| It is of great practical significance to explore the spatial heterogeneity of photosynthetic characteristics in different layers and orientations in the crown for accurately predicting the photosynthetic efficiency of tree canopy,estimating the carbon sequestration capacity,and determining the effective crown of trees.In this study,a 15～17 year old Larix principis-rupprechtii Mayr.forest in Saihanba Mechanical Forest Farm was chosen as the research object,five sampling plots(each was 20 m×30 m)were selected with relatively consistent site conditions and stand factors,and the sample individuals were screened for measuring photosynthetic efficiency.The net photosynthetic rate from different canopy positions of five selected sample trees were recorded by using the portable photosynthetic apparatus(Li-6400).Meanwhile,other environmental factors(e.g.photosynthetically active radiation,water vapor pressure deficit,and leaf temperature)were also recorded in 2018 and 2019.Taking the analysis of variance,we analyzed the spatial heterogeneity and their correlation of photosynthetic physiological and ecological factors.The light response curve model of canopy of Larix principis-rupprechtii was constructed by comparing the fitting effects of various models.According to the distribution rule of photosynthetically active radiation in the canopy,we also established the extinction coefficient estimation models.The main results were as follows:(1)The photosynthetic physiological characteristics of Larix principis-rupprechtii have significant heterogeneity in canopy spatial distribution.Analysis of variance showed that there were significant differences among different layers in the net photosynthetic rate(Pn),the stomatal conductance(Gs),transpiration rate(Tr),carboxylation efficiency(CE),intercellular CO2 concentration(Ci),and photosynthetically active radiation(PAR),respectively.In addition,leaf temperature(Tl),vapor pressure deficit(VPD),and relative air humidity(RH)in the upper layer(UP)was significantly higher than those in the middle layer(MD)and the lower layer(LW),respectively.The chlorophyll content had significant differences in the vertical canopy,and both showed "LW>MD>UP".The leaf mass per area(LMA)was shown as "UP>MD>LW".The maximum net photosynthetic rate(Pmax),the initial quantum efficiency(a),the light saturation point(LSP),the light compensation point(LCP),and the respiration rate in the dark(Rd)showed significant differences among different layers,and were ranked as:"UP>MD>LW".Horizontally,Pn,Gs,Tr,Ci,and CE were significantly different among various orientations,while other indicators(such as PAR,VPD,Tl,RH,chlorophyll content,LMA)had no significant difference.The correlation analysis showed that Pn was significantly positively correlated with PAR and CE(both P<0.05),the correlation coefficients were 0.954 and 0.908,respectively,while it was significantly and negatively correlated with Ci,VPD,and RH(all P<0.05),and the correlation coefficients were-0.833,-0.840,and-0.903,respectively.(2)The rectangular hyperbola model(RHM),the nonrectangular hyperbola model(NRHM),and the exponential model(EM),and the modified rectangular hyperbola model(MRHM)could better simulate the canopy light response curve of Larix principis-rupprechtii.Compared to the results fitted by other three models,the NRHM had the best fitting effect.However,the precision was lower in fitting the light response curve on the lower canopy.(3)The variable,such as environmental conditions(leaf temperature(T1)and vapor pressure deficit(VPD)),leaf morphological characteristics(leaf mass per area(LMA)and chlorophyll content),spatial indicator(relative depth into the crown(RDINC))were parameterized to establish a crown light response curve model.Determinants coefficients(R2)was 0.893,and root-mean-square error(RMSE)was 1.831 μmol·m-2·s-1.The fitting effect of the model was better.(4)Based on Beer-Lambert law,the extinction coefficients(k)in different pseudo-whorls were calculated by the data of PAR of five sample trees.The result showed that k was correlated with RDINC as a power function and was also significantly correlated with solar elevation angle(h),Cumulative leaf area(CLA)and crown length(CL).The k estimated model,which was established considering all the above variables,was superior to the method of estimating k by using constant value or measuring average leaf inclination angle.The R2 and RMSE were 0.685 and 0.019 μmol·m-2·s-1，respectively.The canopy photosynthetic indicators and environmental factors ofthe larch had obvious spatial heterogeneity,the differences in photosynthetic physiological and ecological factors could be ascribed to the synthesis of the vertical and horizontal canopy.Therefore,the vertical and horizontal variation laws of canopy photosynthesis should be fully considered in the estimation of the canopy productivity of Larix principis-rupprechtii plantation. |
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