This article is modified from Perez-Harguindeguy et al (2013). The “New handbook for standardised measurement of plant functional traits worldwide” is a product of and is hosted by Nucleo Diversus (with additional Spanish translation). For more on this and on its context as part of the entire trait handbook visit its primary site Nucleo DiverSus at http://www.nucleodiversus.org/?lang=en
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 RMF 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). 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 Specific root length) 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 Specific root length), 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.
Units, terms, definitions
- RMF – root-mass fraction (synonymous to root-mass ratio, RMR), bounded between 0 and 1
- RSR – root : shoot ratio, is unconstrained and can vary from a tiny to a very large number
Notes and troubleshooting tips
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:
Aerts R, Chapin S III (1999) The mineral nitrition of wild plants revisited: are-evaluation of processes and patterns. Advances in Ecological Research 30, 1-67. doi:10.1016/S0065-2504(08)60016-1
Aerts R, Boot RGA, Van der Aart PJM (1991) The relation between above and belowground biomass allocation patterns and competitive ability. Oecologia 87, 551-559. doi:10.1007/BF00320419
Elberse WTH, Berendse F (1993) A comparative study of the growth and morphology of eight grass species from habitats with different nutrient availabilities. Functional Ecology 7, 223-229. doi:10.2307/2389891
Evans GC (1972) The quantitative analysis of plant growth. Blackwell Scientific Publications: Oxford, UK.
Grime JP (1979) Competition and the struggle for existence. In Population dynamics. (Eds RM Anderson, BD Turner, LR Taylor) pp. 123-139. Blackwell Scientific Publications: Oxford, UK.
Poorter H, Niklas KJ, Reich PB, Oleksyn J, Poot P,MommerL(2012) Tansley review. Biomass allocation to leaves, stems and roots: meta-analyses of interspecific variation and environmental control. New Phytologist 193, 30-50. doi:10.1111/j.1469-8137.2011.03952.x
Reich PB (2002) Root-shoot relations: optimality in acclimation and adaptation or the -Emperor’s new clothes In Plant roots: the hidden half. (Eds Y Waisel, A Eshel, U Kafkafi) pp. 314-338. Marcel Dekker: New York.
Sack L, Grubb PJ, Marañón T (2003) The functional morphology of juvenile plants tolerant of strong summer drought in shaded forest understories in southern Spain. Plant Ecology 168, 139-163. doi:10.1023/A:1024423820136
Veneklaas EJ, Poorter L (1998) Growth and carbon partitioning of tropical tree seedlings in contrasting light environments. In Inherent variation in plant growth. (Eds H Lambers, H Poorter, MMI Van Vuuren) pp. 337-361. Backhuys Publishers: Leiden, The Netherlands.