Using species accumulation curves to ensure adequate sampling effort in root studies

Background/Question/Methods

Roots have long been recognized for their role in allowing plants to meet their resource needs and respond to changing environmental conditions, and roots are now increasingly considered as a critical component when creating terrestrial carbon budgets.  Despite recognition of the importance of below-ground plant biomass, our understanding of roots has long been constrained by difficult, expensive, and time-consuming sampling methods.  Here, we propose a novel application of a long-established ecological statistics tool, species accumulation curves, to determine minimum sampling effort required to capture the full range of root sizes present.   Soil monoliths measuring 8000 cm³(20 x 20 x 20 cm) collected from a loblolly pine plantation in Durham, NC were exhaustively sampled for all roots, and diameters of each segment of each root were analyzed using WinRhizo root analysis software.  Root segments were broken up into distinct diameter classes, and accumulation curves of these diameter classes were created using the vegan package in R statistical software. 

Results/Conclusions

All accumulation curves achieved flattening and approached asymptotes for each soil monolith, indicating that a 8000cm³ sample successfully captures the majority of root diameters present in the soil at this site.  The average sample volume needed to account for 90% of all root diameters present was 3751 cm³ (47% of a monolith).  This volume is substantially larger than the soil volume typically sampled with the most common root-sampling methods, such as minirhizotron imaging and soil cores.  The uneven distribution of root diameters in the soil leads to an inherent risk of skewed data when  insufficient sampling occurs, which is often difficult to detect.  These curves will allow researchers to determine the minimum sample volume needed to capture the entire range of root diameters at a study site and to provide reviewable evidence of sufficient sampling, a tool which has previously been absent from the field of root biology.  Implementation of root diameter-class accumulation curves holds great promise for increasing both efficiency and accuracy in many types of root sampling methodology.