Abstract
There is little comprehensive information on the distribution of root systems among coexisting species, despite the expected importance of those distributions in determining the composition and diversity of plant communities. This gap in knowledge is particularly acute for grasslands, which possess large numbers of species with morphologically indistinguishable roots. In this study we adapted a molecular method, fluorescent fragment length polymorphism, to identify root fragments and determine species root distributions in two grasslands in Yellowstone National Park (YNP). Aboveground biomass was measured, and soil cores (2 cm in diameter) were collected to depths of 40 cm and 90 cm in an upland, dry grassland and a mesic, slope-bottom grassland, respectively, at peak foliar expansion. Cores were subdivided, and species that occurred in each 10-cm interval were identified. The results indicated that the average number of species in 10-cm intervals (31 cm3) throughout the sampled soil profile was 3.9 and 2.8 species at a dry grassland and a mesic grassland, respectively. By contrast, there was an average of 6.7 and 14.1 species per 0.5 m2, determined by the presence of shoot material, at dry and mesic sites, respectively. There was no relationship between soil depth and number of species per 10-cm interval in either grassland, despite the exponential decline of root biomass with soil depth at both sites. There also was no relationship between root frequency (i.e., the percentage of samples in which a species occurred) and soil depth for the vast majority of species at both sites. The preponderance of species were distributed throughout the soil profile at both sites. Assembly analyses indicated that species root occurrences were randomly assorted in all soil intervals at both sites, with the exception that Festuca idahoensis segregated from Artemisia tridentata and Pseudoroegnaria spicata in 10–20 cm soil at the dry grassland. Root frequency throughout the entire sampled soil profile was positively associated with shoot biomass among species. Together these results indicated the importance of large, well-proliferated root systems in establishing aboveground dominance. The findings suggest that spatial belowground segregation of species probably plays a minor role in fostering resource partitioning and species coexistence in these YNP grasslands.
Highlights
IntroductionPlants capture resources and interact with neighbors within the aboveground (sward) and belowground (root) zones that they occupy
As sessile organisms, plants capture resources and interact with neighbors within the aboveground and belowground zones that they occupy
No roots were found in one 50–60 cm core from the mesic site; for most intervals at both sites the most species–depauperate soil volume was occupied by a single species (Table 1)
Summary
Plants capture resources and interact with neighbors within the aboveground (sward) and belowground (root) zones that they occupy. Canopy characteristics, including canopy size, shape, and leaf orientation and density, are relatively easy to measure and have been critical to progress in understanding whole-plant light absorption (Horn 1971, Weiner 1982, Johansson and Keddy 1991, Miller 1994), aboveground intra- and interspecific plant competition (Grime 1977), and plant community assembly and composition (Grime 1977, Givnish 1982, Goldberg and Barton 1992). Manuscript received 6 October 2009; revised 23 February 2010; accepted 12 March 2010.
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