The spatial arrangement of tree roots within the soil volume remains largely obscure despite the critical roles that roots play in both tree stability and resource uptake. Countless studies have focused on a plant’s (Doussan et al., 2003; Fitter, 1987; Guo et al., 2011; Pierret et al., 2007) and in some cases on a tree’s root architectural or morphological factors in response to the soil environment (Comas and Eissenstat, 2004; Pregitzer, 2002). Rightly so, root system morphology and spatial distribution influence a tree’s ability to forage for resources and are essential to a tree’s stability, an important factor in landscape management (Richardson-Calfee et al., 2010; Struve, 2009), agroforestry, and silvicultural practices (Coutts, 1983; Danjon et al., 1999).
Variation naturally occurs in closely related species and can be observed in the intrinsic structural development of a species’ root system (Malamy, 2005). Root exploration and space occupation within the soil volume are largely dictated by the root system’s architecture and morphology. These indices include variables such as root length and diameter, vertical and lateral root expansion, and branching structure (Hodge et al., 2009). Within a woody perennial root system, both the coarse woody roots and fine “feeder” root fractions contribute to these indices. Spatial allocation of a tree’s coarse woody roots is widely accepted as a centrally located root mass (Millikin and Bledsoe, 1999; Ouimet et al., 2008) or a shallow plate extending beyond the canopy dripline (Thomas, 2000) despite a lack of observation for most species (Yanai et al., 2008). However, reports on non-woody and fine root placement is highly variable within a three-dimensional soil volume and has resulted in less than consistent results with studies indicating fine root distribution as centrally concentrated (Leuschner et al., 2001; Yanai et al., 2006) or horizontally even (Millikin and Bledsoe, 1999).
A tree’s root system can be classified a number of ways depending on the level of detail required for study. Experiments range from whole root system biomass to anatomical analysis of individual root segments (Guo et al., 2008; Valenzuela-Estrada et al., 2008). Variations in root function are tightly linked to coarse vs. fine root proportions in the simple sense in that coarse roots of larger diameter and higher order provide structural support, whereas finest roots, primarily of first and second order (usually classified as less than 2 mm), function in water and nutrient uptake. Root diameter classification by class is often an approximation (Pregitzer et al., 2002; Valenzuela-Estrada et al., 2008) and does not represent precise root function as once thought (Guo et al., 2008). Nonetheless, root diameter serves as a means to quantify root distribution through space and time (Danjon and Reubens, 2008) and allows for a whole root system analysis that would be time-prohibitive with an anatomical or root order analysis.
Belowground approaches and protocols for studying root development and architecture are confined by methodological challenges of studying tissues embedded in an opaque substrate matrix. Techniques such as rhizotrons, minirhizotrons, root-exclusion tubes, and ground-penetrating radar have dramatically improved our understanding of root growth (Fang et al., 2012). However, improvement in root sampling methodology must bypass the limitation of highly disruptive root excavation, viewing roots on planar surfaces, and resolution restrictions of bulk-imaging techniques. Recent non-destructive in situ methodologies for studying roots and root systems embedded within a medium currently include magnetic resonance imaging, laser, and ultrasound options (see Fang et al., 2012 for a comprehensive review).
High-resolution X-ray CT scanning offers spatiotemporal imaging of root development (Lontoc-Roy et al., 2006; Tracy et al., 2010). Primary complexities with this method include root organ (root branch) visualization resulting from similarities between the attenuation coefficient of root tissue and organic matter (Fang et al., 2012). Studies to date use large-particle substrate types comprised largely of sand (Kaestner et al., 2006; Perret et al., 2007) and small sampling volumes (Flavel et al., 2012; Pierret et al., 2002; Tracy et al., 2010) as a means to optimize root system visualization. Root visualization through X-ray CT has undergone a number of advances since its first inception by Watanabe et al. (1992) including improvements in pixel resolution, signal noise ratios, and programs available for image slice stacking to produce a three-dimensional sample segmentation (Dhont et al., 2010; Perret et al., 2007) and root tracking (Mairhofer et al.,2012). Although these improvements to the field of CT technology are necessary to advance the field, there remains a lack of replicated studies on the development of tree root systems and the use of growing medium that more closely resembles a “natural” soil. Moreover, we are only aware of two studies that used CT technology to examine the change in root systems through time. Tracy et al. (2012) observed tomato (Solanum lycopersicum L.) and Gregory et al. (2003) observed wheat (Triticum aestivum L. cv. Charger). However, both experiments used extremely young plant material on the scale of a few days old, and the experiments also were relatively short-lived (maximum of 10 d).
Our main aim was to use a non-destructive method to visualize and quantify perennial woody root systems in a growing medium. Specifically, we wanted to shed light on species differences in root system development and production through time and space among economically important shade tree species of a caliper that is common within nursery production and typical for planting in landscapes and urban environments (Watson, 2005). Root growth and developmental patterns have clear implications on how we water, fertilize (Danjon et al., 2007), and manage trees in containers and potentially influence on urban and forest tree stability and performance.
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