Relative growth rates and the grazing optimization hypothesis.

Understanding the influence of different management types on grassland production is essential for improving grassland conservation and management . Previous studies have yielded varying results for aboveground biomass changes. Grazing thus increased or decreased aboveground production in different cases. The fencing optimization hypothesis posits that fencing significantly enhances above- and belowground biomass by means of carbon reallocation to plant growth and promotes increases in soil nutrient concentrations , . Belowground biomass has also been shown to be affected by grazing, with some studies indicating that plants reduce the proportion of aboveground parts and allocate more biomass to belowground parts, so as to geminate and resist environmental stress (i.e., grazing pressure) . Conversely, more belowground biomass was allocated to the surface layer than the subsurface of the soil profile with increasing grazing pressure . For instance, the root to shoot ratio in the grazing pattern was significantly higher than for the mowing and fencing patterns in temperate grassland .

Relative growth rate and the grazing optimization hypothesis

Relative growth rates and the grazing ..

Relative growth rates and the grazing optimization ..

Coastal marshes on the north shore of Akimiski Island constitute the most important brood rearing habitat for geese. In the late 1970s, these coastal marshes were dominated by Puccinellia phryganodes and Carex subspathacea (Martini and Glooschenko 1984, Jefferies et al. 2006), both of which are high quality forage plants for geese (Gadallah and Jefferies 1995a, b). However, these coastal habitats have been progressively degraded over time by the foraging activities of large numbers of staging and nesting geese, and habitat loss was most pronounced from 1985-1993, when more than 5000 ha of coastal marsh were lost on Akimiski Island (Jefferies et al. 2006). Based on the decline in harvest rates of juvenile Canada Geese, we hypothesize that the goose-habitat relationship on Akimiski Island reached a tipping point starting in about 1986, presumably because abundance of Canada Geese and their broods exceeded carrying capacity of brood rearing areas, which led to declines in gosling growth and postfledging survival. Habitat loss may have continued through 2000 (Jefferies et al. 2006), and although some forage species increased in abundance between 1998-2008, the magnitude of changes was small, and above-ground biomass remained low (Kotanen and Abraham 2013). Despite this, we found that first-year survival rates of Canada Geese increased over the course of our study, and increased survival was associated with increases in gosling size. We attribute the increase in gosling size to increases in per capita forage caused by the overall decline in abundance of nesting Canada Geese. The 1990-2010 annual decline was about 585 breeding Canada Geese per year: linear regression with a log link P

M.I DyerRelative growth rates and the grazing optimization hypothesis

In this study, we examined plant traits and soil properties in response to different grassland management strategies in the Northern Tibetan Plateau. The specific aims are: (1) to reveal the impact of different grassland managements on plant traits and soil properties and to test the grazing optimization hypothesis; (2) to test the isometric partitioning theory on different alpine species under varying grassland management patterns; and (3) to explore the effects of grazing stress on clonal growth.

Relative growth rates and the grazing optimization hypothesis
(1981) Relative growth rates and the grazing optimization hypothesis.

hypothesis of grazing optimization ..

Average hatch timing for geese generally coincides with the spring flush of growth of forage species so that newly hatched goslings can take advantage of the high nutritional content and digestibility of new plant growth (Cargill and Jefferies 1984, Sedinger and Raveling 1986). The seasonal decline in forage quality and negative consequences of hatching relatively late within a year compared with conspecifics suggest that differences in hatch dates of as little as a week can result in detectable declines in gosling growth (Cooch et al. 1991a, Sedinger and Flint 1991, Lindholm et al. 1994, Lepage et al. 1998). Person (2001) demonstrated experimentally that late hatched goslings could not compensate for poor forage quality by increasing intake rates and were lighter at 31 days posthatch compared to those that hatched earlier. Therefore, optimizing the time of hatch so that goslings can graze on favorable forage could convey a fitness advantage by facilitating maximum gosling size, because larger size can equate to higher survival and greater fitness (Williams et al. 1993, Sedinger et al. 1995, Hill et al. 2003, Sedinger and Chelgren 2007). Hatching before or after the period of peak forage quality and abundance can be detrimental to gosling growth and may also exacerbate the potential consequences of competition.

Theory of grazing optimization in which herbivory improves photosynthetic ability

The grazing optimization hypothesis predicts a ..

Heuristic models such as the model presented here are generally difficult to parametrise and validate. One reason is that the parameters used in the model describe the aggregated outcome of several underlying ecological processes. Nonetheless, the model could be parametrised and validated by conducting field experiments that measure the transfer rates of metabolites between aboveground and belowground organs. An alternative approach is to parametrise the model indirectly , by fitting the model to data that describe how the abundances of aboveground and belowground compartments of a grassland ecosystem respond to different fire regimes and herbivory . Such a parametrised model would allow us to test the hypothesis that intermediate coupling optimises the cost-benefit relation of persistence and productivity.



A complex relationship may exist between population density and forage availability. For example, foraging by geese can increase plant productivity through a feedback mechanism in which moderately grazed vegetation can produce higher quality forage for longer compared to plants that are not grazed (Harwood 1977, Bazely and Jefferies 1986, 1989, Hik and Jefferies 1990, Person et al. 2003). Similarly, grazing may change the phenotype of some species from a low quality to a high quality forage, thus increasing available feeding habitat (Person et al. 2003, Fondell et al. 2011). However, overgrazing is detrimental (Kuijper et al. 2009) and can result in generally lower gosling growth rates (Cooch et al. 1991a, b, Williams et al. 1993, Sedinger et al. 2001). Thus, densities of geese below a threshold may actually improve foraging conditions, but higher densities are more likely to inhibit gosling growth through exploitive competition (Bazely and Jefferies 1997).