Influence of thinning intensity on canopy structure and light environment inside Larix gmelinii secondary forest

ObjectiveIn order to restore the forest structure and improve the forest growth of Larix gmelinii in Daxing' an Mountains of northeastern China, the different thinning intensities of forest were carried out to find the relationship between canopy structure and the photosynthetic characteristics of the undergrowth vegetation and to optimize the operating effects of secondary forest.

MethodThe experimental plots with thinning intensities of 9.43%, 16.75%, 29.00%, 40.01%, 53.09%, 67.25%, and the control plot were set. The canopy structure parameters selected were the gap fraction, canopy opening, leaf area index, leaf inclination, direct fixed-point factor, indirect fixed-point factor, total fixed-point factor, direct radiation on the crown, indirect radiation on the crown, total radiation on the crown, direct radiation under the crown, indirect radiation under the crown, total radiation under the canopy(13 indexes). The photosynthetic parameters selected 7 indexes, i.e. transpiration rate, photosynthetic rate, CO₂ reference value, leaf surface P.A.R, leaf temperature, intercellular CO₂ concentration, stomatal conductance. They were used to describe the differences between different places and the correlation coefficient method was used to determine the correlation between different indexes. At the same time, the gray correlation method was used to evaluate the light environment of each sample.

ResultThe results showed that: gap fraction, canopy opening, direct fixed-point factor and total fixed-point factor decreased first and then increased by increase of thinning intensity, and the value of the 4th sample with thinning strength of 29% was the smallest. The leaf area index reached the maximum value when the thinning intensity was 29%. There was no significant difference between the radiation direct radiation on the crown and the total radiation on the crown. The direct radiation under the crown and total radiation under the crown also decreased first and then increased by the increase of thinning intensity. With the increase of thinning intensity, transpiration rate, photosynthetic rate and stomatal conductance all appeared first increasing and then decreasing trend, while leaf surface P.A.R, leaf temperature raised afterwards steadily. Thinning increased the concentration of CO₂ in woodland and the intercellular CO₂ concentration decreased at first, then increased, reduced at last by the increasing of thinning intensity. The gap fraction was positively correlated with opening, direct fixed-point factor, total fixed-point factor, direct radiation under canopy and total radiation under canopy, but negatively correlated with leaf area index. There was a positively significant correlation between transpiration rate, photosynthetic rate and stomatal conductance. After removing the strongest correlation index, the gap fraction was negatively correlated with the photosynthetic rate, and leaf area index had a significant positive correlation with photosynthetic rate. Comprehensive evaluation of characteristics of the light environment in woodland was tested by grey correlation and the correlation degree was 29.00%(0.823)>40.01%(0.794)>53.09%(0.739)>0%(0.724)>67.25%(0.713)>16.75%(0.701)>9.43%(0.673).

ConclusionThere was correlation between canopy structure and photosynthetic index of larch, and the light transmittance of canopy had obvious influence on photosynthesis rate of undergrowth. Combining the two characteristics of the light environment to evaluate them comprehensively under different thinning intensities, it was best that 29% and 40.01% of thinning in larch natural secondary forest in the light environment. It indicates that intermediate thinning of 29.00%-40.01% is beneficial to the improvement of light environment in the forest.