Drought event and indices of tree resistance, recovery and resilience
The aridity index named De Martonne Index (DMI) developed by de Martonne (1926) was used to detect the drought events:
\(DMI=\frac{P}{(10+T)}\) (2)
where P and T are total precipitation and mean annual temperature of the growing season (June to September), respectively.
We defined drought events as years when the DMI was less than the average annual DMI minus 1.5 standard deviations. In this way we determined drought events occurring in 1996, 2000 and 2015 (Fig. 1c). As some study species were still at the seeding stages in 1996 and 2000, only in the year 2015 the growth responses to drought event of the studied species were investigated. We calculated the tree growth responses to drought event by using the tree resistance (RT), tree recovery (RC) and tree resilience (RS) indices.
\(\text{RT}=\frac{\text{Dr}}{\text{PreDr}}\) (3)
\(\text{RC}=\frac{\text{PostDr}}{\text{Dr}}\) (4)
\(\text{RS}=\frac{\text{PostDr}}{\text{PreDr}}\) (5)
where Dr indicates tree-ring width during the drought year (2015), PreDr and PostDr indicate average tree-ring width of the two years before and after 2015, respectively. We selected a period of two years before/after drought event for calculating above induces because a recent study reported that drought legacy effects on tree growth generally last for two years and most trees affected can recovery to pre-drought level within two years (Li et al., 2020b). Based on the definition of Fang and Zhang (2018), a species is considered to have high resistance if its RT is greater than 0.75, and high recovering ability if its RC is smaller than 1.25.