Above- and belowground biomass allocation in Tibetan grasslands

Optimal partitioning and isometric allocation are two important hypotheses in plant biomass allocation. We tested these two hypotheses at the community level, using field observations from Tibetan grasslands. Qinghai-Tibetan Plateau, China. We investigated allocation between above- and belowground biomass in alpine grasslands and its relationship with environmental factors using data collected from 141 sites across the plateau during 2001-2005. We used reduced major axis (RMA) regression and general linear models (GLM) to perform data analysis. The median values of aboveground biomass (M-A), belowground biomass (M-B), and root:shoot (R:S) ratio in alpine grasslands were 59.7, 330.5 g m(-2), and 5.8, respectively. About 90% of total root biomass occurred in the top 30 cm of soil, with a larger proportion in the alpine meadow than in the alpine steppe (96 versus 86%). As soil nitrogen and soil moisture increased, both M-A and M-B increased, but R:S ratio did not show a significant change. M-A scaled as 0.92 the power of M-B, with 95% confidence intervals of 0.82-1.02. The slope of the isometric relationship between log M-A and log M-B did not differ significantly between alpine steppe and alpine meadow. The isometric relationship was also independent of soil nitrogen and soil moisture. Our results support the isometric allocation hypothesis for the M-A versus M-B relationship in Tibetan grasslands.