2.2 Forest inventory data
We used a survey database of permanent sampling plotsin natural and planted forests across Sichuan Province, China. The database was based on systematic inventory records of trees, including latitude, longitude, altitude, slope, aspect, soil depth, soil type, human disturbance, tree species, diameter at breast height (DBH), tree height, mean stand age, tree density (individual/ha), stand volume (m3), canopy (%), and disaster (fires, insect infestations). Forest inventory data were obtained for eight years (1979, 1988, 1992, 1997, 2002, 2007, 2012, and 2017). The sample plots were set according to the kilometer grid 2n (km) (n=2,3). Each stem with DBH (1.3 m) ≥ 5 cm was recorded, and trees of height <1.5 m were classified as understory trees. Trees that only reached the DBH threshold after a first (set of) census(es) were classified as recruits.
Permanent sampling plots were set to assess the dynamics of forests of varying ages in the study area by the provincial government of Sichuan. These plots areas were 0.067 ha (25.84 m×25.84 m). There was no massive pest infestation in our study area. We chose permanent plots on the basis of the four criteria for characterizing long-term changes in forests: 1) origin is known, and obvious interfering events have not taken place (replanting, logging, fire, river flood discharges, storm severity); 2) at least more than two censuses have been performed; 3) lack of obvious wrong data and missing data, and 4) mean stand age estimation is available since it was first recorded.
A total of 1,399 permanent plots (ca. 135 ha, 1,248 natural forest plots, 151 planted forest plots) met our inclusion criteria. The latitude of 1,399 plots is from 26˚12ʹN to 34˚14ʹN, the longitude of the plots is from 98˚39ʹE to 104˚24ʹE (Fig. 1) and the altitude is from 400 m to 4460 m above sea level. In the selected plots, 136,988 live trees were measured from 1979 to 2017. The mean measurement interval in natural forests was 6.07±2.08, whereas that of planted forest was 5.75±2.05. The dominant genera across all censuses and plots wereAbies sp. , Picea sp. , Betula sp. ,Tsuga sp. , Sabina sp., Pinus sp., Eucalyptus sp., Populus sp., Tilia sp., and Acer sp. The basic information of permanent plots in natural and planted forest are shown in Table S1.