Browsing Journal of Range Management, Volume 56, Number 4 (July 2003) by Title
Now showing items 12-14 of 14
Ungulate herbivory on buckbrush in an Arizona ponderosa pine forestMonitoring processes that affect plant population dynamics and determine community structure is central in forest restoration ecology. To study effects of mule deer (Odocoileus hemionus) and elk (Cervus elaphus) on buckbrush (Ceanothus fendleri Gray), we built exclosures around 90 plant-centered plots in 3 ponderosa pine (Pinus ponderosa Laws.) forest restoration management units and compared vegetative and flowering characteristics with unprotected plots for 2 years. On unprotected plots, 69% of the current-year branches were browsed during the first year and 44% were browsed the second year. There was no difference in number of aerial stems or current-year branches in the first year, yet stems on protected plots were longer (24.1 cm; P < 0.01) and retained more than 4 times the current-year biomass (1.4 g stem-1; P < 0.01) than those on unprotected plots (12.9 cm and 0.3 g stem-1, respectively). Stem number, length and diameter, number of current-year branches, and current-year biomass on protected plots were all greater (P < 0.01) than on unprotected plots in the second year. Stems on protected plots had significantly higher (P < 0.01) length-diameter ratios and had fewer current-year branches per unit length (P < 0.05) than unprotected stems. Flowering stems were found on significantly (P < 0.05) more protected plots (55%) than unprotected plots (8%) in the second year. Effects of ungulate herbivores on buckbrush size, stem recruitment, morphology, and flowering represent important constraints to early understory development and restoration in this Southwest ponderosa pine forest.
Utilization and grazing distribution of cattle at 4 stocking densitiesThe relationship between stocking density and grazing distribution was studied in eastern Nebraska pastures seeded to a warm-season, tall-grass mixture and grazed at 4 stocking densities: 9, 18, 27, and 54 steers ha-1. Each of 4 pastures was divided into 4 paddocks ranging in size from 0.18 to 1.12 ha. Paddocks within each pasture were grazed rotationally by 10 steers averaging 282 kg during 3 consecutive cycles (12, 36, and 24 days) from early June to late August in 1995 and 1996. Transects 12-m long were established in a grid pattern in each paddock. Six tillers each of big bluestem (Andropogon gerardii Vitman) and switchgrass (Panicum virgatum L.) were marked permanently in each transect. Height and leaf length of marked tillers were measured before and after grazing in the last 2 grazing cycles in both years. Utilization was estimated by the reduction in tiller height or leaf length. Estimates of grazing distribution were based on a uniformity index, which was calculated by summing the absolute differences of tiller height or leaf length between adjacent transects. Stocking density generally did not affect (P > 0.05) tiller height reduction which ranged from 19 to 22 cm and from 29 to 38 cm among the stocking densities in 1995 and 1996, respectively. In most grazing cycles, leaf length reduction for big bluestem was greater (P < 0.05) than for switchgrass while tiller height reduction was similar between species. Spatial grazing distribution was not affected (P > 0.05) by stocking density but big bluestem was grazed more evenly (P < 0.05) than switchgrass in the last cycle in each year. Stocking densities as high as 54 steers ha-1 on warm-season, tall-grass mixtures do not appear to be a major factor in affecting spatial grazing distribution or forage plant selection.
Vegetation indices, CO2 flux, and biomass for Northern Plains grasslandsNative grasslands are a sink for atmospheric CO2 sequestration, but ways for extending site-specific CO2 flux measurements to a regional scale are lacking. Objectives of this study were to determine the utility of using canopy radiometric reflectance for estimating CO2 fluxes for semiarid grasslands. The relationship between the normalized difference vegetation index (NDVI) calculated from spectral reflectance data obtained with hand-held radiometers was compared to CO2 flux calculated from Bowen ratio/energy balance measurements. Carbon dioxide flux was measured during the plant growing season over a nongrazed prairie, grazed prairie, and a shrub dominated prairie site near Mandan, N.D. Measurements were also made of evapotranspiration (ET), green biomass, and green leaf area index (LAI). Correlation coefficients of NDVI with vegetation parameters of biomass and LAI for each site and year exceeded 0.84 in 1999, 0.74 in 2000, and 0.91 in 2001; with CO2 flux correlations exceeded 0.63 in 1999, 0.68 in 2000, and 0.69 in 2001; with ET correlations exceeded 0.91 in 1999, 0.92 in 2000, and 0.90 in 2001. Regression analysis over all years and sites produced a nonlinear relation between NDVI and both biomass (R2 = 0.83) and LAI (R2 = 0.77) and a linear relationship between NDVI and both CO2 flux (R2 = 0.51) and ET (R2 = 0.81). The relationships between NDVI and biomass, LAI, CO2 flux, and ET for the 3 grassland sites, which differed in management and vegetation, were generally quite similar suggesting that NDVI has potential for use in predicting canopy CO2 flux rates for semiarid grasslands in the Northern Great Plains.