Z CLIMATIC CYCLES AND TREE GROWTH 



The Region, Climate, and Trees — Tree-ring data may extend in two ways : 

 Geographically about the earth, and chronologically in past time. In the 

 studies contained in this book a limited geographical area is used, commonly 

 known as the Pueblo area. It consists largely of the Colorado Plateau in 

 northern Arizona, northern New Mexico, and adjacent parts of Colorado and 

 Utah, an extensive area where climatic variations are essentially similar. 

 The Colorado Plateau is not flat like a table but is composed of wide-spread 

 minor variations culminating at the west in the San Francisco Peaks near 

 Flagstaff; thence toward the east it slopes down to the Little Colorado River 

 bottom; rises again to the Chuska and Lukachukai Mountains between 

 Arizona and New Mexico; descends thence to irregular lands extending across 

 the Chaco areas, and rises to Mount Taylor and the Jemez Mountains at the 

 west wall of the Rio Grande Valley in New Mexico. This area described has 

 become especially adapted to our purpose, because in altitude it extends a 

 thousand feet or more above and a thousand feet below the forest-border 

 level. Rainfall is locally dependent upon altitude, and hence at the higher 

 points forests thrive, while at the lower points the desert prevails. We thus 

 have the forest borders and forest areas scattered through the region under 

 consideration and we can find similar border conditions 400 miles apart 

 without important differences in climate. 



The area above described as a climatic unit has two well-defined rainy 

 seasons per year, one in winter and one in summer. The winter rainy season 

 has the characteristics of the temperate zone in type and movements of 

 storms; clouds cover large areas hundreds of miles in extent and precipitation 

 changes from year to year are much alike over a large extent of country. 

 Rainfall is, of course, influenced by altitude and by north and south mountain 

 ranges. The winter storms come from the west and the precipitation is 

 greater on westerly slopes than on easterly. The spring season is exceedingly 

 dry and rain in May and June indicates an exceptional year. The autumn is 

 also marked by diminished rain, but is not so dry as the spring. The summer 

 rains are tropical in their nature and come in July and August, sometimes 

 extending into September. They are largely localized thunderstorms which 

 are very impressive to the beholder and sometimes dangerous in the swollen 

 washes that follow a downpour. However, their contribution to the water 

 supply in reservoirs is small in comparison with that of winter rains. 1 They 

 do, of course, aid the crops and the feed for cattle on the ranges. Summer 

 rains come on southerly winds and are stirred into thunderstorm activity by 

 the interference of mountains and by local convection. In the summer rainy 

 season the mornings are usually clear and subject to enormous evaporation 

 from irrigated or moistened areas. 



In this area the western yellow pine {Pinus ponderosa — now called ponder- 

 osa pine by foresters) follows a contour zone cutting across mountains and 

 table lands between approximate altitudes of 5000 and 8500 feet where it is 



1 Glenton Sykes, Range Studies for Southwestern Range and Forest Experiment 

 Station, Tucson, Arizona. 



