110 CLIMATIC CYCLES AND TREE-GROWTH 



CHANGING CONDITIONS 



The preceding topographic conditions are constant and their 

 effects are sought by comparing trees in one location with those in 

 another. The results are practically constant in any one tree. But 

 changing conditions produce internal alterations in each tree and 

 may often be recognized in the ring record after allowing for the normal 

 change of ring appearance with age. 



Shade — The Vermont hemlocks from the edge of Mount Ascutney, 

 near Windsor, showed a doubling of yearly growth about 1808, due prob- 

 ably to cutting of adjacent trees at that time (Volume I, pages 41, 42). 



Drainage — A small section of Scotch pine in the Berlin Museum 

 shows minute rings for some 40 years and then suddenly the growth is 

 quadrupled. As the history of the tree showed, this was caused by 

 draining the very wet land on which it grew. 



Soil deficiency — A very interesting relationship was recognized 

 by studies in Chaco Canyon in 1926. For 10 years it had been noticed 

 that certain prehistoric or early historic trees showed normal growth 

 to a very good size and then rather quickly the growth dwindled down 

 to a great number of microscopic compressed rings from which there 

 was no recovery. In human language, the tree starved to death. 

 Some of these specimens came from Chaco Canyon and a number came 

 in 1926 from Wupatki, a ruin 35 miles northeast of Flagstaff, in the 

 region of the Lava Beds and volcanic cinders, which suggested showers 

 of volcanic ashes as a means of killing forests. But on the bare rock 

 mesas about Chaco a few pines were found in favorable spots where a 

 little soil covered the bed-rock. Some were dying, some dead, and a 

 very few in good condition, but most of them showed the compressed 

 rings for the last 50 or 100 years. Evidently there was enough soil for 

 small trees, but not enough to support full-grown trees, and the 

 shallow beds of soil were drying out and in many cases blowing away. 

 One small pine in bad condition had 2 feet of horizontal roots bare 

 before any of them were covered by soil. This lack of soil and change 

 in its condition, then, is the common cause of that sort of outer com- 

 pressed rings in this arid area. 



Close grouping — A test for the effect of close grouping of trees was 

 made on the Fort Valley group. These effects have already been 

 described in connection with tree selection, page 12, and eccentricity 

 of ring-growth, page 22. 



Injuries — The injuries chiefly recognized in the western groups are 

 fire and lightning-scars, already referred to in the selection of trees, 

 page 14. 



Pests — This topic is a recognition that such effects are of great 

 importance in the general consideration of tree-rings. Where moisture 



