A fine root is most commonly defined as a plant root that is two millimeters or less in diameter. Fine roots may function in acquisition of soil resources (eg. nutrients, water) and/or resource transport, making them functionally most analogous to the leaves and twigs in a plant's shoot system. Fine-root traits are variable between species and responsive to environmental conditions. Consequently, fine roots are studied to characterize the resource acquisition strategies and competitive ability of plant species. Categories of fine roots have been developed based on root diameter, position in a root system's branching hierarchy, and primary function. Fine roots are often associated with symbiotic fungi and play a role in many ecosystem processes like nutrient cycles and soil reinforcement.
Overview
Fine roots collectively comprise the majority of total length of a root system in many perennial and annual plants. As they age and develop, their function shifts from primarily acquiring soil resources to transporting materials to other parts of the plant body. The primary function of a fine root can be determined based on its functional characteristics.
The traits of a plant species's fine roots are thought to be indicative of that species's evolved strategy for soil resource acquisition. Certain characteristics of fine-root growth and physiology are highly plastic, however, allowing a plant's roots to respond to the nature of the local soil environment. Fine roots have been shown to respond to soil nutrient patches. Responses include the lengthening of root segments and increased total length of fine roots, increased initiation of lateral roots, and increased branching. The effect of these responses on a plant's nutrient uptake is unclear. In multiple ecosystem types and forest stand ages, fine-root biomass has been found to decrease with increasing soil depth. Similarly, fine-root nitrogen concentration has been shown to be lower in deeper soil. These shifts may reflect vertical changes in the nature of soil, as shallow soils may have greater available nutrient content than deep soils. Features that appear to be lateral branch scars have been observed on fine roots, indicating that some fine roots are deciduous.
Classification
Traditionally, fine roots are defined as plant roots with a diameter of two millimeters or less. This size-based definition is arbitrary, as it does not clearly or logically define fine roots based on anatomy, morphology, physiology, and/or function. Early studies that used this definition assumed that all roots in the two millimeter size class are functionally alike, but many successive studies have shown that roots in this size class can have different demographic and functional traits.
Within the two millimeter size class, roots can be highly variable in characteristics and function. To account for this, root biologists have begun to define subcategories of fine roots based on root diameter, position in the root branching hierarchy, and function.
Diameter-based classes
As a group, fine roots are most consistently defined by the diameter cutoff of two millimeters. In recognition of the variety of root traits and functions within this category and the relationship between diameter and function, smaller diameter classes have been used in recent research. Studies focusing only on roots that function in resource acquisition have examined roots under one millimeter or 0.5 millimeters. Roots with a diameter less than 0.5 millimeters have been termed 'very fine roots'. Because fine-root traits like diameter vary by species, and research examining the function of different root sizes in different species is limited, diameter-based classes of fine roots are mostly arbitrary and complicate cross-species comparisons. For example, two-millimeter-diameter fine roots may occur in trees, but would be very large roots in many annual and perennial species of crops.