TREE GROWTH IN VICINITY OF GRINNELL, IOWA 103 



of the settlement of the region by white men. The average annual 

 growth in diameter is found to be — 



0.22 inch for Carya ovata. 

 .29 inch for Quercus velutina at its maximum period. 

 . 16 inch for Quercus velutina after 30 years of age. 

 .30 inch for Quercus macrocarpa. 

 . 49 inch for Ulmus fulva. 

 .63 inch for Acer saccharinum. 



. 27 inch for Primus serotina. 1 



. 34 inch for Jiiglans nigra. 



By Pressler's table, Q. macrocarpa would thus yield 5.4 per cent 

 interest. 



The age at which the maximum growth occurs depends upon a num- 

 ber of circumstances. An elm at 22 years showed an annual increment 

 of 13 per cent; a hickory at 25 years, 8 per cent; bur oak at 40 years, 

 6 per cent. Three black oaks showed their periods of maximum growth 

 at 10 to 20 years, 18 to 28 years, and 90 to 100 years, respectively ; the 

 ages of the trees were 30, 50, and 120 years when cut. Our best figures 

 indicate that Q. macrocarpa makes its most rapid growth from 11 to 30 

 years; Q. velutina, 9 to 30; Prunus serotina, 17 to 36. A single large 

 bur oak 144 years old made its maximum growth between 124 and 132 

 years. This was because it was at the beginning of that time freed 

 from competition by the cutting out of the surrounding timber. Thus 

 a vigorous rejuvenation is possible, even at an advanced age. Attempts 

 to find a relation of rate of growth to wet and dry seasons proved futile. 

 Diflferent trees in a single grove showed maximum growth in all of the 

 years from 1874 to 1911. Evidently the seasonal variation in conditions 

 of growth had less influence than other factors upon the rate of growth. 



In river bottoms on the Kaskaskia loam the principal tree is Ulmus 

 fulva. It grows to large diameter and height. With it occur Populus 

 dclt aides, the tallest and straightest tree of the region ; Acer sacchari- 

 num and Acer negundo ; Juglans nigra and Juglans ciiterea; Ulmus 

 americana; Tilia amcricana; Salix longifolia: Bctula nigra. When 

 these form dense stands, there is no grass on the ground, a thin cover 

 of leaf mold, and more or less underbrush. Often, however, they occur 

 in open parklike formation, with dense grass between. They never 

 make pure stands. Juglans nigra sometimes attains a diatneter of 4 

 feet and Betula nigra 2]/^ to 3 feet. Populus dcltoides is often 4 feet 

 through. 



