Mountain Vegetation - Mountain Geography: Physical and Human Dimensions - page 350

treeline species, as it has in several of the Basin and Range mountains in Nevada and
southeast Oregon (Faegri 1966; Price 1978).
TEMPORAL SHIFTS IN TREELINE
The elevational location of both global and regional treelines is strongly related to tem-
perature and is sensitive to climatic variation. Both arctic and alpine treelines corres-
pond closely with the position of the 10°C (50°F) isotherm for the warmest month (e.g.,
Körner 1998). The ability of mature trees to tolerate extended cool periods and survive
periods of minor climatic fluctuations reinforces treeline stability (e.g., Lavoie and Pay-
ette 1996, Lloyd 1997).
While short-term (i.e., decades to centuries) treeline stability might be argued for
some locations (e.g., Cullen et al. 2001), longer-term evidence of shifting treeline (e.g.,
macrofossil, pollen, charcoal, tree-ring data, and historical measurements) provides
evidence of climate-induced changes in treeline position over periods of decades to mil-
lennia. These data document changing alpine treeline positions during the late Pleisto-
cene in the central Alps (e.g., Tinner and Theurillat 2003), a mid-Holocene treeline ad-
vance in the Andes (Pasquale et al. 2008) and the western United States (e.g., Scuderi
1987), and post-Little Ice Age advance in Sweden (Kullman and Öberg 2009). LaMarche
and Mooney's (1967) classic research on bristlecone pine ( P. longaeva ) in California and
Nevada, for example, documented a 150-m (500 ft) upward advance of treeline during
the Hypsithermal (ca. 7,000-4,000 years before present [YBP]) followed by an elevation-
al depression of treeline during the Little Ice Age (ca. 2,800-120 YBP). Scuderi's (1987)
research on foxtail pine ( P. balfouriana ) in California's Sierra Nevada further suggests a
period of treeline depression occurring between 3,400 and 3,200 YBP, with a maximum
depression of 20 m (80 ft) below current timberline. During the historical period, Kull-
man and Öberg (2009) report a 200-m (660 ft) maximum rise in Swedish treelines since
the end of the Little Ice Age, based on repeated treeline location measurements along
elevational transects. Other research has documented recent and decadal changes in
treeline positioning and tree growth in response to climate change (e.g., Jacoby et al.
1996; Paulsen et al. 2000; Grace et al. 2002; Moiseev and Shiyatov 2003; Díaz-Varela et
al. 2010).
Short-term climate variability can also lead to fluctuations in treeline positions.
Warm temperatures and drought can lead to treeline depression in arid regions (Lloyd
1997; Lloyd and Graumlich 1997), but tree species abundance may serve as a more
sensitive indicator of climate change than treeline position (Lloyd and Graumlich 1997).
These latter results are consistent with those presented by Miller et al. (2004), who
suggest directional, long-term growth responses of high-elevation conifers to long-term
warming, but abrupt and reversible responses to the warm to cool phase shifts of the
Pacific Decadal Oscillation (PDO). Daniels and Veblen (2004) report similar responses
to short-term climate fluctuations in the southern Andes, where Nothofagus seedling es-
tablishment and growth respond more closely to El Niño-Southern Oscillation (ENSO)
conditions rather than general atmospheric warming.