| Qilian Mountains is the source area of the Heihe River, the eco-hydrological processes in Qilian Mountains are crucial for ecological security of the Heihe River vally and the entier Hexi Corridor. Qinghai spruce(Picea crassifolia) forest is an important type of water conservation forest in Qilian Mountains. The structure and growth conditions of Qinghai spruce forest have crucial impacts on mountain hydrological processes. Therefore, to clarify the changes of forest structure and tree growth conditions along altitude increasing will be conductive not only to the analysis of eco-hydrological processes but also to the establishment of the distributed eco-hydrologicla model.Our study area lies in the Pailugou basin, located in in the middle of the Qilian Mountains. In the mountain forest belt(2500-3300 m a.s.l.) of Qinghai spruce, a total of 37 plots were settled. Forest structure and biomass were surveyed and calculated, and then, and the values were calculated with sub-segment elevation(per 100 m a.s.l.). In addition, 4 plots which located at altitiudes of 3300 m, 3200 m, 2700 m and 2550 m were selected, all stands(DBH> 5 cm) in plots were sampled. In addition, 3–4 open growth trees which locted nearly to the plots were sampled. The growth rate of trees, competition effects and growth-climate relationships at different altitudes were analysed. The results are as follows:1) In our study area, the density of the forests were big meanwhile the trees were relatively small. Forest density, diameter at breast height(DBH),tree height(H), and crown diameter were 1550±628(mean±SD) stems?hm-2, 13.9±6.2 cm, 8.1±3.7 m and 3.3±1.7 m, respectively. The young trees(5cm<DBH≤12.5cm) and middle trees(12.5 cm<DBH≤22.5 cm) were predominant in forest. The trees height were mainly about 6-12 m.With altitude increasing, the parametwes i.e. average DBH and crown diameter increased, but forest density, average H and ratio of height to diameter changed in "unimodal" modes, both of which peaked at an altitude of 2800-2900 m. With altitude increasing, proportion of the saplings(DBH≤5 cm) is basically stable, proportion of the young trees decreased slightly, the proportion of the middle changed as "unimodal" mode, with a peake at an altitude of 2800-2900 m, the proportion of the big trees(DBH>22.5 cm) increased. With altitude increasing, proportion of the trees(H≤6 m) changed as a "V" shaped mode, peaking at the elevation of 2800-2900 m. The percentages of the trees(H: 6-12 m) and the trees(H>12 m) changed as "unimodal" modes, peaking at altitudes of 2600-2700 m and 2800-2900 m, respectively.2) Forest biomass were 29.43-199.78 t?hm-2. With elevation increasing, forest biomass showed a "unimodal" change, peaking at an altitude of 2800-2900 m with the maximum was 199.78±45.68 t?hm-2. The minimum value was 29.43±13.50 t ? hm-2, which appeared at an altitude of 3200-3300 m. With elevation increasing, proportions of the young trees and the middle trees biomass in the total biomass decreased, nevertheless, proportion of the big trees biomass increased. At the altitude of 2500-3000 m, the middle trees were accounted for the main part the forest biomass, which proportion was more than 40%, while, the proportion of the young tree and the big tree were about 25% and 20%, respectively. At the altitude of 3000-3300 m, the big tree accounted for the main part of the forest biomass, with its proportion was more than 40%. The middle tree became a secondary source of forest biomass, with the proportion of 20-30%. The proportion of the young trees biomass came down to the lowest, which was 26 percent or less. The changes of forest biomass along altitude were decided by the density, temperature and precipitation, jointly. In our study area, the elevation segment of 2800-2900 m is the most suitable area for the growth of Qinghai spruce forest. There is a following relationship between the forest biomass(B, t·hm-2) with stand density(d, stems ? hm-2), temperature(t, ℃) and annual precipitation(p, mm): tpd-??????689.168854.30430.0036.0B(R2 = 0.616).3) Growth speed of trees at different altitudes changed as 3200 m>3300 m>2700 m>2550 m a.s.l. And, since 1983 when the temperature raised, growth rate of the trees at altitude 3300 m sped up, neverthless, growth rate of the trees at altitudes of 3200 m, 2700 m, 2550 m decreased sharply. Competition will be significantly hinder the growth of DBH and H, and especially the growth of DBH. The denser the forest, the stronger the competitive effects of trees. It was 4.5 time lager of the radial growth rate of open growth trees than the stands in plots, as the forest density was increasing 1000 stems·hm-2. It was shown that, trees meanly competed for water in our study area. And the competition would be stronger due to the increment of temperature or forest density.4) In conclusion, with elevation increasing, there were significant changes in forest structure and tree growth of Qinghai spruce. Therefore, it was demonstrated that it’s necessary to build the functions between forest structure and hydrological processes with elevation increasing. Meanwhile, it will be better that specific value of structure parameters in eco-hydrological models were set for different elevation zone. Moreover, the competition and other biotic factors should be taken into account while buliding the coupling mechanism of tree growth and hydrothemal conditions. |
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