Raices De Palto

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Blackwell Science, Ltd

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Fine roots of trees – a new perspective
The fine roots of perennial plants are a royal pain to study. They seem to be everywhere in the soil, penetrating the rotting leaf litter, wedged between stones and proliferating in worm casts. Millions of lateral branches serve as the carbon depot for mycorrhizal hyphae that ramify away from the root tip toforage widely for essential nutrients. For the sake of convenience, and to promote sanity, this complex network has often been sampled in ways that fail to relate the structure of the intact system to resource acquisition – position and form of the individual root on the branching fine root system have typically been ignored. The traditional view is therefore that all the fine roots of trees can betreated as a coherent mass. But this view now seems deeply flawed. On pages 000–000, King et al. (2002) report on findings in loblolly pine that not only consolidate this view, but also demand a new perspective.

Fig. 1 Image of a very small portion of the fine root system of black spruce (Picea mariana). These intact roots were carefully excavated from the forest litter layer in Alaska (BonanzaCreek Long-term Ecological Research Site, 64°40′-N, 148°15′-W), cleaned of organic matter, and a composite digital image created in Adobe Photoshop (Version 5.5, San Jose, CA, USA) using a Zeiss steromicroscope (Semi 2000-C, Carl Zeiss, Thornwood, New York, USA). All the roots in this image are < 1.0 mm in diameter. 80% of the individual roots in this image are < 0.3 mm in diameter and < 3–4 mm inlength.

‘One of the most remarkable gaps in our knowledge is that we still do not know which fine roots on the branching root system die and what controls the mortality of individual roots’
arise at variable distances from the apical meristem in the pericycle of the parent root, grow through the cortex, and form complex lateral branches. These lateral fine root branches are both morphologicallyand physiologically responsive to changes in water and nutrient availability (Eissenstat & Caldwell, 1988; Pregitzer et al., 1993; Bilbrough & Caldwell, 1997), and the distal root tips are the primary infection point for mycorrhizas.

What are fine roots?
The woody roots of mature trees are widely distributed, with horizontal roots exploring the soil for distances greater than 20 m from the treetrunk and woody tap roots reaching soil depths that exceed 10 m. These woody roots serve anchorage, transport and storage functions and they are the perennial outposts in a vast volume of soil that enable the tree to solve a key ecological challenge: the rapid acquisition of growth limiting water and nutrients. The acquisition of essential resources from the highly heterogeneous soil volume isperformed by small absorbing roots (‘fine roots’) and their mycorrhizal associates. Lateral fine roots of trees

From the pick and weigh legacy to a new perspective
The branching fine root system can be highly complex (Fig. 1), and it is this complexity that has driven traditional sampling procedures. In order to determine biomass, for example, the roots are often sampled destructively, sorted intoarbitrary size classes (e.g. roots 0–1 or 0–2 mm in diameter), dried and then weighed. Other studies of root growth and function have used line intersection methods to quantify root response, traced root growth along glass plates, utilized minirhizotrons, or extracted roots from the soil, cleansed them of rhizosphere complexity and determined their length using image analysis. In other words, every© New Phytologist (2002) 154: 267– 273 www.newphytologist.com

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Commentary

segment of the root system in Fig. 1 has often been assumed to have the same form and the same function. There is now a growing body of evidence that suggests the form of the branching fine root system of trees is directly related to its function. First, smaller diameter loblolly pine roots had...