Mediterranean shrublands growth responses to warming and drought conditions

Elevated temperatures and extended droughts are predicted to have a large influence on the functioning of natural and semi-natural environments both directly and through interactions with land management and pollutant loading. Climatic changes may have particular strong effects on vulnerable ecosystems, which are already subjected to other stresses such as elevated N deposition, intensive grazing or the risk of fire. An experimental manipulation of climatic conditions at field scale was conducted employing a newly developed night-time warming technique and an automated covering system to extend summer drought. The objective of this study was to test how a future extended drought period and an increase in temperature could affect plant response in terms of plant cover and biomass, plant growth and reproductive effort. The study site is located within a nature reserve in north-western Sardinia, Italy (40° 37’ N, 8° 10’ E). Climatic manipulations were carried out on Mediterranean macchia ecosystem which includes sclerophyll species, some scattered shrubs and several herbaceous plants. The set-up of the experimental site was completed on summer 2001. Six experimental plots were manipulated by night-time warming and extending summer drought. The responses to the treatments were compared to three untreated control plots during the years 2001-2003. The warming treatment was obtained covering the vegetation with an automated aluminum curtain at night. In the drought treatment the curtain material is transparent to infrared radiation and the movement of the curtains is determined by rainfall events. The drought treatment was carried out for a 3-month period in autumn. The response of the plants to the treatments was monitored measuring plant cover and biomass, plant growth, and reproductive effort. Pre-treatment measurements were done to identify variability between plots. The pin point method was used for measuring plant frequency, and estimating plant cover, biomass and plant growth. Species types, plant contact and height above the ground for each contact were recorded along four transect in each plot. Different contact types were identified: (1) vegetative parts (green and dead); (2) reproductive structures (flowers, inflorescences, and fruits). In addition, the pin point measurements were calibrated by means of destructive biomass sampling outside the plots. Allometric regressions were estimated for the main species to calculate the pre-treatment value of biomass among the species. Moreover, a direct measurement of plant growth was performed by marking and sampling 10 terminal shoots of the main species for each plot, and measuring for each shoot: length, basal diameter, number of leaves, fresh weight, dry weight, and leaf area. Finally, the percentage of flowering shoots or flowering plants, calculated by the pin point method, was adopted as an indicator of reproductive effort. A one way ANOVAs with differences between years for each measured plant parameter as dependent variable, and treatments as independent factor was performed to test the effects of treatments. Significant differences between means were identified by Duncan’s multiple range test. Although the results obtained are referred to two years of experiments, a general negative trend on plant growth due to drought treatment was observed.