Pollen limitation of plant reproduction occurs in many plant species, particularly those in patched habitats, but the mechanisms are often unknown. We investigated the relationship among pollen limitation, pollinator visitation and breeding system in salt cedar, i.e., the shrub Tamarix ramosissima (Ledeb), which is one of ecologically important species in arid regions of Northwest China. We observed insect visitations to T. ramosissima and estimated pollen limitation. T. ramosissima was pollinated by at least six native bee species, and we observed that pollinator diversity and composition of the pollinator assemblages could affect pollen limitation. Pollinator visitation differed significantly between habitat patches, but did not differ significantly between patch sizes. In general, pollen limitation appeared to be more severe in fruit set than in seed production and the patches with more pollinator visits were less pollen limited in fruit set. However, pollen limitation in seed production was not significantly correlated with pollinator visitation frequency. We concluded that pollen limitation of T. ramosissima reproduction depended on more than one factor, not just pollinator visits. We also found that out-crossing was dominant and self-pollination just played an assistant role to assure reproduction.
