Understory-Canopy Relationships in Oak Woodlands and Savannas1

We summarize available information about the relationships between oak overstory and understory plants for major California rangeland types. Understory biomass, productivity, and plant species composition vary considerably because of geographic location, overstory species composition, and overstory density and distribution. Deciduous oak canopies in areas with less than 50 cm annual precipitation generally have either no effect or enhance understory productivity compared to adjacent grassland. Dense canopies in areas with more than 50 cm annual precipitation generally suppress understory productivity. Forage management implications are summarized for different woodland types around the State. The five major hardwood rangeland habitat types used in the California Wildlife Habitat Relationships System, occupying nearly 4 million ha, have been defined and mapped in California (Standiford and Tinnin 1996). The types, with dominant species in parentheses, are: blue oak woodland (Quercus douglasii); blue oak-foothill pine woodland (Q. douglasii and Pinus sabiniana), valley oak woodland (Q. lobata), coastal oak woodlands (Q. agrifolia, with Q. engelmannii in the south), and montane hardwood (Quercus chrysolepis, Q. wislizenii, Q. garryana, and Q. kelloggii). Cover of the understory of oak woodlands and savannas is predominately annual grasses native to the Old World. Species composition under the canopy differs from that in the adjacent open grassland. Productivity and chemical composition also show striking differences that are important considerations for management. We summarize published information about understory composition and productivity applicable to forage management in oak-dominated vegetation.