Untangling the belowground knot.

The most basic properties of whole-tree root systems have remained essentially impossible to determine non-destructively. The alternative, excavation and destructive root sampling, remains the bane of root physiologists’ existence. In this issue, Ellis et al. (2013) have made progress on electrical capacitance, one of the more promising non-destructive techniques to disentangle the belowground maze. Among their several findings, Ellis et al. (2013) demonstrate that the electrical capacitance technique can (i) separate intermingled tree root systems; and (ii) has the potential for estimating root length across orders of magnitude of root system length. Although it remains to be seen whether this second finding is influenced by allometric autocorrelation, Ellis et al. (2013) have raised the bar on field root research by considering how the physics of both the capacitance method and the root processes themselves operate across scales of organism size, and across individual trees in natural or managed ecosystems. Ellis et al. (2013) build on the electrical capacitance and impedance groundwork laid by previous workers in this area (Chloupek 1977, Dalton 1995, van Beem et al. 1998, OzierLafontaine and Bajazet 2005, Čermák et al. 2006, Cao et al. 2011, Urban et al. 2011, Dietrich et al. 2012). Where Ellis et al. (2013) have gone farther is in the range of plant sizes considered, covering two orders of magnitude in plant size, and an unprecedented species (25) and site (7) diversity. Moreover, this study is the first to apply the electrical capacitance technique in forests. Unlike plant hydraulics, in which analogy is made to electrical circuits, there is no analogy to the electrical capacitance method: the tree and soil are the electrical network. Root system electrical capacitance, defined as the ratio of change in electrical charge per voltage change, is measured in this technique, and physically related to or correlated with other root properties of interest, including root mass, area and length. At an abstract level, the concept is straightforward. A capacitor is simply two electrical conductors separated by an insulator (or more accurately, a dielectric material). In the tree root system, the two conductors are the outside and inside of the roots, and the dielectric is some layer within the root itself. Where things get fuzzier is just where we need them to be clearer: the identity of the specific tissue or tissues constituting the dielectric. Dalton (1995) developed a widely used physical model of root system capacitance, which mentions only ‘root tissue’ as the intermediary insulating layer between the outer (soil matrix) and inner (xylem) conductors. Does this insulating layer consist of the entire root cortex, or just the endodermal layer and casparian strip? Does suberization in endodermal cells, or cutin in epidermal cells, affect the dielectric properties of the insulating layer? Do mycorrhizal associations and structures influence electrical capacitance? Or root membrane ion channels and aquaporins? These are some of the details that probably matter (as emphasized by Dalton 1995), but which remain somewhat of a black box, including in Ellis et al. (2013). Notwithstanding these unknowns, Ellis et al. (2013) set out to test (among a rich set of 10 hypotheses) several predictions of Dalton’s model, assuming the same basic concept of roots as cylindrical capacitors, with the cortex as the insulator. Exemplifying the fortuitous use of objects that is rife (and elegant) in tree physiology research, hand chisels served as stem electrodes (Figure 1). Field studies were conducted in seven ecosystems in Australia and Mexico, covering conifers, evergreen broadleaf, and deciduous tropical trees, and even a previously studied bean plant species (Vicia faba L.). Trees ranged to over a century old and 30 m tall. Among key findings of Ellis et al. (2013) was a simple, but potentially powerful one: intermingled root systems can be Commentary