Effects Of Warming And Nitrogen Addition On Structure And Function Of Leymus Chinensis Community In Songnen Grassland

Over the coming decades the signal of anthropogenic climate change is expectedto become increasingly apparent, with accelerated production of greenhouse gasesfurther warming the earth’s surface and modifying nitrogen deposition patterns.Global warming and nitrogen deposition may interact in their effects on the diversityand composition of natural communities. The two related goals of studying bioticresponses to climate change are to understand changes in species distributions andabundances and to understand ecosystem feedbacks to climate change. Songnenmeadow steppe is the largest, most representative grassland in China, which plays animportant role on response and regulation to global climate change. This studyevaluated Leymus chinensis plant communities, a dominant perennial grass widelydistributed in the Songnen plain, as a model to investigate the effect of increasingambient temperature on community composition, diversity, biomass, dominantspecies phenology, seed production from2006to2009. Our study showed potentialimportance of understanding ecosystem responses to climate change and nitrogendeposition in terms of species responses. The main results can be summarized asfollows:(1) Warming and nitrogen addition may indirectly affect species compositionthrough species numbers variation. Warming increased species numbers, whereasnitrogen addition decreased it. At the same time, community characteristics changedobviously. Warming reduced important value of grass, while enhanced the importantvalue of herbs, the opposite result occurred by nitrogen addition. Both speciesrichness and evenness increased by warming, in contrast, the effect of nitrogenaddition decreased them. Finally, diversity index showed positive dependence onwarming but negative dependence on nitrogen addition.(2) We found significant positive responses of warming to the undergroundbiomass but negative responses to the aboveground biomass. In particular, for theaboveground biomass, a negative impact in dry years and little affects in moreprecipitation years were found. Moreover, nitrogen addition significantly enhancedboth aboveground biomass and underground biomass regardless of rainfall. Theinteraction between them is the same result. (3) Warming accelerated Leymus chinensis spiking stage, flowering stage andseed-setting stage, but had no effect on seed maturity stage. Nitrogen addition hadlittle effect on phenology score of spiking stage, but obviously delayed the floweringstage and setting stage, and seed maturing stage.(4) Warming reduced the mean seed number per square meter and potentialgerminating seeds, and resulted in decreased proportion of light weight seeds butincreased the proportion of heavy weight seeds. The seed germination, germinationrate, and seedling shoot of light weight seeds were reduced by warming but the sameeffects were not observed in heavy weight seeds. Despite of increased proportion ofheavy weight seeds compensated decreased plant number because of reduced seednumber per square meter; a reduction in the potential germination seed number byincreased temperature may imply that climate warming will constrain populationexpansion depending on the viability implications for seeds under further globalclimate warming.Nitrogen deposition cause a decreases in proportion of light weight seeds butresult an increases in heavy weight seeds, which lead to an improving of seedproduction, seed germination success, germination rate and growth of correspondingshoots in Leymus chinensis. These results implied that continued nitrogen depositionwill promote the sexual reproduction process and improve the growth of Leymuschinensis by occupy more space.Overall, warming leaded a decrease in grassland productivity due to the soildrought from warming depspite warming maintained a high biodiversity. Nitrogendeposition leaded an increase in primary productivity, but a decrease of speciesnumber. And it was therefore that the growth and structure of the Leymus chinensiscommunity responded to global warming and nitrogen deposition were sensitive,which might provide a theoretical basis to explain and predict the trends of grasslandecosystems faced with further global changing.