Early cleanings, on the other hand, are effective in molding species' composition 

 only to the extent that the favored individuals are able to retain their advantage. If 

 faster growing species reenter quickly (or are present as "missed" seedlings), they may 

 overtake the leave stand before crovms of the latter close sufficiently to suppress the 

 threat. This hazard becomes relatively greater under the following conditions: (1) The 

 younger the cleaned stand, the less is the size and age disadvantage of potential new 

 competitors; (2) the heavier the cleaning treatment, the longer is the time before 

 crowns of the intended leave stand can cast effective shade; and (3) the greater the 

 disparity in growth rates between slow-growing leave trees and fast-growing invaders, 

 the shorter is the duration of cleaning effectiveness. As illustration of the third 

 point, the performance of released white pine on the study plots was strongly benefited, 

 but the limited response of released western redcedar hardly enabled it to keep up with 

 the other species that reappeared in the understory. 



Of the species on the study areas that were discriminated against during cleaning, 

 western larch has most aggressively regained a dominant or codominant role. Conversely, 

 lodgepole pine, once removed, does not appear able to make a significant reappearance. 

 Grand fir became numerically important again in the Sands Creek study areas in the years 

 after cleaning, but it was unable to escape the growth-retarding shade of taller assoc- 

 iates. This species is one of the most responsive to release from competition, so had 

 it been given any preferential treatment, its growth rate on these sites would probably 

 have about equaled that of the western white pine. 



Western hemlock has shown remarkable ability to recover promptly and strongly in 

 the stands from which it ostensibly was removed by cleaning. Miile these trees 

 generally remain in an understory position, their density and volume indicate an 

 appreciable diversion of the production capacity of the site away from the selected 

 potential crop trees. 



Since these studies were started, western white pine blister rust and the larch 

 casebearer have altered normal development, growth, and management potential of most 

 natural stands in the Tsuga/Paahistima habitat type (Ketcham and others 1968; Tunnock 

 and others 1969). Nevertheless, the study results and conclusions are applicable in 

 principle to other young stands growing under similar conditions. The mixed composi- 

 tion of natural reproduction typical of this habitat type provides ample opportunity 

 to leave well-distributed trees of several species in the cleaned stand; this oppor- 

 tunity was not fully utilized in the described studies. If this were done, there 

 would be greater flexibility at the time of future thinnings for further preferential 

 decisions about species to be left. 



13 



