Optimization of Population Density of Haloxylon spp. Plantations (Phase II)

A general model was devoloped to explain the interrelationships between soil saturation percentage, mean annual precipitation, and population density on growth and vigority of planted Haloxylon trees. Soil saturation percent is an easy measure to determine the water (rain) availability to plants. The main recommendation of the model is thinning the stands in accordance with the above named factors. Calibrating the general model for several regions, and optimizing the stands of Haloxylon plantation are the main objective of the present project. Six thinning treatments were selcted; two densities(T1 and T2) less than estimated number based on the general model(T3), and two(T4 and T5) more than the estimated number, and existing density as the control(T6). The treatments have been compared in a Randomized Complete Block Design, with four replications, in each of the five research sites (Yazd and Kashan, since 1993, Semnan and Sabzevar, since 1994, and Gonabad, since 1995). After thinning practices, height(H) and crown diameter(CD) of the remained trees were measured and These measurments were repeated in the end of next 10 seasonal growth periods. Since both measured parameters(H and CD) have been affected by thinning practises, in order to compare the results, by compiling these two parameters in the forme of H X CD2, as a single parameter, named Size Index(SI), was used. The growth percentage of H, CD, and SI between the first and the last measurement were recorded. In all research sites, the growth percentage of above mentioned factors in thinned plots were more than control. The analysis of variance of these factors showed that there are significant differences between thinned plots and control in Kashan, Sabzevar, and Gonabad, but it was found to be no significat in Yazd and Semnan. The age of trees in these two sites at the bigining of project were 15 and 17 years respectively, but at other sites was less than 10 years. Comparison of the means of measured factors showed that more high growth rates belonged to T1 to T4 thinning treatments which are around T3. These results indicated necessity of thinning of Haloxylon plantation in Iran for revival of vigor and increase of trees growth; it is also indicated accuracy and correctness of the general model for identifying the optimized population density of Haloxylon plantation.