Theory predicts that plants from nutrient-poor sites should allocate a greater fraction of new biomass to roots and maintain a higher proportional distribution of biomass in roots than in shoots. Distribution of biomass to roots can be simply expressed as the root-mass fraction (RMF, synonymous to root-mass ratio, RMR), identically calculated as the proportion of plant dry mass in roots. Note that a true allocation measurement requires quantifying turnover rates as well as standing distributions, which is labour-intensive and rarely carried out. Allocation and distribution are often used synonymously, and whether this is appropriate or not, we follow this convention herein. The RMF is preferable to the often used root : shoot ratio (RSR), because the RFM is bounded between 0 and 1, and can be immediately interpreted and compared, whereas the RSR is unconstrained and can vary from a tiny to a very large number. Notably, root allocation can be highly plastic across light, nutrient and water supplies. Some patterns can be apparently contradictory, because root allocation can allow both greater foraging below ground, which would be an advantage especially when resources are low, and also greater competition below ground, being an advantage when resources are plentiful. In reviews of experimental studies, including those that take an allometric approach, RMF typically decreases with increasing nitrogen availability. However, other studies have reported that for field plants, fast-growing species adapted to nutrient-rich habitats showed higher allocation to roots than did slow-growing species from nutrient-poor sites. Similarly, seedlings showing plastic responses to low light typically decrease their RMF, whereas plants adapted to chronic deep shade in rainforests tend to have higher RMF, apparently to survive periods of low water and nutrient supply in competition with surrounding trees. Note that some reports of differences in RMF across resource gradients are potentially confounded by failure to account for allometry and size (see References on theory, significance and large databases below in the present Section). Additionally, RMF does not directly translate to a high soil resource-uptake rate. Lower allocation to roots may well be compensated by higher specific root length (see Section 5.1) and by higher uptake rate per allocation to root mass, length or surface area.
The RMF can best be used for comparative purposes if measured for plants of similar mass. Alternatively, if plants are harvested of a range of mass, allometries can be used to estimate RMF for plants of a given size.
Care should be taken to harvest all the roots (see Section 5), despite the difficulty of separating roots from soil, particularly fine roots. However, in field studies, sometimes RMF includes only a subset of all below-ground tissues; in such a case, the researcher should be clear about what is included and what is not.
Special cases or extras
(1) Storage organs and root fractioning. RMF should in theory include everything that is plant-developed (so not including mycorrhizae!). However, particular studies can subdivide specific fractions for specific purposes (i.e. fine roots, coarse roots, crowns, rhizomes (for grasses), tap roots (in trees)) to evaluate the relative proportions of each in relation to each other and/or to above-ground biomass.
References on theory, significance and large databases: Evans (1972); Grime (1979); Aerts et al. (1991); Elberse and Berendse (1993); Veneklaas and Poorter (1998); Aerts and Chapin (2000); Reich (2002); Sack et al. (2003); Poorter et al. (2012).