Nutrient Resorption And The Stoichiometry In Planted Forests:A Field Study And A Synthesis

Leaf nutrient resorption(Nu R)is an important essential processes of plant nutrient cycling.It means that the nutrients are resorbed during plant senescence and then conserved or reused for future plant growth.Nu R is an important ecological process for plants to adapt to nutrient limitation,which is driven primarily by genetics,stand age,climate,and soil fertility.Plantation trees are fast-growing and have greater demand for nutrients,however,conclusions regarding the pattern and mechanism of nutrient resorption efficiency(Nu RE)in planted forests remain controversial.The nitrogen(N)enrichment in the terrestrial ecosystems has aroused concern for its effects on ecosystem structure and function.However,a fundamental understanding of the mechanisms that control Nu RE of macronutrients other than N and phosphorus(P)(e.g.,potassium(K),calcium(Ca),and magnesium(Mg))remains limited,under the background of N deposition.In this dissertation,planted forests were selected as the research object and the main contents were to:Part 1,a global data synthesis,by synthesizing results of 146 studies(643 NRE and 539 PRE)and 513 paired observations of leaf NRE and PRE from 116 studies,I explored variations in NRE,PRE,and the stoichiometry among climate zones,stand ages,and tree functional types,as well as the factors that play dominant roles in Nu RE in plantations worldwide.China is experiencing a rapid increase in N deposition and leaf Nu RE is sensitive to changes in soil nutrients.Poplar(Populus deltoids)is one of the most extensively cultivated hardwood species worldwide.However,the effects of N deposition on Nu RE in poplar plantations remain unclear.Part 2,a case study,I conducted a long-term N addition experiment with a series of N addition treatments including N₀(0 kg N ha^-1 yr^-1),N₁(50 kg N ha^-1 yr^-1),N₂(100 kg N ha^-1 yr^-1),N₃(150 kg N ha^-1 yr^-1),and N₄(300 kg N ha^-1 yr^-1)to simulate N deposition.I measured soil and foliar(green and senesced leaves)nutrient concentrations to explore the NRE,PRE,KRE,Ca RE,Mg RE,and NRE:PRE stoichiometric responses to N deposition in two poplar(Populus deltoids cv.(?)L-35‘)plantations(8-and 12-year-old stands)in Dongtai,coastal eastern China.The main results showed that:1.The mean NRE and PRE were 58.98%and 60.21%,respectively.NRE significantly increased from tropical to boreal zones,while PRE did not significantly differ among climate zones,suggesting differential impacts of climate on NRE and PRE.Plant functional types exert a strong influence on Nu RE:Coniferous trees had higher PRE than broadleaved trees,reflecting the adaptation of coniferous trees to oligotrophic habitats;Deciduous trees had lower PRE than evergreen trees that are commonly planted in P-limited low latitudes and have long leaf longevity with high nutrient use efficiency;While non-N₂-fixing trees had higher NRE than N₂-fixing trees,the PRE of non-N₂-fixing trees was lower than that of N₂-fixing trees,indicating significant impact of the N₂-fixing ability on the resorption of N and P.The multivariate regression analyses showed that variation in NRE was mainly regulated by mean annual precipitation(MAP)and latitude,while variation in PRE was dominantly controlled by green leaf nutrient concentrations(N and P).These results,in general,suggest that the predicted global warming and changed precipitation regimes may profoundly affect N cycling in planted forests.In addition,green leaf nutrient concentrations may be good indicators of PRE in planted forests.2.NRE:PRE ratios significantly increased with increasing latitude and decreased with increasing MAP.Among stand ages,NRE:PRE ratios remained constant from 0-20 and 20-40 to40-60 and?60 years old.Additionally,NRE:PRE ratios did not differ between coniferous and broadleaved trees and deciduous and evergreen trees,while they were much higher in non-N₂-fixing than among N₂-fixing trees.Scaling slopes,indicating the proportional changes between NRE and PRE,decreased from tropical regions to subtropical and temperate regions(decreasing MAP)due to high growth rates(more P demand)at high latitudes.The trees in the 20–40 year age range exhibited higher scaling slopes than those in other age ranges.The scaling slopes of coniferous,evergreen,and non-N₂-fixing trees were lower than those of corresponding broadleaved,deciduous,and N₂-fixing trees,given that the former plant groups tend to be in harsh environmental conditions(cold or P-limited).For each functional group,the latitudinal and temporal NRE-PRE scaling patterns were consistent with the pooled data set.Mixed-effects regressions and structural equation models showed that NRE:PRE ratios were negatively controlled by MAP and green leaf N:P concentration ratio,whereas NRE and NRE:PRE ratio negatively regulated NRE-PRE scaling slopes.Consistent with scaling relationships between green leaf N and P contents,these results suggest that NRE generally decouples from PRE in plantations worldwide.3.Consistent for(both)8-and 12-year old stands,N addition did not affect studied Nu RE and stoichiometry(with the exception of Ca RE and Ca RE:Mg RE ratio).NRE-PRE scaling slopes were consistently less than 1.0 under N addition.The results of this study suggest that NRE generally decouples from PRE within each N treatment.Moreover,these results point to robust control of green leaf nutritional status on nutrient resorption processes as indicated by the positive relationships between nutrient resorption efficiency and green leaf nutrient concentrations.This study also provided a direct evidence(increased diameter at breast height with N addition)that growth in 12-year old poplar plantations was N-limited in coastal eastern China.These results are useful for understanding the effects of ongoing global N deposition on plant nutrient resorption and thus ecosystem function.These results may also contribute to our understanding of limiting nutrient resorption strategies and help parameterize nutrient cycling models in plantations worldwide.The implications of this study can fit into a larger framework of research on plant growth rate and ecosystem productivity and nutrient flux under a changing climate.