Site description
The study was conducted in Mediterranean forests in the Alto Tajo
natural park (Guadalajara, Castilla La Mancha, 40.66°N, 02.27°W) in
central Spain, where 30 plots (30m x 30m) within a 20
km2 area were selected from the FunDivEUROPE
Exploratory Platform (Fig. S1, Baeten et al., 2013). To assess the
effect of tree diversity, the plots were established in 2011 in
non-managed mature even-aged (i.e., more than 50 years) forests with
limited variation in altitude (i.e., from 980 to 1300 m a.s.l.),
topography, soil type, and density (see Table S1 and Baeten et al.
(2013) for more details on plot selection). The soils in all plots were
shallow (from 20 to 40 cm) calcic cambisol soils (FAO/UNESCO soil
classification) on a cracked limestone bedrock but with plant roots that
may extend down to several meters through the fractured bedrock
(Peñuelas & Filella, 2003). This area has a continental Mediterranean
climate with hot and dry summers and cold and snowy winters. The
long-term annual precipitation sum was 416 mm (2011-2022), with 516 and
367 mm in 2021 and 2022, respectively. The long-term mean annual
temperature was 11°C (2011-2022), with 12°C and 11°C in 2021 and 2022,
respectively. Maximum daily temperatures were 31°C and 33°C for 2021 and
2022, respectively (Fig.1). The soil aridity index (P/PET) was
calculated monthly using meteorological data from the nearest station
(Molina De Aragon, Castilla La Mancha, ES; 20 km away, Fig. S1). We
first computed the monthly potential evapotranspiration (PET) using the
Thornthwaite equation (Yates & Strzepek, 1994) based on the average
monthly temperature, daylight length, and heat index. P/PET was
calculated by dividing the monthly precipitation sum by the monthly PET.
P/PET varied from 0.34 and 0.38 in May (i.e., corresponding to wet soil
conditions) to 0.11 and 0.05 in July (i.e., the driest and hottest
period) for 2021 and 2022, respectively.