[Gap formation features of humid evergreen broad-leaved forest in central subtropical Wawushan Mountain, Sichuan Province, China].

The primary and secondary subtropical evergreen broad-leaved forests at an altitude of 1720-1750 m in Mt. Wawushan National Forest Park, southwestern China investigated to analyze their canopy gap formation characteristics. The sampling method and canopy formation causes were also discussed. In secondary evergreen broad-leaved forest, the gap density was surprisingly only 9 per hm2 and the size of all gaps was, no more than 10 m2. Almost every gap had only one gap maker, and the gap makers often died standing by suppression. Accordingly successful natural regeneration is difficult in small gap, due to the rapidly lateral growth of canopy trees. In primary evergreen broad-leaved forest, the gap density was 15 per hm2, and 56% of the gaps had an area of no more than 40 m2. The largest gap had an area of 256 m2. The canopy gap accounted for 11.1% and expended gap occupied 19.8% of land area in the forest. Median area of canopy gaps and expended gaps was 59 and 105 m2, respectively. Most gaps had more than one gap maker, and gap makers died falling and were often from mortality events separated in time. Most gaps aged over 10 yr and a few were formed recently and the estimated gap formation rate was 0.01 per year. According to falling direction of trees and the relations with growth process, slope aspect and wind, we deduced gap-maker death resulted from integrated influence among topography, climate particular in wind, tree growth characteristics, and interactions of population under competition. Adapting method of plot sampling and projection drawing used in the paper can improve investigation accuracy and help improving comparison of results in different investigation area.