ABSTRACT

The Hubbard Brook (HBR) Long-Term Ecological Research (LTER) project is an interdisciplinary research program focused on improving the understanding and management of Northern Forest ecosystems. These important natural resources that contribute ecosystem services such as carbon storage, nutrient cycling, water and air purification, and wildlife habitat are impacted by natural and man-made disturbances. Those disturbances include climate variation, air pollution, invasive species and forest harvesting, and this research focuses on how forest ecosystems respond to those disturbances. Despite a long record of research on forest ecosystems, recent observations from HBR of how trees move water through the ecosystem, the role of calcium in forest health, the movement of nitrogen through the ecosystem, and the interactions of plants, insects, and birds within the forest indicate that much is still unknown. Most of the research will occur within the Hubbard Brook Experimental Forest, a site in the White Mountain region of New Hampshire that is operated by the U.S. Forest Service. The research team will measure the forms of nitrogen retained in soils, the growth and death of trees, the fates of germinating tree seedlings, the amounts and types of litter fall, rates of soil respiration, and how fine roots respond to nitrogen and phosphorus additions. They also will test how the exchanges of gases, heat, and water vapor between forests and the atmosphere respond to the timing of leaf out in the spring and leaf fall in the autumn. Information about forested ecosystems and the streams that drain them will be made available to land managers through a Forest Science Dialogues Program and a Science Policy Exchange and will inform policy and management decisions on a regional and national scale. The educational and outreach activities planned, including development of classroom curricula for middle- and high school students, a Research Experience for Undergraduates program that is organized in partnership with Plymouth State University, the training of teachers through the Research Experiences for Teachers program, and development of Waterviz, a water cycle visualization tool, will bring ecological knowledge to the public as well as students and teachers at levels from K-12 to graduate school.

The conceptual model underpinning the HBR-LTER project envisions three principal types of disturbance acting as drivers of change in the ecosystem: atmospheric chemistry, climate, and biota. The effects of these drivers play out on geophysical and historical templates that include variation across the landscape in bedrock, soils, hydrology, climate and history of past disturbance. Within the ecosystem, the disturbances affect the interacting processes of hydrology, biogeochemistry, vegetation dynamics and food web dynamics. Research on the changing atmosphere is focused primarily on the legacies of past air pollution, particularly the depletion of nutrient cations such as calcium from the soil and the accumulation of nitrogen in soil and vegetation. Research on the impacts of climate is focused on causes of the observed long-term decline in evapotranspiration at the site and the effects of changing timing and duration of seasonal transitions on plants, soils, microbes, animals and stream ecosystems. Research on biotic change is focused on the changing composition and structure of the forests caused by multiple interacting stressors including climate, new plant species immigrations, invasive forest pests, and altered disturbance regimes. Research methods include: long-term field measurements with stable isotopes to provide a better resolution of the strengths of nitrogen sinks in soils and metagenomics shotgun sequencing of community DNA (Illumina HiSeq platform) to provide initial characterization of how microbial community composition and function vary with soil depth; field experiments to identify belowground responses that mediate nutrient recycling and plant uptake in response to nutrient additions and the ensuing impacts on soil enzyme activities, microbial nutrient pools and turnover, rhizosphere allocation, and mycorrhizal functional groups; simulation modeling with quantitative models such as stand-level ecosystem carbon and nutrient cycling models, a carbon and nitrogen model parameterized for individual tree species, an ecosystem demography model, and a multiple element limitation model to synthesize data and experimental knowledge will be used to identify knowledge gaps, develop hypotheses, and make prediction of future forest behavior; and laboratory studies including 15N NMR to better determine the forms of nitrogen retained in soil organic matter. Education activities include hosting school groups at the site, hands-on training of teachers in scientific research, mentoring undergraduate student research projects, and facilitating research by graduate students. Outreach activities include synthesizing results of research done at HBR and elsewhere, and communicating the science to local stakeholders through roundtable discussions, to the public through targeted media, and to policy makers through briefings and publications addressing specific policy questions.

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