Abstract:

Objective: To provide a theoretical basis for biodiversity conservation and ecosystem restoration,a study of phylogenetic structure of the undergrowth tree,shrub,and herb layers and their driven ecological processes were conducted across different successional forest communities in the subalpine area of western Sichuan. Method: We used inventory data of tree-shrub-grass species compositions of the undergrowth layers in the subalpine forests,which consists of primary dark coniferous forests(hereafter primary forest) and their secondary forests(hereafter secondary forest) after harvesting in the period 1950 to 1969 and 1970 to 1989. We ignored canopy layer because tree species compositions in this layer were relatively simple. A phylogenetic tree of the understory plant species was rebuilt based on the APG Ⅲ phylogenetic framework for different successional forest communities. Two widely selected phylogenetic diversity indices,net relatedness index(NRI) and nearest taxon index(NTI),were used to quantify and assess the phylogenetic structure of different undergrowth layers and their driven ecological processes during forest succession. Result: For primary forest,a phylogenetic over-dispersion pattern(i.e.,NRI < 0; NTI < 0) was shown in the sub-canopy layer,suggesting that distantly related species coexist in this layer. In contrast,a phylogenetic clustering pattern(i.e.,NRI > 0; NTI > 0) was displayed in the other three layers(i.e.,herb,shrub and small tree),which implies that close related species consist of these layers. The two secondary forests formed in different deforestation phases showed a consistent phylogenetic pattern in the herb layer(i.e.,phylogenetic over-dispersion),which was contrary to that in the sub-canopy layer(i.e.,phylogenetic clustering). However,the shrub and small tree layers showed a similar pattern but inversely expressed on the two secondary forests,which were phylogenetic clustering in 1950—1969 regenerated forest and phylogenetic overdispersion in 1970—1989 regenerated forest. The relationships between NRI and NTI of the undergrowth layers were significantly positive for both primary and secondary forests. Pearson correlation analysis showed that Shannon's diversity index(H') and NRI or NTI were neither significant across undergrowth layers nor across different successional forests. However,the local polynomial regression fitting revealed that the correlations between H' and NRI or NTI displayed identical peak-and-trough changing patterns on primary and 1970—1989 secondary forests. Conclusion: The phylogenetic structure is different across undergrowth layers as well as across primary forests and their secondary forests with different deforestation phases. Environmental filtering is a main ecological process influencing species compositions in the herb,shrub and small tree layers,whereas competitive exclusion has a greater force in constructing phylogenetic assembly of the sub-canopy layer. For secondary forest,the phylogenetic structure of undergrowth layers and their underlying ecological processes are mainly opposite to those in primary forest,that is environmental filtering determines phylogenetic structure of the sub-canopy layer and competitive exclusion determines species compositions of the herb layer. Phylogenetic diversity is independent with species diversity for undergrowth layers across different successional forests in the subalpine of western Sichuan.

Key words: net relatedness index and nearest taxon index, niche theory, tree, shrub, herb layers, phylogenetic structure, dark coniferous forest