| Quercus mongolica secondary forest is widely distributed in the northeast of China,and most of them are forest communities formed after the overcutting of broad-leaved Pinus koraiensis mixed forest.Most of the forest productivity is low and the ecological function is degraded,due to insufficient understanding of Q.mongolica and lack of scientific management.Deepening the research on the short-term and long-term response mechanism of Quercus mongolica secondary forest to different thinning treatment is the basis for improving its quality and exploring reasonable management measures.Twelve permanent plots with an area of 1 ha were established in the Q.mongolica secondary forest in Wangqing,Jilin Province of northeastern China.Four kinds of treatments were carried out by random grouping,namely Control plot(T0),traditional management(T1),crop tree management with density 1(T2)and crop tree management with density 2(T3).The response of growth and structure of Q.mongolica secondary forest on four treatment is studied based on the survey data of 2013 and 2018.Non-linear two-level mixed effect models were constructed to predict the height,diameter and crown growth of single tree using sample plots and tree species as random effects,combined with a variety of stand factors and competitive factors as independent variables.The growth of trees after 20 years is predicted according the established growth models,the growth dynamics and long-term response to thinning are analyzed.The main results are summarized as follows:(1)On the individual tree level,the average annual DBH and volume growth of T1,T2 and T3 were higher than T0 in terms of all trees and dominant tree species(Quercus mongolica,Betula platyphylla and Pinus koraiensis).The growth of the crop tree was higher than that of the non-crop tree.However,the growth rate showed the opposite trend.The promotion effect of T2and T3 on the crop tree's annual growth is obviously better than that of T1 and T0.On the stand level,the promotion effect of T1,T2 and T3 on growth was not as obvious as that of individual tree level.T2 has the most obvious effect on the growth and growth rate of the whole forest and trees of minimal diameter class.T0 has the most significant effect on the growth of small-diameter class trees,and T1 has the best effect on the growth rate of medium-diameter class trees.The annual basal area and volume growth of each forest storey were ranked as upper storey>middle storey>lower storey,and the growth rate showed the opposite trend.T2 has the most obvious promoting effect on the basal area and volme growth rate of each forest storey,and the promoting effect on the growth rate of lower and upper forest storey is also better than other treatments.T0 has the greatest effect on increasing the growth of the middle forest storey.(2)The main tree species in the field are Q.mongolica,B.platyphylla,P.koraiensis,Acer pictum,Tilia mandshurica,Populus ussuriensis,Fraxinus mandshurica,Larix olgensis and Betula dahurica.Q.mongolica secondary forest retains most of the tree species of the zonal vegetation.P.koraiensis,B.platyphylla,P.ussuriensis and A.pictum,which can better adapt to the environment of the study area are the main companion tree species of Q.mongolica.T2 and T3 help to improve the dominance of the target tree species,while reducing the dominance of the main competing forests.T2 and T3 pay more attention to the cultivation of dominant tree species and tree species of superior succession,which is beneficial for improving and maintaining the diversity of tree species in the entire forest.From the aspect of diameter distribution,the diameter distribution of all stands before and after thinning showed the typical characteristics of uneven aged forest:inverted J form,which means small-diameter trees accounted for a large proportion,indicating that the plots are well renewed,and the stand is relatively stable in the early stage of succession of light demanding species such as Q.mongolica,P.ussuriensis and B.platyphylla.The negative exponential function can better fit the diameter distribution of the stand before and after thinning.The changes in diameter distribution indicated that appropriate thinning is very important to maintain the position of small diameter trees in uneven aged forest.The change of q value showed that T2and T3 management was helpful to maintain the stability of diameter structure.From the perspective of spatial structure,T2 and T3 are better than T1 and T0 in the improvement of interspecific isolation,size differentiation and crowding of crop trees.According to the analysis of the comprehensive spatial structure index(CSSI),T2 and T3 are significantly better than T1and T0 in improving the CSSI of the crop trees,while T1 is slightly better than T2,T3 and T0 in optimizing the overall CSSI.(3)By analyzing the competition relationship within the forest and the response to different thinning treatment,it can be seen that the rank of the main tree species competitive intensity is:Q.mongolica>P.koraiensis>A.pictum>T.mandshurica>B.platyphylla>P.ussuriensis>L.olgensis>F..mandshurica>B.dahurica.The ralationship between DBH and competition of Q.mongolica secondary forest and its dominant species all obey the power function relationship.Thinning based on crop tree management has a significant effect on alleviating competition of the upper forest storey and the crop trees,which can maintain the importance of Q.mongolica and P.koraiensis in the stand,and promote the positive succession of the stand.(4)The significant factors related to the height growth of Q.mongolica secondary forest are D,H,CR and SPI.These variables are suitable for constructing the generalized model of height growth.The distance-dependent and distance-independent competition indexes have no significant effect on tree height growth.The fitting accuracy of the two-level mixed-effect model that introduces plots and tree species as random effects has been greatly improved(R~2 increased by 51.8%).The prediction results show that the two trees with the largest DBH have the strongest prediction ability.(5)Initial DBH is the most important variable affecting diameter growth,followed by competition,while the effect of site quality on diameter growth is not significant.Compared with the traditional model,the prediction accuracy of the mixed effect model is improved by 17.69percent,R~2 is 0.6805,indicating that it is suitable for the individual-tree diameter growth prediction of the secondary forest of Q.mongolica.Logistic model is the best fitting model in the relationship between crown and DBH of individual tree in Q.mongolica secondary forest The random effects of tree species and plots are introduced into the basic model.When the random covariance matrix of the random effects of plots and tree species is selected as the diagonal matrix,the model fitting accuracy is the highest,which has a significant improvement over the best basic model,R~2 is 0.7311 and RMSE is 0.8839.The cross-validation method shows that the prediction accuracy based on the tree species level nested under the plot level is the best,R~2 reaches 0.7215,which can be used for the prediction of single tree crown width in the secondary forest of Q.mongolica.(6)Based on the established growth model,the growth dynamic of the stands in 20 years are predicted,and then the growth differences among different thinning treatments are compared.Compared with short-term effect,the annual tree growth has decreased.The growth of individual tree DBH has decreased by 31.07%(T0),28.36%(T1),28.05%(T2)and 26.63%(T3),respectively.The growth of T1,T2,and T3 target trees decreased even more,by 29.73%,30.30%,and 28.15%,respectively.The long-term and short-term effects of various thinning treatment on forest growth are basically consistent. |
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