Root biomass and its distribution of Azadirachta indica and Acacia auriculiformis plantations in the Dry-hot Valley
干热河谷印楝和大叶相思人工林根系生物量及其分布特征

Afforestation is an important tool in ecological restoration in Dry-hot Valleys, a fragile landtype, where vegetation deterioration and soil erosion have developed into a serious problem in southwestern China. However, the root biomass of plantations in Dry-hot Valleys is poorly known and understood. Azadirachta indica A. Juss. is planted extensively in Dry-hot Valleys for its drought-resistance and multifunctionality, and Acacia auriculiformis A. Cunn. ex Benth is a fast growing species which can rapidly improve soil conditions. Establishing plantations with A. indica and A. auriculiformis in Dry-hot Valleys is important for soil and water conservation, soil amelioration, prevention of ecological damage and also helps the regional economy. To improve the understanding nutrient accumulation and distribution, soil moisture dynamics, root competition and evaluate efficiency of mixed plantations, this study investigated root biomass and the characteristics of root distribution in 10 year-old A. indica and A. auriculiformis plantations in the Dry-hot Valley ecosystem in Yuanmou County, Yunnan Province, China. Data from sample trees were averaged to determine root biomass characteristics for a pure A. indica plantation, a pure A. auriculiformis plantation and a mixed plantation of A. indica and A. auriculiformis. A total of 24 mature trees were excavated to a depth of 1.0 m to collect all root biomass for each tree. The results show root biomass in the mixed plantation (2.707 t/hm2) was between the root biomass of a pure A. indica plantation (2.264 t/hm2) and a pure A. auriculiformis plantation (3.405 t/hm2). The taproot biomass in the mixed plantation (1.057 t/hm2) was approximately 69.9% and 69.7% of the taproot biomass in the pure A. indica plantation and the pure A. auriculiformis plantation, respectively, whereas the biomasses of medium, small and fine roots from the lateral roots in the mixed plantation were 228.7%, 120.1% and 450.0% of roots in the pure A. indica plantation and 71.3%, 65.8% and 48.8% of those in the pure A. auriculiformis plantation, respectively. Root biomasses of these plantations were mainly concentrated at a soil depth of 0-0.2 m, which accounted for 63.6%-74.4% of their root biomass within a depth of 0-1.0 m. Compared to the mixed plantation, the pure A. auriculiformis plantation had a shallower root profile while the pure A. indica plantation had a deeper root profile. There was 30.9% of the taproot biomass in the upper 0.4 m soil layer in the pure A. indica plantation, but only 2.8% for A. indica in the mixed plantation. However, there was only 0.3% of lateral root biomass below the 0.4 m soil layer in the pure A. indica plantation, while there were 6.8% and 9.6% of lateral root biomasses in the pure A. auriculiformis plantation and mixed plantation, respectively, which indicated root biomass of A. indica was mainly allocated to lateral roots in the mixed plantation. The relationship between the accumulated root biomass