A global database of plant production and carbon exchange from global change manipulative experiments

Publication collection and data compilation

The detailed methods of publication search and data collection were described in our related work⁷. In brief, 10 databases in Web of Science (WoS; 1 January 1900 to 13 December 2016) including BIOSIS Previews, Chinese Science Citation Database, Data Citation Index, Derwent Innovations Index, Inspec, KCI-Korean Journal Database, MEDLINE, Russian Science Citation Index, SciELO Citation Index, and WoS Core Collection were used for searching peer-reviewed publications that reported GCMEs. The 18 keywords for WoS title search were: global change, climate change, free-air carbon dioxide enrichment, free-air CO₂ enrichment, elevated carbon dioxide, elevated CO₂, elevated atmospheric CO₂, CO₂ enrichment, eCO₂, [CO₂], warming, elevated temperature, changing precipitation, increased precipitation, decreased precipitation, nitrogen deposition, nitrogen addition, and nitrogen application. Through these search, 310,177 publication records that might be relevant to our topic were found.

First, we identified all the 310,177 records via reading each title. Second, we read the abstracts of all the records collected in the first step to further screen publications. During the two steps, we excluded 291,436 records because these studies were reviews/meta-analyses or conducted in non-terrestrial ecosystems such as oceans. Third, we read the methods of the remaining 18,741 publications to identify which of them met the following three inclusion criteria:

1. 1.

   Publications reported results of outdoor GCMEs which had at least three control and global change treatment plots (> = 1 m²).

2. 2.

   The GCMEs were conducted in terrestrial ecosystems except for croplands and lab incubation studies.

3. 3.

   The GCMEs aimed to examine effects of simulated global change drivers on carbon, nitrogen, and water-cycle variables as well as plant and microbial parameters.

During the screening in the third step, 1,290 publications met these defined criteria.

We subsequently cross-checked the list of the 1,290 publications with references cited by the previous review/meta-analysis articles in global change research as well as the 1,290 publications, and collected 756 publications. In addition, 184 studies were collected by searching the websites of ecology laboratories and experiment networks and checking the references of the papers downloaded from these websites. In total, 2,230 publications were collected in the original version of the database⁷. Moreover, another 12 publications were found when we checked and reorganized all the data extracted from the 2,230 publications^{8,9,10,11,12,13,14,15,16,17,18,19}. This database compiled 11 plant production and ecosystem carbon exchange variables including net primary productivity (NPP), above- and below-ground NPP (ANPP and BNPP), total biomass, aboveground biomass (AGB), root biomass, litter mass, gross and net ecosystem productivity (GEP and NEP), and ecosystem and soil respiration (ER and SR). Data of mean values, standard deviations or standard errors, and sample sizes (number of plot replications) of these variables in the control and treatment (e.g., elevated CO₂, nitrogen addition, warming, increased/decreased precipitation, or their combinations) groups were extracted from each publication when possible. The figures were digitized using SigmaScan Pro 5.0 (SPSS, Inc.) and the numerical values were extracted when a publication presented experimental data graphically. Data of the experiments that were conducted over less than one year/growing season were excluded in this database. However, we included short-term data from tundra studies because most of measurements in this ecosystem were performed during July-August. Overall, 5,213 pairs (the control versus global change treatment) of plant production and ecosystem carbon exchange samples were collected in this database, having 2,247, 2,120, 81, and 765 pairs from single-, two-, three-, and four-factor manipulative experiments, respectively (Fig. 1).

Environmental metadata: Climate and vegetation

Information on the locations and altitudes of each experimental site, site climate including mean annual temperature (MAT) and precipitation (MAP) as well as wetness index ((frac{{rm{MAP}}}{{rm{MAT+10}}})), ref. ²⁰, and vegetation types were extracted from each of the 2,242 publications. If a study did not report climate characteristics for its experimental site, data of MAT and MAP were downloaded from Climate Model Intercomparison Project phase 5 (CMIP5; https://esgf-node.llnl.gov/projects/cmip5/) based on the site coordinate. The dataset selection in CMIP5 was “historical (simulation of recent past 1850–2005)” and the climate data averaged from 20 (i.e. BCC_CSM1_1, BCC_CSM1_1_M, CANESM2, CCSM4, CMCC_CM, CMCC-CMS, CNRM-CM5, CSIRO_MK3_6_0, GFDL_CM3, GISS_E2_H, HADGEM2_AO, HADGEM2_ES, INMCM4, MIROC_ESM, MIROC_ESM_CHEM, MIROC5, MPI_ESM_LR, MPI_ESM_MR, MRI_ESM1, and NORESM1_M)²¹, that contained historical climate data, out of the 35 global climate models available in CMIP5 were used in this study. In addition, we downloaded data of climate means at global 1 × 1° land grid cells from Princeton University (http://hydrology.princeton.edu/data/pgf/v3/) to construct global climate space. Moreover, we classified ecosystems subjected to ecosystem manipulative experiments into five typical types: forests (mature forests and tree seedlings), grasslands (grasslands, meadows, short- and tall-grass prairies, temperate/semi-arid steppes, shrublands, savannas, pastures, and old-fields), tundra, wetlands (peatlands, bogs, marshes, and fens), and deserts.

Metadata of experimental facilities and performance

Information on CO₂ enrichment and warming facilities were also extracted from the related publications reporting CO₂ and warming effects on plant production and ecosystem carbon exchange. Facilities used in elevated CO₂ experiments included greenhouse, open-top chamber, free-air CO₂ enrichment, and tunnels. Warming experiments primarily used greenhouse, open-top chamber, soil heating cables, infrared radiator, and infrared reflector to elevate vegetation canopy and soil temperature. In addition, the manipulation magnitudes of global change drivers imposed by manipulative experiments, such as the increases in CO₂ concentrations (ppm) and temperature (°C), the changes in precipitation amount (mm), and the rates of nitrogen input (g N m⁻² yr⁻¹), were also collected and added into this updated database.

Source: Ecology - nature.com