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Project Descriptions
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Prospect Hill soil warming study. Snow melts first on the soil-warmed plots. Various measurements, such as net nitrogen mineralization, carbon flux and carbon and nitrogen in leaves (collected in the laundry basket), allow us to examine nutrient cycling in a warmer world.
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Prospect Hill Soil Warming
In April 1991, we established 18 six by six meter plots assigned to one of three treatments: (1) heated plots in which the average soil temperature at 5cm is elevated 5°C above ambient using buried heating cables; (2) disturbance-control plots that have buried heating cables identical to the heated plots but receive no electric power; and (3) control plots that have been left in their natural state. In the heated and disturbance-control plots, heating cables are buried at a depth of 10cm in rows spaced 20cm apart. Electrical current is supplied only to the cables in the six heated plots.
Over the course of the study, we measured trace gas fluxes (CO2, CH4, N2O), various indices of nitrogen availability, and soil water content (lysimetry). Taken together, these measurements allow us to quantify key biogeochemical responses of this forest ecosystem to soil warming. In addition, we have begun to explore how warming effects changes in soil microbial communities and enzymology. Thus far, we have found that soil warming stimulates the decay of a labile soil carbon pool as well as increasing the availability of inorganic nitrogen to plants. We are continuing to do baseline measurements of CO2 flux from the soil.
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Soil warming research assistants Sarah Butler and Jennifer Johnson taking a soil core. Using the buried bag method, we measure the net nitrogen mineralized in the plots to determine if soil warming alters the nitrogen cycle.
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Barre Woods Soil Warming
In 2001 we established a new larger-scale soil-warming experiment in the Barre Woods area (Slab City tract) at the Harvard Forest where we are tacking the effects of soil warming on respiration. We are looking at the effects of increased nitrogen availability on carbon storage. These larger plots allow us to examine land-atmospheric feedbacks as the world warms.
Our ‘megaplots’ consist of 30 by 30 meter heated and control plots. In the heated plot, over 5 km of heating cable was buried. In April 2002, we began a one-year period of "baseline measurements" before turning on the heat in the new megaplot. These measurements included soil respiration, nitrogen mineralization, trace gas fluxes (CH4 and N2O), tree growth (dendrometer bands), an understory species survey, canopy foliage analysis (C:N, NIR), and soil water content (lysimetry), root respiration and root biomass. Thermistors were installed in both plots to begin tracking soil temperatures. In addition to these baseline measurements, we have started to look at the _15N in soils, roots, and leaves, as well as soil and leaf enzymes to get a better handle on the carbon and nitrogen dynamics in the forest. We hope to gain a better understanding of the re-distribution of carbon and nitrogen in our forested ecosystems in a warmer world.
After six years of warming, we have observed changes in both the carbon and nitrogen cycles on the plots. Soil respiration is elevated by warming, fine root biomass (<1mm in diameter) has decreased substantially in the warmed plots, and root respiration per unit mass has increased in the warmed plots. Carbon storage in trees in the warmed plots has increased in response to increased nitrogen availability and a lengthening of the growing season. While the warmed plots are a net source of carbon to the atmosphere, the carbon accumulation in trees compensates for a substantial portion of the carbon lost from the soil.
Warming increased the fraction of mineralized nitrogen that was nitrified, with approximately 25% of the mineralized nitrogen nitrified. Measurements of natural isotope abundance in canopy dominants indicate that red maple, known to have inducible nitrate reductase capacity, may be taking special advantage of this nitrate resource. During the 2009 growing season, we will continue nitrification measurements, conduct intensive nitrate reductase surveys of canopy dominants, and relate nitrate reductase activity to nitrogen isotopic composition of green leaves.
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Research assistant Chelsea Vario buries heating cable in an open-top growth chamber. In addition to soil warming, these systems will also have an air-warming component. Seeds will be planted to examine the effect of warming on species’ germination, growth and distribution.
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Hot Plants Soil and Air Warming
Climate change will likely cause changes in forest structure and composition over the coming century. However, how the increases in temperature will alter the recruitment, germination, growth and mortality of our forests is uncertain. This study aims to examine how temperate tree species near the northern or southern end of their ranges in the eastern United States will respond to warming. Our study is designed to determine if shifts in species’ ranges will occur in a warmer world. We are conducting air and soil warming experiments in two eastern deciduous forests: the Harvard Forest in central Massachusetts and the Duke Forest in central North Carolina.
At each site, we have two research areas: under the canopy and in a light gap (created by a small clearcut). In each of these areas we have nine 3 by 5 meter open-top chambers arrayed along a naturally occurring moisture gradient. Soil temperature in the heated plots is raised to 5°C above ambient with buried resistance cables, as in the two above ongoing studies. Air temperatures are being raised by a hydronics system that delivers hot air to the chambers, at a target of 3 and 5°C above ambient.
At each site, we have planted seeds and seedlings of selected tree species in temperature-controlled, open-top chambers. We will measure tree species responses such as seed germination, growth, and mortality. In addition, we will make a set of physiological and biogeochemical measurements including leaf-level photosynthesis, net nitrogen mineralization, and plant tissue chemistry. The species under study at each site will include those occurring under the current climate conditions as well as some species native further south.
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