Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/19/2013
Publication Date: 11/1/2013
Citation: Mukherjee, J.R., Jones, T.A., Monaco, T.A. 2013. Biomass and defoliation tolerance of 12 populations of Pseudoroegneria spicata at two densities. Rangeland Ecology and Management. 66:706-713.
Interpretive Summary: Bluebunch wheatgrass is widely used as a restoration species in the Intermountain West. However, this species is regarded as being relatively intolerant of grazing. We evaluated 12 released and experimental populations of this grass for their ability to tolerate defoliation at the critical phenological period when reproductive tillers were elongating in the spring. Control and spring-defoliated treatments were compared at two plant densities because low plant density increases tolerance to defoliation. We found that some populations were more defoliation tolerant than others. Populations with greater defoliation tolerance tended to be less productive when undefoliated, but exceptions were noted to this correlation. Two rhizomatous populations (P-3 and P-7) were more defoliation tolerant than expected on the basis of the undefoliated productivity, while older cultivars (Goldar and Anatone) were less defoliation tolerant than expected on this basis. Considerable variation exists for defoliation tolerance in this species, and germplasm may be improved for this trait.
Technical Abstract: Pseudoroegneria spicata (Pursh.) A Love is an important native grass of the rangelands of the Intermountain West, USA and is widely used in this region for restoration applications. High grazing preference, together with high grazing sensitivity, have reduced the abundance of this species, indicating the need for development of grazing-tolerant plant materials. While a genotype may be defoliation tolerant at low density, e.g., in an experimental setting, an effective grazing-tolerant genotype must also display this trait at higher densities resembling those found in natural and restoration settings. We compared 12 restoration plant materials for response to spring defoliation at high (25 plants m-2) and low (8 plants m-2) plant densities in a field experiment. Two consecutive years of spring defoliation reduced shoot biomass 19% compared to the undefoliated control, and this reduction was similar for the two densities examined. Two populations, P-3 and Acc:238, were able to compensate for shoot biomass after two years of spring defoliation, while the remaining 10 populations undercompensated, as is commonly reported for cool-season grasses in arid and semiarid regions. While the correlation between control and defoliated shoot biomass was marginally positive (r2=0.26; P<0.10), we found a stronger negative correlation (r2=0.36; P<0.05) between defoliation tolerance and control shoot-biomass production. This suggests a possible trade-off between growth and defoliation tolerance among populations. Of the four commercially available plant materials in our study, the more recent pre-variety germplasm, P-7, exhibited higher control shoot biomass and higher defoliation tolerance than the older cultivars, Whitmar and Goldar. Anatone germplasm was intermediate but not statistically different from these other plant materials for these two traits.