Forest Long Term Ecological Research: Harvard Forest, Massachusetts (1989-present

Several Ecosystems Center researchers have been involved in the Harvard Forest Long Term Ecological Research (LTER) project from its inception. This involvement stems from a long history of collaboration between Ecosystems Center scientists Jerry Melillo, Knute Nadelhoffer, and Paul Steudler, working with John Aber, of the University of Wisconsin. Starting in 1978, Melillo and Steudler worked on a US National Science Foundation (NSF) grant with Aber to study the fate of carbon and nitrogen when forests decompose in Wisconsin and Massachusetts. The NSF funded this group with sequential grants from 1978 for about a decade. Aber moved to the University of New Hampshire in 1987, and Melillo, Nadelhoffer, and Steudler continued their collaboration with him on a forest ecosystem project.

Four scientists from the Ecosystems Center were involved when the Massachusetts forest research site, called Harvard Forest, became an official LTER project. The NSF gave a five-year, renewable grant to a group of researchers from five universities and institutions in New England for the Harvard Forest LTER. Ecological research had been going on at this site since 1907. The 1989 LTER grant for Harvard Forest focused on studying how forest ecosystems respond to disturbances. Ecologists use the term disturbance to mean outside phenomena that affect the average conditions of an ecosystem, often producing long-term changes in that ecosystem—examples of disturbances include tornados and forest fires. Because of recent environmental changes, such as acid rain and increased carbon dioxide levels, scientists at Harvard Forest were also interested in the impact of these chemical disturbances on the forest ecosystem.

Researchers from the Ecosystems Center have been involved in two main studies at the Harvard Forest LTER site: adding nitrogen fertilizer to the soil, and heating the soil. Both studies aim to discover the long-term effects of these disturbances. The nitrogen study started in 1988 (before the LTER was started) when Melillo, Steudler, Nadelhoffer, and Aber began adding nitrogen-based fertilizer to large plots of forested land in Massachusetts and Maine. They started this experiment because scientists predicted nitrogen emissions from agricultural fertilizer byproducts and fossil fuel burning would fertilizer forests, causing them to grow faster. In collaboration with Brian Fry, Melillo and his colleagues measured growth in the trees and other plants, gases emitted from the soil, and how dead plant materials decomposed. The fertilizer contained added nitrogen stable isotopes, which the researchers could trace as it moved through the forest ecosystem over time. After eight years of study, in 1996, researchers found that although trees do uptake some of the added nitrogen, allowing them to grow faster, most of the added nitrogen ended up in the soil, where microbes consumed it. This caused microbes to emit carbon dioxide and nitrous oxide, both of which contribute to climate change.

The soil warming experiment started in 1991 at Harvard Forest. Researchers aimed to study the response of forest ecosystems to increased temperatures anticipated as a result of climate change. They buried heating cables under the soil of several six by six meter plots in the forest. Some of the plots were heated, some had buried cable but were not heated (to test the impact of just the buried cables), and some plots had no cables or heat. A computer system was used to turn on the cable heating so that the soil temperature of the heated plots was increased by five degrees Celsius relative to the surrounding soil temperature. The first ten years of the study were funded by the US Department of Energy, the US Environmental Projection Agency, ExxonMobil Corporation, and the US National Science Foundation. After the experiment ran for ten years, researchers found that soil warming stimulated microbes in the soil to increase their activity, which released carbon dioxide. The microbes also made more nitrogen available in the soil, which surrounding trees subsequently absorbed. This caused trees to increase their uptake of carbon dioxide from the atmosphere. In 2003, researchers added 30 by 30 meter heated and control plots to measure the impact of soil warming on larger patches of land that included trees.

The soil warming long-term experiment at Harvard Forest had some unexpected results after 23 years of testing. Microbes initially increased their activity, consuming simple carbon-based materials in the soil. After about ten years, however, they slowed to the levels of activity seen in the control (non-heated) plots. But then starting in the sixteenth year of the study, microbial activity in the heated plots increased again. Researchers hypothesized that the microbe population changed physiologically so that the microbes could “eat” more complex carbon-based materials in the soil, or that the microbe population evolutionary adapted to environmental changes. The Harvard Forest soil warming experiment is one of the longest-running soil warming experiments, and no other researchers have found these long-term results. It is possible that with the predicted impacts of climate change, other ecosystems may start to see changes in microbial populations under increased temperature.