64 BROOKLYN BOTANIC GARDEN MEMOIRS 



contrast between the site producing a forest and a closely adjoining one 

 unable, or only very tardily able to do so. Most other Long Island at- 

 mometer readings show, of course, a decided difference in the evaporating 

 power of the air as between [usually artificial] openings and the forest. 

 But at Montauk, as these and the graphs of 1921 show (Fig. 24) that dif- 

 ference is often two to three hundred per cent. There is involved in the 

 kettlehole readings, as in all forest atmometer records, the question of how 

 much they reflect the effect of the forest canopy, and how much the actual 

 difference in site. In other words, whether the forest readings indicate a 

 contributing cause or merely the effect of the forest itself. An interesting 

 sidelight on this is furnished by the 1920 figures, where, because of the late- 

 ness of spring at Montauk, the forest canopy had only just begun to really 

 intercept the sunlight. Yet the difference between the Open Downs station 

 and the wooded kettlehole, even during this period, is substantially what 

 it proved to be when the canopy had reached its midsummer density. 



The graph for 1921 (Fig. 24) shows an even greater difference between 

 the open Downs and the wooded kettlehole, in some weeks the difference 

 being over five hundred per cent. Taking the figures of the wooded kettle- 

 hole as indicative of a reasonably favorable environment for forest growth 

 and reproduction, those of the open Downs suggest an environment at 

 least five times as severe, so far as evaporation is concerned. Such a 

 contrast of environmental conditions, even excluding the occasional fires 

 that sweep over the Downs, would be more than sufficient to explain the 

 mutual exclusiveness of these two dominant sorts of plant covering at 

 Montauk. The shade of the wooded kettlehole prevents the entrance of 

 grassland (of course certain species of individual grasses are in all kettle- 

 holes, and a few Downs species occasionally do get into them), while the 

 exposure of the Downs to such conditions as these graphs show, effectually 

 prevents nearly all encroachment from the wooded kettlehole out to the 

 open. 



One or two features of the atmometer readings of 1921 demand special 

 mention. While atmometers are not supposed to be a measure of wind 

 velocity that is included in the totality of climatic factors which beats upon 

 these instruments. In the case of Montauk the wind velocity is greater 

 than for any other Lond Island station, where atmometer readings have 

 been made. The total evaporation from white atmometers at these dif- 

 ferent places on the Island, all in the open, and exposed simultaneously was, 

 from July 15 to September 24, 1921 : 



Montauk 1459-9 cc. = 100% 



Crystal Brook, North shore opposite New Haven 1352.2 cc. = 92.6% 



