اثر ده ساله تنك كردن بررشد و سرسبزي تاغكاريهاي سبزوار

Growth of Haloxylon spp. plantation, ten years after thinning, in Sabzevar.

percentage, mean annual precipitation, and population density on growth and vigority of planted Haloxylon trees. Based on the model, estimated number of population density for 7 years old Haloxylon plantation in Sabzevar was 309 trees per hectar; therefore, six density treatments were selected around the estimated density; two density treatments over estimated number, two under it, and existing density as the control treatment. The treatments consisted of Ti=247, T2=278, T3=309, T4=340, T5=371, and existing density(control) T6=500 trees per hectar. The treatments have been compared in a Randomized Complete Block Design, with four replications, since winter of 1994. After thinning practices, height(H) and crown diameters(CD) of the remained trees were measured. This measurement was repeated during the growth period in ten years. Since the CD increment between the first and the 10th measurement was 3 times more than H increment; therefore, a size index(SI) model of the form Hx(CD)2, which was constructed by trial and error, was found to distinguish the best among the 6 treatments. The growth percentage of H, CD, and SI between first and the last measurement were calculated. The analysis of variance of the growth percents showed that there is significant differences between treatments for H, CD, and SI. The highest growth belonged to T2 and lowest belonged to T6(control). The results show that optimized population density of seven years old Haloxylon stands in Sabzevar is 10% less than the estimated number based on the general model.