Woody tissue carbon (C) concentration is a key wood trait necessary for accurately estimating forest C stocks and fluxes, which also varies widely across species and biomes. However, coarse approximations of woody tissue C (e.g., 50%) remain commonplace in forest C estimation and reporting protocols, despite leading to substantial errors in forest C estimates. Here, we describe the Global Woody Tissue Carbon Concentration Database (GLOWCAD): a database containing 3,676 individual records of woody tissue C concentrations from 864 tree species. Woody tissue C concentration data—i.e., the mass of C per unit dry mass—were obtained from live and dead woody tissues from 130 peer-reviewed sources published between 1980-2020. Auxiliary data for each observation include tissue type, as well as decay class and size characteristics for dead wood. In GLOWCAD, 1,242 data points are associated with geographic coordinates, and are therefore presented alongside 46 standardized bioclimatic variables extracted from climate databases. GLOWCAD represents the largest available woody tissue C concentration database, and informs studies on forest C estimation, as well as analyses evaluating the extent, causes, and consequences of inter- and intraspecific variation in wood chemical traits.

Literature review

            Data compilation expanded earlier versions of the GLOWCAD first initiated in 2012²⁶, and more recently published in 2018^20,30 and 2021^31,32. GLOWCAD is therefore based on a systematic search on primary literature of all peer-reviewed papers that cited previously published studies on woody tissue C concentrations^20,24-26. We used key terms “carbon”, “tree”, “wood carbon”, “coarse woody debris”, “dead wood”, and “wood nutrient”, as well as “carbon” aside major tree tissue types (including “wood”, “bark”, “root” and “stem”) to search four web-based platforms (Google Scholar, Web of Science, Web of Knowledge, and Scopus) identifying additional peer-reviewed publications that present species- specific woody tissue C concentration data.

            In addition to peer-reviewed papers, other sources of data included in GLOWCAD include the TRY Plant Trait Database (v. 5.0)³³ and the Global Root Traits (GRooT) Database³⁴. The TRY datasets included are the Subarctic Plant Species Database (dataset ID 105), Plant Traits for Pinus and Juniperus Forests in Arizona (dataset ID 193), Plant Physiology Database (dataset ID 97), Panama Tree Traits Database (dataset ID 230), FRED database (dataset ID 339), and the ECOCRAFT Database (dataset ID 12). While most root C values in GRooT were included in the FRED database (v. 2.0)³⁵, data from 10 papers in GRooT were added to GLOWCAD (n=197 data points): Isaac et al,  2017; Liu et al, 2019; Wang et al, 2018; Minden et al, 2014; Alameda et al, 2012; Aubin et al, 2016; Fernández-García et al, 2014; Grechi et al, 2007; Ineson et al, 1996; and Pregitzer et al, 1995.

Wood C data attributes          

To be included in GLOWCAD, the species-specific binomial nomenclature and tissue-specific information for each woody tissue C sample was required. A detailed field and lab methodology was also necessary, in order to maximize our sample size while permitting reliable species- and tissue-specific analysis. Where a single paper contained multiple tissue- and species-specific woody tissue C records, all the published values were recorded. In the majority of cases, woody tissue C data were extracted directly from published tables or from supplementary data of the articles. In instances where woody tissue C data were published as figures, the data was extracted using the WebPlotDigitizer v4.2 software³⁶. If species-by-tissue-specific woody tissue C data were not published, the corresponding authors were contacted to provide data.

Each published woody tissue C record was then classified according to the forest biome in which it was sampled. A small number of studies (e.g. ³⁷) presented both boreal and temperate data, which were differentiated in our database based on the sampling location coupled with a consultation of species distribution maps. Species taxonomy was first recorded as presented as in published articles. A final list of taxa was then compared with, and resolved according to, the Taxonomic Name Resolution Service v. 4.038. Both original and resolved taxonomy is maintained in GLOWCAD. Inclusion of new published data was halted as of Dec. 31, 2020.

Dead wood C data attributes

When classifying dead wood data, we considered three primary factors associated with woody tissue decomposition and related chemical change: A) decay class (DC), B) position, and C) size (diameter and length). In the majority of publications, dead woody tissue C values were reported along a conventional 1-5 DC scale. These values were included in GLOWCAD as published, while noting the DC scale employed. In cases where DC was reported as a two-category range (e.g. DC 1-2), the higher DC was included in GLOWCAD. In cases where a multiple category DC was presented (e.g. DC 3-5), the middle DC value was used in GLOWCAD. In the few instances DC was reported along a 0-5 point scale (where DC of 0 was defined as dead and not live wood), dead wood reported with a DC of 0 was classified as DC 1. Lastly, in a subset of papers the number of years since tree death (instead of DC) was reported. In these cases, years since death were converted to DC based on published decay class transition metrics (e.g., ³⁹). When classifying position of dead wood, “standing” referred to snags and suspended woody debris, and “downed” referred to anything sampled from the forest floor. The default position was “downed” for the few publications that did not specify position.

GLOWCAD is openly available for use in any application. It can be accessed via 1) the DRYAD Digital Repository (information to be updated upon acceptance), 2) a GitHub repository (information to be updated upon acceptance), 3) the TRY Plant Trait Database (information to be updated upon acceptance), and 4) upon request to the corresponding author. GLOWCAD is licensed under CC-BY 4.0.