Analysis Of Factors Influencing Intrinsic Water Use Efficiency Of Main Forest Types In Subtropical Region

In this study,two main forest types,artificial forest and natural forest in subtropical region were taken as the research object,and the main factors affecting plant water use efficiency were explored.The study was divided into the following three parts.(1)In this part,we studied water use efficiency(WUEi),nitrogen(N)and phosphorus(P)status of leaves at different leaf ages(current year,1-,2-,and 3-year-old foliage)as well as their relationships in a subtropical chronosequence of Chinese fir(Cunninghamia lanceolata)forests(3-,8-,14-,21-and 46-year-old).The results showed that foliar WUEi varied significantly with foliar age in the order of current year foliage>1-year-old foliage>2-year-old foliage>3-year-old foliage,while stand age had no significant impact on foliar WUEi.Foliar N:P ranged from 11.4 to 19.6 and was higher in younger and older stands than in stands at the fast-growing stage.The foliar N and P concentrations tended to display similar trends with foliar ages in the order of current year foliage>1-year-old foliage>2-year-old foliage>3-year-old foliage.WUEi did not change significantly with stand ages,probably because the photosynthetic rates and stomatal conductance decreased simultaneously with stand age.There was no relationship between WUEi and foliar N.WUEi had significant positive correlation with foliar P and significant negative correlation with foliar N:P.It indicated that foliar P concentration would be a key factor affecting WUEi with increasing atmospheric N deposition in subtropical forests.(2)In this part,we determined the water use efficiency and nitrogen and phosphorus concentrations of plants at different altitudes(600 m,900 m,1300 m,1500 m,1800 m,2000 m,and 2100 m)in Wuyi Mountain.The aim of the research was to understand the relationship of water use efficiency with foliar nutrients and soil moisture.The results showed that plant water use efficiency increased with altitudes,and the leaf ?18O of tree showed no significant variance with altitude.On the whole,leaf nitrogen concentration showed no obvious trend,while leaf phosphorus concentration at high altitude was significantly higher than that at low altitude.Lack of significant relationship between water use efficiency and foliar nitrogen concentration was found in this study,but water use efficiency had a positive correlation with foliar phosphorus.In conclusion,the change of water use efficiency was mainly caused by the differences of photosynthetic rate.The effect of water status on plant water use efficiency was not significant.The variances of leaf phosphorus concentrations along the altitudinal gradients may affect photosynthetic rate and therefore the water use efficiency of plant in this area.(3)Plants at same elevation with different life forms,i.e.,tree,shrub and herb,Trom the Wuyi Mountain in subtropical China were used to elucidate the impacts of life form and leaf functional traits(leaf economic spectrum)on plant intrinsic water use efficiency.This chapter aimed to provide a new insight into the plant ecophysiological processes underlying carbon-water cycling.The results showed that plant life form had no significant effect on water use efficiency,despite an increased tendency for different life forms(tree>shrub>herbaceous plants).However,the specific leaf area of these plants was significantly different among life forms and negatively correlated with the water use efficiency.The water use efficiency were also positively correlated with area-based expressions of leaf nitrogen and phosphorus concentrations,but showed no relationships with leaf nitrogen and phosphorus concentrations on a mass basis.Our results linked plant water use efficiency with different life forms to their specific leaf area and area-based leaf nutrient concentration,highlighting the potential of placing leaf functional traits in the context of plant carbon-water cycling,in particular specific leaf area,leaf nitrogen and phosphorus concentrations.In addition,the effect of life form on water use efficiency and associated leaf functional traits might be affected by the pattern of vertical stratification in the forest.This study focused on how the changes in "leaf economics spectrum",in particular specific leaf area,the mass-and area-based expressions of leaf nitrogen,phosphorus and carbon concentration,would affect water use efficiency.