Aboveground productivity and root-shoot allocation differ between native and introduced grass species

Plant species in grasslands are often separated into groups (C-4 and C-3 grasses, and forbs) with presumed links to ecosystem functioning. Each of these in turn can be separated into native and introduced (i.e., exotic) species. Although numerous studies have compared plant traits between the traditional groups of grasses and forbs, fewer have compared native versus introduced species. Introduced grass species, which were often introduced to prevent erosion or to improve grazing opportunities, have become common or even dominant species in grasslands. By virtue of their abundances, introduced species may alter ecosystems if they differ from natives in growth and allocation patterns. Introduced grasses were probably selected nonrandomly from the source population for forage (aboveground) productivity. Based on this expectation, aboveground production is predicted to be greater and root mass fraction to be smaller in introduced than native species. We compared root and shoot distribution and tissue quality between introduced and native C-4 grass species in the Blackland Prairie region of Central Texas, USA, and then compared differences to the more well-studied divergence between C-4 grasses and forbs. Comparisons were made in experimental monocultures planted with equal-sized transplants on a common soil type and at the same density. Aboveground productivity and C:N ratios were higher, on average, in native grasses than in native forbs, as expected. Native and introduced grasses had comparable amounts of shallow root biomass and tissue C:N ratios. However, aboveground productivity and total N were lower and deep root biomass and root mass fraction were greater in native than introduced grasses. These differences in average biomass distribution and N could be important to ecosystems in cases where native and introduced grasses have been exchanged. Our results indicate that native-introduced status may be important when interpreting species effects on grassland processes like productivity and plant N accumulation.