488 



E. P. VAN ARSDEL 



DOOL DOWNDRAFTS 

 ON OPEN SLOPES 





UPDRAFTS 

 OVER 

 FORESTS 



LAKE SUPERIOR 41 55 30 23 17 



RARE NORMAL HIGH RUST NORMAL HIOH 



EL eo2 RUST RUST INCIDENCE RUST RUST 



INCIDENCE (HIOH IN TREES 



Figure 6. Map of eastern Upper Michigan showing areas of high 

 blister rust (high in trees, far from ribes) . Darkest area 

 has high rust. Lower diagram shows cross section of airflows 

 controlling rust spread in this area. 



FOREST EDGES 



A number of tests have shown that a specific air-current relation 

 exists at the edge of a woods. Air flows near the ground from the open 

 area into the woods, up under the crowns in an updraft , and then back 

 out into the open area. Rust spores travel in a warm backflow layer 

 that extends from under the tops of the crowns out into the open, where 

 a down-draft may eventually bring them down (Van Arsdel, 1958). 



For example, in northeastern Minnesota (Fig. 8) , at the top of a 

 divide that slopes down to Lake Superior, we planted white pine trees in 

 an open field surrounded by 35- to 45-ft aspens (Populus tremuloides 

 Michx.). The pines in the center of the field developed 50 times as much 

 rust as those near the edges (Van Arsdel, 1965b). 



A long area of heavy rust concentration in the center of the field 

 paralleled the edges of the taller forests on two sides of the plot. 

 Yet, alternate hosts were distributed throughout the field and in the 

 surrounding woods. The rust distribution in this field was exactly what 

 would be expected if the spores were all carried by air currents in two 

 opposing cells going into the surrounding hardwoods (away from the center 



