Dramatic response to climate change in the Southwest: Robert Whittaker's 1963 Arizona Mountain plant transect revisited

Abstract

Models analyzing how Southwestern plant communities will respond to climate change predict that increases in temperature will lead to upward elevational shifts of montane species. We tested this hypothesis by reexamining Robert Whittaker's 1963 plant transect in the Santa Catalina Mountains of southern Arizona, finding that this process is already well underway. Our survey, five decades after Whittaker's, reveals large changes in the elevational ranges of common montane plants, while mean annual rainfall has decreased over the past 20 years, and mean annual temperatures increased 0.25°C/decade from 1949 to 2011 in the Tucson Basin. Although elevational changes in species are individualistic, significant overall upward movement of the lower elevation boundaries, and elevational range contractions, have occurred. This is the first documentation of significant upward shifts of lower elevation range boundaries in Southwestern montane plant species over decadal time, confirming that previous hypotheses are correct in their prediction that mountain communities in the Southwest will be strongly impacted by warming, and that the Southwest is already experiencing a rapid vegetation change.

Carbon Stocks and Climate Change: Management Implications in Northern Arizona Ponderosa Pine Forests

“…Recent climatic changes have induced shifting forest ecotones and widespread mortality events in the southwestern United States (SW US) [1][2][3][4][5]. Climate in the SW US has become more arid over recent time-scales (i.e., 1970 to present), and this trend is expected to continue into the future [1][2][3][4][5][6].…”

“…Climate in the SW US has become more arid over recent time-scales (i.e., 1970 to present), and this trend is expected to continue into the future [1][2][3][4][5][6]. Atmosphere-Ocean General Circulation Models (AOGCMs) predict mean annual temperature in the SW US to increase between 2 °C and 5 °C in conjunction with an overall decrease in annual precipitation, thus resulting in increased severity and duration of drought-like conditions [4,7].…”

“…Atmosphere-Ocean General Circulation Models (AOGCMs) predict mean annual temperature in the SW US to increase between 2 °C and 5 °C in conjunction with an overall decrease in annual precipitation, thus resulting in increased severity and duration of drought-like conditions [4,7]. Warmer, drier conditions in the SW US have been linked to an increased frequency of catastrophic wildfires [4,8,9], bark-beetle outbreaks [4,10], and widespread mortality events [1][2][3][4][5][6]11]. These amplified rates of tree mortality and extensive mortality events result in large pulses of stored carbon being released into the atmosphere [1,12].…”

“…For example, climate can affect community structure and composition substantially over the range of a couple hundred feet in elevation [2,5,6,14] or over opposite aspects of a hillside [14][15][16]. Transition zones between forest types are particularly sensitive to changes in annual temperature and precipitation and tend to shift towards northern latitudes and/or higher elevations under warmer, drier conditions [2,5]. Understanding spatial patterns of plant mortality and mortality thresholds for many forest and woodland species in the SW US is still poorly understood [19][20][21].…”

“…Uncertainty remains as to which stands are most vulnerable to warmer climates. One study suggests that higher-elevation forests in the SW US will experience larger range contractions than lower-elevation stands [29], while other studies propose that lower-elevation forests are at highest risk in a changing climate [2,5,30].…”