| Driven by solar radiation difference, the change of ecological factors mainly for moisture content on the South-North-facing slope gradients can make corresponding changes in the composition of the species of plant community and abundance along the slope. According to the adaption of physiological and ecological processes and function straits of the community main component species to the moisture change in soil to discuss the causes of species turnover on slope habitat gradients will be the key to clarify the ecological mechanism of mountain vegetation along the slope aspects forming community gradients.Based on the above discussion, we had selected a hill with East-West trend as example in the sub-alpine meadow of the eastern of Qinghai-Tibetan Plateau. According to the results of field community investigation, We had analyzed the change of community composition and species richness along the slope aspect habitat gradients. Simultaneously, we selected five main species (Heteropappus hispidus (Thunb.)Less.,Potentilla fragarioides L.,Tibetia himalaica (B aker) H.P. Tsui, Taraxacum mongolicum Hand.-Mazz, Potentilla fruticosa L.)on each slope aspect, and then investigated the relationship between the relative coverage and soil moisture content. Finally, combining with some laboratory experiments, we had measured the leaf carbon stable isotope (Delta 13C) value, leaf dry matter content (LDMC) and specific leaf area (SLA), analyzed the changes of these leaf traits, water use efficiency and stomatal conductance along with the slope aspect gradients, and discussed the relativity between main species biomass and stable carbon isotope 813C value. Through the above analysis we obtained some results below:1. From south to north-facing slope, the soil moisture content present a linear increasing trend and moisture content increases gradually from 14.68% to 25.96%, and there is extremely significant difference (p<0.0001) between different slope aspects.2. Species richness increases gradually from south to north-facing slopes, and there is significant difference (P<0.05) between different slope aspects. It is a single peak trend of community β diversity that first increased and then decreased from the south-facing slope to the south-north facing slope to the north-facing slope (y=0.0002x2+0.0316x+1.0455, and the community (3 diversity has a highest value on the south-north facing slope. Heteropappus hispidus (Thunb.) Less. is the dominant species in south-facing slope, its relative coverage is significantly and negatively correlated with soil moisture content(y=-0.87x+26.508,r=0.839,p<0.0001); while in the north-facing slope, Potentilla fruticosa L. shrub becomes the dominant species, its relative coverage and soil moisture content have a significant positive correlation(y=13.903x-280.955,r2=0.683,p=0.014); the relationship between relative coverage and soil moisture content of the common species Taraxacum mongolicum Hand.-Mazz is significantly quadratic curve correlation (y=-3.61x2+0.9x+36.719,r2=0.611,p=0.037; and the relative coverage and soil moisture content of common species Potentilla fragarioides L. and Tibetia himalaica (Baker) H.P. Tsui is not significant.3. There is a significant negative relationship between main species leaves Delta 13C value and soil moisture content. Heteropappus hispidus (Thunb.) Less. is the dominant species in south-facing slope whose Delta 13C value is significantly higher than the north-facing slope dominant species Potentilla fruticosa L. shrub and the leaves Delta 13C value of common species Potentilla fragarioides L Tibetia himalaica (Baker) H.P. Tsui and Taraxacum mongolicum Hand.-Mazz have slight difference; leaf dry matter content of main species present a negative correlation with soil moisture content while their specific leaf area and soil moisture content are significant positively correlated.4. Stomatal conductance of main species has significant difference (p=0.002) along with slope gradient varies, both Heteropappus hispidus (Thunb.) Less. and Potentilla fruticosa L. show downtrend along with the south-facing slope to the north-facing slope, Potentilla fragarioides L., Tibetia himalaica (Baker)H.P.Tsui and Taraxacum mongolicum Hand.-Mazz have no significant change regulation along with slope aspect gradient from south to north.5. The biomass of dominant species of south-facing slope Heteropappus hispidus (Thunb.) Less.(y=5.365x-30.056,p<0.0001,r2=0.92) is significantly and positively correlated with the Delta 13C value, the Delta 13C value of common species Tibetia himalaica (Baker)H. P.Tsui (y=4.88x-31.783,p=0.021,r2=0.507) show an increase along with the increased biomass increased, but the biomass of north-facing slope dominant species Potentilla fruticosa L.shrub has a significant and positive correlation with Delta 13C value(y=-4.375x-100.761,p=0.039,r2=0.536), and the other two common species show a non-significant correlation between Delta 13C value and biomass.6. The photosynthetic rate of major species has significant difference on all slope aspects, and it present a declined trend along with the slope aspect gradient from south to north, the dominant species of south-facing slope Heteropappus hispidus (Thunb.) has the largest changes (18.11-3.75). Except for Potentilla fruticosa L., the other four species entirety have little difference, and the photosynthetic rate of Potentilla fruticosa L. is significant less than other major species.7. Water use efficiency of main species has significant difference on different slope aspects (p<0.0001).From south-facing to south-north-facing slope aspect, the water use efficiency of Heteropappus hispidus (Thunb.) Less, is significantly higher than other major species, and there is little difference among Potentilla jragarioides L., Tibetia himalaica (Baker) H.P. Tsui and Taraxacum mongolicum Hand.-Mazz along with the slope aspect gradients change, while water use efficiency of Potentilla fruticosa L. is markedly higher than Heteropappus hispidus (Thunb.) Less, on north-facing slope aspect;According to the above research results, we obtain the following conclusions:1. From south to north-facing slope, the soil moisture content present a linear increasing trend, and the composition of the community major species, the relative coverage and species richness change correspondingly along with the slope aspect gradients and Delta diversity emerge first increase then decrease trend.2. Among species turnover process, on lower soil moisture south-facing slope the community main species regulate the size of stomatal conductance to improve the utilization rate of CO2 and improve the efficient of using limited water resources then increase the biomass accumulation so that against drought stress environment at low soil moisture environment; influenced by both moisture and light, vegetation improves leaf photosynthetic capacity by adding specific leaf area on north-facing slope with relatively abundant water resource environment that make up north-facing slope faultiness of insufficient light.3. On slope aspect gradients, effected by leaf stomatal conductance, the main species biomass and stable carbon isotope δ13C value present a different relationship:the δ13C value of Heteropappus hispidus (Thunb.) Less. and Tibetia himalaica (Baker) H.P. Tsui show an increase along with the increased biomass, on the contrary,813C value of Potentilla fruticosa L.show an opposite situation, the 813 C value of Potentilla fragarioides L.and Taraxacum mongolicum Hand.-Mazz present not really obvious variation along with the increased biomass, which indicate different species have different responses upon stomatal conductance changes... |
PDF Full Text Request