Spatial distribution and growth association of regeneration in gaps of Chinese pine (Pinus tabuliformis Carr.) plantation in northern China

Abstract

Chinese pine is one of the major afforestation species in North China, and plantations of this species are of great significance for improving the ecological environment and for conserving soil and water. However, most of the Chinese pine plantations established by the government in the 1970s are mature or nearly mature. Therefore, how to successfully promote natural regeneration has become a major issue in plantation management. We established four types of gaps with different diameters in a plantation in northern China based on the average canopy height (H), i.e., L-I (0.75 H), L-II (1.00 H), L-III (1.25 H), and L-IV (1.50 H). Seven years after gap creation, each gap was divided into four aspects by vertical lines along the main direction of the gap center. Additionally, three sections were partitioned in each gap according to the mean crown radius of the border trees; these sections were the central area of the gap (section B), the inner edge of the gap (section C) and the outer edge of the gap (section D). The results showed that there were no differences in density among gap size classes and gap aspects, but a positive response was observed for regeneration growth. The maximum growth in L-III implied that this gap was the optimal size for promoting the establishment, survival and development of regenerated trees. The highest density and greatest growth occurred mostly along the gap edge. Spatial patterns of abundance were generally concurrent with patterns of regeneration stature (e.g., height) in all tested gaps. However, the spatial distributions of regeneration density and growth exhibited obvious differences in different gap size classes, which likely resulted from heterogeneity in the micro-environment within the gap and the differences in the regeneration responses to these variations. The mark correlation function indicated that spatial autocorrelation characteristics of regeneration growth within gaps and gap sections were mainly independent. Collectively, our findings suggested that the expansion of gaps with continuous monitoring will likely be necessary to promote further canopy recruitment. Additionally, regenerating trees exhibited different spatial distributions and could be more resilient to various interferences. Factors influencing the differences in spatial distribution need to be further studied in light of the relationship between variations in the micro-environment and regeneration responses after gap creation. Moreover, whether large-scale vegetation displacement and reorganization will result from clear cutting these plantations warrants further investigation.