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INTRODUCTION 



Forests of quaking aspen [Populus tremutoides Michx.) are considered to be predom- 

 inantly subclimax plant communities in the Rocky Mountain Region (Mueggler 1976; Bartos 

 1973) . Mature aspen forests are most often replaced by evergreen conifers (Abies spp. , 

 Picea spp., Pseudotsuga spp., or Pinus spp.) unless some form of major disturbance occurs 

 such as fire, disease, or clearcutting. When an overstory is thus destroyed, prolific 

 root sprouting of aspen generally is initiated and aspen regains dominance on the site. 

 In many areas where natural fires have been curtailed and logging has not occurred, 

 former aspen stands are now dominated by coniferous species. More than 4.1 million 

 acres of commercial aspen forests (Green and Setzer 1974) , and possibly an additional 

 1.5 million acres of noncommercial aspen lands, exist in the Rocky Mountains. Resource 

 managers are concerned that succession of sizable portions of these forests to conifers 

 will have adverse impacts on the water, wildlife habitat, and livestock forage values 

 of the aspen type. 



Because water is a critical resource in the West, it is imperative that we accu- 

 rately assess the impact that succession from aspen-to-conifer may have on water yield. 

 The concept of ecosystem hydrology assumes complex interactions between the ecosystem 

 and the hydrologic cycle, and that a change in one component should effect a change in 

 the other (Huff 1971). With regard to transpiration, for example, Sattcrlund (1972) 

 cited several studies that suggest "...the ecological principle that vegetation replace- 

 ment by better-adapted species will continue until all favorable niches are occupied...." 

 He concluded that "...it appears likely that maximum rates and amounts of transpiration 

 during the drying cycle occur under climax vegetation." 



It has been shown that western aspen may be expected to transpire 3 to 4 inches 

 more water from a 6-foot soil profile than a grass-forb community on a comparable site 

 (Johnston 1969). Douglass (1967) stated that many forest hydrologists believe well- 

 stocked forests use the same amount of water regardless of tree species when end-of- 

 season soil moisture deficits are examined. However, he pointed out that patterns of 

 soil moisture depletion for hardwoods and for conifers are quite different. Because 

 hardwoods begin transpiring later in the growing season than conifers, more water may 

 drain through hardwood soil profiles early in the season. Thus equal soil moisture 

 deficits under hardwoods and conifers may not represent equal amounts of transpiration. 

 Urie (1967) studied the net ground water recharge under hardwood and conifer stands in 

 Minnesota. He found that the net annual water yield to ground water reservoirs from 

 hardwoods exceeded conifers by 2,6 inches. This difference was associated with a greater 

 snowpack under hardwoods and a longer transpiration season for conifers. He found that 

 when transpiration and ground water recharge were combined, the conifers consumed 5.7 

 inches more water than hardwoods on comparable sites. 



In a Colorado study, Dunford and Niederhof (1944) concluded that, from the stand- 

 point of net water available for streamflow, aspen is probably superior to conifers. 

 A most meaningful insight to this problem was provided by Swank and Douglass (1974) who 

 observed a 20 percent reduction in streamflow 25 years after a hardwood stand in North 



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