WHEN TREES GROW 



1387 



open after May 8, and in the extreme northern part 

 after the end of May. On the other hand, factors of the 

 environment, 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 

 two and one-half to three days later with each degree 



of latitude and about two to two and one-half days later 

 with each 350 feet of altitude. White oak begins its 

 growth from seven to fourteen days later on northern 

 than on southern exposures on the Mont Alto State For- 

 est. Trees with small and partially or completely im- 

 bedded buds such as Honey Locust, Black Locust, Ken- 

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

 growth relatively late. Nature seems to protect the 

 tender growing points of these trees from the cold of 

 winter by placing them within small buds which are 

 almost completely imbedded within the twigs. This 

 means of adaptation also protects the tender new growth 

 of spring from late frosts, for the small and deeply im- 

 bedded buds are not stimulated so early in spring as 

 large exposed buds ; hence, the resultant vegetative 

 growth usually appears after the damaging frost period. 



Pennsylvania is the meeting ground of many northern 

 and southern forest tree species. The northern follow the 

 mountains towards the south and the southern extend 

 northward through the valleys. The distinctly southern 

 species, which are decidedly sensitive to Spring frosts, as 

 a rule, begin the elongation of their shoots rather late, 

 that is, after the danger period of frost damage is past. 

 The Eastern Catalpa. supposedly a native of the South 

 Atlantic States, does not leaf out until the latter part of 

 May. Likewise other southern species, such as Persim- 

 mon, Kentucky Coffee-Tree, and- Bald- Cypress postpone 

 the beginning of their vegetative elongation until late 

 spring. 



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. The Sweet 

 Buckeye, Aesculus octandra, usually completes its growth 

 at Mont Alto as early as May 10 to May 15, and by 

 June 15 one can find full-sized winter buds. This species 

 is the first to complete its height growth of the season. 

 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, 1 examined 79 

 different species of trees in the' vicinity of Mont Alto, 

 55 of which, that is 70 per cent, had already ceased 

 growing in height. On June 18 and 19, 1919, I examined 

 50 species of trees in the vicinity of Bedford, Pennsyl- 

 vania, 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 May, followed immedi- 

 ately 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 fac- 

 tors 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 of 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 



THE WHITE OAK MAY TAKE A REST 



The large fully developed leaves are the result of the original growth of 

 the season. After resting for 20 days, growth was resumed, and the ter- 

 minal shoot bearing immature leaves is the result. 



first of August the normal height growth of all the forest 

 trees of Pennsylvania has, as a rule, ceased. 



In order to determine the progress of the height growth 

 each species must be examined by itself, for each indi- 

 vidual species possess distinctive inherent growth charac- 

 teristics. Some place their growth without a break, while 



