TREE RINGS AND HISTORY — SCHULMAN 469 



1,650 years*; (h) in addition to the observed tendency for maximum 

 longevity on the most adverse sites, there seems to be a systematic 

 though probably only indirect relation to latitude in the age limits of 

 various stands of a given species; (c) the median ring width in the 

 lower stem is about 0.30 mm.^; (d) this growth rate is often ap- 

 proached by early maturity — two or three centuries — after which the 

 mean growth rate decreases very slowly; (e) the absolute minimum in 

 total mean radial growth of the lower stem for an entire century is 

 about 6 mm. ; (/) the number of sapwood rings in over-age Douglas fir 

 does not seem to be significantly related to either the number of heart- 

 wood rings or thickness of heartwood (for relatively young trees a 

 systematic relation has been found by Stallings when groups of five 

 or more trees are averaged) ; (g) false rings are almost completely 

 absent in these over-age trees of all species except the scopulorum juni- 

 per, and there is a marked tendency to decreased incidence of locally 

 absent rings in higher latitudes. 



The asymmetric age distribution is most clearly noted in drought- 

 type Douglas fir, which has been sampled on many sites throughout 

 its range of some 30° of latitude in the inland western cordillera from 

 central Mexico to Jasper National Park in Alberta. In a belt roughly 

 defined by latitudes 30° and 40° N. in the Colorado River basin of 

 eastern Utah and western Colorado literally thousands of trees may 

 be found from 700 to 900 years old. Curiously, Douglas fir east of the 

 Continental Divide in these latitudes and also in the Great Basin 

 ranges to the west of the Colorado River basin shows much lower 

 maximum ages. Although the distribution of such trees is, as already 

 noted, highly dependent on local site conditions and therefore very 

 spotty, there is a decided tendency to generally decreased maximum 

 ages both northward and southward. Douglas firs in the 600-year 

 age class may be found in considerable numbers on careful search in 

 southern Utah and southwestern Colorado, but are, in contrast, quite 

 rare in the forests of northern Arizona and New Mexico, and in the 

 writer's knowledge are unreported in the southern areas of these 

 States or in Mexico, where the average maximum age seems to be about 



*The extraordinary longevity in certain stands of the high-altitude pines 

 P. fLexiUs and P. artistata, observed in very recent field sampling by the writer, 

 is being reported in detail elsewhere; the chronologies in the oldest known 

 Juniperus scopulorum (the Jardine juniper of Logan Canyon, Utah, about 1,500 

 years) and in J. osteosperma or utahensis (about 1,050 years) are of doubtful 

 climatic significance. 



• It may be noted that Douglas fir on locally moist sites in this generally dry 

 region may attain growth rates comparable to those in wet-climate regions : the 

 "Hitchcoclc Douglas fir", over 125 feet in height and 7-foot base diameter, a re- 

 cent windfall in the Santa Catallna Mountains near Tucson, Ariz., had an aver- 

 age ring width of 2.53 mm. for the 281 years of growth at the 12-foot level (5.06 

 mm. for the inner 50 years) . 



