Sampling Damage to Tree Fine Roots: Comparing Air Excavation and Hydropneumatic Elutriation

Abstract

Air excavation is commonly used to expose structural roots of trees, but its suitability for sampling fine roots for research has not been closely examined. We compared damage and root-diameter class distribution in roots sampled by air excavation with roots sampled by hydropneumatic elutriation of soil cores. We collected samples from six different tree species with a range of fine root diameters: Amur corktree (Phellodendron amurense Rupr.), apple [Malus sylvestris var. domestica (Borkh.) Mansf.], river birch (Betula nigra L.), striped maple (Acer pensylvanicum L.), swamp white oak (Quercus bicolor Willd.), and western redcedar (Thuja plicata D. Don). Root-diameter class distributions for each species were the same for samples collected by either air excavation or by elutriation. Median root diameter was greatest for Amur corktree and western redcedar (≈0.5 mm), intermediate in striped maple and oak, and least in river birch and apple (≈0.2 mm). Root damage was primarily due to loss of root tips. Although species varied in their susceptibility to root damage and whether air excavation caused more damage than elutriation, root diameter was not a good predictor of damage during sampling. Air excavation caused ≈26% greater damage to root samples of river birch and western redcedar than did elutriation. Both sampling methods caused equivalent root damage in all other species. Root anatomy influenced susceptibility to damage during sampling. Epifluorescence microscopy revealed a root hypodermis in all species except Amur corktree and western redcedar. Without the mechanical support of this suberized layer, the cortex of Amur corktree was easily stripped from the stele, leading to extensive damage by both sampling methods. Hydropneumatic root elutriation conferred some protection to roots relative to air excavation (in two of six species); final choice of root sampling method must depend upon the requirements of the individual study and characteristics of the site.