Data Synthesis
Based on prior analyses (Jackson et al. 1996, 1997), data were compiled from approximately 190 papers from additional journals, book chapters, technical reports, and unpublished manuscripts that included information on live root standing crop and belowground BNPP. The papers described research on every continent except Antarctica, although the majority were from North America. In the database, the plant functional type and biome coverage were most abundant for grasslands and temperate zones. Coverage in South America, Australia, and north central and eastern Asia was particularly sparse.
The data were categorized according to latitude, longitude, mean annual temperature, mean annual precipitation, maximum mean monthly temperature, minimum mean monthly temperature, soil texture, sampling method, plant age, and root diameter class, though not all information was available for every study. In cases where authors did not differentiate between live and dead biomass, we recorded total root biomass. The dominant vegetation type and the vegetation sampled (i.e., forest, grassland, shrubland, wetland) were recorded for each study. When a single paper included information for multiple vegetation types, the species within the type was averaged, and the turnover for each unique vegetation type was reported. Sites were also categorized as tropical, temperate, or high latitude. Sites that included BNPP and standing crop values for more than a single year were averaged for those years. More information and results of this study can be found in Gill and Jackson (2000).
Root turnover, in units of yr-1, is the ratio of Annual Belowground Production to Maximum Belowground Standing Crop. This approach is a modification of the model initially proposed by Dahlman and Kucera (1965). Approximately one-third of the primary literature sources reported mean rather than maximum root biomass; however, these sources did report estimates of both BNPP and root standing crop biomass. In these cases, mean root biomass was transformed to standing crop and then to maximum root standing crop using a regression model based on the relationship of mean root biomass and BNPP. This relationship was derived using the 20 data sets that included both mean and maximum root biomass (Gill and Jackson 2000). The 20 studies used to determine this relationship represented all biome types, and the biomass estimates spanned the range of potential mean biomass reported.
When climate variables were not reported, the CLIMATE database version 2.1 (W. Cramer, Potsdam, pers. comm.) was used to reconstruct mean climate values based on latitude and longitude coordinates.
A PDF copy of the Gill and Jackson (2000) paper is available at http://www.biology.duke.edu/jackson/np00.html.
This data set builds on the initial root data compiled by R. B. Jackson in the mid-1990s (see Jackson et al. 1996; Jackson et al. 1997). The expanded and updated data set (Gill and Jackson 2000) contains measurements for approximately 188 sites from 152 papers that were used to estimate root turnover rates for 341 site-pits. In addition, the Jackson initial data set has been expanded for studies with nutrient concentrations (data for approximately 372 site-pit-depths from 57 papers in Gordon and Jackson 2000) and rooting depth (data for approximately 298 sites with 565 profiles in Schenk and Jackson 2002). The three recent papers include most of the data contained in the initial root data set; however, some observations may have been excluded because of more stringent selection criteria. Many of the source papers provided data for the three recent rooting papers and users are encouraged to review all three data sets.
The Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC) for Biogeochemical Dynamics organized and formatted these data for long-term archive. Appendix 1 and an unpublished file were placed into a spreadsheet format and stored as ASCII tab-delimited (.txt) files. Appendix 1 and an unpublished electronic file were received from Richard A. Gill, Washington State University, Pullman, WA, in March 2002. Appendix 1 was converted from an Excel spreadsheet to a tab-delimited .txt file. A field-labeled flag was added to the file, since the original flagging method could not be transferred to a .txt file. A description of the flags can be found below. In the original Appendix, latitude and longitude were in the same column. They were split into two columns, expressed as decimal degrees, and S and W directions were given negative values and N and E were given positive values. Missing values are represented by -9999. Similarly, the unpublished Excel Spreadsheet was converted to three ASCII tab-delimited files, one for each sheet within the Excel Spreadsheet.
