Spatial Pattern Analysis In A Natural Secondary Betula-Abies Forest At Sub-Alpine Area Of Western Sichuan, China

Spatial pattern is the results of integrated processs of population property, spatial association and environmental conditions. The study of spatial pattern of forest can infer the trend of forest community succession and provide theoretic support for the restoration of degraded ecosystems and sustainable management of forest ecosystem. The dark coniferous forest at sub-alpine area of western Sichuan, has important ecological, economic and social values. Due to extensive deforestation and long-term intensive agricultural land use, the area of primary forests has been reduced dramatically, and resulted in a wide distribution of natural secondary forests of different restoration stages. Therefore, the secondary forests that originated from those disturbed forests have become the main forest landscape in this region. However, the past ecological studies were mainly conducted in primary or old-growth forests, and we still know little about those degraded vegetation after anthropogenic disturbance. In this study, field investigations including individual spatial position of dominant tree species were conducted in a four ha sample plot in a natural secondary Betula-Abies forest. The plot locates in the north aspect at the altitude of 3300 m in Miyaluo, western Sichuan, China (N31°42′18.4″, E102°44′03.7″). Spatial point pattern analysis including Ripley's L-function and O-ring statistic's O-function were mainly adopted, we analyzed: (1) community structure and species composition; (2) spatial distribution pattern and intra-specific association of A. faxoniana population; (3) influencing factors on natural regeneration of A. faxoniana; (4) inter-specific competition of dominant tree species; (5) effects of remnant trees. Driving factors of natural restoration in natural secondary forest were discussed to provide the scientific basis for conservation and restoration of dark coniferous forest at sub-alpine area of western Sichuan. The main results are as follows:(1)B. albo-sinensis and A. faxoniana are the two dominant tree species in the natural secondary Betula-Abies forest. B. albo-sinensis took up 49.0% of total basal area and A. faxoniana took up 28.9%, respectively. DBH frequency distribution of B. albo-sinensis had a unimodal distribution, indicating a declining population with succession. In contrast, A. faxoniana had a reverse J-shaped pattern, showed an increasing population with succession. Point distribution map indicated that spatial distributions varied with size class, but all showed an aggregate and heterogeneous pattern.(2)A. faxoniana distributed as a clumped population, all size-classes exhibited clumped distributions at most spatial scales. Intensity of the assemblage increased firstly, and then decreased with increasing scale and then turned to random distribution. There existed a critical scale in which distribution pattern transfered from clumped to random, for example, small trees: 63 m and medium trees 87 m. All size-classes showed a positive association at small scales. Spatial association turned from strongly positive association to zero with the increasing of scale. However, there were no negative spatial associations in size-classes at all scales. It indicated that the population had a harmonious relationship internally. We found that different size class exhibited maximal intensity of assemblage at a critical scale: seedlings (28 m), saplings (29 m), small trees (15 m), medium trees (34 m), big trees (49 m). Big trees and medium trees had significantly positive association at all scales. Seedlings had significantly positive association with saplings only within the scale of 7 m, but they had no significant spatial associations with big, medium and small trees.(3)The seedlings and saplings of A. faxoniana were very abundant with continuous age distribution in Betula-Abies forest, suggesting its successful regeneration. We did not found alive B. albo-sinensis individuals lower than two m in the plot, suggesting its poor regeneration. As to A. faxoniana, one-year old seedlings accounted for 62.45% of total number of seedlings and saplings, and seedlings more than two-years old accounted for 2.30%. 86.70% of one-year old seedlings and 81.08% of seedlings more than two-years old habitated on moss. Particularly, we focused on spatial association at small scale. Spatial association of different size class of dominant tree species varied with seedlings and saplings. Spatial association of A. faxoniana's seedlings along with its small trees, medium trees and large trees showed significantly negative at the scales of 0~7 m, 0~8 m and 0~8 m, respectively. Spatial association of A. faxoniana's saplings with its small trees, medium trees and large trees showed significantly negative at the scales of 0~11 m, 0~13 m and 0~11 m, respectively. No significant spatial association was detected between A. faxoniana's seedlings and B. albo-sinensis's small trees and between A. faxoniana's saplings and B. albo-sinensis's small trees at all scales. There were significantly negative spatial associations between A. faxoniana's seedlings and B. albo-sinensis's medium trees and between A. faxoniana's saplings and B. albo-sinensis's medium trees at the scales of 0~18 m, 0~34 m, respectively. There were significantly negative spatial associations between A. faxoniana's seedlings and B. albo-sinensis's big trees and between A. faxoniana's saplings and B. albo-sinensis's big trees at all scales. Coverage of moss, stock of logs, density of mother trees of A. faxoniana, coverage of bamboo and coverage of trees were the key factors influencing natural regeneration of A. faxoniana. Coverage of moss, stock of logs and density of mother tree (DBH≥16.5) were favorable to natural regeneration of A. faxoniana, while coverage of bamboo and coverage of trees were obstructive to the natural regeneration of A. faxoniana.(4)The results of spatial pattern analysis revealed severe competition between B. albo-sinensis and A. faxoniana. No association was detected among dominant tree species of the primary forest, while the spatial association between B. albo-sinensis and A. faxoniana exhibited significantly negative at all scales. All big trees of B. albo-sinensis and A. faxoniana were spatially clumped at all scales that were examined. Other size-classes were spatially clumped at small scales, while they tended to be randomly or regularly spatially distributed with increasing spatial scale. Their maximum aggregation degree decreased with increasing size class. Intra- and interspecies spatial associations occurred generally at small scales. A. faxoniana generally showed a positive inner-specific association, while B. albo-sinensis generally showed a negative inner-specific association. For the same size-classes of the two populations, no significant spatial association were found for big trees and small trees, but the negative spatial associations were for medium trees. The negative spatial associations of the two populations at the different size-classes were commonly found, the larger difference size-classes, the stronger negative spatial associations.(5)Large size class of remnant trees had strong inpacts on other size class trees. There were significantly negative spatial associations between B. albo-sinensis's large size class of remnant trees and its small and medium trees at the scale of 0~8 m, 0~9 m, respectively. There were significantly negative spatial associations between A. faxoniana's large size class of remnant trees and its seedlings at the scale of 0~8 m. There were significantly positive spatial associations between A. faxoniana's large size class of remnant trees and its saplings and small trees at the scale of 0~11 m, 0~9 m. No association was detected between A. faxoniana's large size class of remnant trees and its medium trees at 0~14 m scales. Spatial association of B. albo-sinensis's large size class of remnant trees with A. faxoniana's seedlings, saplings, small trees and medium trees showed significantly negative at all scale. There were significantly negative spatial associations between A. faxoniana's large size class of remnant trees and B. albo-sinensis's small trees at the scale of 0~4 m. Large size class of remnant trees facilitated the regeneration and growth of A. faxoniana as late successional and dominant tree species, while impeding the regeneration and growth of B. albo-sinensis as a pioneer tree species.