5.2. Root-system morphology

Characteristics of entire root systems can be independent of individual roots and need to be measured explicitly. There are three main traits of root systems that are best measured: depth, lateral extent, and intensity of exploration. For rooting depth, the simplest metric to determine is maximum rooting depth (maximum soil depth from which resources can be acquired, ranging from a few centimetres to tens of metres). The maximum lateral extent of roots defines the distance from the centre of the plant that roots can acquire resources from. It also determines the ability of plants to interact with spatial heterogeneity in soil resources. The amount of fine-root biomass or root length per unit soil volume indicate the intensity of soil exploration, and the ability of a species to compete for soil nutrients.

The depth distribution of roots combines depth and intensity of utilisation of soil. Depth distributions are a better indicator of the relative reliance of plants on different depths for soil resources and define their vertical distribution of influence on soil activity. In general, it is simpler to determine the root biomass with different depths, whereas understanding root length with depth is likely to be a better metric to understand the competitive ability of uptake capacity, for example. In general, biomass and length with depth would be strongly correlated if there were no change in SRL with depth.

Note that root tissue density and root diameter are positively related to longevity and negatively related to nutrient uptake. In addition, root tissue density is positively related to resistance to pathogens and drought.

Collection and analysis

Determining the maximum extent of roots depends on the species. Excavating entire plants is reasonable for some shallow-rooted species. For more deep-rooted species, a pit must be dug and a cross-section of the soil from a pit face excavated. In some extreme cases, roots have to be accessed from caves or boreholes.

Depth distributions can be determined by digging pits if a known cross-sectional area can be excavated with depth. With pits, a deep pit is dug and one pit face is smoothed vertically. Then a cross-section is removed with a flat shovel. Root systems can be removed in entirety or in sections. In other cases, an auger of 5–10-cm diameter should be used to remove biomass with depth. Typical depth distributions follow a somewhat exponential relationship. A standard set of depths would be 5, 10, 20, 40, 80, 120 and 200 cm for root systems largely confined to the top 2 m of soil. Incomplete root-depth distributions can be used to estimate maximum rooting depths; however, this will depend on the pattern of root biomass with depth. For some species, depth distributions can be determined randomly relative to the individual, or at a point that represents the midpoint of its lateral extent, whereas biomass will have to be determined directly below individuals for species with a tap root. To determine lateral extent of root systems, a horizontal strip of soil can be excavated, starting at the centre of the plant, so as to trace roots outward. In other cases, where individuals are bunched, the lateral extent of roots is likely to be equivalent to half the interplant distance, although this should be verified.

Once soils have been excavated, root storage and washing use the same protocols as described above. Intact root systems are best laid out on a large mesh screen, to be washed out with running water, and/or submerged in large tubs. Confirming the identity of species might require anatomical or molecular comparisons with other roots of that species; however, it is most easily carried out by tracing roots back to their above-ground parts or sampling in conspecific stands. If depth distributions are to be determined, fine (<2 mm) and coarse roots should be separated. Subsamples of cleaned fine roots can then be scanned, if desired, for diameter, length and volume analyses. Regardless, root biomass should be dried and weighed for biomass distributions.

Special cases or extras

(1) Large shrubs and trees. When sampling larger shrubs and trees, the researcher will encounter thicker woody roots. The best way to deal with this is to use a specialised wood-cutting auger. Within the coarse root fraction, those root sections that are obviously particularly important for mechanical support or resource storage, usually exceeding 10 mm in diameter, are best kept separate from the relatively thin sections. They can still be combined for certain analyses later on.

(2) Clayey soil. If the soil is particularly clayey, aggregated, or contains calcium carbonate, consider adding a dispersal agent (e.g. sodium hexametaphosphate) to the washing water. The best washing additive varies depending on the particular condition of the soil.
References on theory and significance: Adiku et al. (2000); Zwieniecki et al. (2002); Hodge (2004); Schenk and Jackson (2002); Dunbabin et al. (2004); Withington et al. (2006); Craine (2009); Lambers et al. (2011).

More on methods: Böhm (1979); Caldwell and Virginia (1989); Jackson (1999); Linder et al. (2000); Schenk and Jackson (2002).