Education & Training

Nutrient Cycle Response to Environmental Change in Subalpine

Update time: 06/08/2010

Global climatic change is directly or indirectly affectting the natural ecosystems, and the forest ecosystems, as the major part of terrestrial ecosystems, which the nutrient cycle between plant and soil will inevitably be affected under changed environments. Abies faxoniana is the dominant tree species of subalpine coniferous forest in Western Sichuan, China, which plays crucial roles in regulating local climate, soil and water conservation, retaining regional ecological stability. In this study, the nutrient cycle between plants and soil in A. faxoniana forest community in response to environmental change which resulting from altitudinal differences was investigated. The results of this study would provide the basic work for predicting the effects of future climate change to subalpine coniferous forest ecosystem in Western Sichuan, and for deciding the ecological safety strategy of this field. The main results are shown as follows:

1. For the whole distribution range of A. faxoniana along the altitudinal gradients, soil was enriched with organic matter (C) and total nitrogen (N), and the differences among altitudes were not significant. Moreover, the lower soil C/N ratio was beneficial to the nutrient release during decomposition process, and transformation rate of organic matter was fast. However, lower temperature and C/N ratio could not explain completely the phenomenon that the soil organic matter and total nitrogen in the high altitude was higher than those in the lower altitude. The soil nutrients of subalpine coniferous forest in Western Sichuan could be more affected by the change of soil types along the altitudinal gradients.

2. Litter biomass of A. faxoniana forest in Wanglang Natural Reserve was higher, compared with the values of the similar communities within this field, and the difference of altitudes was not significant, which indicated that the forest ecosystem had higher productivity. Moreover, the N and phosphorus (P) concentration of litters in A. faxoniana decreased with the increasing altitude, which indicated that the nutrient uptake and reutilization efficiency of A. faxoniana increased with the increasing altitude. In addition, in every season, the nutrient concentration of sundries litter (the litter of broad-leaved species) was significantly higher than that of litter in A. faxoniana, indicating that coniferous species has higher nutrient uptake and reutilization efficiency than broad-leaved species.

3. The nutrient returning mass of litters was mainly controlled by the litter biomass. The annual N and P returning mass were the highest at the 3,000 m altitude for each component of litters and total litter, however, the annual potassium (K) returning mass has the highest value at the 2,500 m altitude. In addition, the proportion of annual N and P returning mass by needle and woody litter in respective total returning mass increased with the increasing altitude, and correspondingly, the proportion of those by sundries litter decreased with the elevating of altitude.

4. The decomposition rate of litter A. faxoniana was different in different altitude, and the condition can be divided into two types at about 3,000 m, below 3,000 m, the needle litter need 13-17 a to decompose the 95%, and that above 3,000 m altitude need more than 20 a. However, the altitudinal difference appeared about two months after decomposing, and the change of decomposition rate could correlate with the releasing (or enrichment) variation of N.

5. C and N of leaves and branches in A. faxoniana were increased with increasing altitude, indicating that the phenomenon of nutrient deficiency in plant tissues at higher altitude was not existent in our study area. Moreover, the C/N ratio of leaves in A. faxoniana was higher than other study areas, indicating that there is higher N utilization efficiency in A. faxoniana than other species. In addition, the C/N ratio and C/P ratio were higher in the higher altitude than lower altitude, which indicated that plants in higher altitude had higher leaf construction efficiency under the unit N and P conditions than that of lower altitude.

6. Non-structural carbohydrate (NSC) at various plant tissues of A. faxoniana showed a stable high level, and the content in leave was higher than the branch with the same age. Moreover, in the whole distribution of A. faxonian, the NSC of leaves and branches in the higher altitude were higher than middle altitudes. Hence, the result did not support “carbon limitation” hypothesis that was used to explain the formation of treeline. In addition, the total NSC of leaves and branches at the beginning of growing season was higher than that at the end of last growing season, which indicated that thecarbonexhausted was not existent during the winter. In addition, although the starch content both of leaves and branches at the beginning of growing season was significant increased than that at the end of last growing season, but the largest increase range of leaves was at 2,750 m, and that of branches was at 3,000 m, which indicated that sensitivity of leaves and branches to phenological phenomena was different, and the carbohydrate dynamics of branches was more sensitive to the variation of temperature than that of leaves.

7. The mean δ13C value both of leaves and branches increased with increasing altitude, indicating that plants has higher water utilization efficiency at the higher altitude. Temperature decreased with increasing altitude was an important reason that resulting in the higher δ13C value of plant at the higher altitude. However, frequent rainfall in summer and lower temperature with frost in winter at the higher altitude may also be two crucial factors.

8. At the whole distribution zone of A. faxoniana along the altitudes, irrespective of tissue age, the branches of A. faxoniana were enriched in 13C compared with the same age leaves, which may result from the variation in biochemical constituents as well as their carbon isotope composition in different plant tissues. The leaf δ13C value was significantly related to the NSC in each age group, and the older of leaf, the stronger of correlation, indicating that sugar and other photosynthates in leaves has important effect on δ13C value. On the contrary, the branch δ13C value was not significantly correlated with the NSC in each age group, which indicated that the carbon isotope discrimination was existent during the use of carbon pool.

Key words: Abies faxoniana; altitudes, nutrient; non-structural carbohydrate; stable carbon isotope composition