Figure 13.— The Populus 

 tremuloidesAbies 

 lasiocarpa/Senecio serra c.t. 

 is a sera! aspen type 

 common throughout the 

 higher mountains of Utah. 

 The aspen will eventually be 

 replaced by A. lasiocarpa. 

 The undergrowth of this 

 stand, on a north exposure 

 in Logan Canyon, Wasatch- 

 Cache National Forest, 

 consists of a mixture of tall 

 and low forbs and 

 graminoids in addition to 

 the conifer reproduction. 



Bromus carinatus, Elymus glaucus, or both (fig. 13). 

 Other forbs often conspicuous in this type are 

 Thalictrum fendleri, Osmorhiza chilensis, Achillea 

 millefolium, and Lathyrus spp. 



The aspen overstory in this serai type will eventually 

 be replaced by A. lasiocarpa during the natural course of 

 succession. The type is most likely a serai stage in the 

 northern Utah A. lasiocarpa/0. chilensis habitat type 

 (Mauk and Henderson 1984) and also probably in the 

 central and southern Utah A. lasiocarpa/Aconitum 

 columbianum habitat type (Youngblood and Mauk 1985). 

 As A. lasiocarpa gains increasing prominence and 

 P. tremuloides decreases in the tree overstory, increased 

 shading will appreciably alter undergrowth production 

 and composition. The tall forb and grass complex will 

 tend to decUne whereas such low forbs as T. fendleri and 

 0. chilensis will gain importance as undergrowth. An 

 aspen-dominated community can be maintained on these 

 sites only if the conifers are removed, usually by such 

 drastic means as burning or clearcutting. When this 

 occurs, P. tremuloides rapidly suckers, usually profusely, 

 from the remnant root system, whereas the conifers can 

 only reestablish from seed. Rapidity of aspen replace- 

 ment by conifers depends to a great extent on the avail- 

 ability of a conifer seed source. Replacement might take 

 place in less than 100 years if abundant conifer seedlings 

 become established from residual seed immediately fol- 

 lowing the disturbance. In other cases, aspen replace- 

 ment may not occur for several hundred years if conifer 

 establishment depends upon gradual invasion from out- 

 side the stand. 



Heavy sheep grazing in this type usually leads to a 

 decrease of many of the palatable tall forbs and an 

 increase in the abundance of the grasses B. carinatus, 

 E. glaucus. and Agropyron trachycaulum. Under heavy 

 cattle use, the grasses and such palatable forbs as 

 A. engelmannii, S. serra, and M. arizonica tend to 

 decrease and species such as R. occidentalis, T. fendleri, 

 A. millefolium, and Lathyrus spp. become more promi- 

 nent. If consistently grazed only in the latter part of the 



growing season, S. serra and M. arizonica may increase 

 substantially. Prolonged abusive grazing could lead to 

 undergrowth dominated by Taraxacum officinale and 

 Poa pratensis, or eventually replacement of perennial 

 herbs by such annuals as Nemophila breviflora, Collomia 

 linearis, and Galium bifolium. 



Productivity of this type for trees is at least moder- 

 ately high. Basal area on 49 stands sampled for produc- 

 tion ranged from 89 to 351 ft^/acre (20.4 to 80.6 m-Iha) 

 and averaged from 177 ft^/acre (40.7 m^/ha). An average 

 23 percent of this basal area was conifers, primarily A. 

 lasiocarpa, and the remainder was aspen. Site index for 

 aspen at 80 years ranged from 33 to 74 ft (10.1 to 

 22.6 m) and averaged a moderate 52 ft (15.8 m). Aspen 

 reproduction averaged approximately 1,300 suckers/acre 

 (3 300/ha). Half of these were in the large 1- to 4.6-ft 

 (0.3- to 1.4-m) size class. Conifer reproduction averaged a 

 little over 890 stems/acre (2 200/ha), of which 96 percent 

 was A. lasiocarpa. About a third of the stems were in 

 the large size class. 



Production of undergrowth in this serai type is greatly 

 reduced as conifers increase in abundance. Undergrowth 

 in the intensively sampled stands ranged widely from 69 

 to 2,289 lb/acre (78 to 2 568 kg/ha) and averaged only 

 767 lb/acre (861 kg/ha). An average 69 percent of this 

 was forbs, 28 percent graminoids, and only 3 percent 

 shrubs. An abundant 55 percent of the undergrowth con- 

 sisted of desirable forage. Only 11 percent was in the 

 least desirable category. The type, therefore, appears to 

 be fairly good livestock range during that time prior to 

 when conifer competition begins to seriously reduce 

 production of the herbaceous undergrowth. Harper 

 (1973) observed that when conifers increased to approxi- 

 mately 20 ft'/acre (4.6 m%a) in an aspen stand, under- 

 growth production was cut in half. Therefore, by the 

 time conifer invasion accounts for 10 percent of the tree 

 basal area, undergrowth production is probably being 

 reduced appreciably. The mixture of conifers and aspen 

 contributes to vegetation diversity, but the lack of a 

 shrub stratum detracts from the type's value as wildlife 



34 



