Plant and insect diversity along an experimental gradient of larch-birch mixtures in Chinese boreal forests

Maintaining and increasing biodiversity level especially in pure plantations is one important way to improve the resistance of forests to pests in Chinese boreal forests. The present study tested the hypothesis that the increased degree of tree species mixture (quantified by the stem proportion of Betula platyphylla Suk. and Larix gmelinii (Rupr.) Rupr.) can affect the species richness and diversity of understory and insects. Twenty-one plots, ranging from pure larch stand to pure birch stand, were sampled in several thinned forests in a Chinese boreal forest area. Data of environmental factors, understory plant species and insect assemblage were collected from the field and connected with historical records, and analyzed using, e.g., multivariate methods and de-trended canonical correspondence analysis (DCCA). The results showed that the variation of plant and insect species was mainly influenced by birch mixture and light conditions in the forest. Species richness and diversity of plants and insects increased with the increasing mixture of birch, and finally declined after passing the peak point in 30% or 50% of birch mixture. The study gives tools to improve the integrated pest management (IPM) especially in man-made pure plantations of boreal forests in China.