Winterfat seed viability and dormant season livestock grazing Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2514nq04s

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  • Krascheninnikovia lanata (winterfat) is a valuable forage species with an average of 10% crude protein during winter when there are few nutritious options for livestock and wildlife. However, excessive grazing throughout the west has negatively impacted survival of winterfat stands. We hypothesized that four years rest from dormant season livestock grazing, along with rest from antelope and rabbit browse, would result in greater seed viability and aboveground biomass relative to grazed plots. In locations across the Catlow Valley, Oregon with winterfat communities, two 40 x 40 m plots, a control and an exclosure, were established at each of 15 locations divided among three pastures: north, middle and south. The control plot could be grazed by livestock and wildlife in all seasons (although cattle were only in the study area during the dormant season). The second plot at each location was a large animal exclosure that prevented grazing by cattle and antelope. At four locations a cattle/antelope/rabbit exclosure was also installed. This total of 34 plots was used to test the effects of location and type of use on winterfat seed viability and aboveground plant biomass. Four years rest from dormant season grazing did not affect the viability of winterfat seed or the levels of aboveground winterfat biomass; however, location across the study area did affect the levels of winterfat aboveground biomass. There was significant variation in the data attributable to unknown factors but represented by the influence that pasture location had on seed state (viable, dead and empty) and winterfat plant biomass. In the most productive locations, increases in winterfat aboveground biomass were correlated with increases in seed viability and decreases in density of winterfat m² while increases in winterfat seed viability were correlated with decreases in the percentage of empty seed. In the locations with the highest levels of dead seed, increases in dead seed were correlated with increases in winterfat density m² and decreases in empty winterfat seed. Plants in the cattle/antelope/rabbit exclosures had the highest winterfat density, the lowest plant biomass, lower levels of empty seed and the highest level of dead seed, the reverse of the most viable plots in the study area. From observation, these plants have a greater amount of woody base growth from prior years and appear more decadent. Further research is needed to see if winterfat seed viability and biomass production is associated with some level of browse. Nuttall's saltbush is the dominant shrub on the site, and its increase is correlated with increases in winterfat aboveground biomass and decreases in the level of dead winterfat seed. These results suggest that resources which vary by pasture location such as precipitation, soil nutrients, texture, or moisture holding capacity determined levels of winterfat aboveground plant biomass, winterfat seed state, and the density of Nuttall's saltbush and that on high resource sites, winterfat allocates resources to existing plants in rather than creating the new individuals The lack of response to four years rest from dormant season grazing, the low levels of viable seed, the low density of winterfat, and the encroachment of Nuttall's saltbush all suggest reduced vigor of winterfat in this study area. One possible explanation is that, since recovery toward a later successional stage would be expected with reduced grazing pressure (Dyksterhuis 1949), these arid lands are significantly degraded by stress or disturbance (Westoby et al. 1989; Laycock 1991) and that recovery is no longer possible without significant intervention. This suggests that the study area has crossed an ecological threshold. An alternate possibility is that since recruitment of winterfat is episodic and driven by precipitation events, climatic fluctuations may have prevented the sequence of good precipitation years needed for stand maintaining winterfat recruitment (Pechanec 1964). Further study is needed to evaluate these possibilities.
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