Numerous physical, chemical, and biological processes operate in positive feedback loops during the invasion of C3 shrubs into C4 grasslands. These processes occur at multiple spatial and temporal scales, ranging from a few mm2 over a period of weeks to hundreds of km2 over a period of millennia. The current C3 shrub invasion at the Jornada Basin LTER has occurred at the landscape scale of tens-of-km2 over a time period of approximately a century as documented by land survey notes beginning in 1858, vegetation maps beginning in 1915, soil maps and landscape photography beginning in 1918, and aerial photography beginning in 1936. In order to investigate broader-scale shrub invasion over longer time periods we have found a “sedoisotopic” technique to be very useful. This technique combines carbon isotopes (13C/12C) in soil organic matter and carbonate with erosion-sedimentary data derived from soil stratigraphy. Based on sedoisotopic records in the Jornada Basin, a major C3 shrub invasion occurred in the middle Holocene around 6,000 years ago and was especially severe on bajada landforms in contrast to topographically low-lying runon areas—a pattern similar to the modern shrub invasion. The sedoisotopic records in the Jornada Basin indicate that this region has generally been dominated by C4 grasslands since the last glacial maximum roughly 20,000 years ago. Like today, however, the middle Holocene was a time during which the Chihuahuan Desert probably expanded northward and up mountain slopes into grasslands and juniper savanna.