As part of the nationwide field campaign (2011-2015) to assess its national terrestrial C storage in China, detailed measurements of vegetation, soil and terrain characteristics were obtained across 7800 forest sites using the same experimental protocol (Fang et al.2012). Details about the measurement protocol can be found in Tanget al. (2018) and Liu et al. (2019). In this study, we focused on 127 undisturbed mature forests representing all six major forest types across China: temperate evergreen needle forests (TENF), temperate mixed forests (TMF), subtropical evergreen needle forests (SENF), subtropical mixed forests (SMF), deciduous broadleaved forests (DBF), and evergreen broadleaved forests (EBF) (see Fig. 1). These forests span across three bioclimatic zones (79 sites in temperate region, 47 sites in subtropical region and 1 site in tropical region) with the MAT from -1.9 ºC to 21.7 ºC, MAP from 378 mm yr^-1 to 2044 mm yr^-1. The dominant soil order is Inceptisols in temperate zones, Ultisols in subtropical regions based on United States Department of Agriculture soil taxonomy (see Figure S1a). Dominant tree genera are Pine (Pinus ), Spruce (Picea ), Oak (Quercus ), Chinkapin (Castanopsis ), and Gugertree (Schima ) (see Table S2 and Figure S1b-c).

We also compiled key variables related to climate, vegetation, soil and terrain based on the measurements at the sites or global datasets using the geographic locations of the sites. They include eight-climate-related, seven-vegetation-related, ten-soil-related and five-terrain-related variables (see Table S3).