The biotic and abiotic interactions that occur between roots and the soil immediately adjacent to roots (the rhizosphere) are easily the most complex and least understood interactions in plants. The opacity of soil and complex nature of the root/rhizosphere biotic system pose unique challenges to tree biologists studying below-ground biodiversity and root system function. Because of these challenges, our understanding of tree root system (roots and rhizosphere biota) structure and function, and below-ground biodiversity and function in forested ecosystems, is based largely on highly controlled seedling and mesocosm studies. However, to scale from seedlings to mature trees, root system biologists must consider how root growth, development and function change as seedlings mature in situ, and how biodiversity of rhizosphere microorganisms in the field alters tree root system function. As seedlings mature, whole-plant source-sink relationships for carbon and nutrients change, ultimately influencing root system function. As seedlings mature into saplings and trees, their roots will modify the physical, chemical and microbiological characteristics of their soil environment to enhance acquisition of limiting soil resources or to survive soil environmental stresses.
The technological difficulty in monitoring in situ root system growth and function of older trees is illustrated below. In this study, we examined whether fast-growing populations of loblolly pine (Pinus taeda L.) allocate their carbon differently to roots and shoots. Destructive harvests are often necessary to collect tissue samples; however, destructive harvests do not allow root biologists to study the same cohort of roots over time, i.e. to examine how root growth and function change temporally over seasons. The larger woody supportive roots extending from the base of a tree are long-lived (often as old as the tree) and comprise most of the lateral root biomass. However, woody supportive roots account for a small proportion of the tree’s total root length and metabolic carbon demand. In contrast, fine roots comprise only 5-10 percent of total root biomass, yet can account for up to 90 percent of the tree’s total root length and metabolic activity. In many forests, the annual carbon cost for fine root system production and maintenance may account for 30-75 percent of net primary productivity, indicating that fine root system carbon demands may represent one of the largest carbon sinks in forested ecosystems.
