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

Authors: WILSEY, BRIAN - IOWA STATE UNIVERSITY; Polley, Herbert

Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/13/2006
Publication Date: 8/23/2006
Citation: Wilsey, B.J., Polley, H.W. 2006. Aboveground productivity and root-shoot allocation differ between native and introduced grass species. Oecologia 150:300-309.

Interpretive Summary: Exotic grasses have been introduced to improve grazing opportunities, but have become the dominant plant species on many grasslands. By virtue of their abundances, exotic grasses may alter productivity (biomass accumulation rates) and rates of nitrogen (N) accumulation by grassland vegetation if they differ consistently from native grasses in growth rates or in the distribution of biomass between roots and shoots. We tested the prediction that rates of aboveground biomass and N accumulation are greater and the ratio of root biomass to shoot biomass is smaller in exotic than native warm season grasses common to central Texas with experimental plots planted to individual grass species (5 species of native grasses and 3 species of exotic grasses). The amount of root biomass to 20 cm depth and tissue N concentrations were comparable between native and exotic grasses. On average, however, the productivity and total N content of aboveground tissues were smaller and root:shoot ratios and the amount of root biomass in deeper soil layers (20 to 45 cm depth) were greater in native than exotic grasses. Because native and exotic grasses differ in biomass distribution patterns and total N content, the two groups of grasses likely impact aboveground and belowground process rates on grasslands differently. Forage production for cattle, for example, may be greater among the generally more productive exotic grasses than native species. Because they produce fewer deep roots, however, exotic grasses also may accumulate less carbon in soil and therefore be less effective than native grasses in slowing the rise of atmospheric carbon dioxide and the associated increase in Earth’s temperature. Our results indicate that it may not be correct to assume that, as a group, native and exotic grasses function similarly.

Technical Abstract: Plant species in temperate grasslands are often separated into groups (C4 and C3 grasses, non-leguminous forbs, and legumes) with presumed links to ecosystem functioning. Each of these groups in turn can be separated into native and exotic (i.e. introduced) species. Although numerous studies have compared plant traits between the traditional groups of grasses and forbs, fewer have compared native vs. exotic species. Exotic grass species, which often are introduced to improve grazing opportunities, have become common and sometimes are the dominant species of grasslands. By virtue of their abundances, exotic species may alter ecosystem process rates if they differ from native grasses in growth and allocation patterns. Introduced grass species were probably selected for forage (aboveground) productivity and N accumulation. We hypothesize that aboveground production and N accumulation will be greater and root:shoot ratios will be smaller in introduced than native species. We tested this hypothesis by comparing root and shoot distribution and tissue quality between exotic and native C4 grass species of Central Texas, and then compared these differences to the more well studied divergence in growth between C4 grasses and forbs. Comparisons were made over two growing seasons in experimental monocultures planted with equal-sized transplants on a common soil type and planting density. Aboveground productivity and C:N ratios were higher, on average, in native grasses than in native forbs, as expected. Native and exotic grasses had comparable amounts of shallow root biomass and tissue C:N ratios. However, on average, aboveground total N and productivity were lower and deep root biomass and root:shoot ratios were greater in native C4 grasses than exotic C4 grasses. These differences in average biomass distribution and N could be important to ecosystem process rates in cases where native and exotic grasses have been exchanged. Differences in deep root growth, for example, could affect C storage by influencing the amount of C entering soil pools with longer turnover times. Our results indicate that it may be incorrect to assume that native and exotic species as a group are functionally similar.