The allometry of reproductive biomass in response to land use in Tibetan alpine grasslands

1. Reproductive biomass (RB) is a key component of plant life history. Because RB is allometric, it remains unclear whether the observed variations of RB in response to land use are driven only by a size dependency effect or if the partitioning of plant biomass may also shift independently of plant size. This question has not been addressed for all the main component species of a given plant community. 2. Here, we report the results of a 3-year experimental study conducted in Tibetan alpine grass-lands. The above-ground vegetative biomass (VB) and the RB of 32 species were measured during 2 years in control, grazed and fertilized plots. We estimated the allometric slope and intercept for 301 VB-RB bivariates, using type II regression analyses in order to disentangle size-dependent and size-independent effects of treatment on RB. 3. Our results indicate significant changes in the VB-RB allometric slope for 26 out of 102 cases in grazed plots, and 16 out of 88 cases in fertilized plots. Moreover, in cases where slopes were not affected by treatment, a significant change in the allometric intercept was found in 50% of the models. For these cases, intercepts which are indicative of the biomass invested in reproductive parts for a given VB - generally increased in grazed plots and decreased in fertilized plots. 4. Grazing and fertilization triggered a significant decreases or increases in the plant size for species, especially graminoids. However, we found no relationship between changes in allometric coefficients and magnitude of the biomass response. 5. At the interspecific level, the intercept, but not the slope, of the allometric VB-RB relationships was significantly affected by land use. Graminoids and forbs responded similarly to treatment. 6. Synthesis. We conclude that (i) size-dependent effects did not explain all the changes observed in RB following grazing and fertilization in alpine grasslands, and that (ii) changes in allometric intercept explained most of the size-independent variations both at the intra- and interspecific level. We discuss why size-independent changes in RB could be important for plant performance in changing environments.