2.1 Experimental design and treatments
The experiment was carried out in the greenhouse of the Swiss Federal
Institute for Forest, Snow and Landscape Research WSL (47°21’48” N,
8°27’23” E, 545 m a.s.l.), Birmensdorf, Switzerland. On February 27,
2018, 3 years old sessile oak and European beech saplings
(~30-40 cm in height) were planted into 10-liter plastic
pots (26 cm in diameter). After transplanting into the pots, the plants
were grown for 4 weeks under well-watered conditions (irrigation every
2-3 days) in the greenhouse to recover from the transplant shock. The
cultivation soil consisted of semi-decomposed humus and commercial
potting soil. The initial soil nitrogen and carbon contents were: 39 mg
N kg-1 soil for
NH4+-N, 573 mg N
kg-1soil for NO3--N,
22.42% for soil C, and 0.82% for total soil N. The greenhouse
temperature and humidity during the period of drought duration treatment
are shown in Fig. S1.
A split-plot experimental design with three blocks was employed in this
study. Each block was divided into two main plots, one of which was
randomly assigned for oak (48 individuals) and the other one for beech
(48 individuals) (Fig. S2). Each plot was then divided into two
sub-plots, one of which was randomly assigned for fertilization (24
individuals) and the other one for the non-fertilization treatment
(ambient, 24 individuals) (Fig. S2). Each sub-plot was further divided
in to four sub-sub-plots (rows) randomly assigned for one of the four
drought duration treatments (6 individuals each). Therefore, a total of
288 plant individuals (144 individuals for each species) was included
(Fig. S2).
The fertilization treatment was conducted on 10 June 2018 (just prior to
the drought duration treatment) (Fig. S2). The fertilizer
(Osmocote
Exact 3-4M Standard, 7.0 % NO3--N,
9.0% NH4+-N, 9 %
P2O5, ICL, Suffolk, UK), equal to 1.68 g
N kg-1 dry soil (0.945 g N kg-1 dry
soil NH4+-N, 0.735 g N
kg-1 dry soil
NO3--N) was added to each pot assigned
for fertilization.
After the N fertilization, plants were exposed to four drought duration
treatments for 4 months in 2018 (Fig. S3). Plants in the well-watered
treatment (D0) were watered thoroughly once a week. There were two
moderate drought duration treatments, one of which was watered
thoroughly biweekly (D1), and the other one was watered thoroughly once
a month (D2) (Fig. S3). The treatment of the
longest drought duration in the
present study was watered thoroughly once two months (D3) (Fig. S3). We
focused on the duration between two watering events and thus did not
measure the soil water condition. After the harvest occurred on 1
October 2018, all remaining plants were treated (e.g. well-watering) in
the same way until the end of the experiment (Fig. S3).