IAB Research Project Description

Ecosystem-level Consequences of Mutualist Partner Choice in Alder across a Forest Successional Sequence in Interior Alaska

Ruess' research field site in Interior Alaska. This project will be the first to study mechanisms for Frankia and ectomycorrhizal fungi (EMF) partner choice in a natural system, or account for mutualist partners in evaluating ecosystem-level N inputs through N fixation. Credit: Roger W. Ruess/Professor of Ecology, Institute

Roger Ruess’ team is investigating whether coordinated changes in host selection for ectomycorrhizal and Frankia partners by alder, specifically Alnus tenuifolia, enables persistence of this keystone N-fixing species throughout forest succession and regulates ecosystem-level N inputs to Interior Alaska river floodplains, such as the Tanana River pictured here. Credit: Roger W. Ruess/Institute of Arctic Biology

Credit: arcticrose.files.wordpress.com/.

Understanding controls over nitrogen (N) fixation by native and invasive plants is a problem of fundamental scientific value, particularly in Alaska, where invasive N fixers are just beginning to appear, and where the range of toothed-leaved trees and shrubs in the birch family called alders, pictured here, has expanded substantially across the boreal/tundra boundary over the past 150 years. Credit: Roger W. Ruess/Professor of Ecology, Institute

Any and all uses of these images must include photographer credit.

Understanding controls over nitrogen (N) fixation by native and invasive plants is a problem of fundamental scientific value, particularly in Alaska, where invasive N fixers are just beginning to appear, and where the range of toothed-leaved trees and shrubs in the birch family called alders has expanded substantially across the boreal/tundra boundary over the past 150 years. This project will be the first to study mechanisms for Frankia and ectomycorrhizal fungi (EMF) partner choice in a natural system, or account for mutualist partners in evaluating ecosystem-level N inputs through N fixation.

Most N-fixing plants form complex mutualisms, with both bacterial and fungal partners, involving the exchange of host carbon in support of nitrogenase and mycorrhizal activities to meet plant nutrient demands. Such plant mutualisms can be modeled as market economies, whereby hosts choose partners based on cost:benefit assessment, and regulate trading partners through a highly evolved sanction/reward system. Although the biochemistry of these associations has been studied extensively, the influence of coupled mutualisms on N fixation rates has never been studied in natural systems, or incorporated into ecosystem-level studies. Yet the strong ecological interdependency between plant-bacterial and plant-fungal mutualisms for the optimization of plant stoichiometry may help explain variation in the growth and N fixation rates of N-fixing species, and associated estimates of ecosystem N input unaccounted for by traditional environmental variables.

Ruess’ team is investigating whether coordinated changes in host selection for ectomycorrhizal fungi and Frankia partners by alder, specifically Alnus tenuifolia, enables persistence of this keystone N-fixing species throughout forest succession and regulates ecosystem-level N inputs to Interior Alaskan river floodplains.

Hypotheses put forth by Ruess’ team addresses mechanisms linking alder growth, N fixation, and mutualist partner choice across early, mid- and late-successional forests where N inputs are naturally up-regulated (high light, low N) and down-regulated (low light, high soil N), and in response to long-term N down-regulation and phosphorus up-regulation fertilization across this successional sequence. Rates and costs of N fixation associated with given Frankia partners will be measured, as will activities of enzymes in both soils and EMF linked with whole plant N:P balance. Shifts in Frankia and EMF species in response to N and P field treatments will be identified using molecular methods, and used to inform greenhouse experiments where partner choice is examined under controlled conditions. Consequences for patterns and shifts in bacterial and fungal partners will be scaled to ecosystem-level N inputs through measurements of nodule biomass and seasonal patterns of N fixation.

The project has strong ties to the Bonanza Creek Long-Term Ecological Research (BNZ LTER) program, where education and public outreach programs will likely include components of this study.

Frankia: a genus of nitrogen-fixing filamentous bacteria that live in symbiosis with plants and which form nitrogen-fixing root nodules.

Mutualisms: beneficial associations between different kinds of organisms.

Mycorrhiza: the symbiotic association of the mycelium of a fungus with the roots of a seed plant.

Nitrogenase: an enzyme of nitrogen-fixing bacteria that catalyzes the reduction of molecular nitrogen to ammonia.

Stochiometry: the quantitative relationship between two or more substances especially in processes involving physical or chemical change.

Project Funding

National Science Foundation
$796,227.00
1 Apr 2007 – 31 Mar 2010
IAB Proposal #07-004
UAF Grant #G3961
IAB Project #171


Media Contact

Marie Thoms
Information Officer
Institute of Arctic Biology
302A Irving I
University of Alaska Fairbanks
Fairbanks, AK 99775-7000
email: methoms@alaska.edu
phone: 907.474.7412
UAF-IAB-News-Info@alaska.edu