Prades

Study site

The study was carried out in a holm oak forest growing at Prades Mountains in North-Eastern Spain (41o 13’ N, 0o 55’ E), on a south-facing slope (25% slope). The soil is a stony xerochrept on a bedrock of metamorphic sandstone, and its depth ranges between 35 and 90 cm. The average annual temperature is 12oC and the average annual rainfall 658 mm. Summer drought is pronounced and usually lasts for 3 months.
Two types of stands can be distinguished in this forest depending on their height. One type has a taller canopy, about 8 or 10 m high, and is dominated by Quercus ilex with abundant presence of other evergreen species (Phillyrea latifolia and Arbutus unedo L.), and occasional individuals of deciduous species (Sorbus torminalis (L.) Crantz and Acer monspessulanum L.). The other type of stand has a lower, more arbustive canopy, about 3 or 4 m high. These arbustive stands are dominated by Quercus ilex and Phillyrea latifolia with an important presence of other species well adapted to drought conditions (Arbutus unedo, Erica arborea L., Juniperus oxycedrus L., Cistus albidus L.). Tree density, is greater in the lower stands than in the higher stands, whereas mean stem diameter (at 50 cm) and aboveground biomass are greater in the higher stands than in the lower stands. No significant differences are observed in the basal area of the two types of stands (Table 1).

Experimental design and measurements

Eight 15 x 10 m plots were delimited at the same altitude along the slope, four plots in each type of stand (high and low stands). Half the plots of each type of stand received the drought treatment and the other half were control plots. The drought treatment consisted of partial rain exclusion by suspending PVC strips and funnels at a height of 0.5-0.8 m above the soil. Strips and funnels covered approximately the 30% of the total plot surface. A 0.8 m deep ditch was excavated along the entire top edge of the upper part of the treatment plots to intercept runoff water supply. The water intercepted by strips, funnels, and ditches was conducted outside the plots, below their bottom edge.
Temperature, photosynthetic active radiation, air humidity, and precipitation were monitored each half-hour by an automatic meteorological station installed in a forest gap between the plots. Soil moisture was measured every two weeks throughout the experiment by time domain reflectometry (Tektronix 1502C, Beaverton, Oregon, USA) (Zegelin et al., 1989). Three stainless steel cylindrical rods, 25 cm long, were fully driven into the soil at four randomly selected places in each plot. The time domain reflectometer was connected to the ends of the rods to determine the soil moisture

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