Distribution of mycorrhiza on competing pines and weeds in a southern pine plantation

Mycorrhiza are important in the uptake of nutrients by plants, yet little is known about the distribution and function of the diverse fungal symbionts associated with competing plant species. The objective of our research was to investigate effects of weed competition on mycorrhizal development in a slash pine (Pinus elliottii Engelm. var. elliottii) plantation in order to better understand the impact of arbuscular mycorrhizal (AM) weeds on ectomycorrhizal (EM) pines. Seedlings were planted in a typical Spodosol and samples were collected to a depth of 100 cm from weed-free (herbicide-treated) and weedy plots in the first and third growing seasons after outplanting. Pine roots were separated from weed roots before samples were assessed for fine root (<3.0-mm diam.) length, surface area, and mycorrhizal status. Fine root length and surface area, as well as EM surface area, of pine generally decreased with depth and increased between Years 1 and 3. In contrast, fine root length of weedy species was fairly constant during the 3-yr period. The AM colonization of weeds followed a trend similar to that of fine root development. The depth distribution of EM pine roots and AM weedy roots exhibited complex interactions with treatment, time, and depth. There was generally greater mycorrhizal colonization of both AM on weedy species and EM on pine in the A soil horizon. Weed competition significantly affected the number of EM root tips and the distribution of EM morphotypes. Thelephora was generally the most abundant morphotype, followed by a white, rhizomorph-producing type (White Rhiz) and Cenococcum. An exception to this pattern was found in the near-surface soil (0-10 cm) of the weed-free plots, where White Rhiz was the most frequent morphotype. A challenge for the future is to evaluate the functional significance of changing patterns of EM in the field.