Forest Investment Account (FIA) - Forest Science Program
FIA Project Y113122

    Contrasting spring and fall grazing regimes for effects on grassland biota
 
Project lead: Jeffrey Lemieux (Thompson Rivers University)
Author: Lemieux, Jeffrey P.
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
Description:
Can operational grasslands be more effectively managed to promote elements of biodiversity? Grasslands in British Columbia are reservoirs of biological diversity, despite their use to graze cattle for well over a century (Pitt and Hooper 1994). Generally speaking, grasslands in strongly degraded condition produce lower yields of plant mass (e.g. Mclean and Marchand 1968), motivating range managers to rotate cattle in a fashion that optimizes both cattle and range condition. Existing grazing studies tend to focus on the differences between heavy grazing and grazing exclusion, bypassing approaches that integrate societal and operational objectives. In this study we contrast different seasonal grazing regimes as operational options to meet grassland conservation and production goals.

Rangeland managers mitigate grazing impacts in part by rotating cattle through seasonal pastures. The cattle cycle in forested grasslands of British Columbia typically sees producers using rangeland in the spring (spring grazing), and then again in the fall (fall grazing) in order to offset potential weight loss incurred in the higher elevation, but lower quality forested range used during the heat of summer. Seasonally grazed pastures are often utilized in one season on a permanent basis. Spring grazing has an inordinate effect on plant fitness by removing photosynthetic tissues at the beginning of the growing season, whereas fall grazing avoids this effect by utilization of plant material imminently destined for the grassland litter layer. Fall grazing also occurs after peak activity for plants and animals, whereby a largely intact canopy is present throughout the summer. However, there are reasons to expect that spring grazing may benefit some components of the biota. These expectations extend from plant structure and soil conditions observed during co-management of grassland pastures by Agriculture and Agri-Food Canada (AAFC) and local producers.

A twenty-year study conducted in these pastures by AAFC demonstrated that fall grazing allowed seral improvement of a bluebunch wheatgrass - sage association when compared against spring grazing at identical stocking rates (Thompson and Quinton 1997). Grass biomass increased dramatically under fall grazing, owing to the retention of late seral structure in dominant species. These trends are consistent with the small body of peer-reviewed literature evaluating seasonal grazing in temperate rangelands (Bork et al. 1998; Naeth et al. 1991; Mueggler 1950). However, effects on other parts of the grassland ecosystem are unstudied in this regard. Forb species have important aboriginal relevance in the Kamloops area, including the mariposa lily and arrow-leaved balsam root. The latter has been shown to improve with fall grazing in areas of the United States (Bork et al. 1998). Both species are infrequent but more abundant with very low levels of disturbance (Thompson pers. obs.). Our previous studies of this area suggested that these and other species (Thompson’s paintbrush, pulse milkvetch, big sage) are favoured by fall grazing, but they were not well quantified with the sampling regime previously used. We expect fall grazing to promote these and other forb species (hypothesis 1), but as possible trade-offs against faunal elements enhanced by spring grazing (hypotheses 2-4 below).

Small mammal and insect species, for example, are abundant and intimate with the plant and soil qualities typically evaluated as range condition. Both groups are poorly studied with respect to range condition, though they can be expected to respond to plant architecture and soil and litter qualities (Dennis et al. 1998; Hayward et al. 1997). Furthermore, arthropods are important prey items for the small carnivores in the grasslands, including the spadefoot toad, yellow-bellied racer (both threatened species), mice, and shrews. Deer mice (an omnivore – see below) numbers also would be expected to respond positively to increased arthropod abundance. To our knowledge, only one other study has simultaneously compared insects and small mammals in temperate grasslands, suggesting trophic interaction (Churchfield et al. 1991).

While fall grazing may allow seral improvement and grass productivity to increase, changes in several structural factors may negatively impact some insect guilds. Possible effects can be outlined as a series of hypotheses: 2) Litter biomass and structure is reduced to a greater degree by fall grazing as cattle consume disproportionate amounts of standing dead litter when feeding in the fall. Litter structure is an important determinant of ground-dwelling arthropod composition and abundance (e.g. Magura et al. 2005), so treatments may cause shifts in ground beetle composition. 3) Grasshoppers are prevalent plant consumers in rangelands. Cattle grazing can increase grasshopper density (Onsager 2000), likely by changes to light and temperature mediated through cano
Contact: Jeffrey Lemieux, (250) 574-1464, jlemieux@tru.ca

    Deliverables:

Executive summary (65Kb)

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Updated May 19, 2011 

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