Road systems, land use, and related patterns of valley oak (Quercus lobata Nee) populations, seedling recruitment, and herbivory.

摘要

This research investigates the interactions of road systems and land use on the population dynamics and recruitment of a long-lived tree in Mediterranean climate California. In the case of Valley oak (Quercus lobata), habitat conversion and limited recruitment of new individuals has resulted in widespread declines throughout Santa Barbara County and California. This pattern contrasts with high recruitment rates along roadsides, offering a unique opportunity to examine the effects of roads on the population dynamics of a native species. The pattern of roadside recruits is described, mapped, and a complete survey of the biophysical environment along 109 kilometers of road was conducted. The biophysical factors of the road and road system were of four types: (1) the general roadside environment; (2) the acorn supply; (3) a measure of ungulate deterrence; and, (4) roadside management. Seven individual or aggregate factors were then related to the pattern of seedling and sapling densities along roads. Univariate analysis and regression trees determined that acorn supply and total woody cover within the roadside plots explained 49% of the variation in Valley oak seedling densities. These results support the conclusion that the recruitment pattern is due to the roadsides serving as refugia from browsers (cattle and deer). The change in Valley oak populations within roadsides, croplands, rangelands, and urban/suburban lands over a 59 year period is examined using georeferenced aerial photos from 1938 and 1997. While population per capita growth rates were less than one (declining) within both rangelands and croplands, rates were greater than one (increasing) in urban/suburban populations. While roadside growth rates were even higher than those in urban/suburban areas, high variance resulted in a rate neither positive nor negative. Finally, seedlings were planted along roadsides and within adjacent grazed and ungrazed uplands to test browsing pressure. Seedlings within roadsides experienced significantly less browsing than those within the adjacent lands, lending support to the theory that roadsides provide refugia from browsing for Valley oak. Results are important for a deeper understanding of the causes and consequences of human land use and for assisting in the management of Valley oak and others similarly affected.