| The crown structure and its fractal characteristics of poplars were studied thoroughly based on the investigation of 3 clonal testing plantations, and the selection methods, the breeding efficiencies and the testing strategies in long-term poplar breeding were also explored theoretically in this paper.The main factors affecting light interception efficiency of both population and individual trees are leaf area index, light extinction coefficient, the capacity of light interception, and lamina angle. A significant difference was found among different layers and parts within poplar crown. The difference of the vertical distribution of foliage was greatly attributed to leaf size, while for the horizontal variation of foliage, the leaf number played a great part. The stemwood growth (D~2H) was positively related to total leaf area (TLA), surface area of crown (TCA) and crown shape ratio (CSR), but negatively related to leaf area index (LAI) and crown layer density (CLD).A software (FDC1.0), which can be used as estimating the fractal dimensions of any 2-dimension pictures, was developed based on the method of box-counting. The fractal dimensions of rooting and crown characteristics for poplar were explored by the means of FDC1.0. The range of variation of fractal dimension for the roots of poplar cuttings were 1.1 -1.8, A significant or stronger positive correlation was found between fractal dimension and rooting traits, thus, fractal dimension could be a substitutive index for rooting characteristics. A similar variation pattern to crown characteristics was found for fractal dimension (FD) of crown, thus, fractal dimension (FD) could be a replacement criteria for crown characteristics. Due to its easy manipulation, rapidity, high repeat, good to stand for crown structure and foliage distribution, box-counting method was the best way among the four methods of fractal dimension estimation for poplar crown compared. Generally, the larger and more even the foliage, the bigger the fractal dimension. At most cases, there were no significant correlation among fractal dimensions and growth, that means a poplar tree with much more foliage may not have bigger growth. A trend was also explored that, compared with height growth, Diameter growth had closer correlation with fractal dimensions of crown. The long-term breeding strategies were also compared based on a theoretical analysis on breeding efficiency by the methods of simulation in this paper. Unrestricted combined index selection was compared to phenotypic selection based on a a forest tree breeding simulation program (POPSIM). The comparison was done keeping budget (plant number) and gene diversity (status number, Ns) constant. At low and medium heritability phenotypic selection was inferior to combined index selection, only when heritability was high phenotypic selection was as efficient as or more efficient than combined index selection. In long-term poplar breeding, both the breeding value and genetic diversity should be considered. Group Merit Gain (GMG) is a weighted average of breeding value and genetic diversity. The overall goal for long-term breeding is to maximize annual progress in Group Merit Gain at a given annual budget (GMG/Y*). To attain this goal, a deterministic simulator BREEDING CYCLE ANALYZER was used . Breeding strategies based on testing of phenotypes, clones or progeny for selection of parent for next breeding cycle were optimized regarding cost, time, GMG and gene diversity loss. Under the most likely parameter values, the highest GMG/Y, and the optimal selection age for clone, phenotype and progeny strategies were 0.7480 %, 0.6989 % and 0.4675 %; 7, 6, and 11 years respectively. The clone strategy was the best over the whole range of considered cases, and the phenotype strategy was almost the second best.
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