Arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) associations are critical for host-tree performance. However, how mycorrhizal associations correlate with the latitudinal tree beta-diversity remains untested. Using a global dataset of 45 forest plots representing 2,804,270 trees across 3840 species, we test how AM and EcM trees contribute to total beta-diversity and its components (turnover and nestedness) of all trees. We find AM rather than EcM trees predominantly contribute to decreasing total beta-diversity and turnover and increasing nestedness with increasing latitude, probably because wide distributions of EcM trees do not generate strong compositional differences among localities. Environmental variables, especially temperature and precipitation, are strongly correlated with beta-diversity patterns for both AM trees and all trees rather than EcM trees. Results support our hypotheses that latitudinal beta-diversity patterns and environmental effects on these patterns are highly dependent on mycorrhizal types. Our findings highlight the importance of AM-dominated forests for conserving global forest biodiversity.

Laboratory and greenhouse studies were conducted to determine the effect of several environmental factors on seed germination and seedling emergence of Crofton weed. Seeds germinated over a range of 10–30 C, with optimum germination at 25 C. High temperature markedly restricted germination, with no germination occurring at 35 C. Crofton weed was moderately photoblastic, with 17% germination occurring in the dark. Crofton weed germinated in a narrow range of pH (5–7). Maximum germination (94%) was observed in distilled water at pH 5.7. Germination was totally inhibited at osmotic stress higher than −0.7 MPa. Germination was greater than 65% at less than 100 mM NaCl, with no germination at 300 mM NaCl. Maximum emergence occurred when seeds were planted on the soil surface. No seedlings emerged when seeds were planted 1.5 cm deep. These results suggest that the future range of Crofton weed in China will be restricted largely to the Yunnan-Guizhou Plateau, which includes major parts of Yunnan and Guizhou provinces, the southwestern part of Sichuan province, and the western part of Guangxi province. Crofton weed also tends to be a sporadic problem in other regions, where the climatic and edaphic conditions are suitable for the seed germination.