Mechanism Of Hydraulic Limitation On Tree Height

The tallest living organisms are trees.While tree height growth slows down as trees grow taller even where resources are abundant,stresses are minor.The maximum tree height is different within different tree species and the same tree species differs in height in different environments.The stop of tree height growth is one of the main factors that would reduce community production.Many researchers focus on tree height limitation factors now.Researches on tree height limitation mechanisms are significant on community ecophysiology,and are also meaningful in improving stand productivity.In order to better understand the determinants of tree height limit,32 tall tree species such as Parashorea chinensis,Sapium baccatum,Garcinia cowa,Eucalyptus citriodora,E.grandis×E.urophylla,Albizia falcataria,Populus tomentosa,Platanus hispanica,Ginkgo biloba,Pinus tabulaeformis,Ulmus pumia,Salix gordeivii,and Elaeagnus angustifolia from typical environments such as Xishuangbanna rainforest,Guangzhou subtropical area,Zhengzhou,Beijing,Helanshan mountain and Maowusu sandland in China were studied.Respectively with the method of porraffin and nail varnish method,acetone extraction,Lior-6400 portable photosynthesis system and pressure bomb and P-V method,morphological,photosynthetic pigments content,gas exchange characteristics and leaf water potential and water parameters with tree height of the 32 tree species were studied and The above indices of 17 tree species that widely spread in different environment gradients were also measured in order to find the environmental limitation factors on tree height.Main conclusions are as follows:(1) changes of leaf morphological and anatomical structures with tree height:25 researched tree species exhibited xeromorphic structures at tree top,that is,toward tree top,leaves were with 1) increased cuticle thickness;2) developed palisade tissue and even double-palisade structure;3) densely arranged tissue cells;4) smaller cells;5) thicker epidermis,smaller and denser stomata;6) smaller and thicker leaf,great LMA value.Xeromorphic structure reflected tree top leaves with stronger adaptability to water stress,and suggested that they were under water stress condition.(2) changes of gas exchange capabilities with tree height:leaf photosynthetic pigments andâ–³value of 32 researched tree species decreased,while leafδ¹³C value increased with tree height. Photosynthetic rate,transpiration rate,stomata conductance and internal CO₂ concentration of 4 researched tree species were negative with tree height.The results were consistent with the result of the changes of stomata structure with tree height and suggested that increased hydraulic limitation with tree height influenced gas exchange characteristics,water use efficiency and photosynthetic capability. (3) changes of water parameters with tree height:water potential of 28 tree species were negatively and linearly related to tree height.Maximum tree height of 86～100m was estimated with water potential in China.Water relations of 22 tree species estimated with P-V method showed that cell wall elasticity,osmotic adjustment,and drought resistant capability of most tree species were positive with tree height,while turgor maintenance capability reduced with tree height,which limit cell division and tree height growth.The trends were consistent with the changes of leaf structure and biophysical characteristics.It's possibly that increase of hydraulic limitation with tree height influenced leaf water potential and turgor as well as leaf structure,which strengthened hydraulic limitation on leaf gas exchange characteristics including photosynthesis and transpiration rate, resultantly influenced carbon assimilation and CO₂ fractionation and increased water use efficiency and drought resistance capability.(4) response of leaf structure and water parameters of widely spread trees species to environmental gradients:from south to north,with the reduction of water and heat condition, thickness of leaf,palisade tissue,cuticle,epidermis and LMA increased or decreased firstly then increased;stomata size and aperture reduced,while stomata density,drought resistance capability andδ¹³C increased.It suggested that worse water and heat condition limited gas exchange capability and increased water use efficiency.Leafδ¹³C value of tree species from different climate condition was influenced by many climate elements and water condition was the most important one.Precipitation and average temperature were negative withδ¹³C value,rainfall and average temperature respectively at 1100mm and 19℃were dividing point for carbon isotope discrimination.Higher temperature increasedδ¹³C,while better water condition influencedδ¹³C less.(5) To sum up,the trend of leaf structure,biophysical characteristics and water parameters with tree height was similar with the change trend of these indices with water gradients.It proves that tree top leaves suffer from water stress and hydraulic limitation becomes greater with tree height.Water stress influence tree height growth in several aspects:firstly is the influence of exterior water condition,and community average tree height is positive with exterior water condition.Secondly, hydraulic limitation influences individual tree structure and function.Xeromorphic structure,Lower water potential and turgor pressure limit cell division and expansion,which restrict gas exchange and carbon assimilation capability.However,respiration consumption increases with the spread of tree crown.Shortage of nutrient limits carbon investigation on new leave growth and may ultimately limit tree growth.Thus,the main limitation of tree height is water.And hydraulic limitation may firstly cause nutrient limitation and ultimately limit tree height.