Abstract

1. Abstract The effects of a fire on aboveground net primary productivity (ANPP) in a southwestern desert grassland were studied. In June, 2003, a prescribed burn was applied to a large area (5,000 ha) of desert grassland and shrubland on the east side of the Sevilleta National Wildlife Refuge. The area was dominated by black grama (Bouteloua eriopoda), blue grama (Bouteloua gracilis), and creosotebush (Larrea tridentata) in pure and mixed stands. The summer growing season following the fire was one of the driest on record (54 mm – mean is 144 mm), and, based on visual inspection, little or no growth occurred on either burned or unburned sites. In the spring of 2004, paired, side-by-side aboveground net primary productivity (ANPP) plots were established in burned and adjacent unburned areas representing the major community types: 1) black grama dominated, 2) a mixture of black and blue grama, and 3) a creosotebush stand with an understory of black grama. The fire did not travel significantly into creosotebush shrublands without grass nor into pure blue grama grasslands, and hence these communities were not sampled. The plots consisted of grids of 40 permanent 1-m2 quadrats at 10-m intervals. They were sampled in the spring and fall of 2004, and in the winter, spring and fall of 2005. Sampling followed the standard Sevilleta ANPP double-sampling protocol in which volume by species was estimated on each quadrat and regressions of weight-to-volume for each species were developed by harvesting the species of various sizes from adjacent areas. While 2003 was dry, both 2004 and 2005 had above average moisture (298 mm and 340 mm – mean is 257 mm) that generated significant growth responses. While the fire generally removed creosotebush canopies, the large majority of plants resprouted from root crowns. Among grasses, black grama mortality was high on burned sites and subsequent production was primarily among bunches that escaped fire. Blue grama was not significantly impacted by the fire and most plants recovered. Overall, on burned plots ANPP was higher in black grama sites, but the relative abundances of the dominants changed following the fire from B. eriopoda to forbs such as Sphaeralcea wrightii and other grasses (Sporobolus spp.) Production in the Creostebush/Black Grama site was still significantly depressed relative to other sites after three years. The timing of the burn at the peak of an 12-month drought may have intensified impacts to production. Indications are that full recovery, if any, may be a decadal process, particularly with respect to black grama and creosotebush communities. Results 2004 2006 • Black Grama Site • Black grama ANPP was significantly reduced on the burned site, primarily as a function of mortality, and there was little recovery through 2005. • In contrast, forb production was significantly higher on the burned site and made up as much as 70% of the ANPP. • As a result, burning resulted in total ANPP that was greater than or equal to that in the unburned plots. Black Grama Prescribed Fire Effects on Above-Ground Net Primary Production in a Southwest Desert GrasslandDouglas Moore, Esteban Muldavin, Scott Collins,and Karen WetherillSevilleta Long Term Ecological Research ProgramBiology Department, University of New Mexico, Albuquerque, NM 87131 Blue Grama/Black Grama • Blue Grama -Black Grama Mixed Site • While black grama ANPP was again significantly lower on the burned site, blue grama productivity was greater than on the unburned plots. • Burning did not result in increased forb production as seen in the Black Grama site • Hence, total ANPP was lower on the burned site than in the unburned site. • Introduction • Fires are thought to significantly impact vegetation community composition and structure in desert grasslands of the Southwest, but their effect on aboveground net primary productivity (ANPP) production has been little studied. • Following a landscape-scale prescribed burn on the Sevilleta National Wildlife Refuge, in June 2003, ANPP monitoring plots were established in burned areas previously dominated by, 1) black grama, 2) a mixture of black grama and blue grama, and 3) creosotebush in a matrix of black grama. Unburned plots of similar pre-burn compositions were established in adjacent sites for comparison. • Here, we evaluate the ANPP responses following the first three growing seasons (through 2005) in terms of the effects on selected dominant species and life forms. • Creosotebush/ Black Grama Site • Creosotebush production was significantly lower on the burned site and attributed mostly to resprouting shrubs. • As with the other sites, black grama ANPP was lower on the burn. • Forb production was only slightly increased on burned plots. • Consequently, total production in the burned site was only about half of that on the unburned site. Creosotebush/Black Grama • Discussion • The effect of fire on total ANPP was different among desert grassland community types. While black grama had significant declines without recovery across all three types in response to fire, this was compensated for by increased forb production on the Black Grama site and increased blue grama production on the mixed Blue Grama-Black Grama site. Yet, there appeared to be no similar compensatory effect on the Creostebush/Black Grama site. • Burn timing is important. The antecedent 12-month severe drought period before the burn in 2003 and the lack of rain in the remainder of the growing season that year may have led to increased mortality for black grama, and possibly even creosotebush, and to intensifying declines in ANPP in two out of the three ecosystem dominants within the burn area. • The sizeable reduction in creosotebush production in the first three growing seasons suggests that fire may be effective for shrub control in this ecosystem, but the lack of compensatory production of either grasses or forbs, along with the creation of more open sites for potential shrub colonization argues for caution in fire use, particularly over the long term. • Precipitation • The burn was conducted at the peak of a severe drought that engulfed the region the previous 12 months. • Summer 2003 was one of the hottest and driest on record and the monsoon rains that did occur arrived late, near the end of the growing season. • 2004 and 2005 were above normal for winter precipitation while summer precipitation was close to normal. • ANPP Sampling • Three grassland community types were sampled with paired burned and unburned plots (Fig. 1B): • Black Grama, burned and unburned • Blue Grama-Black Grama mix, burned and unburned • Creosotebush/Black Grama mix, burned and unburned • Each plot had 40 permanent 1-m2 quadrats either on a 10-m grid or as part of a set of circular rodent-sampling webs (Black Grama Unburned). • Sampling followed standard Sevilleta ANPP double-sampling protocols in which volume by species was estimated on each quadrat and regressions of weight-to-volume for each species were developed by harvesting the species of various sizes from adjacent areas. Positive change in green biomass for each species from one season to the next on each quadrat was considered NPP. • Plots were sampled in the spring and fall of 2004, and in the winter, spring and fall of 2005. In 2003, sampling was not conducted because the lack of rainfall during the growing season precluded any significant production. Methods • Site and Fire Characteristics • A prescribed burn of approximately 5,000 ha of desert grassland was conducted from June 19-22, 2003 on McKenzie Flats on the Sevilleta National Wildlife Refuge in central New Mexico (Fig. 1a). • The mid-June timing mimics when lightning-caused fires are most likely to occur, i.e., the hottest and driest time of the year and when dry-lightning associated with the beginning of the Southwest monsoon provides an ignition source, typically without rain. • Wind condition were moderate to high, and the fire generally moved quickly across the landscape with low residence times, but due to patchy fuels and unexpected transient increases in humidity, much of the total area still did not burn. However, study plots were completely burned. Acknowledgements This research was made possible by NSF Long Term Ecological Research Program and its support of the Sevilleta LTER. We especially thank those people who have helped collect and manage the data: Maya Kapoor, Tessa Edelman Julieta Betinelli, Charity Hall, Yang Xia, Michell Tohmey, Jay McLeod. A special thanks to the U.S Fish and Wildlife Service and especially the Sevilleta National Wildlife Refuge and the care taken by its fire team led by Don Kearny. Figure 1A. 2003 prescribed-burn location on Sevilleta National Wildlife Refuge, NM. Figure 1B. Location of aboveground net primary productivity (ANPP) plots on McKenzie Flats.