Invasive fig trees (Ficus carica) in Central Valley riparian forests: Community impacts, expansion rate, and spatial distribution

The common edible fig (Ficus carica) is a shade-tolerant, sub-canopy tree that has invaded the remnant riparian forests of California’s Central Valley. Fig is unusual in its ability to invade low-light, low-disturbance, native-plant-dominated environments. A series of plant community surveys along several watersheds in northern California found that, at the local scale, fig dramatically reduces plant species richness and significantly alters community physiognomy and litter cycling rates. Dendrochronology combined with regression and spatial analyses allowed us to empirically quantify the rate and pattern of fig expansion into the native plant community over a 70-year invasion period at Caswell Memorial State Park. Fig utilizes a combination of short- and long-distance dispersal that results in constant, linear expansion at source population sites as well as rapid recruitment of satellite populations once it reaches ideal environments. Although fig initially experienced a long lag in its invasion rate, it is now expanding at an exponential rate at the landscape scale in Caswell. We identified a number of characteristics intrinsic to the fig population (shade suppression, pollinator presence, highly-specialized reproduction, and propagule pressure) as well as extrinsic characteristics of the receiving environment (safe sites and abiotic heterogeneity) that have influenced the rate and pattern of fig invasion. Central Valley riparian forests have been reduced to less than five percent of their original area, and invasive fig is a significant threat to the remaining fragments of this important habitat. We conclude with suggestions for effective fig eradication based on knowledge gained in this study.