22 CLIMATIC CYCLES AND TREE-GROWTH. 



this yellow pine under all conditions. Without doubt it is here due to 

 homogeneous climatic conditions in a uniform topography and a tree 

 sensitive to varying moisture-supply. 



In a good many cases where the number of trees in a group has 

 decreased in earlier years, it has been found (by carrying overlapping 

 curves through a considerable period) that a few trees give essentially 

 the same curves as a large number. From the entire experience I have 

 been led to assign a minimum preferably of 5 trees in any one group, 

 while in some groups (notably the yellow pine of Arizona and the 

 sequoias of California, together with the Scotch pine in central Sweden 

 and in north Germany), 2 trees would give a very excellent record. In 

 only one group have 5 failed to give a satisfactory record, and that was 

 the set of Scotch pines from the outskirts of Christiania. The cross- 

 identification of this group was not felt to be satisfactory, and a double 

 number of trees from that locality would have been an advantage. This 

 failure was thought to be due in part to the rugged character of the region. 



Direction of maximum growth. The maximum trunk-growth was 

 observed to occur a little east of north. The average difference between 

 the radii was 12 per cent. An explanation of this increased growth to 

 the north is to be found in the increased amount of moisture on that 

 side, due to the slower melting of snow and decreased evaporation in the 

 shade. For nearly all these trees the ground had a gentle slope toward 

 the south, so that moisture working down hill reaches the north side 

 of the root system first. 



Rate of growth and age. The relation of average ring-width to 

 radius was found to be intermediate between an inverse proportion to 

 the radius and an inverse proportion to the square of the radius. If 

 the tree merely increased in diameter without growing upward, the 

 width should be roughly inversely proportional to the radius. If the 

 tree is increasing in height at the same time, we should expect an 

 inverse proportion to the square of the radius. We find the relation to 

 be between these. 



Growth and soil. In early studies of 25 yellow pines at Flagstaff 

 it was noticed that a certain subgroup of 6 trees dropped to its strong 

 minima in 1780 and 1880 more promptly than the others. This 

 appears to be connected with the soil upon which the trees grew. This 

 subgroup stood on a limestone formation where the soil is porous and 

 the rock below full of cracks. The other two subgroups grew on 

 recent lavas, very compact and unbroken, covered with a rather thin 

 layer of clayey soil. With the former, therefore, the rain passed 

 quickly through the soil and away, and we do not find so much con- 

 servation of moisture as in the latter, where the water could find no 

 convenient outlet. On the whole, the growth seems to be more rapidly 

 influenced by changes of moisture on limestone than on volcanic rocks. 



