Spatial influence of the Asian Summer Monsoon on pollen assemblages of the Tibetan Plateau and its potential implication for the interpretation of fossil pollen records

The direction and intensity of Asian Summer Monsoon (ASM) circulation in the Tibetan Plateau (TP) region strongly influence pollen dispersal patterns; however, this spatial effect has been rarely quantified. In this paper, we explore the relative importance of this directional spatial effect. Specifically, we construct a wind connec-tivity diagram for modern pollen sample sites according to the prevailing monsoon direction, and extract latent spatial variables representing the directional process of the ASM, using the method of asymmetric eigenvector maps. We fit redundancy analysis models using both spatial variables and local climate factors, and conduct variation partitioning to quantify the effect of directional spatial effect by the ASM. Taking into account the joint effect with local climate, the directional spatial process induced by the ASM significantly explains the variation of pollen assemblages on the TP. The effect is detected in pollen groups with contrasting functional types and dispersal modes; for example, the composition of arboreal pollen and non-arboreal pollen are both significantly affected by the ASM, yet best explained by models assuming different monsoon directions. Anemophilous pollen shows a higher percentage of variation explained by the ASM than non-anemophilous pollen. Our results indicate that directional processes induced by the monsoon is an important influence on pollen assemblages in the TP and support the idea that deposition patterns of pollen, especially arboreal pollen is a useful proxy for studying monsoon evolution. A similar approach can also be applied to paleomonsoon simulations using fossil pollen records. The role of directional dispersal in affecting fossil pollen composition and abundance by prevailing winds deserves further investigation when undertaking Quaternary paleoenvironmental reconstructions using pollen.