Spatial Distribution and Structural Characteristics for Haloxylon ammodendron Plantation on the Southwestern Edge of the Gurbantunggut Desert

Haloxylon ammodendron (C.A.Mey.) Bge. is crucially important for stabilizing sand dunes in the desert area of the Junggar Basin and has thus been widely planted in the oasis-desert ecotone for windbreak and sand fixation purposes since the 1980s. The spatial distribution and structural characteristics of Haloxylon ammodendron plantations of three different ages-planted in 1983 (36a), 1997 (22a), and 2004 (15a)-on the southwestern edge of the Gurbantunggut Desert were studied. The results showed that the spatial distribution patterns for the different stages of growth showed a trend of cluster that was random during the transformation from seedlings to juvenile and mature trees. Forest density for the 15a, 22a, and 36a plantations was, respectively, 1110, 1189, and 1933 plants ha(-1); the base stem diameter for the main forest layer was 5.85, 8.77, and 6.17 cm, respectively, and the tree height was concentrated in the range of 1.5-3.0 m, 2.0-3.5 m, and 1.5-2.5 m. In the regeneration layers, the proportion of seedlings was the largest in all three stand ages, followed by juvenile trees, and mature trees only appeared in the 22a plantation. The proportion of deadwood in the 36a forest was the highest, and there were no mature trees in the regeneration layer. These results indicate that the three Haloxylon ammodendron plantation stages were in the period of rising at 15a, stable and degenerate with increasing age at 22a, and at 36a the regeneration ability was very weak and presented degradation due to species competition for soil moisture, because of too many seedlings and mature plants. In this case, measures such as thinning could be taken to prevent rapid degradation and to accelerate regeneration when the stand age exceeds 20 years. Considering the sand fixation effect, the pressure of competition for water resources, and forest capacity for renewal and sustainability, the most suitable forest density in the Haloxylon ammodendron plantation would be 8.5-9 m(2) per plant.