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Grants:"Acclimation of a Forest Ecosystem to Elevated Carbon Dioxide", DOE - PER, $150,000, 2001 - 2004 (Subcontract from Forest-Atmosphere Carbon Transfer and Storage (FACTS) - renewal 2)
"Acclimation of a Forest Ecosystem to Elevated Carbon Dioxide", DOE - PER, $495,824, 1995 - 1998
With our initial funding (DE-FG02-95ER62127 and DE-FG02-95ER62124) we found that exposure of an intact pine forest (Duke FACTS1 experiment) to an increase in atmospheric CO2 of 200 m l-1, operating through a sustained stimulation of photosynthesis (Myers et al. 1999), caused a 27% stimulation in net primary production (NPP; DeLucia et al. 1999) and a 41% stimulation in net ecosystem production (NEP; Hamilton unpublished). A stimulation of global forest NEP of this magnitude would store only ~10% of the fossil fuel CO2 in the atmosphere by the year 2050, and there is evidence that this stimulation may be abating as the forest enters a phase of soil nitrogen limitation. Recent data from our experiment (App. I), as well as a report from a nearby-unreplicated prototype (Oren et al. 2001) indicate that the growth response is slowing. The observation that N demand by trees under CO2 enrichment now exceeds N supply (Finzi et al. 2001) suggests that the forest is becoming N limited. Physiological acclimation to low N supply, operating at the foliage and canopy level, may drive this loss of responsiveness. Moreover, in constructing the C budget for this forest we have identified key uncertainties in our ability to accurately extrapolate respiratory processes to the ecosystem level. The objectives of this research are to 1) through detailed photosynthetic and biochemical measurements of different needle ages and canopy locations, and at different scales from leaf to canopy, diagnose the onset of N limitations to tree growth; and, 2) to apply new methods to address important assumptions in our estimates of the carbon budget for forest plots exposed to elevated CO2.
We propose to make physiological and chemical measurements at the leaf, branch and canopy level to determine if an N limitation affects the growth response to elevated CO2. Because N is highly mobile in plant tissues, we predict that an N limitation will be evident first in older foliage cohorts, as N is moved to support new growth. This prediction will be tested by gas-exchange analysis and direct measurement of N content of different leaf ages, combined with a demographic analyses of needle longevity in control and enriched plots. A novel analysis of canopy N content will capture putative N limitation and changes in its distribution at the ecosystem level. We expect a linear relationship between the cumulative distribution of N and leaf area with depth in the canopyan increase in the slope or intercept of this relationship will indicate N dilution, independent of changes in specific leaf area, under elevated CO2. The detailed analysis of canopy leaf area and N content, and the relationship between photosynthesis and leaf N, will also provide the values to drive PnET, a process-based ecosystem model. With this model, we will calculate the potential affects N redistribution and limitation on gross primary production in control and enriched plots.
Our most recent estimates of gross primary production (GPP) and ecosystem respiration (Re) for loblolly pine are high relative other reports in the literature. These discrepancies may call into question some of the key assumptions in extrapolating tissue-specific fluxes to the ecosystem level. The second thrust of our proposed research is to examine two of these assumptions for canopy and wood respiration. Estimates of canopy respiration (~25% of Re) rely on the assumption that day respiration is a fixed proportion of night respiration. No data on Rday are available for loblolly pine and changes in leaf N and carbohydrate status under elevated CO2 may change the relationship between Rday and Rnight. Using the "Laisk" method we will determine these values for trees in control and enriched plots at different time during the year. CO2 efflux from woody tissues also contribute about ~25 to Re, and a substantial proportion of the respiration may be contamination from soil CO2 moving in the transpiration stream. This "contamination" would cause an overestimate of plant respiration and thereby increase our estimate of GPP. We propose novel simultaneous measurements of CO2 and O2 exchange from tree boles, and to use the unique C isotope signature in FACTS1 to determine the magnitude of this contamination.
The coordinated series of physiological measurements we have proposed will provide new insight into processes regulating the forest C cycle under elevated CO2. Combined with ongoing measurements of tree growth, data from this research will contribute to a dynamic forest carbon budget that will provide a benchmark for other modeling and empirical studies.