Multiple constraints on ecological restoration: Re-establishment of black grama grass (Bouteloua eriopoda) after intensive grazing

Background/Question/Methods
Climate change may alter potential ecological restoration of semi-arid rangeland. Bouteloua eriopoda (black grama, BOER) is a perennial C4 grass that usually fails to re-establish in degraded rangeland and may be constrained by increased atmospheric CO2 and reduced or seasonally redistributed rainfall. Eighteen contiguous 0.5-ha plots were established in a remnant black grama grassland at the Jornada Experimental Range in the Northern Chihuahuan Desert. Removal of mesquite shrubs (Prosopis glandulosa) and intensive seasonal grazing (none, winter, summer) were applied in a randomized complete block design. From 1996-2000 annual grazing removed 65-85% of the available forage in 24-36 h, a grazing intensity 30X greater than in the previous 40 years. In 2002 the mean percent cover of BOER was 5.82±0.94 in grazed plots compared to 27.3±1.33 in ungrazed plots. By 2005 BOER had not re-established. A pair of circular 1 m diameter, 6.35 mm wire mesh exclosures, were randomly located in each of the 18 plots. Annually from 2005-2007 the treatment exclosure of each pair had non-BOER above-ground biomass removed and viable BOER seed planted and fertilized.
Results/Conclusions
BOER seedlings germinated and established in every treated exclosure in every year (2005 mean 24.4±3.7 SE seedlings/exclosure, 2006 111.8±8.2). Only three BOER plants established in the control exclosures. The long-term (1918-2007) mean annual rainfall was 232 mm, with 320 mm in 2004, 189 mm in 2005, 447 mm in 2006, and 235 mm in 2007. Biomass (g) of BOER in the treated exclosures (mean 154 ±16g SE) was four times greater than in the untreated exclosures (35 ±12g). The biomass of BOER in control exclosures located in untreated plots (115.±32g) was greater than the biomass in control exclosures in intensively grazed plots (21±9g), but less than in treated exclosures (163±21g), suggesting that restoration treatments compensated for prior intensive grazing. In 2007 there were no effects of prior grazing season or shrub removal on the number, biomass or nitrogen concentration of BOER in the treated exclosures. Ambient precipitation and atmospheric CO2 did not prevent re-establishment of BOER. Protection from vertebrate grazing was not sufficient to allow re-establishment of BOER. Rapid restoration of BOER after intensive grazing was successful when viable seed, soil nutrients and protection from plant competition and grazing were supplied. Multiple constraints on re-establishment of species should be excluded before failures of restoration are attributed to climate change.