Effects of Increasing Shrub Abundance on Arctic Tundra
Global climate change is shifting the distribution and abundance of species, effects which are expected to be particularly prevalent in the Arctic. For example, global change has resulted in a ‘greening’ of the Arctic -- an increase in the abundance of a number of deciduous shrub species. These large-scale changes in plant composition have the potential to affect a number of other trophic levels (from microbes to vertebrates) as well as physical ecosystem properties. Much of my current research has focused on the effects of increases in shrub abundance on arctic tundra. This project takes advantage of a chrono-sequence of long-term fertilization experiments (ranging from 5 to 22 years of fertilization) with corresponding levels of shrub density.
This is a collaborative project with Laura Gough (University of Texas at Arlington), John Moore (Colorado State University) and Matt Wallenstein (Colorado State University). The ultimate goal of the larger project is to parameterize a soil carbon model of the arctic tundra using field-derived data from these experiments.
A more detailed description of some aspects of this project I have been focusing on are below.
This is a collaborative project with Laura Gough (University of Texas at Arlington), John Moore (Colorado State University) and Matt Wallenstein (Colorado State University). The ultimate goal of the larger project is to parameterize a soil carbon model of the arctic tundra using field-derived data from these experiments.
A more detailed description of some aspects of this project I have been focusing on are below.
Variation in Thaw Depth with Shrub Abundance
Permafrost is a layer of permanently frozen soil found in arctic ecosystems. The depth-to-permafrost (thaw depth) determines the amount of soil available for plant growth and influences both plant and microbial communities. I am examining influences of the plant community (shrub tundra vs. tussock tundra) on thaw depth through a variety of mechanisms including insulation by litter, shading of soil by vegetation, and changes in rooting depth.
Watch an International Polar Year FrostByte Video about this permafrost depth project on YouTube
Seasonal Nitrogen Dynamics
I am exploring differences in seasonal variation between the different plant communities along a chronosequence with weekly measures of soil-available nitrogen (in collaboration with Mike Weintraub, University of Toledo) using microlysimeters (pictured left) to repeatedly measure soil water.
Effects of Betula Litter on Decomposition Processes
As shrub species change in abundance in the arctic tundra, so does the abundance of their litter. We are examining the effects of mixing litter of Betula (one of the shrubs increasing in abundance) on the decomposition of litter of other species. We are conducting this study using both leaf and root material, and are investigating seasonal changes in decomposition by measuring both microbial respiration (instantaneous decomposition) and mass loss (cumulative decomposition) throughout the growing season.This experiment is in collaboration with Marjan Van de Weg (VU University Amsterdam) and Kate Buckeridge (University of Kansas).
Effects of Functional Group Identity on Ecosystem Functioning
Ecosystems worldwide are losing some species and gaining others. Concern that loss of species would result in ecosystems that perform less efficiently than the species-rich ecosystems from which they are derived has resulted in a prolific body of research known as biodiversity effects on ecosystem function. Changes in biodiversity necessarily involve changes in species composition and my research uses a novel experimental approach, plant functional group knock-outs, to examine the consequences of these changes in species identity on communities and ecosystems. Using this approach, we determine the effects of plant identity by removing independent functional groups from intact ecosystems and then examine a variety of ecosystem processes. Some of the specific results are described below.
Mass Ratio Hypothesis
My studies using functional group knockouts represent one of the first experimental tests of the mass ratio hypothesis, which predicts that ecosystem properties are driven by the traits of the dominant plants in the community. We found that the mass ratio hypothesis cannot be used to predict effects of species loss, and showed that grasses had much stronger effects on ecosystem properties than would be expected based on their dominance in the community.
Litter Decomposition
I also conducted a number of studies on the impact of plant identity on decomposition processes. In one study, I examined the effects of plant identity on leaf and root decomposition; roots are a rarely examined, but extremely important, factor in determining ecosystem carbon cycling. A second decomposition study tested for the interaction between two different mechanisms through which plant community composition may affect litter decomposition: (a) effects on the decomposition micro-environment and (b) effects of the litter community composition.
