Belowground carbon allocation (BCA) in forests regulates soil organic matter formation and influences biotic and abiotic properties of soil such as bulk density, cation exchange capacity, and water ...holding capacity. On a global scale, the total quantity of carbon allocated belowground by terrestrial plants is enormous, exceeding by an order of magnitude the quantity of carbon emitted to the atmosphere through combustion of fossil fuels. Despite the importance of BCA to the functioning of plant and soil communities, as well as the global carbon budget, controls on BCA are relatively poorly understood. Consequently, our ability to predict how BCA will respond to changes in atmospheric greenhouse gases, climate, nutrient deposition, and plant community composition remains rudimentary. In this synthesis, we examine BCA from three perspectives: coarse-root standing stock, belowground net primary production (BNPP), and total belowground carbon allocation (TBCA). For each, we examine methodologies and methodological constraints, as well as constraints of terminology. We then examine available data for any predictable variation in BCA due to changes in species composition, mean annual temperature, or elevated CO2 in existing Free Air CO2 Exposure (FACE) experiments. Finally, we discuss what we feel are important future directions for belowground carbon allocation research, with a focus on global change issues.
Climate change predictions derived from coupled carbon-climate models are highly dependent on assumptions about feedbacks between the biosphere and atmosphere. One critical feedback occurs if C ...uptake by the biosphere increases in response to the fossil-fuel driven increase in atmospheric CO{sub 2} ('CO{sub 2} fertilization'), thereby slowing the rate of increase in atmospheric CO{sub 2}. Carbon exchanges between the terrestrial biosphere and atmosphere are often first represented in models as net primary productivity (NPP). However, the contribution of CO{sub 2} fertilization to the future global C cycle has been uncertain, especially in forest ecosystems that dominate global NPP, and models that include a feedback between terrestrial biosphere metabolism and atmospheric CO{sub 2} are poorly constrained by experimental evidence. We analyzed the response of NPP to elevated CO{sub 2} ({approx}550 ppm) in four free-air CO{sub 2} enrichment experiments in forest stands. We show that the response of forest NPP to elevated CO{sub 2} is highly conserved across a broad range of productivity, with a stimulation at the median of 23 {+-} 2%. At low leaf area indices, a large portion of the response was attributable to increased light absorption, but as leaf area indices increased, the response to elevated CO{sub 2} was wholly caused by increased light-use efficiency. The surprising consistency of response across diverse sites provides a benchmark to evaluate predictions of ecosystem and global models and allows us now to focus on unresolved questions about carbon partitioning and retention, and spatial variation in NPP response caused by availability of other growth limiting resources.
We investigated the influence of elevated CO
2
and soil N availability on the growth of arbuscular mycorrhizal and non‐mycorrhizal fungi, and on the number of mycophagous soil microarthropods ...associated with the roots of
Populus tremuloides
. CO
2
concentration did not significantly affect percentage infection of
Populus
roots by mycorrhizal or non‐mycorrhizal fungi. However, the extra‐radical hyphal network was altered both qualitatively and quantitatively, and there was a strong interaction between CO
2
and soil N availability. Under N‐poor soil conditions, elevated CO
2
stimulated hyphal length by arbuscular mycorrhizal fungi, but depressed growth by non‐mycorrhizal fungi. There was no CO
2
effect at high N availability. High N availability stimulated growth by opportunistic saprobic/pathogenic fungi. Soil mites were not affected by any treatment, but collembolan numbers were positively correlated with the increase in non‐mycorrhizal fungi. Results indicate a strong interaction between CO
2
concentration and soil N availability on mycorrhizal functioning and on fungal‐based soil food webs.
Small diameter (<1.0-mm) Acer saccharum Marsh roots were separated into white, brown and woody development state classes and analyzed for total N and C concentrations in April, July and October of ...1988. White roots had greater concentrations of N and C than either brown or woody roots at each sampling date, and the N concentration of brown roots was consistently greater than that of woody roots. There were no temporal changes in N concentrations in any of the roots. C was slightly elevated in mid-summer in all three classes of roots. The data suggest the possible existence of an N translocation mechanism in ageing and developing fine roots. More research should be undertaken to establish the mechanisms of N loss in developing fine roots.
The study involved the delineation and analysis of forest ecosystems in the Cyrus H. McCormick Experimental Forest, Upper Michigan, U.S.A. There were three general objectives: to develop a local ...ecosystem classification using a multifactor approach, to determine the most efficient method of predicting ecosystem membership, and to study the ability of ground flora to predict edaphic factors. Special attention was given the role of physiography and soils in ecosystem classification and the functional relations between plant and soil. The study area was stratified into biologically equivalent ecosystems using a special reconnaissance field technique. Sixty-six plots were sampled using stratified random sampling. Detailed observations were recorded on physiography, soils, and vegetation. Standard soil laboratory analyses included particle-size distribution, organic matter, pH, and macronutrients. Statistical analyses were used to compare ecosystems and test the validity of the field classification. The analyses included multiple linear regression, analysis of variance, principal component analysis, and numerical clustering methods. Multivariate discriminant analysis and canonical ordination were used to predict ecosystem membership. Unbiased probabilities of misclassification based on discriminant analysis were calculated using the jackknife method. Empirical distribution functions and rank order correlation were used to study the distribution of ground flora species over environmental gradients. Eleven dryland forested ecosystems were identified using a multifactor approach. Each ecosystem occupied a characteristic topographic position within the landscape. Individual ecosystems significantly differed in many of their biophysical properties such as slope, aspect, soil texture, soil drainage and soil fertility. The ecosystems were characterized by distinctive potential overstory and ground flora compositions. Deep, well drained soils with more than 10% silt and clay had overstories strongly dominated by Acer saccharum Marsh. Extremely xeric sites, either very well-sorted sands or rocky soils, supported overstories dominated by Pinus banksiana Lamb., Pinus strobus L., Quercus rubra L. and Acer rubrum L. Imperfectly drained sites had a diverse overstory vegetation. Differences in forest composition and environmental factors showed close correspondence over the late successional forests of the study area. Placing the emphasis of ecosystem classification on the biophysical structuring of the local landscape greatly facilitated the understanding of the functional relations among plant and environment. The most reliable and efficient method of local ecosystem classification was the use of a combination of physiography, soils and vegetation. The combined approach classified ecosystems 18% more efficiently than the use of vegetation alone and 5% more efficiently than the use of physiography and soils alone. Relations among ecosystems using ordination techniques depend fundamentally on the data used to produce the ordination. Ordinations differed when comparing physiography and soils data with vegetal data. A combination of both produced the best ordination. Certain ground flora species, combined into ecological species groups, were found to be accurate and effective indicators of soil moisture, soil texture, soil pH and soil total nitrogen. It was possible to make probability statements about specific edaphic conditions merely by noting the presence or absence of selected ecological species groups.
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