Clarifying the spatial and temporal variations in precipitation-use efficiency （PUE） is helpful for advancing our knowledge of carbon and water cycles in Tibetan grassland ecosystems. Here we use an integrated remote sensing normalized difference vegetation index （NDVI） and in-situ above-ground net primary production （ANPP） measurements to establish an empirical exponen- tial model to estimate spatial ANPP across the entire Tibetan Plateau. The spatial and temporal variations in PUE （the ratio of ANPP to mean annual precipitation （MAP））, as well as the relationships between PUE and other controls, were then investigated during the 2001- 2012 study period. At a regional scale, PUE increased from west to east. PUE anomalies increased significantly （〉 0.1 g.m^-2.mm^-1/10 yr） in the southern areas of the Tibetan Plateau yet decreased （ 〉 0.02 g. m^-2. mm 1/10 yr） in the northeastern areas. For alpine meadow, we obtained an obvious breaking point in trend of PUE against elevation gradients at 3600 m above the sea level, which showed a contrasting relationship. At the inter-annual scale, PUE anomalies were smaller in alpine steppe than in alpine meadow. The results show that PUE of Tibetan grasslands is generally high in dry years and low in wet years.
Aims Both dominance distribution of species and the composition of the dominant species determine the distribution of traits within community.Leaf carbon(C)and nitrogen(N)isotopic composition are important leaf traits,and such traits of dominant species are associated with ecosystem C,water and N cycling.Very little is known how dominant species with distinct traits(e.g.N-fixing leguminous and non-leguminous trees)mediate resource utilization of the ecosystems in stressful environment.Methods Leaves of 81 dominant leguminous and non-leguminous trees were collected in forest(moist semi-deciduous and dry semi-deciduous ecosystems)and savanna(costal savanna,Guinean savanna and west Sudanian savanna ecosystems)areas and the transitional zone(between the forest and the savanna)along the transect from the south to the north of Ghana.We measured leaf traits,i.e.leafδ13C,leafδ15N,leaf water content,leaf mass per area(LMA)and C and N concentration.Correlation analyses were used to examine trait–trait relationships,and relationships of leaf traits with temperature and precipitation.We used analysis of covariance to test the differences in slopes of the linear regressions between legumes and non-legumes.Important Findings Leafδ13C,δ15N,leaf water content and LMA did not differ between leguminous and non-leguminous trees.Leaf N concentration and C:N ratio differed between the two groups.Moreover,leaf traits varied significantly among the six ecosystems.δ13C values were negatively correlated with annual precipitation and positively correlated with mean annual temperature.In contrast,leafδ15N of non-leguminous trees were positively correlated with annual precipitation and negatively correlated with mean annual temperature.For leguminous trees,such correlations were not significant.We also found significant coordination between leaf traits.However,the slopes of the linear relationships were significantly different between leguminous and non-leguminous trees.Our results indicate that shifts in dominant trees with distinc
