Background

1. How Does Belowground Carbon Allocation Change Along A Resource Gradient? Kathryn A. Berger Department of Natural Resources Complex Systems Research Center, Institute for the Study of Earth, Oceans and Space University of New Hampshire, Durham, NH 03824 katie@guero.sr.unh.edu • Objectives • To determine if changes in belowground carbon allocation occur along a resource gradient. • To determine if PnET correctly captures belowground carbon allocation dynamics predicted by the root-to-shoot ratio. • Introduction • Concentrations of carbon dioxide (CO2) in the atmosphere have increased significantly over time resulting in deleterious effects on the environment. • Increased levels of CO2 in the atmosphere caused by anthropogenic sources have increased approximately 35% since the industrial revolution (Luo, Hui, and Zhang, 2006). • Of all the carbon released through anthropogenic sources (e.g., fossil fuel combustion, deforestation) approximately 45% remains in the atmosphere, while the remaining 55% is either absorbed into the ocean or terrestrial ecosystems (Norby, 1997) • The challenge is to identify the missing carbon sink and to better explain the pools and fluxes in the terrestrial ecosystem. • PnET Model • PnET is a simple, monthly time-step model that uses very few input parameters to predict carbon and water dynamics in forest ecosystems. • PnET is able to demonstrate important interactions between foliar nitrogen availability (measured by foliar nitrogen concentrations) and leaf physiology as they effect photosynthesis and transpiration (Aber and Federer, 1992). • The model measures plant productivity in the plant pool through allocation by tissue type: foliage, wood, and fine-roots. • There are currently three models of PnET: • PnET models fine-root production by an equation based upon the relationship between aboveground litter production and carbon allocation to roots developed by Raich and Nadelhoffer (1989): • Rs – Pa ≈ Pb + Rr (Raich and Nadelhoffer, 1989) • The Raich and Nadelhoffer (1989) relationship is based upon the assumption that the forest is at steady-state. • Proposed Research • The PnET model’s mechanism of belowground carbon allocation has not been examined along resource gradients. To validate PnET’s predictions of this pool, I will examine the relationship along two resource gradients: elevated levels of atmospheric CO2 and nitrogen availability. • Part I: PnET under elevated CO2 • Compile published data from FACE experimental forests: • New climate, vegetation files and parameterizations for each site • Run sensitivity analysis for unknown parameter values • Compare results of PnET-Day to published values from eddy flux towers • Run PnET-CN for FACE sites • Compare PnET-CN results with published FACE site experimental data • Determine if root-to-shoot mechanism in PnET creates valid predictions • Part II: Nitrogen availability • Create a database based on published values of foliar and soil metric measurements in New England forests • Potential measurements include: soil respiration, litterfall, nitrogen mineralization, foliar nitrogen concentrations, and C:N ratios • Analyze database for trends between belowground carbon allocation and nitrogen availability • Background • The extent to which terrestrial ecosystems are able to store excess carbon is debated in literature. • Soils accumulate two thirds of all carbon allocated to terrestrial ecosystems, making it the largest pool of carbon in forests (Canadell, Pitelka, and Ingram, 1996). • The soil carbon pool has one of the longest residence times, making it possible for long-term carbon sequestration in the future (Canadell, Pitelka, and Ingram, 1996). • Many environmental factors influence belowground carbon allocation: • Elevated levels of atmospheric CO2 • Nitrogen availability • Elevated levels of atmosphericCO2 have repeatedly demonstrated increased photosynthesis in young forests and growth enhancement of mature forests in open-air CO2 enrichment experiments (Ollinger et al., 2002). • The scale by which increased photosynthesis increases carbon storage over the long-term will be highly dependent on a variety of feedback mechanisms between plant carbon-nitrogen (C:N) rations, rates of litter decomposition, and availability of nitrogen in the soil (Ollinger et al., 2002). • The PnET model, developed at UNH by Aber and Federer (1992), has never been run for long-term free-air CO2 enrichment (FACE) sites and their modeling of belowground carbon allocation dynamics. Figure 2.0 Sample output from PnET-CN model for Harvard Forrest, Petersham, MA Figure 1.0. PnET model interface. Includes all three nested PnET models on a selection of validated sites Figure 3.0. Experimental design of Oak Ridge National Laboratory’s free-air CO2 enrichment (FACE) experimental forest. References Aber, J.D., and A. Federer. 1992. A Generalized, Lumped-Parameter Model of Photosynthesis, Evapotranspiration and Net Primary Production in Temperate and Boreal Forest Ecosystems. Oecologia 92: 463-474. Canadell, J.G., L.F. Pitelka, J.S.I. Ingram. 1996. The Effects of Elevated [CO2] on Plant-Soil Carbon Below-Ground: A Summary and Synthesis. Plant and Soil 187: 391-400. Luo, Y., D. Hui, and D. Zhang. 2006. Elevated CO2 Stimulates Net Accumulations of Carbon and Nitrogen in Land Ecosystems: A Meta-Analysis. Ecology 87 (1): 53-63. Norby, R. 1997. Inside the Black Box. Science 388: 522-523. Ollinger, S.V., et al. 2002. Interactive Effects of Nitrogen Deposition, Tropospheric Ozone, Elevated CO2 and Land Use History on the Carbon Dynamics of Northern Hardwood Forests. Global Change Biology 8: 545-562. Raich, J.W., and K.J. Nadelhoffer.1989. Belowground Carbon Allocation in Forest Ecosystems: Global Trends. Ecology 70 (5): 1346-1354. (Photo Credit: http://www.esd.ornl.gov/facilities/ORNL-FACE/expdes.html) Acknowledgements NASA/UNH Research and Discover Fellowship Advisor: Scott Ollinger (CSRC-EOS, UNH) Thesis Committee: Christy Goodale (Cornell University), Andrew Richardson (US Forest Service, Durham, NH) & Mary Martin (CSRC-EOS, UNH)