Geographic variation in seed mass within and among nine species of Pedicularis (Orobanchaceae): effects of elevation, plant size and seed number per fruit.
We examined geographic variation in mean individual seed mass (MISM) among 38 populations within and across nine Pedicularis species in the eastern Tibetan Plateau, measuring the effects of one extrinsic factor (elevation) and two intrinsic factors (plant size and mean seed number per fruit). Across all populations, elevation is negatively correlated with MISM; within eight of nine species, this pattern is also observed among populations. This relationship, however, is not significant when controlling for variation in plant size or seeds per fruit. High-elevation populations produce smaller plants with more seeds per fruit than low-elevation populations; controlling for these variables eliminates the negative correlation between elevation and MISM. Across all populations, the predicted trade-off between MISM and seeds per fruit is consistently strong, with or without controlling for the effects of plant size. By contrast, the trade-off between MISM and total seasonal fecundity can be detected only when controlling for plant size. Independent of plant size, populations that produce small seeds do not support individuals with particularly low reproductive yield (fecundity × MISM). Accordingly, high-elevation populations exhibit neither lower reproductive yield nor smaller seeds than expected given their lower biomass. Synthesis. In Pedicularis, elevation, plant size and seeds per fruit are all correlated with MISM among populations across species. Elevation is less important, however, than intrinsic factors in determining the MISM of a population; the effect of elevation on MISM disappears when the effects of intrinsic factors are controlled statistically. The observed decline in MISM with increasing elevation is therefore partly mediated by the decline in plant size and partly by an increase in mean seed number per fruit with elevation. Altitudinal variation in MISM across populations or species has been described before, but this is the first study to control for the effect of intrinsic factors simultaneously. This result calls into question the conclusions of studies that have detected geographic variation in MISM without controlling for variation in intrinsic factors.