Appendix A. Fossil records documenting late Pleistocene Joshua tree distribution and Holocene migration rates.
Joshua tree fossils have been found frequently in packrat middens from across the Sonoran Desert of southern Arizona, the Colorado Desert of southern California, and the Mojave Desert of California, Nevada, and northwestern Arizona (Fig. 2; TABLE A1).
The southern portions of broad regional late Pleistocene distribution rapidly disappeared after the start of the Holocene, leaving only the northernmost localities intact. However a few Holocene records near what had been these northerly late Pleistocene stands allow some inferences of possible Holocene, post-ground sloth, movements.
One of Joshua tree's most northerly Pleistocene locations is in the Alabama Hills of California's Owens Valley (Koehler and Anderson 1995), about 10 km to the north of its current northern limit within the Owens Valley. These fossils could infer that its distribution has retreated southward sometime during the Holocene at a rate that averaged 0.9 m/yr over the entire period (10 km over 11,700 years). But, the rarity of midden records make it impossible to determine whether this was a slow, steady contraction, an instantaneous change, or the end product of many expansions and contractions through time. Whichever it was, the northern population limit did not change much during this period.
Alternately, a series of fossils from Eureka Valley, CA, near Joshua tree's current northern extent just east of the Owens Valley, could infer a northward Holocene expansion. Joshua tree is contained in one of four recent assemblages from the area, dated at < 200 years (Spaulding, 1980), but not in several older middens. The site is about 35 km northwest of its nearest recorded Pleistocene fossil locality in Death Valley (Wells and Woodcock, 1986). But Joshua tree is not contained in 3 other nearby Eureka Valley assemblages also dating to < 200 years, suggesting that its inclusion into these middens from on rocky slopes is infrequent despite its occurrence on the alluvial slopes well below the site (Spaulding 1980). Even if Joshua tree did migrate northward to this site over the Holocene, it is less than 20 km northwest of portions of Death Valley where it could reasonably be expected to have occurred considering the high elevation (~1500 m) of its Pleistocene occurrences nearby in Owens Valley (FIG. 2). Using this 20 km distance between the northernmost late Holocene midden record in Eureka Valley and the nearest likely Pleistocene occurrence in Death Valley, a maximum migration rate of 1.7 m/yr (20 km over 11,700 years) could be calculated. Thus a range of possible northward Holocene migration rates could be -1 to 2 m/yr, much slower than any other species yet studied (McLachlan et al. 2005; Yansa 2006; Cole et al. 2008a).
TABLE A1. Fossil records of Joshua tree associated with radiocarbon ages > 10,000 yr B.P.
|Alabama Hills, Owens Valley, CA||
|36.63||-118.13||Koehler and Anderson 1995|
|Death Valley, CA||19,550||36.58||-117.33||Wells and Woodcock 1985|
|Scodie Mts., CA||
|35.58||-117.95||McCarten and Van Devender 1988|
|Joshua Tree National Park, CA||12,015||33.99||-116.07||Holmgren et al., 2010|
|Granite Mts., CA||24,400||35.45||-116.54||Koehler et al. 2005|
|Marble Mts., CA||10,555||34.67||-115.58||Spaulding 1980|
|Whipple Mts., CA||10,840||34.27||-114.42||Rowlands 1978|
|Whipple Mts., CA||
|34.21||-114.37||Van Devender 1990|
|Picacho Peak, CA||12,500||32.87||-114.83||Cole 1986|
|Amargosa Desert, NV||
|36.57||-116.09||Spaulding 1985, and Personal Communication|
|Amargosa Desert, NV||
|36.63||-116.27||Spaulding, Personal Communication|
|Specter Range, NV||28,460||36.66||-116.20||Spaulding 1985|
|Frenchman Flat, NV||> 40,000||36.63||-115.93||Wells 1983|
|Sheep Range, NV||19,200||36.70||-115.27||Spaulding 1981|
|Sheep Range, NV||
|Sheep Range, NV||
|Arrow Canyon Range, NV||
|Gypsum Cave, NV||
|36.22||-114.90||Laudermilk and Munz 1935, Steadman et al. 2005|
|Rampart Cave, AZ||
|Artillery Mts., AZ||18,320||34.37||-113.62||King and Van Devender, 1977|
|Artillery Mts., AZ||
|34.33||-113.58||Van Devender and King 1971.|
|Kofa Mts., AZ||11,450||33.43||-114.10||Van Devender 1973|
|Kofa Mts., AZ||13,400||33.40||-114.02||King and Van Devender 1977.|
|Tinajas Altas Mts., AZ||> 43,200||32.35||-114.09||Van Devender 1990|
|Puerto Blanco Mts., AZ||14,120||31.95||-112.78||Van Devender 1987|
|Ajo Mts., AZ||
|32.12||-112.70||Van Devender 1977|
|Waterman Mts, AZ||
|32.35||-111.46||Van Devender 1990, Anderson and Van Devender 1991.|
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