Marshall R. Haferkamp

Carbon fluxes on North American rangelands

Abstract Rangelands account for almost half of the earths land surface and may play an important role in the global carbon (C) cycle. We studied net ecosystem exchange (NEE) of C on eight North American rangeland sites over a 6-yr period. Management practices and disturbance regimes can influence NEE; for consistency, we compared ungrazed and undisturbed rangelands including four Great Plains sites from Texas to North Dakota, two Southwestern hot desert sites in New Mexico and Arizona, and two Northwestern sagebrush steppe sites in Idaho and Oregon. We used the Bowen ratio-energy balance system for continuous measurements of energy, water vapor, and carbon dioxide (CO2) fluxes at each study site during the measurement period (1996 to 2001 for most sites). Data were processed and screened using standardized procedures, which facilitated across-location comparisons. Although almost any site could be either a sink or source for C depending on yearly weather patterns, five of the eight native rangelands typically were sinks for atmospheric CO2 during the study period. Both sagebrush steppe sites were sinks and three of four Great Plains grasslands were sinks, but the two Southwest hot desert sites were sources of C on an annual basis. Most rangelands were characterized by short periods of high C uptake (2 mo to 3 mo) and long periods of C balance or small respiratory losses of C. Weather patterns during the measurement period strongly influenced conclusions about NEE on any given rangeland site. Droughts tended to limit periods of high C uptake and thus cause even the most productive sites to become sources of C on an annual basis. Our results show that native rangelands are a potentially important terrestrial sink for atmospheric CO2, and maintaining the period of active C uptake will be critical if we are to manage rangelands for C sequestration.

Interactive Effects of Drought and Grazing on Northern Great Plains Rangelands

Abstract Drought is common in rangeland environments and an understanding of its impacts on the structure and function of rangeland ecosystems is paramount for developing effective management strategies. This research was the second of a series of studies investigating the impacts of varying seasonal droughts on northern Great Plains rangelands. Research was conducted on native rangeland during the 1998 through 2001 growing seasons. Study plots were twelve 5 × 10 m nonweighing lysimeters. An automated rainout shelter was used to establish drought conditions on 6 lysimeters during April, May, and June of 1998 and 1999. Single-day, flash grazing events were imposed at the beginning of May, June, and July. Grazing treatments were 1) graze during the 2 years of drought and the year after; 2) graze during the 2 years of drought and rest the year after; and 3) rest all years. Results showed that the intense spring drought reduced soil water content in the upper 30 cm of the soil profile and subsequently reduced total herbage production 20% to 40%; cool-season perennial grasses were the primary contributor to the reduction and cool-season annual grasses were secondary. Periodic grazing during drought had minimal impact on herbage production, whereas impacts on nondrought plots ranged from moderate enhancement to moderate suppression, with effects varying depending on functional group. Results also showed that substantial recovery occurred during the 1st postdrought year, with near full recovery realized within 2 years.

Drought and grazing. I. Effects on quantity of forage produced.

This research addresses the hypothesis that grazing intensity during and following drought can dramatically alter community level, post-drought recovery patterns. Research was conducted during the 1993 through 1996 growing seasons at the Fort Keogh Livestock and Range Research Laboratory located near Miles City, Mont. Study plots were twelve, 5 × 10-m non-weighing lysimeters constructed in 1992 on a gently sloping (4%) clayey range site. An automated rainout shelter was constructed to control the amount of precipitation received on 6 lysimeters during the 1992 growing season. We conclude from study results that the independent and combined effects of the imposed late spring to early fall drought and associated grazing treatments were minimal relative to soil water dynamics and aboveground net primary production although both grazing treatments reduced herbage standing crops. We attribute the absence of a strong response to the drought to its timing (i.e., late growing season) in that most herbage production in these cool-season dominated grasslands is completed by early summer. Thus, annual production processes in these grasslands avoided the major impacts of the drought. The results do not provide convincing evidence, however, that would lead us to completely reject our original hypothesis. Rather, they simply provide evidence that these grasslands are well adapted to surviving late growing season drought with or without intensive grazing by ungulates.

Effects of mechanical treatments and climatic factors on the productivity of Northern Great Plains rangelands.

Impacts of 7 range treatments and climate on late spring herbage standing crops (SC) were measured in rangelands near Miles City, Mont., from 1983 to 1990. Treatments, established in 8 pastures at 2 sites, were: (1) untreated control + season long grazing (SL); (2) soil tillage (ST) + SL; (3) ST + drill seeding legumes (DS) + SL; (4) brush control (BC) + ST + DS + switchback grazing (utilizing 2 pastures); (5) BC + ST + DS + SL; (6) ST + nitrogen fertilization + SL; and (7) contour furrowing (CF) + aerial seeding legumes + SL. Data were analyzed using years as a repeated measure. Treatments increased (p less than or equal to 0.05) total SC 320 kg/ha over controls, but did not affect species/species group composition. Treated pastures produced similar (p greater than or equal to 0.10) SC of 881 kg/ha. Total SC averaged 490 kg/ha more (p less than or equal to 0.05) in 1983, 1986, 1987, 1989, and 1990 than in 1984, 1985, and 1988. Perennial cool-season grass SC was greatest in 1986 (651 kg/ha). Peak annual grass SC (337-506 kg/ha) occurred in 1983 and 1984, the 2 years following ST or CF, and 1989 and 1990, the 2 years following severe drought. Although regression analyses showed fall, winter, and spring precipitation and temperature were closely related to spring SC, less than 50% of the variation in SC was accounted for when precipitation and temperature were summed on a 1-month, 2-month, or 3-month basis. Above-average fall and spring precipitation (September and April) resulted in the greatest total SC. Species composition varied temporally with changing weather conditions and management strategies.

Effects of defoliation, shading and competition on spotted knapweed and bluebunch wheatgrass.

Spotted knapweed (Centaurea maculosa Lam.) is a noxious plant that has invaded many native ranges in the Northern Intermountain Region. Although the use of livestock to control knapweed is intuitively appealing, feasibility of the strategy has received little attention. This greenhouse study was conducted to evaluate response of spotted knapweed to defoliation, light, and competition. Although total knapweed biomass (g/plant) was not altered by defoliation treatments, several of the more severe treatments adversely affected root, crown, and final harvest foliage. Root and crown growth were also adversely affected by increasing competition from bluebunch wheatgrass (Pseudoroegneria spicata). Foliage, root, and crown growth of spotted knapweed increased significantly when plants received full, rather than half light. Spotted knapweed was less sensitive to defoliation than was bluebunch wheatgrass. Although the feasibility of using livestock to control spotted knapweed cannot be completely disregarded, data suggest that the knapweed would have to be selectively and repeatedly grazed during the growing season.

Mineral dynamics in forages of the Northern Great Plains.

Mineral concentrations of range grasses are often below that required by grazing livestock. Limited information is available on forage mineral concentrations for the Northern Great Plains and there is little data on factors influencing concentrations of forage minerals throughout the year. Therefore, a study was conducted to evaluate special and temporal variations in mineral concentrations of major forage species in the Northern Great Plains. Herbage was sampled from 4 replicates on each of 2 soils in July, August, and September 1991, April, June, July, August, and September 1992, and April 1993. Herbage was sorted by species grouping and by live and dead tissue classes. Analyses on herbage included Ca, P, Mg, K, Na, Zn, Cu, Mn, and Mo. For western wheatgrass [Pascopyrum smithii (Rydb.) Love] and annual bromes [Bromus spp.], Zn and all macrominerals except Na were greater in live than in dead tissue. Live tissues of all other species groupings contained greater amounts of P and K than did dead tissue. Live tissue Mg concentrations were greater than dead tissue concentrations for other cool-season and warm season grasses. Manganese concentrations were greater in live than dead annual brome tissues, while Cu was greater in dear than live tissue. Dead sedge tissue contained greater concentrations of Ca than live tissue. Soil type affected several nutrient but this was partially related to soil effects upon composition of species groupings and live:dead ratios. Minerals most likely to be found in quantities less than required for animal production were P, Na, K, Zn, and Cu.

Herbage dynamics on 2 Northern Great Plains range sites.

Quantity and quality of forage produced are primary determinants of level of livestock production derived from grazing lands. Moreover, species composition of herbage is often considered a primary determinant of the ecological condition of rangelands. The broad objective of this study was to quantify the productivity, growth dynamics, and quality of herbage growing on 2 Northern Great Plains range sites and to concurrently relate magnitude and composition of production to the ecological condition of the sites. Using frequent harvest techniques, the 2-year study showed herbage production on the highly productive silty range site averaged 219 g m-2 as compared to 218 g m-2 on the supposedly less productive clay pan range site. The primary reason the clay pan site proved to be as productive as the silty site was attributed to the greater amounts of introduced annual grasses on the clay pan (148 g m-2) than the silty site (104 g m-2). The annual grass component on the clay pan was a near equal mix (71 vs 51 g m-2) of Japanese brome (Bromus japonicus Thunb.) and cheatgrass (B. tectorum L.) whereas the overwhelming dominant on the silty site was cheatgrass (73 g m-2). Western wheatgrass [Pascopyrum smithii Rydh. (Love)] was the dominant perennial grass on both sites averaging 49 g m-2 on the clay pan site and 57 g m-2 on the silty site. There were minimal differences between sites in terms of nutrient quality values (i.e., crude protein, acid detergent fiber, neutral detergent fiber) with results showing clearly that age of tissue was the major factor altering seasonal forage quality values. Range condition analyses revealed the clay pan site was in fair ecological condition and the silty site was in good condition. Study results demonstrate the need for land management agencies to continue to refine productivity estimates as well as adopt new techniques for assessing the ecological condition of rangelands.

Growth Regulator Herbicides Prevent Invasive Annual Grass Seed Production

Abstract Auxinic herbicides, such as 2,4-D and dicamba, that act as plant growth regulators are commonly used for broadleaf weed control in cereal crops (e.g., wheat, barley), grasslands, and noncroplands. If applied at late growth stages, while cereals are developing reproductive parts, the herbicides can reduce seed production. We tested whether growth regulators have this same effect on the invasive annual grass Japanese brome. The herbicides 2,4-D, dicamba, and picloram were applied at typical field use rates to Japanese brome at various growth stages in a greenhouse. Picloram reduced seed production nearly 100% when applied at the internode elongation, boot, or heading stages of growth, whereas dicamba appeared to be slightly less effective and 2,4-D was much less effective. Our results indicate it may be possible to control Japanese brome by using growth regulator herbicides to reduce its seed production, thereby depleting its short-lived seed bank. Nomenclature: 2,4-D; dicamba; picloram; Japanese brome, Bromus japonicus Thunb.; barley, Hordeum vulgare L.; wheat, Triticum aestivum L.

Drought and grazing. IV. Blue grama and western wheatgrass.

An understanding of the impacts of grazing during and following drought on rangeland ecosystems is critical for developing effective drought management strategies. This study was designed to examine the effects of drought and grazing on blue grama [Bouteloua gracilis (H.B.K) Lag. ex Griffiths] and western wheatgrass [Pascopyrum smithii Rydb. (Love)] tiller growth dynamics. Research was conducted from 1993 to 1996 at the Fort Keogh Livestock and Range Research Laboratory located near Miles City, Mont. An automated rainout shelter was used during 1994 to impose a severe late spring to early fall (May to October) drought on 6 of twelve, 5x 10-m non-weighing lysimeters. Twice replicated grazing treatments were: 1) grazed both the year of (1994) and the year after (1995) drought; 2) grazed the year of and rested the year after drought; and 3) no grazing either year. Drought had minimal impact on tiller relative growth rates of plants grazed twice, although it reduced (P s 0.01) rates of axillary tiller emergence for blue grama (79%) and western wheatgrass (91%), respectively. Defoliation periodically increased relative growth rates (P 0.05) and tiller emergence (P 0.01) of both species. Neither drought nor grazing affected tiller densities or tiller replacement rates of either species nor did they affect productivity of blue grama. Drought, however, reduced (P 0.01) productivity of western wheatgrass 50% in 1994 whereas grazing reduced productivity (P 0.01) by 46% in 1994 and 69% in 1995. Moderate stocking levels (40-50% utilization) during and after drought did not adversely affect the sustainability of these dominant native grasses.

Effects of defoliation and competition on total non-structural carbohydrates of spotted knapweed.

Spotted knapweed (Centaurea maculosa Lam.) is replacing native bunchgrasses and interfering with resource management objectives on many ranges in the northern Intermountain Region. Herbicides, biological control agents, and fire have not successfully contained spotted knapweed. Since knapweed is grazed in some situations, effects of defoliation and competition on total nonstructural carbohydrates (TNC) of spotted knapweed plants were determined in this study. Transplanted knapweed plants were grown for 6 months in a greenhouse under 3 levels of competition with bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] Love) and 3 levels of defoliation. Total nonstructural carbohydrates (pool and concentration) were determined at the end of the experiment. Concentration and pools of carbohydrates generally varied among stems, roots, crowns, and with monthly defoliations. Importance of stems for carbohydrate storage was more evident in analyses of pools rather than concentrations. Monthly defoliations decreased carbohydrate concentrations by about 50%, and pools by about 80% respectively within stems, roots, and crowns. While competition from bluebunch wheatgrass influenced total nonstructural carbohydrates concentrations, it did not influence pools. Although frequent defoliations of spotted knapweed reduced carbohydrates, other factors probably limit the feasibility of using grazing animals to control spotted knapweed on native bunchgrass ranges in western North America.