Print PageForests near the Mediterranean coast have been shaped for millennia by human disturbance. Consequently, few natural forests remain, and it is difficult to determine how forests respond to climatic change in this densely populated and ecologically-diverse region. We combined a landscape vegetation model with paleoecology to examine the causes of forest expansion and collapse during the Holocene, and to predict the response of coastal ecosystems to future climate. Dynamic modeling allows us to isolate human and climatic impacts, and paleoecology provides empirical data to validate simulation output. We contiguously sampled sediment from Gorgo Basso, a small lake in southwestern Sicily to produce decadally-resolved pollen records for two intervals: 8600 - 6300 cal yr BP when forests expanded, and 2800 - 1800 cal yr BP when forests disappeared. We simulated vegetation dynamics in the landscape surrounding Gorgo Basso with the LandClim model using a sensitivity-test approach. We evaluated soil-moisture limitations on forest expansion by varying annual precipitation between 50 and 100% of the modern abundance, and mimicked increasing human disturbance by varying ignition frequency. In a final step, we modeled future vegetation with mean monthly temperatures 4° C above the historical average, consistent with the IPCC A2 scenario for Sicily.
Results/Conclusions
Pollen analysis reveals that evergreen shrublands surrounded Gorgo Basso before 7000 cal yr BP. This result matches simulations with <80% of modern precipitation. Forests of Quercus ilex and Olea europea replaced the shrublands by 7000 cal yr BP. These forests have no modern analogue but are consistent with vegetation simulated with modern precipitation. Thus, increasing moisture availability probably caused forest expansion during the middle Holocene. Broadleaved evergreen forests persisted until 2700 cal yr BP when shrublands and herbaceous species typical of human disturbance expanded. A contemporaneous increase in charcoal influx indicates fire was important to this change. LandClim run with modern precipitation produces a similar shrub-dominated landscape when the simulated ignition frequency matches historical fire return intervals. Forests also disappear from simulations with predicted future temperatures, even with modern precipitation abundance and low ignition frequency. We conclude that human disturbance caused forests to collapse in southwestern Sicily during the late Holocene, and that evergreen forests remain possible if fires are not too frequent. Such forests are a desirable management goal because of their low flammability and positive impacts on soils and hydrology. However, a warmer future climate could eliminate the potential for forests to re-expand along the Mediterranean coast.
