Isometric biomass partitioning pattern in forest ecosystems: Evidence from temporal observations during stand development

Knowledge of biomass partitioning is essential for estimating spatial patterns and temporal dynamics of root biomass in terrestrial ecosystems. The isometric hypothesis predicts that aboveground biomass scales isometrically with belowground biomass across both individual plants and community types (i.e. the slope of the log-log relationship between above- and belowground biomass is not significantly different from 1.0), and that the isometric pattern is independent of variation in environmental conditions. However, current evidence primarily comes from observations over space. It is still unknown whether biomass partitioning patterns occurring over time are similar to those observed over space. In this study, we explored biomass partitioning patterns in forest ecosystems over space and time by synthesizing biomass measurements made in 112 stands extracted from 16 age sequences around the world. We characterized biomass partitioning patterns in forest ecosystems using both root: shoot ratio and the relationship between above- and belowground biomass. Data across various individual studies were organized to reflect biomass partitioning patterns over space, while data within each individual study were used to illustrate biomass allocation patterns over time. Our results showed that root:shoot ratio did not exhibit any significant trend with stand age over space. Similarly, root:shoot ratio remained relatively constant over time in 10 out of 16 age sequences. Reduced major axis analysis indicated that the slope of the log-log relationship between above- and belowground biomass did not reveal a significant difference from 1.0 over space, supporting the isometric hypothesis. Likewise, the slope was not statistically different from 1.0 in 10 out of 16 age sequences, nor did it show any significant change with climatic factors and stand age. Thus, the isometric hypothesis also most probably holds true during forest stand development. Synthesis. Our results demonstrate that biomass partitioning patterns occurring over time are consistent with those observed over space, suggesting that belowground biomass dynamics in forest ecosystems may be reliably estimated from aboveground biomass using biomass partitioning patterns generated over space.