Looking back

  • Motivation: Diagnosing and predicting the effects of global warming on terrestrial ecosystems is challenging because terrestrial ecosystems both respond to and feed back on the atmosphere. Since the thermal optimum of photosynthesis is lower than that of respiration, it has been widely anticipated that warming will result in a positive feedback on the carbon-climate system.

  • Background: The core expectation is that when temperatures reach the point where photosynthesis starts to fall but respiration continues to rise, the net loss of carbon dioxide to the atmosphere will tend to drive continuing warming. However, many potential interactions have been identified that could either amplify and/or suppress this pattern.

  • Approach: To understand the direction and magnitude of the most likely net effect, researchers have started to test hypotheses about the temperature response with “ecosystem experiments” that have much larger spatial scales and longer temporal scales than traditional biological experiments. The one that got me hooked on this approach is described below.

  • 1. The long-term soil warming experiments

    To study warming-driven feedbacks, Jerry Melillo of the MBL Ecosystems Center designed a series of experiments that have been implemented at the Harvard Forest LTER. In 2007-2008, I worked as a technician to support these projects, focusing primarily on the Barre Woods experiment. Pictured here are my colleagues Frank Bowles and Sarah Butler at Barre Woods.

  • 2. Barre Woods: large-scale and long-term

    The Barre Woods experiment started in 2002 and uses buried resistance cable to warm a forest soil 5°C above ambient over a 30 x 30 m plot. This manipulation is unique because it is large enough to enclose the entire above- and below-ground structures of mature canopy trees, and has been sustained for long enough to trace feedbacks throughout the ecosystem.

  • 3. The direct effects of warming

    The experiment has clearly shown all of the expected direct effects of warming: stimulation of soil microbial respiration, acceleration of the decomposition of soil organic matter, and release of carbon dioxide to the atmosphere as well as inorganic nutrients to the soil solution.

  • 4. The indirect effects of warming

    It has also permitted quantification of several indirect effects of warming: uptake of the additional inorganic nutrients from the soil solution by the tree root systems, and use of these nutrients to drive increased photosynthetic uptake of carbon dioxide from atmosphere and storage in woody tissues.

  • 5. The overall net effect of warming

    In this ecosystem, the redistribution of nitrogen from the soil organic matter to new woody growth results in an order of magnitude increase in the ratio of carbon stored per unit nitrogen. Due to this mechanism, the warming-driven carbon losses from the soil were almost exactly compensated by the carbon gains in the vegetation over the period I worked here.

The full story, and its significance

  • Full story: The full details of our findings can be found in these references:

    • References: (1) J. M. Melillo, S. Butler, J. E. Johnson, J. Mohan, P. Steudler, H. Lux, E. Burrows, F. Bowles, R. Smith, L. Scott, C. Vario, T. Hill, A. Burton, Y. Zhou, and J. Tang. 2011. Soil warming, carbon-nitrogen interactions, and forest carbon budgets. Proceedings of the National Academy of Sciences 108: 9508-9512; (2) Butler, S. M., J. M. Melillo, J. E. Johnson, J. Mohan, P. A. Steudler, H. Lux, E. Burrows, R. M. Smith, C. L. Vario, L. Scott, T. D. Hill, N. Aponte, and F. Bowles. 2012. Soil warming alters nitrogen cycling in a New England forest: implications for ecosystem function and structure. Oecologia 168: 819-28

    • Article DOI: 10.1073/pnas.1018189108 and 10.1007/s00442-011-2133-7

    • Press: Discover Magazine article

    • More info: Harvard Forest LTER page on soil warming experiments

  • Significance: Prior to these experiments, the idea that warming-induced redistribution of nitrogen from soil to vegetation could alter the carbon budget of nitrogen-limited forests was an untested hypothesis. These experiments provided empirical support for this hypothesis and motivation for representing carbon-nitrogen interactions in Earth system models.