| Summary: | Ecologists have long been searching for the mechanisms that control the distribution and abundance of species. There are numerous examples of studies that have investigated the influence of local and regional climates, intra- and interspecific competition for limiting resources as well as predation. Recently, increased attention has been devoted to the effects of climate change on species distributions, abundance and community composition. Plants have been shown to respond to climate change in various ways. For instance, primary production, phenological progression, reproduction as well as plant community composition have all been shown to respond to a changing climate. However, herbivory has also been shown to have strong impact on plants and plant communities. But, the influence of herbivory has been excluded from the majority of published studies on climate change and the effects on ecosystem properties. This may have led to inaccurate predictions about how ecosystems respond to climate change. The Arctic is expected to experience the earliest and most pronounced changes due to increasing global temperatures. My research was conducted in a low-arctic ungulate grazing system in West-Greenland occupied by large ungulate grazers like caribou (Rangifer tarandus) and muskoxen (Ovibos moschatus). The goal of this research was to investigate the influence of herbivory on how arctic vegetation responds to a warmer climate. Twelve pairs of permanent vegetation plots were randomly selected inside and outside of herbivore exclosures in 2003. An open top chamber (OTC) was randomly assigned to each pair of permanent plots, to simulate climate warming. I investigated the influence of ungulate herbivory on the biomass production of arctic vegetation to simulated climate warming. By the use of the non-destructive point-intercept method (PIM) I acquired a biomass index that was converted to biomass values. By monitoring changes in aboveground biomass from 2003 until 2006 I investigated the potential effect of climate warming on plant biomass production in an arctic grazing system. I did not find any strong evidence of climate warming effects on the aboveground plant biomass production. Neither did I find any strong effects of ungulate herbivory on plant production although there was indications that large herbivores could mitigate the predicted increase in shrub biomass over graminoids and forbs. However, the effect of my treatments was probably concealed by intense caterpillar foraging during a population outbreak of the moth Eurois occulta in 2004 and 2005 that severely affected above ground biomass in the study area. In a related study, I investigated the effects of climate warming and ungulate herbivory on plant community composition and the abundance of shrubs, graminoids and forbs. By converting plant intercepts from the PIM to vegetation cover values, I monitored changes in abundance from 2003 until 2006. The intent was to investigate if ungulate herbivores could mitigate the predicted increasing dominance of shrubs in the Arctic as a response to a warmer climate. I did not detect any significant changes in the abundance of shrubs, graminoids or forbs to either the warming treatment or exclosure treatment. But, there appeared to be higher abundance of shrubs where ungulate herbivores were excluded and higher abundance of graminoids in grazed areas, suggesting potential negative effect of ungulates on shrub dominance. However, the lack of responses observed in plant abundance to the treatments was most likely affected by the caterpillar outbreak during 2004 and 2005. Climate warming has been shown to affect plant reproduction. Also, theory suggests that if herbivory is predictable in time and space, plants have the ability to compensate for tissue lost. Several plant species have been shown to compensate for tissue lost to herbivory. I investigated the possible influence of ungulate herbivory and climate warming on the flower and fruit production of two co-occurring forb species, Cerastium alpinum and Draba cana. The objective was to investigate potential compensatory responses in these two forbs species to grazing exposure and climate warming that could lead to changes in interspecific competition. Forty eight ramets from each species were permanently marked in the vegetation plots. Biometric measurements were performed on each ramet throughout the growth season. Results indicate a reduced reproductive success in D. cana to climate warming when exposed to herbivory while C. alpinum experienced increased reproductive success when compared to ramets in plots excluded from herbivory. This suggests that climate warming could affect the competitive outcome between plant species and hence affect future community composition. Finally, I investigated the effects of climate warming and ungulate herbivory on plant emergence and phenological progression of arctic plants during the 2004 and 2005 growth seasons. Higher air and soil temperatures have been shown to advance plant emergence and the phenological progression of arctic and alpine plants. It has also been shown that plants have the ability to compensate loss of tissue to herbivores by compensatory growth. However, such allocation of resources to compensatory growth responses has been suggested to delay phenological progression. By monitoring plant emergence and phenological progression of plants in the permanent vegetation plots in 2004 and 2005, I investigated if herbivory could mitigate an advanced phenological progression of arctic plants as a response to climate warming. In 2004, the results indicated that a warmer climate advances the emergence of arctic plants. Furthermore, ungulate herbivory delayed plant emergence, but it appeared that the plants compensated a later emergence with a faster phenological progression. However, during the caterpillar outbreak in 2005, the relationships found in 2004 were offset by severe plant defoliation that caused fewer plant species to emerge during that year. The results found in this study did not find sufficient evidence that climate warming and the effects of herbivory have strong influence on aboveground plant production or plant community composition. However, the outbreak of the moth E. occulta severely affected aboveground biomass and plant cover and most likely concealed any treatment effects from the experiment. However, effects of both climate warming simulations and ungulate herbivory significantly affected plant reproduction, plant emergence and phenological progression. Based on the findings in this study, it is strongly advised that the influence of herbivory should be taken into account when effort is made to make prediction about how plants respond to a changing climate. Throughout the development of my dissertation work, I have been involved in and contributed to research closely related to the issues investigated in this thesis. These publications are found in Appendix A-D.
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