Biased estimates of pollen limitation due to resource reallocation across modular levels in the alpine herb Veratrum grandiflorum

Pollen limitation, a decrease in reproductive success via the female function due to deficient pollen receipt, may influence plant demography and the evolution of sexual systems. Nonetheless, empirical estimation of pollen limitation by pollen supplementation is prone to overestimation due to potential resource reallocation among resource pools within an individual, e.g., among flowers and inflorescences. Yet little empirical research has simultaneously evaluated the effect of resource reallocation across different modular levels within a single species, especially in alpine plants. We estimated pollen limitation at three modular levels (i.e., flower, raceme, and whole-plant) in the perennial alpine herb Veratrum grandiflorum in the eastern Himalayan Hengduan Mountains, using fruit set and seed production as two proxies for female reproductive success. Based on seed production, we found that pollen limitation differed significantly among modular levels, with the highest and lowest values being at the flower and whole-plant levels, respectively; this result points to resource reallocation among modules within individuals. In contrast, fruit set was relatively insensitive to pollen supplementation, so estimates of pollen limitation were low and did not differ among modular levels. These results highlight the importance of multi-modular and multi-metric approaches for accurately assessing pollen limitation, which may often be overestimated at lower modular levels.