Do Svalbard reindeer regulate standing crop in the absence of predators?

The Svalbard reindeer is the only mammalian herbivore in Adventdalen (78°N), Svalbard, where it has no natural predators. To test if herbivores in the absence of predators regulate standing crop to a level independent of productivity, which is one of the predictions of the “exploitation ecosystems” model, herbivore exclosures were set up in 1992 in Salix heath, Luzula heath, Cassiope heath, and Alopecurus meadow in Adventdalen. Standing crop of vascular plants was harvested and measured inside and outside the exclosures in 1994, when the reindeer population was at peak density (ca 5.4 animals km⁻²), and in 1996, when the reindeer density was about 30% lower (ca 3.7 animals km⁻²). Standing dead material was reduced by grazing in the Luzula heath in 1994. However, we found no effect of grazing, year, or interactions between grazing and year on live standing crop. Also contrary to the predictions from the model, differences in standing crop between vegetation types were highly significant. Mean biomass of plant material was lowest in the Alopecurus meadow (36 g m⁻²), two fold higher in the Luzula heath, and about threefold higher in the Salix heath and Cassiope heath, indicating that reindeer do not regulate standing crop to the same level on a local scale. The predictive power of the “exploitation ecosystems” model is low due to lack of recognition of the importance of plant chemistry, plant compensation ability, variation in forage availability during the year, parasites functioning as predators, and adverse weather conditions, which may cause density-independent variations in fecundity and mortality of reindeer. Received: 28 December 1997 / Accepted: 6 April 1998     

Effects of simulated herbivory on tillering and reproduction in an annual ryegrass,Lolium remotum

This study examined the overall impact of simulated herbivory on tillering and reproductive performance of an annual ryegrass,Lolium remotum. The interaction between herbivore damage and intraspecific competition and the effect of the timing of damage were also studied. The experimental plants were sown at two densities and were randomly assigned to eight different damage treatments consisting of artificial leaf area removal by clipping with scissors or removal of one-third of the ripening seeds. The treatments were executed at two flowering stages. The pattern of tiller development differed significantly among treatments and between densities. At the lower density, earlier treatments delayed tiller development more than the same treatments executed later. At the higher density, all treatments delayed tiller development. The density effect was significant for all reproductive traits measured. The reproductive output of plants grown at the higher density was lower and the negative treatment effects were stronger than at the lower density. The treatment effect was significant for seed dry weight per plant and individual seed weight but not for number of seeds per plant. There were no statistically significant interaction effects between the damage treatments and density, suggesting that the plants responded to the damage similarly, irrespective of the density. The plants did not totally compensate for losses due to damage at either density, even though they slightly increased their resource allocation to sexual reproduction at the higher density.     

Determinants of C3 forb growth and production in a C4 dominated grassland

Forbs are the most abundant species within the vascular flora of tallgrass prairie and they make the greatest contribution to biodiversity of all growth forms. However, little is known about the factors that determine their productivity and growth rates. The objective of this study was to assess the controls of forb growth (absolute and relative) and production in tallgrass prairie from a long-term burning experiment at the Konza Prairie in NE Kansas. Over the 14-year study, forbs comprised 9% of the total biomass production on sites with a high fire frequency vs. 29% on the low fire frequency site, with gramminoids accounting for the remainder. Although interannual variations in peak biomass of the grasses was strongly correlated with environmental variables related to water availability, there were no similar relationships for forbs, suggesting that production of forbs and grasses responded to interannual variations in climate in different ways. Multivariate analysis of climatic controls on growth rates of grasses and forbs yielded similar results. Although forbs had low biomass and absolute growth per unit ground area in frequently burned prairie, their relative growth rates were highest in such sites. Thus, it appears that reduced growth rates of individual forbs per se do not limit forb success in annually burned prairie. Instead, direct negative effects of fire on forbs (increased mortality) may be responsible. Determinants of forb growth and productivity in unburned prairie remain unresolved.