Raymond F. Angell

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.

Long-Term Dynamics of Production, Respiration, and Net CO2 Exchange in Two Sagebrush-Steppe Ecosystems

Abstract We present a synthesis of long-term measurements of CO2 exchange in 2 US Intermountain West sagebrush-steppe ecosystems. The locations near Burns, Oregon (1995–2001), and Dubois, Idaho (1996–2001), are part of the AgriFlux Network of the Agricultural Research Service, United States Department of Agriculture. Measurements of net ecosystem CO2 exchange (Fc) during the growing season were continuously recorded at flux towers using the Bowen ratio-energy balance technique. Data were partitioned into gross primary productivity (Pg) and ecosystem respiration (Re) using the light-response function method. Wintertime fluxes were measured during 1999/2000 and 2000/2001 and used to model fluxes in other winters. Comparison of daytime respiration derived from light-response analysis with nighttime tower measurements showed close correlation, with daytime respiration being on the average higher than nighttime respiration. Maxima of Pg and Re at Burns were both 20 g CO2·m−2·d−1 in 1998. Maxima of Pg and Re at Dubois were 37 and 35 g CO2·m−2·d−1, respectively, in 1997. Mean annual gross primary production at Burns was 1 111 (range 475–1 715) g CO2·m−2·y−1 or about 30% lower than that at Dubois (1 602, range 963–2 162 g CO2·m−2·y−1). Across the years, both ecosystems were net sinks for atmospheric CO2 with a mean net ecosystem CO2 exchange of 82 g CO2·m−2·y−1 at Burns and 253 g CO2·m−2·y−1 at Dubois, but on a yearly basis either site could be a C sink or source, mostly depending on precipitation timing and amount. Total annual precipitation is not a good predictor of carbon sequestration across sites. Our results suggest that Fc should be partitioned into Pg and Re components to allow prediction of seasonal and yearly dynamics of CO2 fluxes.

Research ArticlesLong-Term Dynamics of Production, Respiration, and Net CO2 Exchange in Two Sagebrush-Steppe Ecosystems

Abstract We present a synthesis of long-term measurements of CO2 exchange in 2 US Intermountain West sagebrush-steppe ecosystems. The locations near Burns, Oregon (1995–2001), and Dubois, Idaho (1996–2001), are part of the AgriFlux Network of the Agricultural Research Service, United States Department of Agriculture. Measurements of net ecosystem CO2 exchange (Fc) during the growing season were continuously recorded at flux towers using the Bowen ratio-energy balance technique. Data were partitioned into gross primary productivity (Pg) and ecosystem respiration (Re) using the light-response function method. Wintertime fluxes were measured during 1999/2000 and 2000/2001 and used to model fluxes in other winters. Comparison of daytime respiration derived from light-response analysis with nighttime tower measurements showed close correlation, with daytime respiration being on the average higher than nighttime respiration. Maxima of Pg and Re at Burns were both 20 g CO2·m−2·d−1 in 1998. Maxima of Pg and Re at Dubois were 37 and 35 g CO2·m−2·d−1, respectively, in 1997. Mean annual gross primary production at Burns was 1 111 (range 475–1 715) g CO2·m−2·y−1 or about 30% lower than that at Dubois (1 602, range 963–2 162 g CO2·m−2·y−1). Across the years, both ecosystems were net sinks for atmospheric CO2 with a mean net ecosystem CO2 exchange of 82 g CO2·m−2·y−1 at Burns and 253 g CO2·m−2·y−1 at Dubois, but on a yearly basis either site could be a C sink or source, mostly depending on precipitation timing and amount. Total annual precipitation is not a good predictor of carbon sequestration across sites. Our results suggest that Fc should be partitioned into Pg and Re components to allow prediction of seasonal and yearly dynamics of CO2 fluxes.

Variability of crude protein in crested wheatgrass at defined stages of phenology.

Variability of crude protein concentration in crested wheatgrass (Agropyron desrrtorwn (Fisch. ex Link)Schult) is an important consideration in the development of graxing programs. Crude protein (CP) concentration in crested wheatgrass was monitored at specific stages of phenology for 5 years. During that time Sep tember through August precipitation varied from 68 to 142% of the 37-year mean. Vegetation was clipped once at 10 phenological stages beginning in April. At the last clipping date, in mid-August, regrowth accumulated after prior clippings was collected. Over the 5-year period, CP of vegetation clipped during mid vegetative growth in late April varied 35%, relatively, from 14.7% in 1983 to 9.5% during 1985. Even though 1984 crop year precipitation exceeded 1983 by 84 mm, maximum topgrowth biomass was 449 kg/ha lower in 1984. Also, CP percentage of vegetative growth in April was 1.8% lower in 1984 than in 1983. Regrowth CP was positively correlated (r=0.98) with June precipitation, and with the number of rain events in July (r=0.97). Plants clipped in the boot stage had greater forage CP in August than plants clipped prior to boot stage. However, regrowth biomass was affected by soil water availability and was highly variable. Crude protein in vegetative growth was marginal for growing steers in 1985. Although growing stock are often supplemented in late summer, these data are htterpreted to show that spring supplementation may be needed some years.

Clipping date effects on soil water and regrowth in crested wheatgrass.

Although extensive work has evaluated plant response to season of defoliation, few studies have evaluated the influence of season of defoliation on soil water depletion, amount of regrowth, and total seasonal biomass production. This 5-year study evaluated the effect of clipping date and yearly climatic variation on soil water depletion, amount of regrowth, and total seasonal forage production. Timing of clipping significantly (P?0.05) affected soil water depletion patterns. Clipping at the early vegetative stage had little effect on soil water potential unless soil water potentials were below-0.03 MPa. In mid June soils beneath plants defoliated during the boot stage were consistently wetter than soils beneath undefoliated plants. However, total seasonal soil water depletion was usually similar among treatments by the end of the growing season. Phenology and the amount of standing crop present when defoliation occurred were significantly (P?0.05, R2 = 0.877) correlated with regrowth. Date of defoliation also significantly (P?0.05) affected total production in wet years. Total seasonal forage production on plots clipped during the boot stage was generally lower than on plots clipped during the vegetative or late-flowering stages

Precipitation Regulates the Response of Net Ecosystem CO2 Exchange to Environmental Variation on United States Rangelands

Abstract Rangelands occupy 50% of Earths land surface and thus are important in the terrestrial carbon (C) cycle. For rangelands and other terrestrial ecosystems, the balance between photosynthetic uptake of carbon dioxide (CO2) and CO2 loss to respiration varies among years in response to interannual variation in the environment. Variability in CO2 exchange results from interannual differences in 1) environmental variables at a given point in the annual cycle (direct effects of the environment) and in 2) the response of fluxes to a given change in the environment because of interannual changes in biological factors that regulate photosynthesis and respiration (functional change). Functional change is calculated as the contribution of among-year differences in slopes of flux-environment relationships to the total variance in fluxes explained by the environment. Functional change complicates environmental-based predictions of CO2 exchange, yet its causes and contribution to flux variability remain poorly defined. We determine contributions of functional change and direct effects of the environment to interannual variation in net ecosystem exchange of CO2 (NEE) of eight rangeland ecosystems in the western United States (58 site-years of data). We predicted that 1) functional change is correlated with interannual change in precipitation on each rangeland and 2) the contribution of functional change to variance in NEE increases among rangelands as mean precipitation increases. Functional change explained 10–40% of the variance in NEE and accounted for more than twice the variance in fluxes of direct effects of environmental variability for six of the eight ecosystems. Functional change was associated with interannual variation in precipitation on most rangelands but, contrary to prediction, contributed proportionally more to variance in NEE on arid than more mesic ecosystems. Results indicate that we must account for the influence of precipitation on flux-environment relationships if we are to distinguish environmental from management effects on rangeland C balance.

Crested wheatgrass and shrub response to continuous or rotational grazing

A four-year study was conducted to investigate effects of continuous and short duration grazing in spring on standing crop and tiller density of crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult], along with changes in cover and density of Wyoming big sagebrush (Artemisia tridentata Nutt. subsp. wyomingensis Beetle and Young) and green rabbitbrush [Chrysothamnus viscidiflorus (Hook.) Nutt.]. Eight pastures were each stocked with 10 steers (224 kg) beginning in early May. Four grazing treatments consisted of continuous grazing at 0.6 AUM/ha (CONT) or short duration grazing management at 0.6, 0.9, and 1.2 AUM/ha for LOW, MED, and HIGH treatments, respectively. After 4 years, mean tiller density was greatest on LOW paddocks (P=O.lO) (707 tillers/m’). Tiller density on HIGH paddocks did not differ (P>O.O5) from CONT. Density of large (> 15cm tall) Wyoming big sagebrush increased (PrO.05) across years, but did not vary (P>O.O5) among treatments, at about 9 plants/100 m2. Sagebrush plants <15-cm tall responded differently (P=O.O2) in CONT compared to HIGH. Small sagebrush density increased under short duration grazing at doubled stocking rate (HIGH) compared to CONT, but LOW and MED did not differ from CONT. We concluded that short duration rotation grazing at a conventional stocking rate decreased neither tillering nor yield of crested wheatgrass. Shrub density and cover changes on LOW were similar to CONT. It does appear, however, that short duration grazing at the doubled stocking rate has the potential to limit crested wheatgrass productivity over time because of enhanced sagebrush seedling survival.

Diets and Liveweight Changes of Cattle Grazing Fall Burned Gulf Cordgrass

We investigated effects of fall burning of gulf cordgrass (Spurtina spartime) rangeland on winter diets and liveweight gains of cattle on the Texas Coastal Prairie during 1979-1981. Gulf cordgrass dominated steer diets (21-76s) regardless of burning treatment. However, Texas wintergrass (Stipa leucotricha) on adjacent upland sites accounted for 13 to 36% of animal diets during winter growth periods. Burning increased dietary crude protein content from January to March in all years and increased in vitro organic matter digestibility during February and March. Cattle gained or maintained weights on burned pastures but maintained or lost weight on unburned pastures. Weight gains of animals with access to burned gulf cordgrass, but not Texas wintergrass, equaled gains of animals grazing unburned gulfcordgrass and Texas wintergrass. Burned gulf cordgrass can provide alternative green forage that will improve diet quality of cattle when cool-season species are absent. Gulf cordgrass (Spartino spartinae), a perennial warm-season species, provides abundant but poor quality forage for cattle grazing the Texas Coastal Prairie (Gould 1975, McAtee et al. 1979 a, b). Oefinger and Scifres (1977) and McAtee et al. (1979 a,b) determined prescribed burning or shredding could enhance nutritional value of gulf cordgrass without damaging stand integrity. However, it remains uncertain whether burning results in direct benefits to grazing livestock. This study was undertaken to test 3 hypotheses: that gulf cordgrass rangeland burned during fall provides greater dietary crude protein and energy during winter than unburned gulf cordgrass; that fall burning increases the proportion of gulf cordgrass in the diet of free-ranging cattle during the winter; and that cattle grazing fall burned gulf cordgrass rangeland have greater gains in liveweight than cattle grazing unburned areas.

The effect of grazing duration on forage quality and production of meadow foxtail

For the past 50 yr, meadow foxtail (Alopecurus pratensis L.) has been invading native flood meadows throughout the Harney Basin in southeastern Oregon. The expansion of this grass species has been the result of its broad climatic adaptation and ability to withstand drought while thriving in saturated soil conditions for a large part of the growing season. The growth of meadow foxtail starts as soon as adequate soil moisture exists. Managing this early-maturing hay species can prove to be a challenge because soil saturation and elevated water tables make it difficult to harvest hay when forage quality and yield are maximized. The purpose of this study was to evaluate whether planned grazing would retard maturation and thus prolong forage quality. Treatments included a non-grazed control and grazing durations of 2, 4, 6, and 8 wk. Grazing was initiated in May of 1998 and 1999 on six replications of each treatment arranged in a randomized block design. Within each treatment/replication combination, ten 0.2-m...

Effect of seasonal herbage allowance on bolus weight of cattle.

Technology has recently made it possible to count forage boli consumed daily in ruminants. This tec+ique can presently be used to measure grazing time and rates of nutrient intake. However, to determine total daily intake, the ifiuence of inherent variations in bolus weight and associated forage conditions must be known. An attempt was made to determine if degree of uniformity in forage boli weights were statistically nonsignificant to be counted and used as a measure of daily dry matter intake of an animal. Herbage allowances varying from 15.4 to 3.4 kg DM/lOO kg BW/day did not have a significant effect on bolus weights in mature cows grazbg bahiagrass pastures during mid-summer and early winter (PrO.05). Cow size and season of the year also had no significant effect on bolus weight. Bolus weight of the cows averaged 4.4fO.l g across seasons and cows. Much effort has been directed toward relating forage intake of livestock to sward characteristics by measurement of size and rate of biting by the grazing animal (Allden and Whittaker 1970, Stobbs 1975, Chacon et al. 1976, Chaconand Stobbs 1976). Allden and Whittaker (1970) noted that as tiller height increased, size or weight of bite increased and rate of biting decreased. Hence, intake remained static over nonlimiting range forage availabilities. It appears that an animal has a strong tendency to maintain a given level of daily intake by varying bite size and grazing time (Spedding et al. 1966). The end result of these prehensile measurements is the swallowed bolus. Since rate of intake (g/min) by the animal was static in the Allden and Whittaker (1970) study, it would appear that the animal can maintain a static intake by creating boli that have little variation in dry matter weight. Thus, rate of swallowing boli and time spent grazing would determine daily intake of an animal. This hypothesis would imply that the animal gathers forage in its mouth until a “critical” density or weight is attained to form the bolus. The shorter the vegetation the greater time spent forming the bolus. If grazing time does not exceed some inherent behavioral limitation of the animal, intake can be maintained at a stable level under nonlimiting forage conditions by adjusting grazing time. If the forage bolus is uniform across a wide array of forage conditions, then measurements of daily swallowing events of boli should provide a technique which measures daily intake and avoids the fluctuations in size and bite and rate of biting associated with variable forage availabilities. Information to date on swallowed boli is limited to confined animals fed known feed sources (Schalk and Amadon 1928, Bailey 196 I, Gill et al. 1966). Boli in these studies were collected by partial evacuation of the rumen and collection by hand of the swallowed bolus at the cardia in the rumen. Access for collection was provided by a rumen fistula. Bolus weight varied among studies, cows, and feed source. In general, as initial dry matter density of a feed source increased, bolus weight increased. The boli in these studies were reasonably uniform within each feed source and cow. Authors are assistant professor and research technician, Department of Range Science, Texas Agricultural Experiment Station, Texas A&M University System, College Station, respectively. Published with permission of the Director, Texas Agricultural Experiment Station as TA 16095. Received for publication May 5, 1980. JOURNAL OF RANGE MANAGEMENT 35(2), March 1982 Recently, Stoner et al. (1979, 1980) and Stuth et al. (1981) described systems capable of measuring the various swallowing events of cattle, goats, and whitetailed deer. They were able to identify and distinguish among swallowing events of green forage, hay, concentrate feed, water, and regurgitation. In light of these findings, it appears that more information is needed to characterize dynamics of bolus weight under actual grazing situations in order that this information could be used to develop a new technique for determination of intake of animals. It is the purpose of this paper to test the hypothesis that bolus weight is reasonably uniform throughout a nonrestrictive range of herbage allowance and that season has a negligible effect on weight of dry matter in the bolus.