Consumer control of oak demography in a Mediterranean-climate savanna

Abstract

Tree recruitment is rare and oak populations are declining in many Mediterranean-climate oak savannas. Factors affecting acorn production, seedling establishment and initial seedling growth have been much studied in short-term experiments. However, fecundity and early survival rates have not been placed in a demographic framework that also considers sapling survival and growth and adult tree mortality.We use matrix models and life table response experiments to analyze long-term experimental and observation data on California valley oak (Quercus lobata Nee ´) near its southern range limit in Santa Barbara County to answer three questions: (1) How sensitive is oak population growth rate to variation in acorn production and initial seedling establishment vs. sapling survivorship and growth? (2) How do mammalian consumers—specifically, cattle, deer and rodents—impact valley oak seedling establishment and sapling survival and growth? (3) Can vertebrate consumers account for the observed population decline of valley oak in savannas in its southern range? We find that population growth is far more sensitive to consumer- mediated variation in sapling survivorship and growth than to variation in fecundity or seedling establishment and that consumers exert strong influence on the demography of the species. Deterministic, finite population growth rate (k)is ,1 for unprotected plants and for plants that are protected from cattle but still exposed to mule deer and rodents. Population growth rate increases to 1.03 with protection from both cattle and deer, mainly because plants are able to quickly reach the tree layer when they are protected from ungulate browsing. Population growth rate jumps to 1.15 for plants protected from both ungulates and rodents as a result of increased survivorship and height growth of established individuals and because of increased seedling establishment during the first year. Our experimental findings are consistent with observed patterns in natural populations in Santa Barbara County, where tree recruitment is rare in both grazed and non-grazed savannas but more common in areas such as roadsides that are refuges from cattle and