  The nuclide $^{14}$C can be produced in the atmosphere by high energy
particles and $\gamma$-rays from high-energy phenomena. Through the carbon
cycle, some of $^{14}$CO$_2$ produced in the atmosphere can be retained in
annual tree rings. Four events of rapid increase of the $^{14}$C content
occurred in AD 775, AD 994, BC 660 and BC 3371 were found. Recently, the data
of Jull et al. (2020) was inconsistent with our records around BC 3371. We
measured our sample again and found the $^{14}$C records are consistent with
the value in Wang et al. (2017). Therefore, our $^{14}$C records are robust.
The inconsistency may be caused by the difference of calendar ages for the wood
samples, or the physical origin of the event. First, crossdating on ring width
can be performed only between trees whose growth has the same environmental
conditions. Because the master tree-ring for dendrochronology is lack for
Chinese trees. The master tree-ring from California has to be used. Therefore,
the calendar ages derived from dendrochronology may be not precise. Second, the
$^{14}$C even may be not global. One evidence is the variation of $^{14}$C
content around AD 1006. The $^{14}$C contents of Californian trees increase
12\textperthousand~ in two years, while Japanese trees show no $^{14}$C
increase.
