Terrestrial carbon sink predictions, particularly as related to soil organic carbon storage, presents uncertainties going into the future (Todd-Brown et al., 2014). This uncertainty is influenced by plant allocation strategies under environmental changes. Plants allocate the carbon gain harvested by foliage to grow leaves, stems, and roots (Poorter et al., 2012). Currently, much research has been done on the spatial distribution and changing trends of aboveground biomass worldwide based on satellite and field-measured data. However, surveys of belowground biomass are relatively scarce. It is difficult for satellites to detect belowground conditions and site-scale sampling can be time-consuming and destructive. Temporal and spatial variations in global belowground biomass have not been adequately studied. Recently, studies have attempted to integrate data from various datasets and different regions of the world to analyze the underlying mechanistic relationships between environmental factors and vegetation allocation strategies (Ma et al., 2021; Poorter et al., 2012; Qi et al., 2019; Reich et al., 2014).
These studies reveal that the strategies of vegetation allocation are trade-offs between the uptake of light and CO2 by aboveground tissues and the absorption of water and nutrients by belowground roots. In resource-rich environments, where water and nutrients do not limit plant growth, plants tend to allocate less carbon to roots. Adequate water or nutrient access will allow for greater aboveground biomass and reduce the need for roots. On the contrary, belowground biomass increases when belowground resources are scarce. Colder ecosystems such as the Qinghai-Tibet Plateau and higher northern latitudes prefer larger root systems because low temperatures reduce the ability of roots’ nitrogen uptake (Reich et al., 2014). The study of Porter et al., 2011 suggests that nutrients and light are key factors controlling biomass allocation strategies. The fraction of total biomass distributed in leaves was strongest with increasing nutrients and decreased most strongly with increasing light. Other studies have shown that temperature and/or precipitation control plant allocation (Ma et al., 2021; Reich et al., 2014). But all these studies agree that grassland and tundra tended to have the highest RMF (>60%) and forests have the lowest (<30%).
Table 1: Changes in carbon allocation strategies of plants to different stress (Modified from Bonan, 2015)
| Stress | Belowground Allocation | Aboveground allocation |
| Water stress | increased | reduced |
| Nitrogen deficiency | increased | reduced |
| Cold temperature | increased | reduced |
| Lack of light | reduced | increased |
Despite our general understanding of how environmental factors control plant allocation, theories and frameworks for the quantitative prediction of allocation strategies are still scarce. Going forward, on the one hand, it would be very useful to establish a global biomass observation network to enhance the availability and accuracy of both aboveground and belowground data. On the other hand, the mechanistic theories of relationships between vegetation allocation strategies and environmental factors should be further developed. Only then can we improve the accuracy of Earth system models’ terrestrial belowground carbon storage predictions.
Reference
Bonan, G. (2015). Ecological Climatology. Cambridge: Cambridge University Press.
Ma, H., Mo, L., Crowther, T. W., Maynard, D. S., van den Hoogen, J., Stocker, B. D., Terrer, C., & Zohner, C. M. (2021). The global distribution and environmental drivers of aboveground versus belowground plant biomass. Nat Ecol Evol, 5(8), 1110-1122. doi:10.1038/s41559-021-01485-1
Poorter, H., Niklas, K. J., Reich, P. B., Oleksyn, J., Poot, P., & Mommer, L. (2012). Biomass allocation to leaves, stems and roots: meta-analyses of interspecific variation and environmental control. New Phytol, 193(1), 30-50. doi:10.1111/j.1469-8137.2011.03952.x
Qi, Y. L., Wei, W., Chen, C. G., & Chen, L. D. (2019). Plant root-shoot biomass allocation over diverse biomes: A global synthesis. Global Ecology and Conservation, 18. doi:ARTN e00606
10.1016/j.gecco.2019.e00606
Reich, P. B., Luo, Y., Bradford, J. B., Poorter, H., Perry, C. H., & Oleksyn, J. (2014). Temperature drives global patterns in forest biomass distribution in leaves, stems, and roots. Proc Natl Acad Sci U S A, 111(38), 13721-13726. doi:10.1073/pnas.1216053111
Todd-Brown, K. E. O., Randerson, J. T., Hopkins, F., Arora, V., Hajima, T., Jones, C., Shevliakova, E., Tjiputra, J., Volodin, E., Wu, T., Zhang, Q., & Allison, S. D. (2014). Changes in soil organic carbon storage predicted by Earth system models during the 21st century. Biogeosciences, 11(8), 2341-2356. doi:10.5194/bg-11-2341-2014