| Net primary production （NPP） is an essential processin natural terrestrialecosystems.Production process, on the one hand, are controlled by the external environment of vegetation.Onthe other hand, the function and traits of photosynthetic organs, such as N:P stoichiometry, alsoconstrain this process. Thus, investigating the pattern, influence, and relationship of NPP and N:Pstoichiometry could improve our understanding of ecological process. Temporal variation in NPPand N:P stoichiometry not only reflects how communitiesrespond to environment fluctuation,butalso has important implicationsforpredicting regional patterns in the future.Unfortunately,studiesbased on fieldmeasurement in the extreme environment of alpinegrasslandsto directly address this issue are rare.Here, we used data of aboveground NPP（ANPP）, leaf N and P stoichiometry at communitylevel of40sites across the Tibetan alpine grasslands obtained from annual-repeated field surveysduring2006–2009to investigate thespatial patterns and temporal variations of ANPP, N and Pstoichiometry, the influence factors, and their relationships. Considering the Tibetan Plateau as amajor pasturing area in China, we also analysed the pattern of forage quality and environmentalfactors that determine it,explored the relationship between forage quantity and quality, andestimated the proper livestock carrying capacity.The main results and conclusions are as follows:1.From2006to2009, the averageANPP varied1.5fold, from83.9to125.7g m-2, with acoefficient of variation （CV） of temporal variationof36.6%across the40sites. Throughcomparing with other studies, we found that results from this study do not support the claim that alpine grasslands are more vulnerable to climate than other grasslands types generally. The spatialpatterns of ANPP were consistent during the four years, possibly due to similar spatial patternsand comparable trends in the climateover the four years on the plateau. Climatic fluctuation hada significant effect on temporal variation of ANPP. However, in contrast to traditional belief, wefound that rainfall, but not temperature,drive the spatial patterns of alpine grassland production,suggesting alpine grasslands are more constrained by water condition.Moreover, our studyprovides direct evidence that species richness （SR）play a role instabilizing community productioninthe alpine grassland of high altitudes.2. Over4years, the mean value of leaf N, P and N:P at community level were24.13mg g-1,1.78mg g-1and14.18respectively, with temporal CV of13.3%,18.9%and18.6%respectively, allexhibiting significant temporal variaotions. The spatial pattern of leaf N had significantinterannual variation, while the spatial pattern of P kept consistent during the four years. This mayreflect P limitation are stronger than N in the Tibetan alpine grasslands. At large scale, soil andclimate factors contribute to the spatial patterns of N and P stoichiometry. However, climaticfluctuations are in relation to none of temporal variations of leaf N, P and N:P, suggesting thatclimatic fluctuations are not the direct driver for local community. Relative growth rate ofcommunity related postively to both leaf N and P, while negatively to N:P, reflecting that thegrowth rate hypothesis hold in true at community level. Moreover, the fact that nutrion useefficiency （NUE） are controled by soil moisture prove our opinion that Tibetan alpine grasslandsare more constrained by water condition. Finally, the positive relationship between SR and NUENmay be due to either complementar effect or sampling effect.3. In general, contrasting to temperate grasslands, Tibetan alpine grasslands had higher crudeprotein （CP） and nitrogen free extract （NFE）, while lower crude fiber （CF） and ether extract （EE）.Alpine meadow had higher forage quality than alpine steppe. PCA showed that climate and soilhad significant effects on forage quality patterns. CF increases while NFE decreaseswith theincrease of temperature, and CP increases while EE and NFE decrease with the increment of soilnutrition condition. Negative relationship was found between forage quality and quantity: higherforage quantity means more CF, less EE and NFE. Finally, we built an empirical model for estimating the livestock carrying capacity at large scale. The model showed that the properlivestock carrying capacity in Tibetan alpine grasslands should be0.68sheep unit pre ha. Theactual carrying capacity is0.79sheep unit pre ha, surpassing16.7%of proper livestock carryingcapacity. |
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