Public rangelands in North America are typically managed under a multiple use policy that includes livestock grazing and wildlife management. In this article we report on the landscape level extent of grassland loss to shrub encroachment in a portion of the Rocky Mountain Forest Reserve in southwestern Alberta, Canada, and review the associated implications for simultaneously supporting livestock and wildlife populations while maintaining range health on this diminishing vegetation type. Digitized aerial photographs of 12 km of valley bottom from 1958 and 1974 were co-registered to ortho-rectified digital imagery taken in 1998, and an un-supervised classification used to determine areas associated with grassland and shrubland in each year. Field data from 2002 were over-layed using GPS coordinates to refine the classification using a calibration-validation procedure. Over the 40-year study period, open grasslands declined from 1,111 ha in 1958 to 465 ha in 1998, representing a 58% decrease. Using mean production data for grass and shrub dominated areas we then quantified aggregate changes in grazing capacity of both primary (grassland) and secondary (shrubland) habitats for livestock and wildlife. Total declines in grazing capacity from 1958 to 1998 totaled 2,744 Animal Unit Months (AUMs) of forage (-39%), including a 58% decrease in primary (i.e., open grassland) range, which was only partly offset by the availability of 1,357 AUMs within less productive and less accessible shrubland habitats. Our results indicate shrub encroachment has been extensive and significantly reduced forage availability to domestic livestock and wildlife, and will increase the difficulty of conserving remaining grasslands. Although current grazing capacities remain marginally above those specified by regulated grazing policies, it is clear that continued habitat change and decreases in forage availability are likely to threaten the condition of remaining grasslands. Unless shrub encroachment is arrested or grassland restoration initiated, reductions in aggregate ungulate numbers may be necessary.
This study evaluated the impact of shrub encroachment within cool-season floodplain meadows in a portion of the Rocky Mountain Forest Reserve in southwestern Alberta, where nearly half of open grasslands have been lost since 1958. Sample transects situated in meadows, known formerly to be in open grassland, were assessed in 2001 (n 5 21) and 2002 (n 5 33) in areas ranging from 0% to 92% shrub cover. Shrub cover, density, and height were correlated to understory parameters, including herbaceous production, bunchgrass cover and density, and species richness and diversity. Additional data on soil horizon depths were obtained. Analysis was conducted using regression to assess empirical relationships between understory or soil characteristics and the overstory. Stronger empirical relationships (i.e., greater R 2) were found in 2002, coincident with larger sample sizes and greater rainfall. Among independent overstory variables, aggregate shrub cover for all species accounted for the greatest variation in understory characteristics. Significant (P , 0.01) negative nonlinear relationships were observed between shrub cover and herbage production, with the latter declining from 6 629 kg ? ha-1 in meadows containing less than 12% shrub cover, to 2 797 kg ? ha-1 in areas where shrub cover exceeded 35%. Negative linear trends were found (P , 0.01) for bunchgrass density and cover in relation to increasing shrub abundance in 2002. Understory diversity increased with shrub abundance, peaking at 64% shrub cover in 2002. Meadows with low and high shrub cover also coincided with thicker Ah and litter, fibric, and humic soil layer (LFH) horizons, respectively. Collectively, these results indicate shrub encroachment is threatening the sustainability of native bunchgrass communities and reducing forage availability. Meadow conservation in this area will require proactive management to protect or restore open grasslands. Resumen Este estudio evaluó el impacto de la invasión de arbustos dentro de la temporada fría de praderas inundables en una porción de la Reserva Forestal de las Montañ as Rocallosas en el suroeste de Alberta, donde se han perdido cerca de la mitad de los pastizales abiertos desde el 1958. Los transectos de muestreos situados en praderas conocidas antiguamente como pastizales abiertos, fueron evaluadas en el 2001 (n 5 21) y 2002 (n 5 33) en áreas con rangos de 0% a 90% de cobertura arbustiva. La cobertura arbustiva, la densidad, y la altura fueron correlacionadas con varios parámetros del sotobosque que incluyen la producción de herbáceas, la densidad, y cobertura de pastos, y la diversidad y riqueza de especies. Adicionalmente se obtuvieron datos de la profundidad del suelo. El análisis se condujo usando regresión para evaluar las relaciones empíricas entre el sotobosque o características del suelo y el dosel. Las relaciones empíricas más fuertes (i.e., más grande R 2) fueron halladas en el 2002, coincidiendo con las muestras de mayor tamañ o y con mayor precipitación. Entre las variables independientes del dosel, l...
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