In the Iberian Peninsula, the cork oak woodlands are of great ecological and socio-economic importance. These savanna-type woodlands are characterized by an herbaceous understorey, dominated by C3 annual species. The productivity and related ecosystem processes of this understorey are highly dependent on timing and magnitude of precipitation events.

Climate change scenarios for the Iberian Peninsula suggest not only increasing air temperatures, but also the possibility of decreasing spring precipitation, accompanied by an increase in the interval between precipitation events, which might cause drought conditions to occur, with more severe effects on productivity and ecosystem processes.

An experiment was carried out in southern Portugal to assess the effect of a drought treatment, with a subsequent extreme precipitation event, on the herbaceous component of managed cork oak woodland. The facility consisted of five plots with rainout shelters (2.5 x 2.5 m), which exclude normal rainfall inputs (drought treatment), and five non-sheltered control plots (control treatment). In the drought treatment, 193 mm of precipitation was excluded in the period from mid March to mid May, with a subsequent precipitation pulse of 50 mm in the middle of May. Variables measured included productivity, plant community composition, soil respiration and soil microbial activity.

Results/Conclusions

Our results indicated that volumetric soil water content in the control plots followed precipitation patterns. In the drought treatment, soil water content decreased from 23 to 10% (v/v) during the period of rain exclusion. The subsequent precipitation pulse resulted in an increase to 16% (v/v).

In the drought treatment, aboveground productivity was significantly reduced, with a decrease in species diversity. Belowground productivity remained unaltered, but root to shoot ratio increased. Drought treatment significantly decreased LAI, accompanied by a decrease in SLA, a widely recognized drought effect.

Until application of the precipitation pulse, soil respiration and microbial activity were higher in the control plots. The precipitation pulse resulted in a pronounced increase in soil respiration and microbial activity in the drought treatment, which reflected the increase in soil water content, showing evidence of the ‘Birch effect’.

The present study showed that a precipitation pulse, after a drought period, resulted in a burst of decomposition, microbial activity and release of CO₂. Decreasing precipitation, accompanied by an increase in the interval between precipitation events, as forecasted by climate change scenarios, can increase the amount of CO₂ released to the atmosphere, with concomitant decrease in aboveground biomass and species diversity.