Comparing two field protocols to measure individual shrubs’ root density distribution

Purpose: A large fraction of a plant’s biomass is belowground, especially in shrublands that typically occur in episodically water-limited climates. Nonetheless, we have no standardized method to map the distribution of the root density (i.e., biomass per soil volumetric unit) of plant individuals (hereafter, Individual-level Root Density Distribution, IRDD). This type of information is difficult to collect, especially in woody plant communities in natural conditions where roots of different individuals can be highly intermingled. Methods: We assess three methods to map IRDD of field shrubs: soil drilling to extract roots, plant injection with dyes, and microsatellite analysis for individual-level root identification. Using the resulting data, we fitted IRDD models obtaining comparable predictions of the root density of shrubs for each method. Results: The proportion of identified roots was higher using plan injection, but the cost per linked roots was two orders of magnitude higher using microsatellite. Model results show that microsatellite markers had a similar success as compared to plant injection for those plant individuals for which it worked well, but it failed completely for several genotypes or individuals. Conclusions: Core drilling machines and plant injection with dyes of different colors to link root fragments from the sample pool to plant individuals represent an affordable, reliable way to study the foraging behavior of woody plants which roots are highly intermingled.