The mutable nature of the climate has never been more apparent than today, with human inputs to the atmosphere rapidly increasing greenhouse gas levels and global temperatures. Although the concept of “climate” is one that suggests long-term stability, climates are constantly changing, and climatic variability throughout the Holocene Epoch has had major effects on YPMC forests in the assessment area. It is also important to note that temporal changes in climate have not been uniform across the assessment-area landscape, and the timing of changes in temperature and precipitation, as well as biotic responses, differ from one area to another. Nonethe-less, some useful generalizations can be made.
The Holocene Epoch is now considered to have begun about 12,000 YBP (years before present). The entire epoch falls within a broadly defined “intergla-cial” period. The Earth’s climate has been in “gla“intergla-cial” periods for about 90 percent of the past 850,000 years, and interglacial periods as warm as the Holocene are
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GENERAL TECHNICAL REPORT PSW-GTR-256
relatively rare (Tausch et al. 2004). Overall, mean annual temperatures have fluctu -ated by 3 to 6 °C through the Holocene, and precipitation has also risen and fallen.
Researchers generally divide the Holocene into three periods that are defined by broad (and somewhat ill-defined) changes in temperature and precipitation. The Early Holocene stretches from the beginning of the epoch to 8,000 or 7,000 YBP, and was characterized by post-glacial warming, but generally cool and moist con -ditions compared to today; however, data from the northern Great Basin portion of the assessment area (e.g., Warner Mountains) suggest that post-glacial conditions were somewhat warmer and drier than today (Minckley et al. 2007). Many closed basins east of the Sierra Nevada crest supported large rain- and snowmelt-fed lakes at the beginning of the Holocene, but most of these dried or decreased greatly in size over the ensuing couple of millennia (Minnich 2007). At the beginning of the Holocene, elevations that currently support YPMC forests were largely vegetated by high-elevation sagebrush and grass species, with a minor presence of pine and juniper (Anderson 1990, Woolfenden 1996). By 9,000 to 10,000 YBP, however, conifer forests had established themselves in most of these areas (Minnich 2007), except in the Great Basin portions of the assessment area (Minckley et al. 2007).
The Middle Holocene, about 8,000 to 4,000 YBP, is also often referred to as the “Xerothermic,” “Hypsithermal,” or “Altithermal” Period. Climates became much warmer and drier, with the driest and warmest conditions occurring around 6,000 YBP. Glaciers completely disappeared from the Sierra Nevada (Clark and Gillespie 1997), and lake levels dropped precipitously. For example, Lake Tahoe was apparently permanently below the Truckee River outlet elevation during the period between about 6,300 and 4,800 YBP (Lindström 1990). Fire frequency also increased during the Middle Holocene, at least in places that were able to main -tain plant cover (Beaty and Taylor 2009, Woolfenden 1996). Paleoecological data suggest that forests of fir and pine were replaced by oak, sagebrush, and juniper in many areas, and forest structure was likely very open, with abundant understory shrubs. Conifers invaded formerly moist areas of meadow, and desert plant and animal taxa migrated upslope (Anderson 1990, Minnich 2007).
The Late Holocene (4,000 YBP to present) has been generally characterized by cooling, with some warmer periods. Precipitation increased, and small glaciers began to form again in the Sierra Nevada. Millar and Woolfenden (1999) suggested that the basic spatial and compositional outlines of modern Sierra Nevada ecosystems developed by the beginning of the Late Holocene (note that Minckley et al. [2007]
suggested that ecological conditions similar to today were already in place in the northwestern Great Basin by the Middle Holocene). As temperatures cooled, available moisture rose, and fir and incense cedar abundance increased relative to pine and oak;
giant sequoias began to colonize their current groves (Millar and Woolfenden 1999).
White fir was a mostly minor component of assessment area YPMC forests before the general cooling and increase in precipitation seen over the past 4,000 years. Oaks have declined in importance since the end of the Middle Holocene. Earlier in the Holocene, sagebrush was a major component of low-elevation west-side landscapes, but now it is primarily restricted to higher elevations on the east side of the assess -ment area (Anderson 1990). Fire has been present as an important ecosystem process ever since deglaciation at the beginning of the epoch, but the presettlement period, characterized by frequent fire and large areas of fire-adapted vegetation, began during the Late Holocene for most of the assessment area (Millar and Woolfenden 1999).
The past 1,000 years of the Holocene have been marked by short-term changes in temperature and precipitation that have had major impacts on assessment-area eco-systems (Millar and Woolfenden 1999, Minnich 2007, Woolfenden 1996). Between about 900 and 1100 CE, and from 1200 to 1350, two long drought periods (the “medi -eval droughts,” or collectively, the Medi-eval Warm Period) led to very low levels in lakes and streams (Stine 1994) and increased fire frequencies. This was followed by a shift to cooler temperatures known as the Little Ice Age, initiated apparently by a series of massive volcanic eruptions that caused atmospheric reflection of solar radia -tion (Miller et al. 2012a). The Little Ice Age lasted from about 1400 to 1880, and the period between 1650 and 1850 was the coolest since the Early Holocene (Stine 1996).
Glaciers expanded in the Sierra Nevada, tree line dropped, and fire frequencies mod -erated. Minnich (2007) noted, however, that there is little evidence for major changes in vegetation composition during the Little Ice Age. It is important to underline that the period of the Little Ice Age is also the period most commonly used as an histori-cal reference period for restoration planning in the Western United States.
Most recently, human emissions to the atmosphere have resulted in renewed warming, even though Earth orbital cycles should be resulting in a cooling trend (Rud -diman 2005). Current temperature trends include increased temperatures especially at nighttime, a decrease in the number of days with below-freezing temperatures, and an increase in the number of extreme-heat days. The 20th century was one of the wettest centuries in the Late Holocene (Stine 1996), and recent precipitation trends in most of the assessment area have been steady or positive (Safford et al. 2012b). Interannual variability in precipitation is up at many stations in the assessment area, and the pro -portion of precipitation falling as rain versus snow is increasing; as a result, the depth of the winter snowpack is decreasing, except in the southern assessment area, where mountain elevations are very high (Safford et al. 2012b). Recent trends in fire activity are positive, with burned area, fire size, and fire frequency all rising in assessment area YPMC forests, accompanied by an increase in fire severity on lands where fuels have accumulated as a result of long-term fire exclusion policies (Mallek et al. 2013, Miller and Safford 2012, Miller et al. 2009b, Steel et al. 2015).
The presettlement
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