After finishing his PhD at Duke University in 1960, Hal Mooney became an instructor at the University of California, Los Angeles and began researching the iconic vegetation of California. In 1961 Hal Mooney and colleagues surveyed the vegetation of the subalpine and alpine zones in the White Mountains; an arid mountain range in the rain shadow of the Sierra Nevadas, located on the western edge of the Basin and Range province. They documented species elevational range limits, and showed that in many cases they were jointly influenced by microclimate and soil type.
In 2010, we resurveyed the Mooney et al. transects between 2900 and 3500 m. Over nearly five decades temperatures in the White Mountains increased by 0.98 degrees C, and precipitation declined by 53 mm. We expected to find upward elevational range expansions consistent with regional increasing temperatures. To evaluate whether the shifts in species abundances observed over time could be attributed to warming, we conducted a passive warming experiment from 2012-2016, at two elevations (3100 m and 3500 m). To evaluate whether species responses over time were influenced indirectly by climate change via altered species interactions, we placed passive warming chambers in areas both with and without sagebrush.
Between 1961 and 2010 on granitic and quartzite soils, one shrub species, Artemisia rothrockii (sagebrush), expanded its elevational range upward into the alpine and became increasingly abundant in both the subalpine and alpine zone. This range expansion corresponded with declines in the abundance of three species of cushion plant and one grass species that had been abundant in the alpine zone in 1961. There were fewer shifts in abundance on dolomite soils, which Mooney et al. showed were buffered from high temperatures due to high albedo.
The warming experiment showed that the alpine species were negatively impacted both directly in response to rising temperatures, and indirectly in response to the range expanding sagebrush. There was a statistical interaction, whereby species abundances and flower production declined more in plots warmed with sagebrush present, than in warmed plots in the absence of sagebrush.
These results demonstrate the invaluable contributions of ecologists such as Hal Mooney, who both documented patterns of species abundances across environmental gradients, and developed frameworks for predicting how climate change would impact plant species. They also highlight how shifting species composition, through processes such as invasions and range expansions, can alter plant species and community responses to climate change.