Variation in shrub structure and species co-occurrence in the Mediterranean maquis.

The structure of the Mediterranean maquis developing inside the Presidential Estate of Castelporziano (S-SW of Rome) was due to mixture of different evergreen species. On an average six species co-occurred in shrub formation, the highest frequency being of Cistus incanus (80%), Erica multiflora (73%), Smilax aspera (53%) and Quercus ilex (50%). Discriminant analysis indicated that shrub volume was the most discriminating trait among the analysed shrub traits (height, volume, crown height, crown depth, major axis and minor axis of the crown, crown projection at soil, crown volume, leaf area index). On the base of shrub volume, three different sizes of shrubs were defined by cluster analysis: small (S), medium (M) and large (L) shrubs and they were characterised by a volume of 1.9±1.5m3, 13.1±4.8 m3 and 34.9±8.8 m3 respectively. The considered maquis stand was constituted by 1050±300 shrubs ha-1, corresponding to a total shrub volume of 7793±1401 m3 ha-1; the frequency of S, M and L shrubs was 63%, 31% and 6% respectively. Number of species increased from S to L shrubs: S were constituted by 4.8±1.7 species, M by 7.4±1.6 and L shrubs by 8.3±0.8. Cistus incanus, Daphne gnidium, Erica multiflora, Pistacia lentiscus and Rosmarinus officinalis of small height (< 1 m) occurred mainly in S shrubs, while Arbutus unedo, Erica arborea, Phillyrea latifolia and Quercus ilex (height = 1.4±0.3 m) in medium and large shrubs. The impact of shrub size on the Mediterranean maquis resulted in a patchiness of microclimatic variations showing the buffering effect of the shrub size: 78% of the study area was covered by shrubs having 4% and 94% reduction of air temperature and light transmittance respectively, and 10% and 165% increase of air humidity and soil water content respectively. The remaining 22% of the study area was uncovered and directly subjected to the climate of the zone.