Evaluation Of Degraded Subalpine Dark Brown Coniferous Forests Based On Ecology In Western Sichuan, China And Implications For Restoration

By adopting the concept of space as a substitute for time, based on bamboo and mosssecondary forests (20a, 30a, 40a, 50a) located the north or northwest aspect at the altitudesbetween 3 100-3 600m in Miyaluo, western Sichuan, China, which came from naturalregeneration when the old-growth bamboo dark brown coniferous forests and old-growth mossdark brown coniferous forests harvested between 1950s and 1980s, division of restorationstages, species diversity, community structure, soil ecohydro-function and the effect factors ofAbies faxoniana natural regeneration were studied. The similar old-growth dark brownconiferous forests with age of 160-200 years old were selected as the reference sites forcomparisons. At last, the restoration degree of different restoration stages was evaluation andsimulation. The results show as follows:(1) During the period of 20-40 years restoration, the secondary regenerated forests weredominated by broad-leaved tree species, such as Betula albo-sinensis. There were two maintypes of the forests including moss-B. albo-sinensis forest and bamboo B. albo-sinensis forest.Through 50 years natural succession, the secondary regenerated forests turned into theconifer/broad-leaved mixed forest dominated by B. albo-sinensis and A. faxoniana. The mainforest types included moss B. albo-sinensis-A.faxoniana forest and bamboo B. albo-sinensis-A.faxoniana forest. The remaining dark brown coniferous forests composed mainly of A.faxoniana without cutting proceeded into the old-growth stage at the age ranging from 160 to200 years old. Their main types were moss A. faxoniana forest and bamboo A. faxonianaforest.(2) Species richness increased significantly when broad-leaved forest restored to conifer/broad-leaved mixed forest, but decreased significantly in the old-growth dark brown coniferousforest. As for the bamboo secondary forest and old-growth dark brown coniferous forest, themaximum value of evenness index and Shannon index were recorded in the conifer/broad-leaved forest, but the minimum value of dominance index of tree layer was recorded in theconifer/broad-leaved forest. As for the moss secondary forest and old-growth dark brownconiferous forest, the maximum value of evenness index and Shannon index were recorded in the broad-leaved forest, but the minimum value of dominance index of tree layer was recordedin the broad-leaved forest. Species richness, evenness index, Shannon index and dominanceindex were not significantly different between bamboo and moss secondary forest.Dominance index was not significantly different among forest restoration stages, while thereverse occurred at the species richness, evenness index and Shannon index. With forest ageincreasing, the value of Bray-Curtis index between secondary forest and old-growth darkbrown coniferous forest had an increasing trend, indicating the similarity increasing in speciescomposition between secondary forest and old-growth dark brown coniferous forest.(3) There was significant difference in density of main dominant tree species ofB.albo-sinensis and A. faxoniana, respectively, both between two forest types and among forestrestoration stages. When broad-leaved forest restored to conifer/broad-leaved mixed forest, thedensity of tree species of B.albo-sinensis decreased, and ultimately disappeared at theold-growth dark brown coniferous forest stages, the reverse occurred at the tree species of A.faxoniana. The stand structure of broad-leaved forest with age of 20-40 years had restored to ca.one third compared with the old-growth dark brown coniferous forest. The stand structure ofconifer/broad-leaved mixed forest with age of 50 years was ca. one second compared withold-growth dark brown coniferous forest. There was similar speed at stand structuredevelopment between bamboo broad-leaved forest and moss one. But the speed of mossconifer/broad-leaved mixed forest structure to old-growth forest was faster than bamboo one.There was significant correlation (negatively) between density of all trees and old-growthindex except other stand structural variables. Proportion of conifer species was the componentmost strongly (positively) correlated to old-growth state index. Thus it is possible thatimproving regeneration of A. faxoniana and enhance the proportion of conifer species andlimiting tree density may speed the development of old-growth state structure.(4) When forest restored to bamboo and moss broad-leaved stage with age of 20-40 yearsold, the value of soil ecohydro-function index was 0.56 and 0.55, respectively. When forestrestored to bamboo and moss conifer/broad-leaved stage with age of 50 years old, the value ofsoil ecohydro-function index was 0.68 and 0.67, respectively. There was not significantdifference on the value of soil ecohydro-function index between two forest types, butsignificant difference among forest restoration stages. Basal area was the component moststrongly (positively) correlated to soil ecohydro-function index. There was significant positivecorrelation between basal area and root density, maximal water holding capacity of moss, litterand soil. There was significant negative correlation between basal area and trees density, andno significant correlation between basal area and combined variable of soil bulk demsity andinfiltration rate. Thus it is possible that through thinning and cultivating big DBH class trees to decrease trees density and increase basal area may accelerate the development of soilecohydro-function.(5) As for the bamboo secondary forest and old-growth dark brown coniferous forest,density of mother tree of A. faxoniana, stock of logs and cover of bamboo were the directeffect factors to A. faxoniana natural regeneration. Among the three effect factors, density ofmother tree and stock of logs make the significantly positive direct effect. However, cover ofbamboo makes the significantly negative direct effect. Thus it is possible that increasingmother tree of A. faxoniana, stock of logs and decreasing cover of bamboo may make for thenatural regeneration of A. faxoniana. Thickness of moss makes the significantly negative directeffect to the natural regeneration of A. faxoniana. Indirectly affected by other effect factors,thickness of moss is significant (positively) correlated to natural regeneration of A. faxoniana.As for the moss secondary forest and old-growth dark brown coniferous forest, cover of shrubmakes the significant negative direct effect to A. faxoniana natural regeneration, and being themost important effect factor. Thickness of moss just makes the significant positive direct effectto A. faxonian seedlings, and indirectly little affected by other effect factors. But thickness ofmoss makes the significant negative direct effect to A. faxonian saplings and small trees, andindirectly more affected by other effect factors. This indicates that moss make for the seedlingsof A.faxonian coming out but make against the development of saplings and small trees.(6) The account formula of restoration degree of community was constructed withBray-Curtis index, old-growth state index and soil ecohydro-function index. With forest ageincreasing, the restoration degree increased and the degradation degree decreased. Therestoration degree at the bamboo and moss broad-leaved forest stage with age of 20-40 yearsold was ca. 0.35, with degradation degree ca. 0.65. The restoration degree at the bamboo andmoss conifer/broad-leaved forest stage with age of 50 years old was ca. 0.46, with degradationdegree ca. 0.54. The restoration speed of moss broad-leaved forest and mossconifer/broad-leaved forest was litter faster than that of the bamboo one, respectively. Thesequence of restoration speed of species composition, community structure and soilecohydro-function as follows: community structure> soil ecohydro-function> speciescomposition. Use the Logistic model to simulate restoration process, after 130 years torestoration, the restoration speed was very slow, with restoration degree below 0.01 per 10 years. At the tine, the restoration degree was 0.9969, which may believe that it has restored tothe reference sites, i.e., the old-growth dark brown coniferous forest.From the results above, implications for accelerating the secondary forests to the referencesites may be by the following ways: Promote the natural regeneration of A. faxonian byman-made measures, so that to increase the similarity, on species composition and old-growthforest state of secondary forest in compare with reference site; On the other hand, cultivatingthe big DBH classed by thinning, and to make for soil ecohydro-function and old-growth foreststate.