I am broadly interested in interactions between different species, particularly in how such relationships change with changing environmental conditions and across evolutionary time. The interaction I have studied for my dissertation work occurs between alder plants and Frankia bacteria in different forest habitats in Alaska. This symbiotic relationship, in which the plant houses and grows the bacteria inside specialized root organs called nodules, is usually considered a mutualism, meaning both the plants and the bacteria benefit from the interaction. This assumption is based on the fact that Frankia are able to convert nitrogen from the air into ammonia that the plant can use in its own metabolism, while the plant is capable of photosynthesis which provides the bacteria with food. However, as with most relationships, the nature of the alder-Frankia symbiosis can change drastically under different conditions and over time. For example, when nitrogen is available from other sources such as the soil, the benefit of the symbiosis for the alders may be outweighed by the cost to the plant in terms of photosynthate.

Alaska is an ideal place to study such changes in the alder-Frankia relationship. Alder is widespread over the whole state and occurs in several different types of habitat with very different levels of light, water and soil nutrients, including nitrogen. On the Tanana River floodplain these different habitat types often occur right next to each other on the landscape and are repeated over large areas, so it is possible to examine multiple examples of a given habitat while controlling for any effect of location. In some habitats two different species of alder - thinleaf alder (Alnus incana ssp. tenuifolia) and Siberian alder (A. viridis ssp. fruticosa) - occur in the same microhabitats, making it possible to examine differences between the two species without any confounding effect of environmental differences.

My dissertation has centered on describing how different genotypes of Frankia are distributed between these two relatively distantly related species of alder across a variety of habitats in the Tanana Valley. Using molecular tools such as PCR and DNA sequencing to look at genetic variation in Frankia within alder nodules, I have found that different alder species associate with distinct types of Frankia and that differences in habitat for single alder species can also result in very different mixtures of Frankia genotypes inhabiting plant nodules. The remainder of my dissertation examines the hypothesis that these environmental differences are primarily due to an ability of plants to actively choose the most beneficial Frankia genotypes from the soil.

Publications:

Anderson MD. In press. Two in the far north: the Alnus-Frankia symbiosis with an
           Alaskan case study. Invited chapter in Ecological Aspects of Plant Nitrogen
           Metabolism. Polacco JC and Todd CD, eds. Wiley-Blackwell.

Anderson MD, Ruess RW, Myrold DD and Taylor DL. 2009. Host species and habitat affect nodulation by specific Frankia genotypes in interior Alaska. Oecologia 160: 619-630. pdf

Ruess RW, Anderson MD, Mitchell JS, McFarland JW and Trummer LX. 2006. Effects of defoliation on growth and N-fixation in Alnus tenuifolia: consequences for changes in disturbance regimes at high latitudes. Ecoscience, 13: 404-412. abstract

Anderson MD, Ruess RW, Uliassi DD and Mitchell JS. 2004. Estimating N₂ fixation by two species of Alnus in interior Alaska using acetylene reduction and ¹⁵N₂ uptake. Ecoscience, 11: 102-112. pdf