Reproductive allometry in Pedicularis species changes with elevation

1. Plants produce biomass and then allocate some of it to reproductive structures, so the relationship between reproductive (R) and vegetative (V) mass is a fundamental aspect of a plants reproductive strategy. 2. Differences among populations or taxa in the allometric relationship between R and V have been attributed to environmental conditions. We hypothesize that populations and taxa living at high elevations should exhibit a lower log R vs. log V slope than those in more benign environments, because the environmental limits on size in alpine environments should favour a relatively large reproductive allocation at smaller sizes and a smaller investment in reproduction per additional unit of biomass accumulated. 3. We investigated variation in the allometric relationship between R and V among 44 naturally occurring populations representing 24 species of Pedicularis in the Tibetan Plateau, to test the hypothesis that the slope of the relationship declines with increasing elevation. 4. There was a significant negative relationship between the slope of the log R vs. log V relationship and elevation among populations, although the relationship among populations within species varied. We interpret this in terms of abiotic limitations on size and decreasing efficiency of resource allocation to reproduction with increasing plant size (measured as vegetative biomass) at high elevations. We also found a significant positive relationship between the gamma-intercept of the regression of log R on log V and elevation across species, but the relationship was not significant among populations within species. The combination of the lower slopes and the higher gamma-intercepts for high-elevation populations means that plants growing at high elevations allocate proportionately more biomass to reproduction at smaller sizes and less at larger sizes than plants growing at lower elevations. 5. Synthesis. The allometric slope (exponent) of the R-V relationship decreases with increasing elevation among Pedicularis populations and species, reflecting fundamental changes in the costs and benefits of increased vegetative biomass with elevation.