| Pruning has been widely recommended to encourage the early formation of clear and knot-free wood with high economic value.Although the principal objective of pruning is to improve wood quality,the question that most frequently arises during the design of pruning treatments is the following:how much of the live crown can be removed without significantly reducing tree growth?The functional crown is an accurate and reasonable guidance for pruning,but it has lacked a scientific determination method.However,the the boundary of the functional crown(BFC)was usually dynamic,which may increase the cost of management.Thus,a scientific and effective pruning treatment is necessary to maximize the economic benefits of timber.In this study,the data of photosynthetic factors,leaf characteristics,environmental factors and tree attributes for Larix olgensis plantation from Maoershan Forest Farm of Northeast Forestry University was collected during the growing season of 2016-2018.The spatial variation of photosynthetic physiological characteristics of Larix olgensis was discussed and the main factors affecting photosynthetic physiological characteristics were analyzed.On this basis,the crown photosynthetic light-response(PLR)curve model was established.The extinction coefficient(k)estimation model was established according to the distribution of photosynthetically active radiation(PAR)in the crown,and the dynamic estimation of PAR within the crown was established.Combined with above two models,the instantaneous net photosynthetic rate(Pn)in different pseudo-whorls within the crown was further simulated.Then,the photosynthetic accumulation during the growing season of different pseudo-whorls were calculated using interpolation and numerical integration and the mechanism of photosynthate allocation was analyzed.Finally,the effective crown was determined by analysing the carbon contribution in different pseudo-whorls to the trunk,and meanwhile the accumulate contribution rate of carbon in different pseudo-whorls were also calculated.The specific contributions and main conclusions of this paper are as follows:1.Spatial variation of photosynthetic characteristics and environmental factors in tree crown.the spatial variations of main photosynthetic indicators,including Pn,stomatal conductance(gs),intercellular CO2 concentration(Ci)and transpiration rate(Tr);and photosynthetic physiological parameters,including maximum net photosynthetic rate(Pmax),dark respiration rate(Rd),apparent quantum yield(AQY),light compensation point(LCP)and light saturation point(LSP);and environmental factors,including photosynthetically active radiation(PAR),relative humidity(RH),temperature(T)and vapor pressure deficit(VPD)were analyzed.Results showed that,in the vertical direction of crown,photosynthetic indicators and photosynthetic physiological parameters both showed significant differences among upper,middle and lower crowns,however gs and Tr did not differ significantly between middle and lower crowns.PAR,T and VPD showed significant differences,while RH did not.In the horizontal direction of crown,the differences of photo synthetic indicators and photosynthetic physiological parameters are significant,which is more obvious in the upper crown.The PAR,T and VPD are only significant in the upper crwon,but are not significant in the middle and lower crown.Therefore,it is necessary to consider the spatial heterogeneity of each factor in estimating crown photosynthesis.2.Establishment of crown PLR curve model of Larix olgensis.Data of 752 PLR curves of 5 Larix olgensis plantations from different seasons were used to modify the Mitscherlich model to establish the crown PLR curve model.Re-parameterization was used by analyzing the relationship between the model parameters of PLR model and the corresponding environmental factors,leaf morphological characteristics and leaf positions.The adjusted coefficient of determination(Ra2)and root mean square errors(RMSE)were 0.884 and 1.51 μmol m-2 s-1 respectively.The model was validated with independent data and the mean error(ME)and estimation accuracy of the model were 0.097 μmol m-2 s-1 and 95.6%respectively.It is concluded that the model which considering environmental factors,leaf morphological factors and leaf spatial positions can effectively simulate the PLR curves at any positions within the crown during the growing season.3.Dynamic simulation of light distribution in the crown of Larix olgensis.The extinction coefficients(k)in different pseudo-whorls were calculated based on Beer-Lambert law based on the data of PAR and LAI and the main influencing factors were also analyzed.Result showed that k were not a constant value in different trees,different pseudo-whorls and different solar elevation angle.It was correlated with relative depth into crown(RDINC)as power function,and was also significant correlated with solar elevation angle and cumulative leaf area(CLA).The k estimated model considered all the above variables,which conformed to the reality.The coefficient of determination(R2)and RMSE were 0.569 and 0.16 μmol m-2 s-1 respectively,which indicated that the k estimated model performed well in simulating the crown PAR.4.Calculation of annual photosynthetic accumulation of carbon.Estimation model of leaf area and specific leaf weight of Larix olgensis based on their seasonal variations in growing season.Then,the dynamic simulation of crown Pn was simulated by combining the crown PLR curve model,the dynamic estimation model of PAR and the corresponding dynamic meteorological data.Finally,the daily accumulated carbon in each pseudo-whorl was calculated by using numerical integration.The photosynthetic accumulation of carbon during the whole growing season was calculated by adding all the daily accumulated carbon.The allocation proportion of photosynthate for trunk,branch,leaf and root are about 37%,29%,21%and 13%,respectively.5.Determination of effective crown for Larix olgensis.The estimated photosynthetic accumulation of carbon was corrected by the total carbon increment which was calculated by using the tree analysis data.Then the contribution from each pseudo-whorl to the trunk was obtained,and the crown with positive contribution was determined as the effective crown.The effective crown of different sample trees differed greatly,but the relative cumulative leaf biomass was stable between 0.80 and 0.90,with an average of 0.86.Therefore,it is feasible to use 0.86 of relative cumulative leaf biomass to determine the effective crown.Estimation model of cumulative carbon contribution rate of tree crown was established to provide reference for intensity and scheme of artificial pruning.6.Suggestions on artificial pruning of young Larix olgensis forest.The height of artificial pruning was greatly affected by the size of trees themselves,so it is more reasonable to make the pruning plan based on the diameter class.For Larix olgensis,when 85%cumulative contribution rate was retained,the artificial pruning effect can last for two pruning cycles,which will reduce the operating cost. |
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