| The study of plant ecological stoichiometry is not only important for understanding ecosystem functions and biogeochemical cycles,but also can be used to assess the nutrient limits of ecosystems,and thus provide important references for predicting the responses of ecosystems in different regions under the background of global nutrient changes.Therefore,plant ecological stoichiometry has gradually become a research hotspot in recent years.The change of altitude gradient covers the change of a series of environmental factors such as temperature,humidity and soil nutrients,as well as the change of biological factors such as vegetation type and species characteristics.Therefore,the vertical pattern of plant nitrogen and phosphorus stoichiometry under natural gradient has attracted much attention.Yarlung Zangbo Grand Canyon has a climate zone from the tropical to the cold zone,and also forms the most complete and abundant mountain vertical natural vegetation zone in the world.However,its phytostoichiometric characteristics have not been reported so far.In order to understand the ecological adaptability and N and P nutrient utilization characteristics of plants in Yarlung Zangbo Grand Canyon,367 plant leaf samples were collected along the natural gradient of elevation,including133 samples from broadleaf forest（BF）,185 samples from coniferous forest（CF）,and 49 samples from shrub and spindling shrub（SC）.The stoichiometric characteristics of nitrogen and phosphorus were analyzed,and the main results were as follows.（1）At the altitude of 780-4396m,leaf N concentrations ranged from 6.6 to49.4 g kg-1,with an average of 22.6 g kg-1.leaf P concentrations ranged from0.58 to 4.08 g kg-1,with an average of 2.34 g kg-1.The N/P range was 3.1-23.7,with an average value of 10.2,indicating an N-restricted ecosystem.The content of N and P and the content of N/P in plants decreased with the increase of altitude,and reached a significant level.In terms of vegetation types,leaf N content in broad-leaved forest area was significantly higher than that in coniferous forest area and shrub meadow area,with mean values of 26.4,20.6and 20 g kg-1,respectively.The P content in broad-leaved forest and coniferous forest was significantly higher than that in shrub meadow,with the mean values of 2.46,2.46 and 1.92 g kg-1,respectively.N/P was significantly higher in broad-leaved forest and shrub meadow than in coniferous forest,which were11.4,8.94 and 10.9,respectively.From the perspective of life type,there was no significant difference in the content of N in leaves of herbs and woody plants,which were 22.6 g kg-1and 20 g kg-1,respectively.There was a significant difference in P content between herbs and woody plants,which were 2.56 and1.93g kg-1,respectively.The difference of N/P between herbs and woody plants did not reach a significant level,with the mean values of 9.80 and 11.6,respectively.The woody plants were divided into evergreen and deciduous plants for further analysis.The N content of evergreen and deciduous plants was significantly different,which was 15.1 g kg-1and 24.0 g kg-1,respectively.The P content of evergreen and deciduous plants was 1.39 and 2.42 g kg-1,respectively.The N/P values of evergreen and deciduous plants were 10.8 and 9.9,respectively.The stoichiometric analysis of plant N and P in the study area was conducted with the factors of altitude gradient,life type and species as influencing factors.The results showed that plant species was the determining factor of plant N and P content and N/P.（2）The mean C concentration,N concentration,P concentration,C/N,C/P and N/P were 452 g kg-1,26.4 g kg-1,2.46 g kg-1,19.7,212 and 11.4,respectively.In terms of life type,the C concentration of woody plants in the study area was higher than that of herbaceous plants,the N concentration was not significantly different,and the phosphorus content was lower than that of herbaceous plants.There was no significant difference in the stoichiometric characteristics of carbon,nitrogen and phosphorus in the leaves of woody plants among different sampling sites,and no significant difference in carbon and nitrogen in the leaves of woody plants,but the difference in phosphorus content of herbaceous plants reached a significant level among some sampling sites.Correlation analysis showed that the correlations between C and N and between C and P were not significant,but the correlations between N and P reached a very significant level.The content of C in leaves of the evergreen broad-leaved forest of Metoo was slightly lower than that of the other evergreen broad-leaved forests in southern China,the content of nitrogen was slightly higher than that of the other regions,but the content of P was significantly higher than that of the other regions,so the C/N,C/P and N/P were all lower.（3）The power index of nitrogen and phosphorus in plants is an important parameter in the study of nitrogen and phosphorus stoichiometry in plants.The linear fitting results showed that the power index relationship constants of shrub meadow were higher than those of broadleaf forest and coniferous forest,which were 0.769,0.588 and 0.605,respectively,and the fitting degree of high altitude area was higher.In terms of life type,the relationship constants of nitrogen and phosphorus power index of woody plants were higher than those of herbaceous plants（0.739 and 0.465,respectively）,and the coupling relationship of nitrogen and phosphorus of woody plants was better than that of herbaceous plants.At the same time,the N and P power index relationship constants of evergreen plants were higher than those of deciduous plants（0.854 and 0.323,respectively）,and the coupling relationship of N and P of evergreen plants was better than that of deciduous plants.In conclusion,this study confirms that not all low-latitude tropical and subtropical forest ecosystems are limited by P.Although the annual precipitation and average annual temperature are extremely high,tropical and subtropical evergreen broad-leaved forests are dominant,but from the perspective of plant leaf nutrients,the ecosystem in this region should be limited by P.Therefore,it is uncertain to predict the nutrient limitation of ecosystem based on temperature,annual precipitation and vegetation type.From the perspective of nutrient restriction of ecosystem,this study confirmed that the different vegetation communities in the Yarlung Zangbo Grand Canyon should be nitrogen restricted ecosystems.At the same time,the nitrogen-phosphorus coupling relationship increases with the increase of plant leaf life.Under the coupling effect of increasing global atmospheric nitrogen deposition and temperature,the nutrient cycling and carbon uptake potential of different ecosystems in this region may be promoted to a certain extent.Therefore,attention should be paid to the nutrient uptake and carbon fixation potential of vegetation in different ecosystems in this region by nitrogen deposition and warming. |
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