352 



Canadian Forestry Journal, Scplcrnber, J919 



a comprehensive subject, and some phases of it 

 so technical that I have decided to Hmit myself 

 to the question When Trees Grow? 

 200 Trees Measured Daily. 



The data which will be presented herewith 

 were collected near Mont Alto, Pennsylvania, 

 by myself and by several of my former students 

 under my direction during the past four forest 

 tree growing seasons. The field work embraced 

 the measuring of many trees at rather regular, 

 usually weekly, intervals, and the keeping of 

 meteorological records, especially of tempera- 

 ture and rainfall. In order to obtain detailed 

 field data over 200 trees were measured daily 

 during the 1918 forest tree growing season, and 

 in a number of cases specific groups of trees 

 were measured both in the morning and in the 

 evening. 



The belief is prevalent that trees grow 

 throughout the general growmg or vegetative 

 seasons, which embraces m Pennsylvania from 

 150 to 200 days, and extends from the last kill- 

 ing frost in the spring, that is, when the leaves 

 of the larches, birches, cherries, and maples 

 appear, to the first killing frost in the autumn 

 when the leaves exhibit their autumnal colora- 

 tion. This, however, is a mere supposition, for 

 most of the native and introduced forest trees 

 in the vicinity of Mont Alto make 90 per cent 

 of their height growth in less than 90 days.* 



The date when the different species start the 

 elongation of their twigs depends upon the in- 

 herent tendency of the species and the factors 

 of the environment. The late opening of the 

 buds of Norway spruce is not a local character- 

 istic, but an inherent tendency for records from 

 Germany show that they usually open after May 

 8th, and in the extreme northern part the end of 

 May. On the other hand, factors of the en- 

 vironment, such as latitude, altitude, exposure, 

 shade and shelter, also have a strong influence 

 on the starting time of the season's growth. As 

 a rule, buds open about 2'/2 to 3 days later 

 with each degree of latitude, and about 2 to 

 2'/2 days later with each 350 feet of altitude. 

 White oak begins its growth from 7 to 14 days 

 later on northern than on southern exposures on 

 the Mont Alto State Forest. Trees with small 

 and partially or completely imbedded buds such 

 as Honey Locust, Black Locust, Kentucky 



♦Editor's Note. Prof. Illick's table of representa- 

 tive Pennsylvania trees, -indicates that white pine 

 starts its growth on April 18th, attains 92 per cent 

 of growth on .June 1, and ceases growth by July 1. 

 Norway Spruce starts growth May 6th, achieves 

 7-t per cent of development by June 1. Growth 

 ceases approximately at the same time as with 

 white pine. 



Coffee-Tree, Tree of Heaven, and Catalpa, begin 

 growth relatively late. Nature seems to pro- 

 tect the tender growing points of these trees 

 from the cold of winter by placing them within 

 small buds which are almost completely im- 

 bedded within the twigs. This means of 

 adaptation also protects the tender new growth 

 of spring from late frosts, for the small and 

 deeply imbedded buds are not stimulated so 

 early in spring as large exposed buds; hence, 

 the resultant vegetative growth usually appears 

 after the damaging frost period. 



All Growth Ceases by August. 



The range of the period during which the 

 height growth of forest trees ceases is longer 

 than that during which height growth starts in 

 the spring. Most species of forest trees in 

 southern Pennsylvania cease growing during the 

 latter part of May and the early part of June. 

 Only a few species continue their growth into 

 July. On June 10, 1919, I examined 179 

 different species of trees in the vicinity of Mont 

 Alto, 55 of which, that is 70 per cent, had al- 

 ready ceased growing in height. On June 18 

 and 19, 1919, I examined 50 species of trees 

 in the vicinity of Bedford, Pennsylvania, and 

 found that the height growth of 40 had already 

 stopped. This is an unusually high percentage 

 of growth cessation, and is probably due to the 

 extremely cold period during the early part of 

 extremely cold perio dduring the early part of 

 May, followed immediately by an unusually hot 

 period during late May and early June. Such 

 extreme temperatures and the abrupt transition 

 from one extreme to the other are potent factors 

 in retarding growth and in extreme cases may 

 cause entire cessation of growth. The white 

 pine, which usually stops growing in the vicinity 

 of Mont Alto about June 15, but may continue 

 to grow as late as June 30, ceased growing this 

 year (1919) about June 3. It is the writer's 

 belief that 85 per cent of the forest trees of 

 Pennsylvania have already (June 20, 1919), 

 completed their normal height growth for the 

 season. Of the remaining 15 per cent the Tulip 

 tree. Sycamore, and the Larches are prominent 

 species, which may continue to grow until the 

 middle or latter part of July. By the 1st of 

 August the normal height growth of all the 

 forest trees of Pennsylvania has, as a rule, 

 ceased. 



Trees Rest Between Spurts. 



In order to determine the progress of the 

 height growth each species must be examined by 

 itself, for each individual species possess dis- 



