Vegetation structure and biodiversity in Mediterranean ecosystems: A comparative study from Lebanon and California

The Mediterranean type ecosystems in Lebanon and California were compared to investigate the distribution of plant communities along an altitudinal gradient. Comparable climates and nutrient poor soils appear to produce similar ecosystems in both regions. The Mediterranean plant growth forms and vegetation formations are thought to be the result of convergent evolution. Meanwhile the physiognomies of various herbs, shrubs and trees that dominated vegetation are similar in both regions, whereas the underlying floras are quite distinct. There is some evidence supporting the convergent evolution theory at community level on the different continents. Growth form spectra of the two Mediterranean floras also show large differences. California has significantly smaller fractions of annuals and geophytes but larger fractions of sub-shrubs, shrubs and trees. Shrubs and trees with evergreen sclerophyllous leaves dominate California chaparral and Lebanese maquis, however, the details of canopy structure are different. Chaparral is generally 1-3 m tall with a closed canopy. It is essentially a one-stratum vegetation with an absence of herbaceous elements beneath the evergreen canopy. On the other hand, the maquis canopy generally averages 4-5 m in height and is frequently open. There is a spring peak in community flowering in both Mediterranean regions. There are no obvious differences in flowering patterns between communities at different elevations and soil nutrient contents. This can perhaps be attributed to predominance of insect pollinated species in Lebanon and California. Disturbance has not only contributed to biological diversity of Mediterranean plant communities, but also to their composition, structure and niche differentiation.