Long Term Ecological Research (LTER)

Long Term Ecological Research

The Hubbard Brook Long Term Ecological Research (LTER) program is part of a national network of long-term ecological research sites. Established as an LTER in 1988, the focus of research has consistently been on the impact of disturbance on the Northern Forest. The administrative home of the Hubbard Brook LTER project is the Cary Institute of Ecosystem Studies, and the project has 45 co-Principal Investigators at more than 20 collaborating institutions.

The mission of the Hubbard Brook (HBR) LTER program is to improve understanding of the response of northern forest ecosystems to natural and anthropogenic disturbances. The conceptual model guiding our research envisions disturbance playing out on a biogeophysical template that influences the biogeochemistry, vegetation, hydrology, and food webs of the northern forest. Our research focuses on three drivers of disturbance: (1) atmospheric chemistry change, (2) climate change, and (3) biotic change. These drivers interact with a biogeophysical template (i.e., topography, geology, soils, vegetation, disturbance history), producing spatial and temporal variation within our site and across the broader region. 

Our conceptual model and research program have continually evolved as unexpected observations emerge from our long-term studies, raising questions that can only be addressed with continued long-term research. Recent surprises that we are currently investigating include:

1. The elevational distribution of some tree species has shifted upward, as expected in response to climate warming. However, other species have surprisingly migrated downward in response to succession, recovery from acid deposition (i.e., deacidification), and pests and pathogens. Interacting responses to multiple environmental changes complicate predictions of future community structure and function. 
2. Rates of evapotranspiration abruptly increased by about 30% over the past 10 years (since 2010). The drivers of this fundamental change in ecosystem function involve complex interactions among climate, biota, and atmospheric chemistry. 
3. Exports of nitrogen from the reference watershed (W6) continue to decline (since the 1970s), contrary to established theory that the opposite should be occurring in these aging forests. A “nitrogen oligotrophication” process, driven by declines in atmospheric nitrogen deposition, increases in atmospheric carbon dioxide concentrations and growing season length and temperature, and decreases in acid deposition may be driving these declines. This process may affect ecosystem recovery from disturbance, and drive declines in insect and bird populations, forest productivity, and carbon sequestration.
4. We have observed significant declines in birds, salamanders, caterpillars, and beetles since the 1970s, but not all species have declined in all places. This variation may be linked to factors as diverse as forest maturation, deacidification, regional species declines, and changes in seasonality and snow cover.
5. Analysis of long-term (>50 year) biology and biogeochemistry data from Hubbard Brook for early warning signals of state change suggest declining resilience in watershed nutrient retention and tree, bird, and insect populations. Changes in early warning signals were correlated with precipitation chemistry and temperature, suggesting that acid deposition and climate change have reduced ecosystem resilience and compromised biological and biogeochemical functioning of the ecosystems at Hubbard Brook.

Our work also includes educational and outreach efforts aimed at undergraduates, graduate students and informal science education for interested adults in the region. We hold roundtable discussions with stakeholders to discuss the results and implications of our science, and we engage with policy makers in the state and federal government.

Our approach to broader impacts has also evolved. New ideas about engagement with community members bring scientists and stakeholders together to share knowledge and perspectives about socio-scientific issues, resulting in scientific synthesis papers and  professional development programs for young scientists. A new “Young Voices of Science” program began in 2020 and follows a participant-centered cohort model that pairs a semester-long series of online, skills-based seminars with one-on-one mentorship for outreach projects and policy engagements. 

Another new effort will incorporate a “critical ecology” framework which merges critical theories of power, privilege, and human/environment interactions with existing frameworks for ecological perturbations. This new approach will allow us to expand our understanding of the societal roots of disturbances such as acid rain and climate change, address past, current, and future interests of Indigenous people in the northern hardwood forest, and increase diversity, equity and inclusion in our research group.