484 



E. P. VAN ARSDEL 



RADIATION EFFECTS- -FOREST OPENINGS 



The opening in the crown cover of the forest serves as a good 

 example of the influence of radiation patterns modified by vegetation on 

 the epidemiology of the disease. An opening in the forest is subject to 

 strong radiational influence because the surrounding forest breaks the 

 wind and minimizes its effect (Geiger, 1950) . 



A forest opening with a diameter greater than the height of the 

 surrounding trees is hotter by day and cooler by night than either open 

 fields or the surrounding forest. The extent of this diurnal temperature 

 variation depends on the ratio of the diameter of the opening to the 

 height of the surrounding trees (Fig. 4) (Van Arsdel , Stearns, and Main, 

 1968; Geiger, 1950). Frequently, the daily temperature range is 

 increased by 8°C. Ribes spp. (gooseberries and currants) growing in 

 these larger-sized openings do not have blister rust present on their 

 leaves. In the northern Lake States, minimum summer night temperatures 

 in such openings are usually below 5°C and often below freezing. This 

 temperature is too low for either aeciospores or urediospore infection 

 of the ribes. Daytime temperatures are often greater than 35°C (see 

 Fig. 5) (Van Arsdel et al . , 1968). The blister rust mycelium in the leaf 

 generally does not survive under such high temperature regimes. These 

 larger openings are both too hot and too cold for rust infection on 

 ribes. They are also too hot for sporidial germination and infection on 

 white pines. 





- 





...— o.f JULY. 1966 



30 



- 





\> — ^ 





- 





jT '-- Q N<" AUGUST TEMP. 

 ? / '"•'.... N. «+6.7°C or 12° F " 



28 



- 





;// x. - - 



!// ( " : 



: / J SEPTEMBER TEMP. 



26 







[ J J +7.8° C or 14° F 







**0. 



{ J { 



24 



,^"' 



I 1 



V /' 



, , 1 i 1 I , 1 , , 1 i i ! ! i i 1 ; i i i 



89 



85 



80 



• I .5 .8 I 2 4 7 10 20 50 



FOREST OPENING SIZE (AS RATIO OF TREE HEIGHT) 



75 



100 



Figure 4. The relative effects of circular forest opening 

 diameter and air temperature in the center of the opening 

 (18" above the ground) . 



