The Effects of Different Moso Bamboo Densities on the Physiological Growth of Indocalamus latifolius Cultivated in Moso Bamboo Forests

Cultivating Indocalamus latifolius in moso bamboo (Phyllostachys edulis) forests is a technique in a compound economical and ecological agroforestry system. However, the impacts of different moso bamboo densities on the physiological growth of I. latifolius remain unclear. The aim of this study was to elucidate the adaptation mechanism of I. latifolius to the environment in forests with different moso bamboo densities. One-year-old I. latifolius seedlings were planted in moso forests with four different densities (CK: 0 plants·ha−1; T1: 1050 plants·ha−1; T2: 2100 plants·ha−1; T3: 3150 plants·ha−1) for two years. The biomass and contents of nitrogen (N), phosphorus (P), potassium (K), starch (ST), and soluble sugars (SSs) in old leaves, new leaves, stems, rhizomes and roots of I. latifolius, as well as leaf functional traits [leaf length (LL), leaf width (LW), leaf thickness (LT), leaf area (LA), specific leaf area (SLA), and leaf tissue density (LTD)] and root morphology [root surface area (RSA), root length (RL), root diameter (RD), and specific root length (SRL)] were measured. With the increase in moso bamboo density, the biomass of various organs of I. latifolius showed a trend of first increasing and then decreasing, and all reached the highest level under treatment T1. Compared with the CK, treatments T1, T2, and T3 significantly increased the LL, LW, LT, LA, RL, RSA, RD, and length ratio of thicker roots (diameter > 2 mm) of I. latifolius, while significantly decreasing the SRL, SLA, and length ratio of finer roots (diameter ≤ 0.2 mm). Treatments T1, T2, and T3 significantly reduced the N content in the stems and rhizomes, the P content in the old leaves, and the SS content in the new leaves, and they increased the P content and K content in new leaves, stems, rhizomes, and roots; the N content in roots; and the starch contents in old leaves and new leaves. Treatment T1 significantly increased the N content in old leaves and the SS contents and the SS/ST of old leaves, roots, and rhizomes, and it decreased the N content in new leaves and the ST contents in roots, rhizomes and stems. Our results indicated that moso bamboo forests with low density can effectively promote the growth of I. latifolius in the forest. I. latifolius adapts to the shading and the root competition of moso bamboo by expanding the leaf area and promoting root growth. In this process, it supports the morphological plasticity of leaves and roots through the mechanisms of reabsorbing P and K and the directional transportation of photosynthetic products.