Abstract
The close-to-Nature management method of interplanting broad-leaved trees after thinning of monoculture plantations is an important mixed species restoration model to improve the ecological service and functions effectively as well as to reduce the productivity decline of the multi-generation continuous planting of monoculture. Thus, the selection of tree species for establishing mixed forest and its ecological adaptability are the key issues. In this study, we conducted thinning experiment in an 11-year-old Chinese fir plantation with retention density of 900 trees/ha, 1200 trees/ha and 1875 trees/ha, and then underplanted four broad-leaved species, Schima superba, Phoebe bournei, Tsoongiodendron odorum and Michelia macclurei. After three years, we analyzed the growth rate and leaf functional traits of the broad-leaved species and their correlations with stand characteristics. The results showed that growth rate of seedlings of the four broad-leaved species were significantly different (p < 0.05) among different tree density levels and species. Low tree density favored seedling growth compared with high tree density and seedlings of T. odorum and S. superba performed best. However, leaf functional traits varied significantly (p < 0.01) among species only, and T. odorum had the largest specific leaf area, the smallest leaf mass per unit area, the smallest leaf tissue density, relatively large leaf thickness, and relatively small dry matter content. The leaf C content varied significantly among tree density levels and species; leaf N content varied significantly among species only; and leaf p content varied among tree density levels only. Correlation analyses between growth characters and leaf functional traits showed that height growth was significantly correlated with leaf N content (r = 0.686; p = 0.041) and with C:N ratio (r = −0.682; p = 0.043). Root collar diameter growth was significantly correlated with specific leaf area (r = 0.820; p = 0.007), leaf N content (r = 0.685; p = 0.042), leaf thickness (r = −0.706; p = 0.034) and leaf mass per unit area (r = −0.812; p = 0.008). Thus, leaf functional traits possibly predict diameter growth better than height growth. As a whole, growth rate and leaf functional traits could be used as a guide for selection of species for under planting in thinned pure monoculture plantations to establish conifer-broadleaved mixed forests. Based on growth rate and leaf functional traits, T. odorum appeared to be suitable for planting under low tree density stands where the degree of shading is low.
Highlights
Restoration of mixed species forests has gained increasing recognition as a strategy to spread the risks associated with predicated climate changes that will alter precipitation regimes and a rise in air temperature in the hundred years and provision of several ecosystem services [1]
Annual growth rate in root collar diameter showed significant variation among different tree density levels whereas significant difference in annual height growth was observed both among tree density levels and species, but no interaction effects was observed (Table 2). Both height and root collar diameter growth was higher in low tree density stand than high tree density stand
Leaf thickness was low for S. superba followed by P. bournei compared with T. odorum and
Summary
Restoration of mixed species forests has gained increasing recognition as a strategy to spread the risks associated with predicated climate changes that will alter precipitation regimes and a rise in air temperature in the hundred years and provision of several ecosystem services [1]. Reasonable thinning plays a vital role in forest management as it improves the growing space of the forest canopy and the supply of nutrients and groundwater for individual trees, resulting in increased production of the forests [2,3]. This management model promotes nutrient cycling, and improves soil fertility and the diversity of soil microorganisms through diversified litter fall [4]. Using existing growth models, mixed forests are shown to be more suitable for certain sites and climatic conditions [7] and may circumvent the down sides of monocultures, such as decline of forest production through successive plantation of monocultures on the same place and decline of soil nutrient availability [8]
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