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Studies of soil productivity must compensate for the effects of temporal trends in order to examine the pattern of crop yields along spatial gradients. A n analysis of the published yield estimates for 30 soils in 233 counties, however, did not find consistent yield increases over the past three decades. O n the contrary, the yield estimates for many soils weTe markedly uniform since 1972. The uniformity appears to have two cawes: the acknowledged difficulty of making yield estimates in a time of increasing variability in soil and crop management, both within and between regions, and surveyors' awareness of data stored in a national soils data base. The effect is to cast doubt on soil productivity data reported in county soil surveys published between 1973 and 1988. Key Words:crop yields, geographic information systems, soil productivity, soil surveys, SOILS-5. Gersmehl and Brown (1986) demonstrated that spatial variability in soils and soil management makes a simple ranking of soils according to crop yield problematic in most areas of the United States. One way to understand factors that affect productivity would be to investigate trends in crop yields across spatial gradients. Time can be a factor in yield studies, because the available records have been collected over a period of years, and per-acre crop yields in the United States have generally increased in this century. The effect of time on yields must be therefore isolated as a factor before we can draw conclusions about other factors that may affect yields across spatial gradients. Our purpose in this paper is to examine the ways in which temporal factors can confound the analysis of yield according to 'This research was accomplished under support from National Science Foundation grant SES 8618417. Dwight Brown, Tom Fenton, Herb Huddleston, and Harvey Terpstra provided helpful comments during the research. The Cartography Lab at the University of Minnesota assisted with the illustrations.

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Fragmentation of forests in the eastern United States is strongly suspected to have led to the decline of forest bird populations. A similar trend may be occurring in the rangelands of coastal California. Rangelands have been fragmented by exurban development, with its houses, other structures, and landscaping. A prime concern in fragmentation is the creation of edge habitat. Edges favor the activities of nest parasites and nest predators. The buildings associated with rangeland development harbor mammalian nest predators such as striped skunks, raccoons, Virginia opossums, and domestic and feral cats. The vegetation and structures also attract avian nest predators, especially American crows and scrub jays. Analysis of historical aerial photographs of Sonoma County demonstrates the invasion of range and pasture lands by exurban residences. Sample sites show the decline of range and pasture areas, and the increase in density of residences and other buildings and associated vegetation. The trend in California is toward increasing human densities in rural landscapes. Conservation of rangeland birds may depend upon directing development to less-sensitive sites, and upon landscaping and construction that are less favorable to predators.