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.