Effects of soil properties, topography and landform on the understory biomass of a pine forest in a subtropical hilly region

Soil conservation will remain an essential subject for forest in slope land. However, the interactions between soil, vegetation and complex topography are not fully understood. To address this concern, field studies were performed in a typical small watershed in the subtropical hilly red soil region of China. This site covers an area of approximately 3.2 ha, and the mean slope steepness of the land is 15 degrees. Soil properties (including available and total N, P, K, soil organic carbon (SOC), CEC, soil texture), understory vegetation biomass and diversity index on different slopes (different slope steepness, slope aspects, and slope positions) were measured. Meanwhile, interrelationships between understory vegetation biomass and soil properties, slope steepness and altitude were evaluated via redundancy analysis. The study results showed that the soil nutrients and SOC contents were greater in the down-slope and north and west slopes. And the soil properties displayed high spatial heterogeneity and had semi-variance structure, which were mainly derived from random factors. Due to obvious erosion characteristics and the negative correlations between soil properties and slope positions, we suspected that soil erosion was the primary random factor. In addition, the mean total understory vegetation biomass in the pine forest was 283.58 g m(-2) (ranging from 66.60 to 573.79), and higher vegetation biomass was found in down-slope and north and west slopes. Further analyses indicated that topography (slope steepness and altitude) and soil properties combined contributed to 58.7% of the variations in the understory biomass, and they individually had a contribution of 17.3% and 41.4%, respectively. Moreover, altitude (height of slope positions) alone explained 15.9% of the variation of the vegetation biomass. This study indicated that soil properties, which were highly affected by slope aspect and slope position, were the most important factor in influencing understory vegetation biomass in subtropical pine forest. Slope position also had a tremendous direct influence on soil vegetation. Controlling understory soil erosion is essential for conservation of pine forest land in subtropical China.