| The subalpine coniferous forest (the southwestern forestry area) is the secondly largest forest in China. It plays an important role in the national economic development, biodiversity conservation and water conservation by preventing soil erosion and in keeping the ecosystem stable. One of the typical vegetation types in this forest in western Sichuan is bamboo-Abies-Picea mixed forest, which is also the important habitat of the giant panda. However, in nowadays, the survival and reproduction of the giant panda have been seriously threatened by the degradation of the environment and the ecological functions of this ecosystem, which is attributed to the influence of natural and human disturbances. Moreover, Abies faxoniana Rehd. et Wils., the constructive species in this ecosystem, is hard to regenerate naturally. On the one hand, the regeneration of A. faxoniana is associated with canopy gaps. The seedlings in the understory have low survival rates, and are difficult to develop into the sapling stage. On the other hand, the dwarf bamboo, Fargesia denudata Yi, usually impedes the establishment and growth of A. faxoniana regeneration. Therefore, study on the regeneration mechanism of A. faxoniana in this place should pay more attention to the ecological relationship among’gap disturbance-A. faxoniana regeneration-bamboo’. The related study will be very important and meaningful for the maintenance, conservation and restoration of the ecosystem, especially the giant panda’s habitat, in subalpine coniferous forest in southwestern Sichuan.Nowadays the interspecific relationship between tree regeneration and other plants is increasingy important in gap regeneration research at home and abroad. The scientific issue of this paper is’How does gap disturbance influence A. faxoniana regeneration’. The research object in this paper is A. faxoniana, the dominant tree species in subalpine coniferous forest in southwestern Sichuan. Ten located transect belts were established randomly on Dawodang in Wanglang Natural Reserve. In order to study the influence of gap size and age, F. denudata density and coverage, and microhabitats on the recruitment densities of A. faxoniana, the size and age of each gap in transecting belts were recorded. The number and recruitment microhabitats (bare soil bed, moss-cover ground, decaying logs, or stumps) of A. faxoniana seedlings and saplings, as well as the number and coverage of F. denudata, were counted. To describe their growth situations in detail, the individuals of A. faxoniana seedlings and saplings were classified according to their height classes:seedlings:0.05-0.18 m,0.19-0.35 m,0.36-0.50 m; saplings:0.50-1.40 m,1.41-2.10 m,2.11-3.50 m. This paper would help to discover the recruitment mechanism of A. faxoniana regeneration, and the coexistence relationship between A. faxoniana regeneration and F. denudata. It will also provide theoretical foundation and scientific reference for the artificial regeneration of A. faxoniana and the habitat conservation and restoration of giant panda. The main results are as follows:(1) The sizes of canopy gaps in the studied area were investigated and the influence of gap size on the densities of F. denudata, A. faxoniana seedlings and saplings were analyzed. The main results showed that among the 103 gaps, the small gaps (<=200 m2) accounted for 72% of the total gaps, while the medium-sized gaps (200-1000 m2) were around 26% and the large gaps (1000-2000 m2) were less than 3%. With the gap sizes increased, the total amount and coverage of F. denudata significantly increased, while the densities of A. faxoniana seedlings of 0.19-0.35 m m in height decreased. It is possible that small gaps were relatively suitable for A. faxoniana seedlings in this height class.(2) The ages of canopy gaps in the studied area were investigated and the influence of gap age on the densities of F. denudata, A. faxoniana seedlings and saplings were analyzed. The main results showed that among the whole gaps, the medium aged gaps (20-40 years) accounted for the largest proportion of gaps (50.49%), while the young gaps (<=20 years) accounted for 23.30% and the old gaps (40-60 years) accounted for 26.21%. The density and coverage of F. denudata barely varied in young and medium-age gaps, while they significantly increased in old gaps. The total amounts of A. faxoniana seedlings or saplings had insignificant difference among three aged gaps. It implied that gap age had no significant influence on the densities of A. faxoniana regeneration.(3) The effects of F. denudata coverage on the densities of A. faxoniana seedlings and saplings on four microsites were analyzed. The main results showed that with the F. denudata coverage increased, the densities of A. faxoniana seedlings and saplings on bare soil-bed and stumps significantly decreased. In A-gaps (without F. denudata) and Ss-gaps (there were A. faxoniana regeneration, and with F. denudata coverage>=17%), A. faxoniana seedlings and saplings showed no preference on microhabitats. However, in Sn-gaps (there were A. faxoniana regeneration, and with the F. denudata coverage<=17%), the density of A. faxoniana regeneration on decaying logs was highest. It showed that the preference of A. faxoniana regeneration on microsites may result from the pressure from F. denudata.(4) The effects of gap size and age on the densities of A. faxoniana seedlings and saplings on four microsites were analyzed. The main results showed that in gaps without F. denudata (A-gaps), A. faxoniana seedlings distributed more on soil, while with significant limitation of gap size. The densities of A. faxoniana saplings were relatively even among different microsites, and were not influenced by gap size. In gaps where A. faxoniana regeneration and F. denudata coexisted (AF-gaps), A. faxoniana regeneration distributed more on decaying logs, but with the gap sizes increased, the densities also decreased. Gap age had significantly negative influence on the densities of A. faxoniana saplings on decaying logs.(5) The influence of gap size and age on the densities of A. faxoniana seedlings and saplings in gaps with different F. denudata coverage was analyzed. The main results showed that in A-gaps, the total amount of A. faxoniana seedlings significantly decreased with the gap size increased. With the gap size of<=200 m2 or with the gap age<=20 years or over 40 years, it was obvious that F. denudata would exert negative effect on the A. faxoniana seedling amounts. It implied that the limiting effect of F. denudata coverage on the A. faxoniana regeneration would be influenced by gap size or age. Meanwhile, F. denudata with a coverage of<=17% may provide desirable living environment for A. faxoniana seedlings in larger gaps.(6) The effects of gap size, gap age, F. denudata density and coverage, and microsites on the densities of A. faxoniana seedlings and saplings were analyzed by PCA and correlation analysis. The main results showed that the most important influential factors of A. faxoniana regeneration density were gap size and F. denudata (density and coverage). Small gaps (<=200 m2 in size) with a small amount of F. denudata (with a coverage<=17%) were most suitable for A. faxoniana recruitment. Therefore, in the future ecological conservation and restoration work in the A. faxoniana forests, it is important to keep implementing the’gap disturbance-A. faxoniana regeneration-bamboo’principle. |
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