Rachel K. Thiet, PhD

Core Faculty
Department of Environmental Studies

Research Projects


Biological soil crusts

(Funded by a Garden Club of America grant to Alexis Doshas)

Biological soil crusts (BSCs) are communities of green algae, cyanobacteria, lichens, and bryophytes that colonize the top 5 mm of soil in arid and semi-arid soils worldwide. BSCs reduce soil erosion, increase soil moisture, enhance seed germination, and increase the nutrient content of soils they colonize. Most work in BSC ecology has been conducted in the desert southwest of the United States; however, BSCs colonize soils in temperate ecosystems such as coastal sand dunes and inland sand plains and pine barrens. For example, BSCs significantly increase N inputs to sand dune soils at the Indiana Dunes National Lakeshore (Thiet et al. 2005), and increase N, P, and soil moisture in sand dunes at Cape Cod National Seashore (Smith et al. 2004). Recently, Conservation Biology student Corey Miles and I, working in collaboration with Cape Cod National Seashore ecologist Steve Smith, found that green algal BSCs enhance overland flow of rainwater and decrease infiltration to plant rooting zones in the Province Lands sand dunes on the outer Cape. Currently I am working on a project with Conservation Biology student Alexis Doshas to evaluate the effects of algal, moss, and lichen BSCs at the site on seed germination and seedling productivity of two important dune plants at the site, northern bayberry and wavy hairgrass.

Interactions between ants and broom crowberry in Cape Cod heathlands

(Funded by the 2012 Cape Cod National Seashore Nickerson Graduate Student Fellowship to Erin Hilley)

The field of pedology relies heavily on Duchochev’s five-factor model of soil formation to explain soil development and describe soil physical features; however, bioturbation of soils by invertebrate and vertebrate animals has a strong influence on soil development and soil biological and biochemical properties in many terrestrial and aquatic soils. Further, ants are important seed dispersers for many terrestrial plant species. Currently I am working with Conservation Biology student Erin Hilley to examine the interactions between ants and seed dispersal of broom crowberry (Corema conradii) in the Marconi heathlands on Cape Cod National Seashore. Broom crowberry is endemic to the northeastern US and is listed as a species of concern in the state of Massachusetts, and Cape Cod National Seashore is managing to restore and protect native heathlands. Thus, understanding more about the relationship between ants, seed dispersal and germination of broom crowberry, and soil properties in the Marconi heathlands will provide important information for protecting crowberry and other important heath species at this site.


Mollusk community recovery

Since 2004, two Conservation Biology students and I have characterized and quantified mollusk community recovery in East Harbor, a partially restored salt marsh on Cape Cod National Seashore in North Truro, MA (Thelen and Thiet 2008a, Thelen and Thiet 2008b). Currently I am working with Conservation Biology student Erica Kidd to continue to monitor mollusk community recovery throughout the marsh along environmental gradients of salinity, sediment particle size, submerged aquatic vegetation biomass, and proximity to tidal exchange with Cape Cod Bay. Seashore ecologists and managers are using results from our on going monitoring to make decisions about restoration initiatives for the marsh. Opportunities exist for incoming Master’s and Doctoral students to continue and expand these mollusk community recovery studies. Specifically, I would like to develop a study with an Antioch student to characterize and quantify the larval stages of mollusks in this salt marsh, to better predict future mollusk species composition. I would also like to develop studies that examine the effect of predation by blue crabs and moon snails on mollusk community composition and diversity at this site.

Mechanisms and mitigation of problematic algal blooms

In recent years, East Harbor restored salt marsh on Cape Cod National Seashore has experienced severe blooms of macroalgae that have resulted in anoxia and extensive fish kills. Research in the Great Lakes suggests that zebra mussel invasion may facilitate algal blooms by decreasing phytoplankton biomass in the water column and concentrating nutrients in the benthic layer. Conservation Biology students Vicki Rubino and Rebecca Clark and I are collaborating with plant ecologist Steve Smith to examine whether mollusk community recovery in East Harbor may be facilitating algal blooms in this system through the same mechanism at work in the Great Lakes. We are using a combination of field and laboratory studies to quantify the effects of soft-shell clams (Mya arenaria) on phytoplankton biomass, porewater chemistry, benthic nutrient levels, and macroalgal biomass. These experiments are in their fourth year, and our results strongly suggest that soft-shell clams at the site are contributing to problematic macroalgal blooms. In a simultaneous study, Conservation Biology student Erica Kidd and I are working with Steve Smith to conduct experiments in East Harbor to better understand why important molluscan algal grazers like periwinkles and mud snails are absent from the site.

Invasive species management and native halophyte re-establishment

(Funded by the 2008 Cape Cod National Seashore Nickerson Graduate Student Fellowship to Jesse Wheeler)

One of the biggest obstacles to salt marsh restoration on Cape Cod National Seashore is the presence and persistence of the invasive giant reed grass Phragmites australis. In many restored salt marshes on the Cape, restoring tidal flow increases water and sediment salinity enough to kill Phragmites, thereby permitting native halophyte re-establishment. However, one of the biggest salt marshes under management by the National Seashore, Hatches Harbor in Provincetown, MA, remains partially restricted to tidal flow. In areas of Hatches Harbor that remain restricted to significant tidal inputs from Cape Cod Bay, Phragmites retains its stronghold and halophyte re-establishment is spotty at best. Conservation Biology students Jesse Wheeler, Megan Boyle, and I are collaborating with National Seashore plant ecologist Steve Smith on a study to evaluate the effects of Phragmites removal and planting of halophyte seedlings on Phragmites productivity and halophyte re-establishment along environmental gradients of salinity, porewater chemistry, and proximity to the tidal creek. In this study we are evaluating both plant and seed bank community dynamics. Our results will help inform Cape Cod managers of best practices for controlling Phragmites at this and other salt marsh restoration sites.

If you are a prospective student and are interested in being involved in any of these research projects, please contact Dr. Rachel Thiet.