Fires were once a natural part of most savanna ecosystems, but lack of fine fuel and an active suppression policy have changed fire frequency and seasonality. Re-introducing fires to these systems has been touted as a cost-effective means to reduce woody cover while increasing herbaceous growth. We compared the effects of single, recurring annual, and biennial fires on the vegetation dynamics of a mixed mesquite/acacia (Prosopis/Acacia) savanna in southern Texas. Fires were conducted either during the growing season or the dormant season from 1991 through 1995. Some of the fire treatments were statistically unreplicated to permit a sufficient plot size for natural fire behavior. All plots showed a successional trajectory from short-grasses towards mid-grasses regardless of the fire season or frequency. The population size structure of perennial grasses was unaffected by the fires, with the basal area of most plants being less than 25 cm2. Forb diversity was high with over 100 different species identified in the experimental area. Forb composition, however, was unaffected by either the season or frequency of fire, but was related to the year of observation. Shrubs in this community typically sprout after disturbance, so little mortality was expected. The only observed mortality was of small shrubs or saplings (diameter < 3 cm) and of large trees which had woodrat (Neotoma micropus) nests at the base. Reintroducing growing season fires into mesquite/mixed acacia shrublands did not conclusively alter plant community composition. Burning during the growing season when environmental conditions were hotter and drier did not accelerate succession toward grass-dominated communities.

Precise estimates of diet composition are useful to assess herbivores impact on rangelands and to make management decisions. Since the variability within- and between-samples affect precision of estimates on diet studies, we studied this variability in diets of the rodent vizcacha (Lagostomus maximus Blainv.). We analyzed fecal pellets using a microhistological technique and we estimated the number of samples and subsamples required to achieve given confidence levels. Diets of this herbivore, which is thought to compete with cattle for forage, were studied in November 1994, May, July, and October 1995 in a mixed shrub-grassland community of the southern caldénal in central Argentina. Most grasses, the main components of the diets (> 80%), were estimated with high precision (confidence interval: CI = 10%, p = 0.05) by observing 14 samples and 5 slides per sample. Forbs (5-6%) and shrubs (12%) were estimated with this same number of samples and slides, but yielded a lower level of precision (CI = 20%, p = 0.10). Although our results may not be directly applicable to other vegetation or herbivores, the procedures may be used in other situations to improve precision of diet estimates through microhistological analysis of feces.

Accurate and efficient monitoring of habitat structure on rangelands is important for understanding wildlife responses to land management practices. Unfortunately, studies of wildlife responses to changes in habitat structure often use monitoring techniques that fail to measure variation in multiple structural dimensions. Our objectives were to evaluate relationships between measures of habitat structure in a shrubland community and to discuss the usefulness of several techniques in integrating multiple structural dimensions into a single index of habitat structure. We evaluated relationships between shrub cover, herbaceous cover, shrub patch number, average shrub patch size, average vegetation height, visual obstruction across multiple strata of a profile board, cone of vulnerability, and angle of obstruction using a principle component analysis. Many of these variables were redundant with each other. Average visual obstruction estimates, using a profile board, were associated with variability in vertical structure as indicated by its association with height. Coefficients of variation for cone of vulnerability and visual obstruction were dependent upon their means and of limited use in describing horizontal patchiness. In contrast, shrub patch number was not linearly correlated with any other single measure in our analysis, and may be useful in describing horizontal patchiness. Cone of vulnerability and angle of obstruction are recently developed techniques that provided useful, single indices of multidimensional habitat structure. Efficient monitoring of wildlife habitat structure should employ multiple, independent techniques that measure distinct dimensions of habitat structure or a single measure that integrates multiple dimensions.

Increased demand for available water supplies necessitates that tools and techniques be developed to quantify soil water reserves over large land areas as an aid in management of water resources and watersheds. Microwave remote sensing can provide measurements of volumetric water content of the soil surface (theta(vSL)) up to about 10 cm deep. The objective of this study was to examine the feasibility of inferring the volumetric water content of the soil profile (theta(vBL)) by combining remotely sensed estimates of theta(vSL), in situ measurements, and modeling techniques. A simple soil water budget model was modified to estimate theta(vBL) from assimilated values of theta(vSL). Four modeling scenarios were evaluated at 4 tallgrass prairie sites located in central and south central Oklahoma: 1) unmodified model, 2) assimilation of field-measured theta(vSL) at 2-day intervals, 3) assimilation of field-measured theta(vSL) matching dates of remote sensing data acquisitions during the study period, and 4) assimilation of remotely sensed theta(vSL). The unmodified model (scenario 1) underestimated measurements with root mean square errors (RMSE) between 0.03 and 0.06 m3m(-3) and mean errors (ME) between 0.02 and 0.04 m3m(-3). Model output from scenario 2 agreed well with measurements at all study sites (ME less than or equal to 0.01 m3m(-3), RMSE less than or equal to 0.03 m3m(-3)). The RMSE and ME values from scenario 3 were comparable to those of scenario 2. Simulations from scenario 4 agreed well with measured data at 2 study sites (0.00 m3m(-3) greater than or equal to ME less than or equal to 0.02 m3m(-3), RMSE less than or equal to 0.03 m3m(-3)) but underestimated measurements at the remaining sites, in one case by as much as 0.15 m3m(-3). The underestimation was due largely to inaccurate remotely sensed theta(vSL) values. These preliminary results suggest that it is feasible to infer theta(vSL) in tallgrass prairies by combining remotely sensed estimates of theta(vSL), in situ field measurements, and modeling, provided that the remotely sensed data correctly estimates surface conditions.

The terms of grazing lease contracts potentially influence a tenant's incentive to preserve the vegetation resource. Annual stocking rate decisions dictate the degree of overgrazing, which can be cumulative over long periods of time. The objective of this study was to identify the impact the tenant's lease length and lease type has on profit maximizing stocking rates. A multi-period nonlinear programming model was developed to identify economically optimal stocking rates each year over a 24-year period. The model was solved under 1-, 4-, 8-, and 12-year leases on a "per ha" and "per head" basis. The relative importance of each lease alternative and other input values in explaining the tenant's optimal stocking rate was ranked based on standardized ordinary least squares coefficient estimates. Lease length and lease type had a minor impact on optimal stocking rates relative to non-lease factors such as livestock prices and production costs. Holding lease length constant, per ha leases generated a 2% higher average stocking rate than per head leases. Optimal stocking rates were inversely related to the length of the lease. Twelve-year leases generated 18 and 13% lower optimal stocking rates than the 1-year per ha and per head leases, respectively. The optimal stocking rate difference between an 8-year and a 12-year lease was negligible, suggesting the 8-year lease would provide a similar incentive to protect vegetation as a lease with a longer planning horizon.

Knowledge of livestock population dynamics is important to better understand functional attributes and development potential of pastoral production systems. With a focus on the Borana system of semi-arid Ethiopia for 1980-97, the main objectives of this research were to: (1) Characterize cattle population trends; (2) determine associations of rainfall and stocking rate with change in cattle numbers; and (3) estimate economic losses from cattle mortality. We predicted that the regional cattle population trend would consist of a "boom and bust" cycle with long periods of gradual herd growth punctuated by drought-induced losses. We expected that cattle losses would occur when high stocking rates were combined with large rainfall deficits. Such observations would refute the idea that cattle numbers were erratic and purely controlled by rainfall variation, as predicted by non-equilibrium theory. Cattle dynamics were quantified using herd histories from interviews of 56 households living in 4 sites. Data were aggregated to portray regional cattle population trends and quantify economic losses. Regression analysis was employed to assess associations of rainfall variation and stocking rate with cattle dynamics using 2 approaches: (1) Regional using aggregate herd data, empirical rainfall records, and calculated estimates for stocking rates; and (2) local using site-specific herd data along with recall of rainfall and stocking rate dynamics. Overall, results confirmed that cattle numbers followed a boom and bust cycle. Average cattle holdings dropped front 92 to 58 head/household between 1980 and 1997, respectively. Droughts in 1983-5 and 1991-3 resulted in the deaths of 37 to 42% of all cattle, respectively, up to 15-times higher than net sales. Over 17 years our target population of 7,000 households lost 700,000 cattle with a capital asset loss valued at USD 45 million. Statistical results were more difficult to interpret. Our regional approach indicated neither rainfall nor stocking rate were significantly associated with cattle mortality. We felt this interpretation was erroneous, however, due to a probable-but unmeasured-decline in key grazing resources that lowered carrying capacity, increased herd instability between successive droughts, and undermined relationships among model parameters. Our local approach was somewhat clearer in that results indicated cattle losses were significantly and consistently associated with rainfall deficits, and occasionally associated with high stocking rates that varied by site. We were concerned, however, about respondent bias and possible error in these results. We concluded that the strongest information we had was simply the aggregate pattern of herd dynamics. When aligned with empirical rainfall records and augmented with data from another dramatic cattle crash in 1998-9, we make the case that stocking rate indeed appears to influence the likelihood that a dry year will reduce cattle numbers. We concluded that the Boran live in a dynamic and productive equilibrial system where land-use change has interacted with rainfall variation to create a vicious cycle of massive cattle losses every 5 to 6 years. Improving human welfare under such circumstances should focus on creating a virtuous cycle based on more timely livestock sales, alternative investment of revenues, and sustainable economic diversification.