Respond Of Leaf Economical Spectrum Of Woody Plants In Subtropical Evergreen Broad-leaved Forest To Canopy Gradeints

The leaf economics spectrum?LES?is one of the most important examples of using traits to explore fundamental ecological relationships,and it is crucial to understand the response of ecosystem processes and functions to environmental changes.However,LES mainly focused on the functional traits of canopy surface leaves,and it is unclear whether the LES of shade leaves is congruent or differs from that of sun leaves.Such facts will lead to the great uncertainty in predicting the response of plant ecosystem to future environmental changes through LES.Although LES quantifies the trade-off strategies among leaf funcational traits,its formation mechanism is still unclear.The leaf vein and stem play an important role in transport and support in the plant.There is still much controversy about whether the vein and stem traits drive the synergistic changes of all LES traits.Here,we reported the LES for leaves sampled in different canopy position?i.e.the top and bottom canopy?for 75 woody species in subtropical evergreen broad-leaved forest.?1?Compared with the sun leaves,the shading significantly reduced the leaf mass per area?LMA?(i.e.,sun leaves:83.68±3.20 g m^-2,shade leaves:75.25±2.60 g m^-2),net photosynthetic rate(A_mass)(i.e.,sun leaves:113.79±6.51 nmol g^-11 s^-1,shade leaves:66.51±3.71 nmol g^-11 s^-1)and dark respiration rate(R_mass)(i.e.,sun leaves:9.50±0.78 nmol g^-11 s^-1,shade leaves:6.25±0.50 nmol g^-11 s^-1);however,shading did not significantly alter the nitrogen content(N_mass)?i.e.,sun leaves:2.04±0.07%,shade leaves:1.99±0.06%?and phosphorus content(P_mass)?i.e.,sun leaves:0.11±0.004%,shade leaves:0.12±0.004%?.A_mass showed the largest plasticity at canopy scale?i.e.37.99±2.99%?,indicating that the key metabolic traits of the leaves had stronger plasticity than the morphological and chemical traits.There were significant allometric relationships among the five quality-based leaf traits(LMA,A_mass,R_mass,N_mass,P_mass)in different canopy positions,and they were consistent with the trend of global LES.The sun and shade leaves manifested the same mass-based function-trait relationships?i.e.,the numerical values of the scaling exponents of LES relationships did not differ between sun and shade leaves?.There are significant differences in the scaling constants.The shade leaves showed LES strategy consistent with the sun leaves.?2?Compared with the sun leaves,the shading significantly reduced the leaf angle?LA??i.e.,sun leaves:34.93±1.17°,shade leaves:16.66±0.50°?,and the light response curve parameters:the light saturation point?LSP?(i.e.,sun leaves:950.85±25.87?mol m^-22 s^-1,shade leaves:405.92±16.27?mol m^-22 s^-1),light compensation point?LCP?(i.e.,sun leaves:27.09±0.94?mol m^-22 s^-1,shade leaves:12.49±0.37?mol m^-22 s^-1),apparent quantum efficiency(q_LCP)?i.e.,sun leaves:0.03±0.001,shade leaves:0.02±0.001?,and the shading significantly increased the length of the leaf drip tip?LDT??i.e.,sun leaves:4.26±0.15mm,shade leaves:7.18±0.22 mm?of shade leaves.There was a significant correlation between the A_mass predicted by the N_mass and LMA based Mitscherlich model and the measured A_mass?i.e.,sun leaves:r²=0.62,P<0.001;shade leaves:r²=0.47,P<0.001?,indicating leaf economic traits can be used to predict the light response curve within the canopy.Isometiric relationships were found among the LDT,LA,LSP and LCP in different canopy positions,indicating that there was a trade-off between leaf interceptions in the canopy and drying of the leaf surface.A single axis of variation could explain the traits variation?i.e.,sun leaves:53.24%;shade leaves:49.09%?,indicating that the light response parameters and LA,LDT could be incorporated into the LES traits,but the sun and shade leaves were in the axis of variation.The scores on the scores were significantly different?P<0.05??3?Compared with sun leaves,the shading did not significantly change the leaf vein density?VD?(i.e.,sun leaves:13.84±0.69 mm mm^-2,shade leaves:14.27±0.74 mm mm^-2)and vein spacing?IVD??i.e.,sun leaves:24.98±1.04?m,shade leaves:24.15±1.02?m?of the shade leaves.Leaf vein traits at canopy scale could be used to predicted LMA?i.e.,sun leaves:r²=0.28,P<0.001;shade leaves:r²=0.44,P<0.001?and N_mass?i.e.,sun leaves:r²=0.26,P<0.001;shade leaves:r²=0.33,P<0.001?,supporting leaf vein traits driving leaf economic trait variation.Shading increased the shade stems density(i.e.,sun leaves:0.95±0.08 mg mm^-3,shade leaves:1.17±0.08 mg mm^-3),but did not significantly change other stem traits?P>0.05?.There was a significant relationship between the stems and leaves of the canopy N_mass and P_mass,indicating the co-variation of stem and leaf in the distribution of nutrients.However,there was a decoupling between stem traits and LES.The sun leaves and stems tended to acquire strategies,and the shade leaves and stems tended to be conservative strategies,indicating that strategies of different organs in different canopy positions are heterogeneous.In summary,in the subtropical region,there is a convergent LES strategy at canopy scale,and the leaf vein trait is the intrinsic formation mechanism of the trade-off,but the stem and leaf traits are decoupled.In the future,we should explain the differences in key traits between sun and shade leaves in terms of mechanism.Important advances will result from further tests the coupling trade-off relationship between carbon,nutrient and water resources in the canopy and variations on the spatial scale.