Fine Root Architecture of Nine North American Trees

The fine roots of trees are concentrated on lateral branches that arise from perennial roots. They are important in the acquisition of water and essential nutrients, and at the ecosystem level, they make a significant contribution to biogeochemical cycling. Fine roots have often been studied according to arbitrary size classes, e.g., all roots less than 1 or 2 mm in diameter. Because of the size class approach, the position of an individual root on the complex lateral branching system has often been ignored, and relationships between the form of the branching root system and its function are poorly understood. The fine roots of both gymnosperms and angiosperms, which formed ectomycorrhizae (EM) and arbuscular mycorrhizae (AM) fungal associations, were sampled in 1998 and 1999. Study sites were chosen to encompass a wide variety of environments in four regions of North America. Intact lateral branches were collected from each species and 18 561 individual roots were dissected by order, with distal roots numbered as first-order roots. This scheme is similar to the one commonly used to number the order of streams. Fine root diameter, length, specific root length (SRL; m/g), and nitrogen (N) concentration of nine North American tree species (Acer saccharum, Juniperus monosperma, Liriodendron tulipifera, Picea glauca, Pinus edulis, Pinus elliottii, Pinus resinosa, Populus balsamifera, and Quercus alba) were then compared and contrasted. Lateral roots 75% of the total number and length of individual roots sampled in all species except Liriodendron tulipifera. Both SRL and N concentration decreased with increasing root order in all nine species, and this pattern appears to be universal in all temperate and boreal trees. Nitrogen concentrations ranged from 8.5 to 30.9 g/kg and were highest in the first-order “root tips.” On a mass basis, first-order roots are expensive to maintain per unit time (high tissue N concentration). Tissue N appears to be a key factor in understanding the C cost of maintaining first- and second-order roots, which dominate the display of absorbing root length. There were many significant differences among species in diameter, length, SRL, and N concentration. For example, two different species can have similar SRL but very different tissue N concentrations. Our findings run contrary to the common idea that all roots of a given size class function the same way and that a common size class for fine roots works well for all species. Interestingly, fine root lateral branches are apparently deciduous, with a distinct lateral branch scar. The position of an individual root on the branching root system appears to be important in understanding the function of fine roots.