Broadleaf trees mediate chemically the growth of Chinese fir through root exudates

Tree performance in mixed-species forest plantations is ultimately the net result of positive and negative interactions among species. Despite increasing knowledge of interspecific interactions, relatively little is known about the chemical mechanisms mediating such interactions. We constructed mixed planting systems with two species including Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) and broadleaf species Cinnamomum camphora L. Presl, Elaeocarpus decipiens Hemsl, Liquidambar formosana Hance, or Michelia macclurei Dandy. Based on a series of manipulative experiments, we investigated the performance of Chinese fir and analyzed root placement patterns and the composition of main soil microbial groups. The broadleaf trees influenced the growth of Chinese fir roots more than the growth of shoots. Furthermore, C. camphora roots released allelochemicals into the soil environment, resulting in growth inhibition of Chinese fir and changes in main soil microbial groups. However, when grown with E. decipiens and M. macclurei, the growth of Chinese fir was consistently promoted. It responded by enhancing its root growth and altering root behavior, resulting in a shift from growth inhibition to chemical facilitation. These positive inter-specific interactions also stimulated changes in the composition of soil microbes. Complementary experiments indicated that non-toxic signaling molecules in the root exudates of E. decipiens and M. macclurei may be responsible for mediating positive root-root interactions and regulating the composition of main soil microbial groups. Thus, our study demonstrated that broadleaf species chemically mediate the growth of Chinese fir through root exudates. Such a novel mechanism offers many implications and applications for reforestation programs undertaken to rehabilitate forest plantations that suffer from declining productivity related to densely planted monocultures.