Relationships between expanding pinyon–juniper cover and topography in the central Great Basin, Nevada

Aim Increasing geographical range and density of conifers is a major form of land‐cover change in the western United States, affecting fire frequency, biogeochemistry and possibly biodiversity. However, the extent and magnitude of the change are uncertain. This study aimed to quantify the relationship between changing conifer cover and topography. Location The central Great Basin in the state of Nevada, USA. Methods We used a series of Landsat Thematic Mapper satellite images from 1986, 1995 and 2005 to map change in pinyon–juniper woodlands (Pinus monophylla, Juniperus spp.) in the montane central Great Basin of Nevada. We derived fractional greenness for each year using spectral mixture analysis and identified all areas with an above average increase in greenness from 1986 to 1995 and 1995 to 2005. Results Areas with high fractional greenness in 2005 were most likely to occur at elevations between 2200 and 2600 m a.s.l. Increases in fractional greenness between 1986 and 2005 were most likely to occur at elevations below 2000 m a.s.l. and on south‐facing slopes. However, relationships between elevation and increasing greenness for individual mountain ranges varied considerably from the average trend. Fractional greenness values measured by Landsat suggest that the majority of pinyon–juniper woodlands have not reached their maximum potential tree cover. Main conclusions Expansion of pinyon–juniper at low elevations and on south‐facing slopes probably reflects increasing precipitation in the 20th century, higher water use efficiency caused by increasing atmospheric CO₂ in the late 20th century and livestock grazing at the interface between shrubland and woodland. Identification of the spatial relationships between changing fractional greenness of pinyon–juniper woodland and topography can inform regional land management and improve projections of long‐term ecosystem change.