Effects of temperatures on the spatial arrangement in Mediterranean annual dry grasslands

Some preliminary results of diachronic researches on the spatial arrangement and species richness in Mediterranean annual dry grasslands belonging to the class Tuberarietea guttatae are presented. Our results are based on 12 permanent plots, set in three places at different heights in Southern and Central Sardinia. Sites were chosen to have annual species richness and plant density not significantly different. The relationship between patchiness and temporal stability was investigated at the scale of the square plots. We used the mean abundance divided by the SD in abundance (S = xm/r), as the measure of temporal stability of populations (Tilman 1999) and the Shannon-Wiener index to evaluate the patchiness in the therophytic layer (Whittaker 1972, 1979). According to our preliminary results, among the factors influencing the spatial arrangement in the Mediterranean dry grasslands, the average temperatures and seasonality could play an important role for the following reasons: - Higher temperatures amplify the effects of the seasonal drought in the thermomediterranean, therefore promoting the specialization of annual plants and pulling down the competition. In the Mediterranean region, annual dry grasslands typically form a mosaic with perennial plant communities and they occur mainly in patches corresponding to ‘gradient wells’, characterized by a severe summer drought. In these xeric situation, patches would remain a longer time devoid of perennials (Madon & Médail, 1997). - Higher temperatures can affect the fire frequency, resulting in variations in the soil nutrients and microbial activity (Grasso et al., 1996; Heike, 2007). Periodical severe disturbances caused by fires may give more chances to the poorly competitive but highly versatile therophytes, therefore increasing their population stability, that in xerothermophilous therophytic vegetation is positively related with the community diversity and population size (Valone & Hoffman, 2003). - In the meso-mediterranean bioclimate, the summer drought becomes less severe, although summer temperatures keep relatively high. On the other hand, the winter cold stress becomes relatively more severe (Mitrakos, 1980). Density-dependent seed predation by ants may govern population stability to such an extent as to override the potential effects of relatively lower mean temperatures on the annual plant communities, that in the Mesomediterranean context resulted to be the most stable and least patchy. - In the supramediterranean bioclimate the summer drought stress is buffered quite well by the cooler temperatures and by the moisture condensation (Guarino, 2001). This would lengthen the growing season beyond the optimum for most annual competitors and provide conditions adequate for perennial seedling survival and development (Jackson and Roy 1986). In this context, the affirmation of annual plants largely depends upon the extensive land use, related to the stock raising and to the use of fire for enhancing the development of rangelands.