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Environmental factors have a full impact on species distribution and diversity of forest community.The relationship between environmental factors and species patterns suggests important implications for developing optimal strategies for conservation of species diversity,sustainable managing and utilizing plant resources.Various types of plant communities have been documented in regard to the relationship of environmental factors to species distribution and diversity;however,little attention has been paid to Abies fargesii-Betula utilis mixed forest,the typical plant community functioning critically in soil and water conservation.We investigated characteristics and environmental factors of the Abies fargesii-Betula utilis mixed forest located within an elevation of 2700-3100 m in Taibai Mountain,China from July through September 2009 and July through August 2010.We established 3 transects along elevation gradient,and set up 10 plots(20m×20m,for trees) in each transect.In the corner of each plot,we set up one subplot(10m×10m) for shrubs and herb.We recorded the community characteristics including species name,number,cover,height,tree basal diameter and diameter at breast height(DBH1cm) as well as the environmental factors including elevation,slope,stone cover,soil pH,organic matter,total nitrogen and total phosphorus.Three analysis procedures were carried out: canonical correspondence analysis(CCA) to examine relationship between species distribution and environmental factors,partial CCA to assess the respective importance of environmental factors on species distribution,and generalized additive model(GAM) to fit response of species richness to various environmental factors.The results showed that soil pH,elevation and stone cover had significant effect on species distribution(P 0.05) with the impact intensity being elevation stone cover pH.Nevertheless,the other environmental factors(soil organic matter,total nitrogen,total phosphorus content and slope) had no significant effect(P 0.05).Some species,such as Ribes glaciale,Lonicera webbiana,Rubus amabilis,Chrysosplenium pilosum var.valdepilosum,and Kobresia myosuroides,were associated with low stone cover,acidic and total phosphorus poor-soil;otherwise,Betula utilis,Sabina pingii var.wilsonii,Potentilla glabra,Ajuga ciliate and Ajania variifolia were more prominent in high stone cover,low acidic and total phosphorus rich-soil.Besides,Spiraea mongolica,Pleurospermum heterosciadium,Carex filamentosa,Poa nemoralis and Draba ladyginii var.trichocarpa occurred at a high elevation.However,Lonicera szechuanica,Rubus pileatus,Fragaria gracilis,Viola biflora and Carex capilliformis var.major were found in all plots since all environmental factors had little effects on their distribution.GAM analysis indicated that the major environmental factors associated with species richness were soil pH,stone cover,elevation and total nitrogen content(P 0.01).Species richness increased with increasing pH and decreasing stone cover and elevation,and richness changed intricately with changes of total nitrogen content;in contrast,there were no significant effects on species richness(P 0.05) from soil organic matter,total phosphorus content and slope.Thus,species distribution and richness pattern of Abies fargesii-Betula utilis mixed forest were influenced by a variety of environmental factors in which elevation,stone cover and soil pH played the most important role.

Changes in land use might affect the combined C, N and P stoichiometry in soil. The Grain-to-Green Program (GTGP), which converts low-yield croplands or abandoned lands into forest, shrub, and/or grassland, was the largest land reforestation project in China. This study collected the reported C, N and P contents of soil in GTGP zones to achieve the factors driving the changes in the C:N, C:P and N:P values. The results showed that the annual average precipitation exerted significant effects on the C:P value and on the N:P value became significant 20 years after the change in land use. The annual average temperature was the main factor affecting the C:N value during the first 10 years, while the annual average precipitation strongly affected this value afterwards. In addition, “Redfield-like” interactions between C, N and P in the soil may exist. A linear regression revealed significant positive correlations between the C:N, C:P and N:P values and the restoration age, temperature and precipitation after a change in land use. Therefore large-scale changes in land use under the ‘GTGP’ program might significantly affect the C:N, C:P and N:P ratios in soil.