Effects of target tree management density of Michelia macclurei on understory plant diversity and soil nutrients

This study explores the effects of different management densities of Mytilaria laosensis as a target tree on the diversity of understory vegetation and soil nutrients, providing a scientific basis for the ecological management and sustainable operation of mixed forests with M. laosensis as the target species. The study selected a mixed forest of Mytilaria laosensis, Erythrophleum fordii, and Castanopsis hystrix as the research subject, with M. laosensis as the target tree. Four management density treatments were established: no thinning (CK), target tree densities of 180 trees/hm⁻² (T1), 120 trees/hm⁻² (T2), and 60 trees/hm⁻²(T3). After five years of thinning, the diversity of understory vegetation and soil nutrient characteristics were systematically analyzed. The results showed that: (1) A total of 120 shrub and herb plots were surveyed, recording 56 species of understory plants belonging to 31 families and 52 genera. Among them, 36 species were found in the shrub layer and 20 species in the herb layer, with Euphorbiaceae, Hamamelidaceae, Poaceae, and Pteridaceae being the dominant families. In the shrub layer, M laosensis had a dominant ecological position, with an importance value exceeding 50%. In the herbaceous layer, Blechnum orientale was dominant in the CK treatment (importance value: 44.78%), while Pteris semipinnata was dominant in the T3 treatment (importance value: 54.44%). (2) The Margalef richness index of the shrub layer in the T1 treatment was significantly higher than that in CK (P<0.05). The Shannon-Wiener index and Margalef richness index of the herbaceous layer in the T2 treatment were significantly higher than those in CK (P<0.05). (3) Overall, the contents of total phosphorus, available phosphorus, available potassium, total nitrogen, and ammonium nitrogen decreased with increasing soil depth. The T2 treatment showed relatively high nutrient contents in both soil layers and significantly increased the levels of total phosphorus, available phosphorus, available potassium, and total nitrogen (P<0.05). The T1 treatment significantly enhanced the surface soil contents of available phosphorus and available potassium but had a negative impact on total nitrogen content in the deeper soil layer (P<0.05). There were no significant differences in soil nutrients between the T3 and CK treatments (P>0.05). (4) Correlation analysis showed that total phosphorus in the soil was significantly positively correlated with available phosphorus and available potassium (P<0.05). In the shrub layer, the Shannon-Wiener index was significantly positively correlated with the Simpson index, Margalef index, and available potassium (P<0.05). Pielou evenness was extremely significantly positively correlated with available phosphorus and total potassium (P<0.01), and significantly positively correlated with total nitrogen (P<0.05). The Margalef index showed a non-significant negative correlation with total nitrogen (P>0.05). In the herb layer, the Shannon-Wiener index was extremely significantly positively correlated with the Margalef index and available potassium (P<0.01), and significantly positively correlated with the Simpson index (P<0.05).(5) Redundancy analysis (RDA) indicated that soil nutrients explained 65.37% and 65.39% of the variation in diversity indices of the shrub and herb layers, respectively. In the shrub layer, total nitrogen and available potassium were identified as significant factors, while in the herb layer, available potassium was the only significant factor.In conclusion, maintaining a target tree management density of approximately 120 trees/hm⁻² for M. laosensis is more conducive to promoting understory plant diversity and soil nutrient accumulation.