Belowground processes and global change. Invited Feature

Authors: Jackson RB, LO Hedin

Articles in the Feature:

Trumbore S 2000. Age of soil organic matter and soil respiration: radiocarbon constraints on belowground C dynamics. Ecological Applications 10:399-411.

Ehleringer JR, N Buchmann, LB Flanagan 2000. Carbon isotope ratios in belowground carbon cycle processes. Ecological Applications 10:412-422.

Jobbágy EG, RB Jackson 2000. The vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecological Applications 10:423-436.

Allen AS, JA Andrews, AC Finzi, R Matamala, DD Richter, and WH Schlesinger 2000. Effects of free-air CO2 enrichment (FACE) on belowground processes in a Pinus taeda forest. Ecological Applications 10:437-448.

Daly C, D Bachelet, JM Lenihan, RP Neilson, W Parton, D Ojima 2000. Dynamic simulation of tree-grass interactions for global change studies. Ecological Applications 10:449-469.

Jackson RB, HJ Schenk, EG Jobbágy, J Canadell, GD Colello, RE Dickinson, CB Field, P Friedlingstein, M Heimann, K Hibbard, DW Kicklighter, A Kleidon, RP Neilson, WJ Parton, OE Sala, MT Sykes 2000. Belowground consequences of vegetation change and their treatment in models. Ecological Applications 10:470-483.

Feature description:

The evidence for global change is increasingly apparent. Population growth and increased resource consumption are altering the composition of the atmosphere and transforming landscapes. While many aboveground changes are fairly obvious, there are important changes belowground that are less visible and perhaps equally important. This Feature examines interactions between belowground processes and global change, highlighting feedbacks between them. It also emphasizes the importance of belowground processes in successful predictions of global change. The goal for the Feature is to help provide frameworks for such predictions, present evidence from recent experiments, and examine the use of models for predicting the extent and consequences of global change.

Two aspects of global change most closely link the six papers in this Feature - soil carbon dynamics and vegetation change. Trumbore begins the Feature by discussing the importance of soil organic matter for the global carbon cycle. She highlights the use of radiocarbon data to estimate the proportion of soil organic matter that is relatively young (decades or less), the turnover time of that carbon, and the extent to which relatively young soil organic carbon contributes to soil respiration. Such questions are important for a basic understanding of soil processes, but they have practical relevance for the Kyoto accord and global carbon sequestration. Insight into soil and ecosystem carbon fluxes is also the theme of Ehleringer et al., but using stable isotopes rather than radioisotopes. They discuss global patterns of ecosystem respiration, including an analysis of how to partition fluxes into heterotrohic and autotrophic components. General patterns of soil organic carbon, climate, and vegetation are the subject of the contribution by Jobbágy and Jackson. Their analysis, using data from several thousand soil cores in two global soil databases, shows that climate is a predictably dominant control of the amount and vertical distribution of soil carbon, but vegetation type modifies the observed patterns significantly. Consequently, changes in plant functional types (e.g., grasses, shrubs, and trees) with vegetation change may have important consequences for organic carbon distributions and longer-term carbon storage at different depths in the soil, particularly relatively deep layers.

The second group of papers builds upon the first to examine experimental and simulated responses to global change. Allen et al. describe results from the first Free Air CO2 Enrichment (FACE) experiment in a forest ecosystem. They examine changes in belowground properties, including root and microbial biomass, litterfall, net N mineralization, and CO2 flux from the soil. They also use isotopic tools, as discussed in Trumbore and Ehleringer et al., to estimate inputs of new carbon to the soil (d13C and 14C soil signatures). Climate change, and its interaction with soil and vegetation, is the subject of the paper by Daly et al. They describe a new dynamic vegetation model that simulates vegetation distributions and carbon and nutrient fluxes in response to climate scenarios. It combines aspects of two widely used models (MAPSS and CENTURY as biogeography and ecosystem models) to address how changes in belowground attributes affect predicted vegetation type, productivity, and trace gas emissions. A fire module stresses interactions among plant functional types at the landscape scale. For the final paper, Jackson et al. examine belowground consequences of vegetation change. Deforestation, afforestation, and woody plant encroachment typically alter the relative abundance of grasses, shrubs, and trees. These changes in plant functional types, in turn, can change ecosystem properties, including soil nutrient distributions, the water balance, and plant primary productivity. Since models are an increasingly important tool for predicting the effects of vegetation change, the paper discusses ways current models differ in their treatment of belowground processes and how these differences may affect model outputs.

Several workshops and integrated activities contributed to the ideas presented in this Feature. These include workshops at the National Center for Ecological Analysis and Synthesis, the Dahlem Conference "Integrating Hydrology, Ecosystem Dynamics, and Biogeochemistry in Complex Landscapes" (organized by J. Tenhunen and P. Kabat), and the Global Change and Terrestrial Ecosystems (GCTE) open science conference in Barcelona, Spain. The papers presented here contribute to the efforts of GCTE, a core project of the International Geosphere Biosphere Programme (IGBP), whose twin goals are to predict the effects of changes in climate, atmospheric composition, and land use on terrestrial ecosystems and to determine how these effects lead to feedbacks with the atmosphere and climate.

Ultimately what we seek as ecologists is a better understanding of ecology as an integrated science. Dichotomies of "plant and animal" or "belowground and aboveground" are conveniences, allowing processes to be broken down conceptually. Reassembling the parts is eventually necessary - be it combining root and shoot functioning, the exchange of material among plants, the soil, and the atmosphere, or other convenient divisions. The challenges of global change research have made that reassembly especially important, blurring traditional distinctions among ecology, biogeochemistry, hydrology, atmospheric science, and other disciplines. That blurring is, in my opinion, productive and essential for solving today¹s global problems and for continued growth of the diverse science of ecology.
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