ARCTIC VEGETATION RESPONSES TO GLOBAL CHANGE: REPRODUCTION, GROWTH, AND CONSEQUENCES FOR BIODIVERSITY

Open Access
Author:
Watts, David Ami
Graduate Program:
Ecology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 25, 2016
Committee Members:
  • Eric S Post, Dissertation Advisor
  • Eric S Post, Committee Chair
  • Tomas A Carlo-Joglar, Committee Member
  • Margot Wilkinson Kaye, Committee Member
  • Erica A H Smithwick, Outside Member
  • Tracy Lee Langkilde, Committee Member
Keywords:
  • climate change
  • global change
  • seed ecology
  • biodiversity
  • ecosystem function
  • clonal plants
  • genotyping
  • plant herbivore interactions
  • Greenland
  • Arctic
  • tundra
Abstract:
Understanding how biological systems respond to local and global environmental change is necessary to make useful and reliable predictions of future community dynamics. Multiple facets of environmental change, such as climatic shifts and increases or declines in large fauna, can interact to produce additive or counteracting effects on vegetation. Altered composition and dominance patterns in plant communities can, in turn, have consequences for ecosystem functioning through, for example, modification of biogeochemical processes or regional climate patterns. Global scale climate change has already strongly impacted the makeup and dynamics of ecological systems. The northern tundra biome is expected to undergo large changes, which previously have and likely will affect human wellbeing into the future. Thus, the tundra ecosystems of the Low Arctic provide the context within which my dissertation expands our understanding of ecological responses to environmental change. Understanding how facets of global change, such as warming or human overexploitation of animal resources, affect plant communities in arctic tundra requires understanding responses across life history stages. In this dissertation, I have sought to address gaps in our knowledge related to the role of reproductive processes in contributing to the dynamical patterns that have become evident. In the chapters that follow, I identify the importance of accounting for individualistic species responses to change, and I further suggest that the similar patterns among arctic plant species within a functional group may be the consequence of different underlying processes. I begin this dissertation by providing some background on global change and the ensuing shifts in the Arctic biome. I then discuss how a warming climate influences the niches of arctic plants and what the consequences of these changes are for biodiversity in tundra ecosystems. One key demographic parameter that can mediate species’ responses to change is the regeneration niche. In Chapter 2, I addressed two of the earlier stages of the regeneration niche, the production of viable seed and germination, in an experimental context to bridge the gap between the effects of environmental change on established and establishing individuals. I hypothesized that the reproductive responses of arctic plant species to change would be species-specific and poorly predicted by plant functional type. I manipulated temperature and precipitation over three years in a factorial design and also sampled grazed and exclosed plots. The interannual variation in seed mass and germination appeared to reflect weather in those years for several species, and the effect sizes of this variation were similar to those of the warming, watering, and herbivore exclusion treatments. Grazing only affected graminoid species, but these effects were positive for some graminoids and negative for others. Both warming and watering as well as their interaction led to individualistic effects among species belonging to several different functional types, suggesting that some uses of these latter classifications are unlikely to result in reliable predictive generalities. Additionally, a few species demonstrated contrasting effects of warming and watering across years, suggesting these species are near thresholds for successful sexual reproduction. Such thresholds can occur when environmental conditions preclude plants or plant parts from attaining a minimum size required to complete reproduction. The potential existence of such thresholds indicates that amelioration of abiotic stressors, such as temperature minima that can reduce survival or fecundity, as a result of warming in the Arctic may lead to increases in sexual reproduction and recruitment. In Chapter 3, I addressed the relative contributions of sexual recruitment and asexual spread in populations of Salix glauca, a deciduous shrub increasing in abundance in many tundra landscapes, distributed along an environmental gradient comprising a shift maritime to continental climates. I hypothesized that landscapes with warmer summers would have greater evidence of sexual recruitment. To test this, I located sites that appeared to have increased in shrub cover in recent decades and collected leaf samples from neighboring ramets (stems). I found evidence that sexual recruitment occurs universally across this gradient, but the proportion of sexual recruits, inferred from the uniqueness of their genotypes, was greater in the inland landscapes that experience a continental climate, supporting my hypothesis. Additionally, I demonstrated that the probability of a ramet arising from clonal growth increases where neighbors are larger and vegetative cover is greater, suggesting light is a limiting factor for recruitment from seed. The above results suggest that the responses of arctic plant communities to climatic change are shaped by the variable responses in growth and reproduction among species. This interspecific variation can lead to shifts in patterns of plant community diversity. While recent syntheses suggest species diversity at small spatial scales has not declined in a range of ecosystems in recent years, I documented the potential for notable declines in the diversity of local assemblages of mixed shrub-graminoid tundra. In Chapter 4, I demonstrate that warming increases the accumulation of biomass, resulting in eventual declines in taxonomic diversity approximately one decade after a disruptive caterpillar outbreak. In particular, shrub abundance was negatively associated with forb richness under warming, but only when large herbivores were experimentally excluded. Additionally, the nature of the relationship between biomass and taxonomic richness went from positive in grazed and unwarmed plots to unimodal in exclosed, warmed plots. Warming thus led to increased biomass, but this only resulted in decreased local diversity in the absence of ungulate herbivory. The expansion of woody shrubs in the Arctic has become a particular focal point of research, but there remain many unknowns. In Chapter 5, I highlight the contributions of these studies to our current understanding and put them in the context of ongoing research on the causes and consequences plant community responses to global change. This dissertation adds to our understanding of the possible changes in tundra vegetation that may ensue in a warmer Arctic. It thus contributes to the broader body of knowledge on the ecology of clonal plants and the link between ecosystem function and biodiversity.