Spatial Analysis of a Haloxylon Ammodendron Plantation in an Oasis-Desert Ecotone in the Hexi Corridor, Northwestern China

Haloxylon ammodendron is a commonly used sand-fixing species in the desert area of northwestern China; it has been abundantly planted in areas where annual precipitation is about 120 mm in the Hexi Corridor since the 1970s. Spatial patterns and associations of an H. ammodendron plantation in five stages of community development were analyzed in an oasis-desert ecotone to gain insights into population dynamics over a course of succession. Five 0.3-ha (50 m x 60 m) permanent plots were established in each of five developmental stages; H. ammodendron was classified as seedlings, juvenile and mature trees, and all individuals were measured and stem-mapped. The univariate spatial analysis by the L-function and the bivariate L-12-function were used to describe the spatial patterns of all trees and examine the spatial association among trees between different tree size-classes. Results showed that at scales >2 m, the spatial pattern of H. ammodendron shifted from initially clustered to random, and back to clustered; at scales <2 m, a transition from uniform to clustered was observed with stand age. In 5-10-year, 10-20-year and 20-30-year stages, competition between conspecifics may be the dominant factor which influenced plant survival. In 30-40-year and >40-year stages, interactions between conspecifics may be the dominant factor in conditions of tree-size-asymmetric competition, but abiotic stress may be more important in tree-size-symmetric competition. The H. ammodendron plantation experienced highest mortality at the 5-10-year stage as a result of fierce competition for soil water, while with respect to growth, it entered into a relatively stable stage, where the gaps generated due to mortality of adult trees and improved soil conditions provided opportunities for regeneration. In the >40-year stage, the regeneration experienced a decline under enhanced competition for water, and the plantation showed a clustered pattern at all scales due to water stress.