Distance-weighted size ratio competition index in the forest of Picea crassifolia in Dayekou, Gansu Province of northwestern China

The study was conducted to compare the differences of distance-weighted size ratio indices to reflect the effect of competitive pressure when using different methods to determine the competitive tree and parameters, and provide a reference for its future application in forestry practices, especially in forestry remote sensing. In this paper, taking Picea crassifolia natural forest as an example, we used Pearson correlation and variation partitioning analysis to compare 15 kinds of competition index, which used fixed area method, crown overlap method and vertical search cone method to select competitors, respectively, and adopt DBH, tree height, crown width as size ratio parameters, respectively, and a fixed radius, the sum of tree height, and the sum of crown width as relative distance parameters, respectively. All the above was based on distance-weighted size ratio index generalized model. Results showed that: 1) there was a significant positive correlation (P < 0.01) between competition index and height-diameter ratio (height/diameter), and the correlation with the gradual exclusion of understory trees from the samples was improving. 2) For stands in the study area, the vertical search cone method was the best in three methods on choosing competitors, followed by a fixed area method and the crown overlap method was the worst. 3) Excepting fixed area method, the correlation was stronger when choosing the sum of tree height as the relative distance parameters than choosing the sum of crown width. 4) The correlation coefficient was the highest when using the diameter at breast height as size ratio parameter, followed by using the crown width and the tree height as size ratio parameter, respectively. 5) Excluding the synergies section with growth factor, the competition index adopting height ratio as the size ratio had the largest independent explanatory power (15.7%) in explaining variation of height-diameter ratio, followed by diameter ratio (7.3%), crown ratio least (3.6%). According to the analysis above, we can speculate that for the stand in the study area, distance-weighted size ratio index can effectively reflect the competitive pressure situation, but it is different when using different methods to determine the competitive tree and parameters. In three methods on choosing competitors, the vertical search cone method is the best. The relative distance parameter adopting the sum of tree height is better. Selecting tree height as the size ratio parameter will have more ecological significance.