of Ranunculaceae, Gentianaceae and Scrophulariaceae accelerated first flowering date; legumes did not change in flowering time.4. For most species, both first flowering date and reproductive allocation (RA) significantly responded to nitrogen addition. Species are different either in the response direction or in the change of the FFD and RA. All graminoid species trends to reduce RA in response to nitrogen addition, The response of forb species to nitrogen addition was more variable. Nitrogen addition significantly delayed the FFD and reduced the RA for all graminoid species, but accelerated flowering and increased RA for most forb species.5. Species that advanced their flowering time with nitrogen addition increased their reproductive allocation, whereas those that delayed flowering time tended to decline in reproductive allocation with nitrogen addition. That changes in FFD significantly negatively correlated with the changes in RA over species in response to nitrogen, which indicated a positive relationship between flowering response and plant performance in reproductive allocation.6. In alpine meadow community, the plant height and plant stem allocation is positively correlated with peak flowering date in different nitrogen addition treatments. This positive relevance increased in nitrogen added plots.7. Nonmetric multidimensional scaling (NMDS) ordination of phenologically-weighed community similarity among nitrogen addition gradient showed that, the community level phenology similarity and species composition similarity were significantly decreased after nitrogen addition. With the nitrogen addition gradient the change of both community level phenology and community structure were more intense.