## Materials and Methods  
### Research Area and Site Measurement Design  
The research was carried out in pedunculate oak stands of Jastrebarsko forest, located approximately 30 km SW from Zagreb, Croatia, near the town of Jastrebarsko (Figure 1). Jastrebarsko forest is part of the Pokupski basin (i.e., river Kupa basin) forest complex, which is, with its area of approximately 13,600 ha, the second largest complex of pedunculate oak forests in Croatia. The river Spacva basin in the east, spanning over 40,000 ha, is the largest forest complex of pedunculate oak in Croatia and in Europe [34] (Figure 1). The area covered with forest in Croatia is 2.493 Mha or 44% of the total land area [35]. Pedunculate oak forests are among the most productive as well as ecologically and commercially most important stands, with the pedunculate oak alone having a standing stock of 48.4 Mm\({}^{3}\) (11.6% of the total growing stock in Croatia) [35]. The terrain of the Pokupski basin is mainly flat, with the elevation ranging from 106 m above sea level at the central part of the basin up to 120 m and 130 m above sea level in the south-western and northern parts, respectively. The soil is deep (>5 m), acidic in the top 1 m layer (pH in H\({}_{2}\)O 4.9-5.7), but it becomes neutral (pH in H\({}_{2}\)O 6.9-7.5) at depths > 1.2 m. It is mainly composed of heavy clay (54% clay, 18% silt, 28% sand) with low vertical water conductivity, and according to the Food and Agriculture Organization World Reference Base (WRB) for soil resources [36], it is classified as luvic stagnosol. Parts of the forest are waterlogged or flooded with stagnating water during winter and early spring, while during summer the soil dries out. The basin is intersected with several natural creeks as well as a network of human-made drainage canals. The average groundwater table depth in the basin ranges from 0.6 m to 2 m, with the average of \(\sim\)1.5 m at the study site, and seasonal oscillation in between 0 m and 4 m [37]. According to the Koppen classification, the climate of the area is temperate oceanic climate (Cftb), with a mean annual air temperature of 10.6 \({}^{\circ}\)C for the period 1981-2010 (data from the Croatian Meteorological and Hydrological Service for the nearest meteorological station in Jastrebarsko). Average annual precipitation during 1981-2010 is 962 mm year\({}^{-1}\), out of which around 500 mm falls during the vegetation season (April-September). The dominant tree species in the basin  
Figure 1: Map (**left**) shows a distribution of pedunculate oak stands in Croatia (marked in green) with the enlarged areas of the Pokupski basin and the location of the study site with the installed eddy covariance tower (**right**). is pedunculate oak (_Quercus robur_ L.), with a significant share of other species, namely black alder (_Almus glutinosa_ (L.) [PERSON]), common hornbeam (_Carpinus betulus_ L.), and narrow-leaved ash (_Fraximus angusifolia_ [PERSON]). Frequently present in the understory are common hawthorn (_Crataegus monogyna_ [PERSON]) and hazel (_Corylus avelhana_ L.). The study site is located in managed, young stands (during the time of the study 35-44 years old) dominated by pedunculate oak (Figure 1, left). Stands are the result of regeneration cuts of old (\(\sim\)140 years) pedunculate oak stands in the early 1970s. The EC tower (coordinates: 45 deg37 deg10\" N and 15 deg41'16\" E) was erected in May 2007 as a part of the Carbon-Pro project [38] and from 21 September 2007 provides continuously meteorological and EC measurements (Figure 1, right). At the time of installation, the measurement height was 23 m (3-5 m above canopy). Because of tree growth, the tower was upgraded and the measurement height was elevated to 27 m on 1 April 2011. A network of permanent circular plots was set up according to a 100 \(\times\) 100 m grid (i.e., one plot per ha) around the EC tower during 2007 and winter months of 2008 (Figure 11). All trees were permanently marked with numbers and measured.