Great Basin Naturalist
Volume 52 Number 4 Article 4
12-30-1992
Plant age/size distributions in black sagebrush (Artemisia nova): effects on community structure
James A. Young USDA, Agricultural Research Service, Reno, Nevada
Debra E. Palmquist USDA, Agricultural Research Service, Reno, Nevada
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Recommended Citation Young, James A. and Palmquist, Debra E. (1992) "Plant age/size distributions in black sagebrush (Artemisia nova): effects on community structure," Great Basin Naturalist: Vol. 52 : No. 4 , Article 4. Available at: https://scholarsarchive.byu.edu/gbn/vol52/iss4/4
This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Great Basin Naturalist 52(4), pp. 313--320
PLANT AGE/SIZE DISTRIBUTIONS IN BLACK SAGEBRUSH (ARTEMISIA NOVA): EFFECTS ON COMMUNITY STRUCTURE
1 James A. Young and Debra E. Palmquist1
ABSTllAC'T.-The demography of black sagebrush (Artemisia nova Nelson) was investigated in the Buckskin Mountains ofwestern Nevada to detennine patterns ofstand renewal in sagebrush communities currently free from wildfires. Biomass samplingwas conducted to develop growth classes that reflected apparent age ofthe shrubs. The density ofblack sagebrush plants was twice that ofbasin big sagebrush (A tridentata ssp. tridentata Nutt.) in adjacent communities on contrasting 2 soils (2.2 versus 1.1 plants per m ). Black sagebrush accumulated only 75% as much woody biomass as big sagebnlsh. Regression equations were developed and tested for predicting total woody biomass, current annw.J growth (CAe), and leaf weight of black sagebrush plants. Apparent age classes were developed both for the black sagebrush plants l;md the SUb-C<'UlOPy mounds on which they grew. Discriminant analysis was used to test this classification system. Plant succession, apparently controlled by nitrate content of the surface soil, appeared to eliminate the successful establishment of black sagebrush seedlings on the mounds. After the shrubs die. the mounds eventually deflate. We propose that mounds reform around shrub seedlings; but because seedling establishment is so rare in these comlUunities, this could not be verified.
Key words: biomass. shrub SflCC8ssion, desert soil forrTULtion. 80il nitrate, black sagebrush, Artemisia nova.
Black sagebrush (ArtemL,ia nova Nelson) is west central Nevada is characterized by black one of the dwarf sagebrush species which col sagebrush/desert needlegrass (Stipa speciosa lectively constitute about half the sagebrush Trin. & Rupr.) plant communities. The Buck vegetation in Nevada (Beetle 1960). Black sage skin Mountains are located 100 km southeast of brush plays a dominant role ina numberofplant Reno, Nevada, in the rain shadow of both the communities in the Great Basin (Zamora and Sierra Nevada and Pinenut Mountains. This is a Tueller 1973). Rarely does black sagebrush portion ofthe Carson Desert in which Billings share dominance \ivith another species of Arte (1945) suggested that Atriplex-dominated salt misia. In the section Tridentate of the genus desert shrub vegetation occurred because of Artemisia, black sagebrush is perhaps the spe atmospheriC drought rather than occurrence of cies most adapted to arid environments. Black soluble salts in the soil. Ifwe compare the black sagebrush is closely associated with shadscale sagebrush communities ofthe Buckskin Moun [Atriplex confertifolia (Torr. & Frem.) Wats.] tains with those described in the regional study dominated landscapes (Blaisdell and Holmgren conducted by Zamora and Tueller (1973), we 1984). The browse of black sagebrush is highly find that the highest-elevation, north-facing preferred by domestic sheep (Ovis aries), slope communities of the Buckskin Mountains pronghorn (Antilocarpa americana), and Sage correspond to the most arid communities pre Grouse (Centrocereus oroph08ianus). From the viously described. From this we assume the 1890s until thelate 1950s, blacksagebrush plant black sagebrush communities in this study rep communities in the Carson Desert of Nevada resent an arid extension ofthis type. were a vital partofwinter range for the domestic Only recently have occasional wildfires of range sheep industry. Years of excessive brows any extent occurredin black sagebrush commu ingbysheepactually shapedthe outline ofblack nities in western Nevada. The fires that have sagebrush shrub canopies; Zamora and Tueller occurred have been associated with the recent (1973) reported they had difficulty in finding spread of the alien annual cheatgrass (Bromus relic communities in high range condition. tectorum L.) into these arid environments Vegetation of the Buckskin Mountains of (Young and Tipton 1990). Apparently for much
I USDA, Agricultuml nesenr<::h Sel">~<:e, 920'/:111£)' Boad, Reno, Nevada 89512.
313 314 GREAT BASIN NATURALIST [Volume 52 of the twentieth century these communities loc-ated along the western flank of the Buckskin have not been subject to wildfires because of Mountains. The five stands, located on the same lack ofherbaceous vegetation to carry the fire. outcroppingofMickey Pass tuff, were separated Because ofthe lackoftreesto produce fire scars, by small canyons where the westerly tilted ash it is difficult to determine whether these sites flows ""ere broken by faulting. All sites were were subject to periodic burning under pristine west facing and located in a band along the conditions. This is in sharp contrast to basin big mountainside at 172Q-1780-m elevation. sagebrush communities where periodic cata Astarting point was located on aerial photo strophic stand renewal by burning from wild graphs in each stand, and 10 plots, each 10 m' fires has been common. The lack ofcatastrophic in area, were located randomly along line tran stand renewal in black sagebrush communities sects parallel to the slope. A total of 50 plots should be reflected in the age/size class struc were established (5 stands x 10 plots perstand). ture ofthe communities. In each plot the follOwing were determined: (a) Our purpose was to determine the age/size shrub density by species, (b) crown cover of distribution ofblack sagebrush plants to deter shrubs (ocular estimate), (c) shrub height, (d) mine commnnity structure. area of mound aud interspaces, and (e) herba ceous cover (ocular estimate). Mound cover MATERIALS AND METHODS refers to the slightly raised areas heneath shruh canopies where subaerially depOSited soil and Studies were conducted from 1984 through saltation depOSits accumulate. 1988 in the Buckskin Mountains located about 100 kin southeast of Reno, Nevada. The geo At each plot location the herbaceous vegeta lOgiC features of this mountain range have been tion frequency was sampledwith 100 step points described in detail by Hudson and Oriel (1979). arranged in 4 lines of 25 points each follo"'"ng Vegetation and soils of the range have been tl,e procedures of Evans and Love (1957). The mapped and related to the geologiC map of the herbaceous vegetation was resampled annually. area (Lugaski and Yonng 1988). The plant com Using the same startingpoint, but by placing munities used in this study were located on the the transects up and down the slope, 25 black Guild Mine member of the Mickey Pass tuff. sagebrush mounds were located in each stand. This geolOgiC unit consists ofcI)'Stal-rich, mod The shrubs rooted on each mound were mea erately to poorly welded ash flow tuff (Proffett sured for (a) height, (b) maximum and mini and Proffett 1976). It has been proposed that mum crown diameter, (c) stem number (as black the soils (a) developed in/lace, (b) developed sagebrush ages the cambium splits, forming from subaerially deposite material from long multiple-stemmed plants), and (d) stem diame distance transportation, or (c) developed from a ter at the soil surface (diameter of the group of combination of residual and subaerially depos split stems). The aerial portion of the plant was ited material (unpublished research, ARS subdivided by clipping into the follmving cate USDA). The bulk of the prome is an argillic ~~~:es: (a) coarse stems, 2.5 cm or larger in horiwn, about 50 cm thick, which consists of . leter; (b) fine stems, 0.25 to 2.4 cm in diam 50% or more clay-textured material. It is pro eter; (c) current annual growth; and (d) leaves. posed that this clay horizon is arelic ofa soil that The material was dried at 80 degrees C for 24 developed on the siteandwhose original surface hours and weighed. horizon has been removed bv, erosion. The After the aerial portion of the shrub was important point is that the clay horiwn, which removed, the litter beneath the canopy was col is intermittently exposed on the soil surface, lected and screened through a 2-mm screen. developed under different environmental con The material too coarse to pass through the ditions from the current surface horizon. The screen was saved, dried, and weighed. The max current surface soil consists of a relatively imum and minimum diameters of the mound recently deposited layer, apparently from sub were measured, and the height of the mound aerial deposition, that is largely confmed to was determined by digging to the clay horizon. mini-mounds beneath the canopies ofthe black The number ofperennial grasses rooted on the sagebrush plants. The soil is classified as a fine, mound was counted by species, and the cover of iridic, montmorillonitic, Typic Paleargid. cheatgrass was estimated ocularly per mound. Spatial structure oftheblacksagebrush com A series of age/size classes WdS established munities WdS determined by sanlpling five stands for the black sagebrush plants sampled. These
,'. , . " , '" ," ' 1992J BLACK SAGEBRUSH DEMOGRAPHY 315
2 2 TABLE 1. Mean blus standard error (SE) for shrub density per m , percent projected canopy cover, frequen<.y (lO_m plot) within stands N"" 10), and constancy among stands (N '" 5).
Species Density SE Cover SE Frequency SE Constancy SE (m') (%) (%) (%)
Artemisia nova 2.2 0.40 22 2.4 100 0 100 0 Chrysothamnus viscidifloros 0.7 0.10 2 0.4 40 8 80 8 Ephreda nevadensis 0.3 0.08 1 0.4 64 10 100 0 Tetradym/a glabrata 0.2 0.04 - " 32 5 60 4 , EriogOflU.m microthecum 0.1 0.04 - 5 1 20 2 , EriogonuJn umbeUotum 0.2 0.00 - 8 1 20 I
'lndicates less than 1% l\lIemge cover, classes were based on the size, growth form, RESULTS A D DISCUSSION percentage dead canopy, and apparent age of the plants. The classes were (a) seedling, (b) Community Competition young plant, (c) mature plant, (d) patriarch, (e) The plant communities of the Buckskin senescent, and (I) dead. Mountains dominated by black sagebrush are Soil samples from the surface 5 cm were low in diversity (Table 1). Green rabbitbrush taken (a) next to the shrub stem, (b) at the [Chrysothamnus viscidiflorus (Hook.) Nutt.] canopy edge, and (c) 10 em beyond the edge of occurs in patches in the community. Nevada the shrub canopy. These samples were dried, ephedra (Ephedra nevadensis Wats.) is rather screened, and shipped to a CXlmmerciallabora evenly distributed through the black sagebrush tory for nitrate nitrogen analysis. communities, but at a Tow density. Littleleaf A two-way analysis ofvariance and post hoc horsebrush (Tetradymia glabrata Gray) is a rel Duncan's Multiple Range test were performed atively infrequent component ofthe communi to analyze differences in soil nitrate concentra ties. The two species of Eriogonum are tion between sagebrush age/size classes and semiwoody species that also occurred in the sample location. A series ofstepwise regressions most arid black sagebrush CXlmmunities that was performed, utilizing the general hnear Zamora and Tueller (1973) reported. model, wherein a subset ofvariables was chosen Squirreltail (Elymus hystrix Scribn.) and that would best predict plant weight, annual cheatgrass are the most frequent herbaceous growth of plant (weight), and leaf weight of species (Table 2). The relative frequency ofthe black sagebrush. Separate step-up regressions two species reverses from year to year depend ing on available moisture for plant growth. were performed for plantand moundcharacter Cheatgrass is abundant only in years with ade istics (Neter and Wasserman 1974). The inde quate moisture during the spring. The density pendent variables that were significant ofsquirreltail plants remains relatively constant. contributors to predicting age/size classes for In dry years, squirreltail is virtually the only these two groups were chosen as ruscliminant herbaceous species in these communities. vaJiablesto beusedintwo separatediscriminant analyses. The age/size classes of the sagebrush Biomass plants were used as the grouping structure in Along the western margin of the Buckskin the discriminant analysis. The plant character Mountains, black and basin big sagebrush com istic variables selected as significant contribu munities OCCW' side by side on sharply contrast tors to classification into age/size classes were ing soils. The basin big sagebrush communities (a) weight ofCXlarse stems, (b) number ofstems, have been burned in wildfires, based on historic (c) plant height, and (d) plant diameter. The records andfire scaranalysis (Youngetal. 1989). mound characteristics (ranked in order of Our analysis ofblack sagebrush communities is importance) used were (a) litter cover, (b) litter essentially based on aboveground woody mate weight, (c) soil nitrate CXlncentration, and (d) rial accumulation of the dominant shrub. We cheatgrass cover on the mound. had previousl)' conducteda study ofthe biomass 316 GREAT BASIN NATURALIST [Volume 52
-r.o\IlIJo: 2. Mean plus standard error (SE) for freqllcn<.; of herbaceous SJ.lecies for an averag:e of four years' sampling (avcr..t~c precipitatioll 175 mm). for a dry spring (1989, no April precipitation), ,mel a year with above-avemgc lTIoisture availahle for ptmt grO\vth (L9S6, 225 mm precipitation). Ba..ed on 5000 sample points per year.
Frequency
Dr)' (1989) Wet (1986)
Four yem'S 5E Spring SE Spling SE
GH()Wfll j<'ltOM ~PI':(;II';S ------..------9,0--.------.------.------PEJIENNIAI.(:BhSS Et!jlllJ/s h!i~trix StilJrt .~p(,'dO.Wf Stillrl tlmrherimlf/ PO(/ scc1mda OrY;:'()/~~i.~· hljlTu:rlOides
ANI\UA I. (:1lA.'l."i Bnllllfl$ fectontm 44 6." 14 2.8 76 3.8
PI<:liENl\lAI, H!HB Cast-,Ilcja ch nJIIIO!)'(1 I 0.2 1 0.2 SJ,/uwmlwa p{froifolia 2 0.3 Ph/m luHHlii 3 0.3
ANNtJAl,FOlll1 J';mdiuln dClltar1111n 08 2 0.8 [)(1.~ctlmi'ji(f piHl/ala 5 0.7 Si.~Y'JJJd"'lllJti....<;illntm 5 0.9 )0 0.8
of htl'iin hig sagebrush adjacent to the western wide difference bet\veen the two communities ed~e of the Buckskin Mountains (Young ct al. is apparently due to the higher woody biomass 1989). This allowed comparison ofthe produc ofmorc mature ba.liin big sageblush plants. tion of hiomass of hasin Lmu hlack sagebrush Woody biomass of black sagebrush was best hom the same area. The hasin hig sagebrush predicted by the equation: commnnity had a sandy loam surface soil and a greatersoil depth (llaplargids deJived from meta Y = 9.87 + 1.21 • Xl + 1.12 • X2 + 0.88 • X3 volcanic sources). Big sagebrush ages were where Y = total woody biomass (gmms), Xl = dumped at 55--60, 4tJ...4.5, and 10--15 years old. fine stems, X2 :::: coarse stems, and X3 :::: root The general aspect of the two communities 2 crown. R = .96 for this determination. Yearly is strikin~ly different, with the maximum height growth increment was predicted by the equation: of the black sagebrush being 60 em and that of the big sagehrush over 1 m. In contmst to the Y = 16.96 + 0.26 • Xl + 0.16 • X2 centralwoody stems ofthe big sagebrush plants, 0.73' X3 - 0.14' X4 black sagebrush plants appear multi-stemmed. where Y :::: current growth, Xl = fine stems, X2 Despite the differcnce in height, the two com , :::: coarse stems, X3 = plant height, and X4 == root • munities have similar biomass because of the 2 • higher density of plants in the black sagebrush crown. R = .57 for this determination, despite community. There is more coarse and fine the inclusion of a fourth variable. Our third woody material in the basin big sagebrush com equation predicted leafweight: munity (Table 3). Y = 23.53 + 0.29 • Xl - 0.93 • X2 + If\ve assume hoth populations are the same 0.2 • )(,1 - 0.35 • X4 age (assumption is necessary because actual age of black sagebmsh plants could not be esti where Y = leafweight, Xl = c.'Oarse stems, X2 = mated), the mte of woody biomass accumula height of plant, X3 =fine stems, and X4 =plant tion wus 13.2 glm'iyear and 64.5 gtm'/year for density. These four variables in the equation black and basin big sagebrush, respectively. The accounted for 64% ofthe variability in the data. ,
,• • I 1992] BLACK SAGEBRUSH DEMOGRAPHY 317
30 •
• • • • • • • •a a • a 20 • • a a
• ·10 • :f. .!1'" § 8 a
-10 = 0
-20
-4 -3 -2 -1 0 1 2 3 4 Canorical Fnc. 2
Fig. 1. Plot ofblack s..,gebrush group membership based on plantcharocteristic discriminant equations where 0 '" young, @ =:: m Age/Size Classes material in the canopy (to 30%) and complete separation of the stems. The separated stems The selected variables for both plant and formed V-shaped flutes with the open end of mound charactelistics were important contrib the U toward the former center of the stem. It utors in distinguishing between age/size classes was not possible to establish the maximum age and were good indicators ofgroup composition ofthe class because the center ofthe stem was (Fig. 1). Very few misclassifications occurredby missing. The individual section had at least 40 use ofthe resulting discIiminant functions. growth rings. The bulk of the black sagebrush stands was Senescent plants formed the next, appar composed of mature plants 20-60 em tall with ently older, age/size class. In this class at least canopies 20-50 em in diameter (Table 4). This 50% ofthe canopy was dead. Older black sage is a wide range in height and canopy size, but brush plants do not get taller, probably because the matureage/sizeclass was distinguished from they have no central stem to support th~ canopy. young plants by the presence ofup to 10% dead The diameter of the crowns does increase. material in the canopy and the beginning ofthe There is a marked increase in woody biomass separation ofthe stem into individual cambium between the patriarch and senescent classes. bundles. The patriarch class was distinguished Seedlings and young plants constituted only from the mature class by an increase in dead 6% ofthe black sagebrush populations (Table 4), 318 GREAT BASIN NATURALIST [Volume 52 2 2 TABlE 3. Mean density (stemslm ) plus st:mdard error (SE) and oYen-dry bioma~ (gIm ) of Artemisia tlOtx! and A. tridetttata subsp. trl{!entata, Datu [Ol' A ttidentllta subsp. tridentata fTom a previous study (Young et aI. 1989). a-lornass per m• Spedes Density Coarse Fine CAG Leaves Total m' SE g SE g SE g SE g SI': g SE Artemisin rIlJ(.l(1 U 0.4 750 90 520 60 180 45 130 40 1550 240 Artemisitl tridenuaa subsp. lridfmtata 1.1 0.3 850 100 970 110 170 40 130 50 2120 420 TABLE 4. Artemisill nova crown and biomass c-haracteristics for individual age/size dasses. Demographic breakdown of blnck sagebnlsh (."Ommunitics by growth classes. Classes are related to age for younger P~lOts, but oncc stems separate, ages are not hased on allnual rings. Crown characteristics Biomass charactelistics Dead Coorse Fine Age/si/..e Ireight Diameter Density branchlets siems stems CAG Leave Stem Percentage Age class (em) (em) (%) (%) (g) (g) (g) (g) number ofstand (l"""') Seedlin~ 5 5 30 0 0 15 5 10 1 >1 2-5 Young pant 10--20 5-10 50 0 28 64 30 40 1 5 5--30 Mature 20-60 10-50 80 10 140 120 80 60 Multiple 60 30-,5() l'atriurch 20--60 20-RO 60 30 560 420 140 100 Multiple 17 40+ Senescent 20-60 20-100 30 60 9RO 64.0 60 30 Multiple 12 'I Dead 20-00 20-100 0 100 910 320 0 0 Multiple 5 ? with seedlings being very rare. The separation surface of the mound was disturbed, the dark between seedling and young plants was based color was replaced by a grayish shade. Mounds on the occurrence of coarse, woody biomass in appear to reach their maximum height with this the latter class. Young plants had entire stems growth stage of black sagebrush. Mouods of with no evidence ofdivision ofthe cambium, mature plants had perennial grasses associated with the sub-canopy area, The most frequent Mound Types perennial grass was squirreltail. Each black sagebrush age/sil.e class had a Litter accumulations increased with the corresponding type ofsuh-canopy mound. The patriarch age/size class, but height ofthe mound only seedling found in the entire study was did not increase. Apparently, trapping of sub ,; locatedin an interspace between mounds, Obvi aerial deposition material and saltation particles ously, one seedling is not a valid sample, but the must he related to growth stage of black sage lack ofseedlings is a critical factor in the dynam brush plants in terms of crown architecture. , ics ofthe communities studied. The first detect Subaerially deposited p"'ticles are obviously , able mound occurred under young plants, Only very unstable and subjectto redeposition ifthey 5-10% ofthesub-canopyarea under bJack sage fall in the largely bare interspace among shrub brusb plants in the young plant age/size class mounds (Young and Evans 1986).lflitter accu was covered with litter (Table 5). The litter was mulation increa"es on patriarch mounds, why composed offragments ofhlacksagebl1lsh leaves. do they not trap these secondary erosion prod In the mature plant age/sil" class the cover ucts and the mound keep growing in height? oflitter and the weight oflitter increased (Table Canopy structure changes with the patriarch 5). The mounds were easily distinguished by age/size cla'\s, with increa.sing bare stems and both height aod surface soil color and texture. spreading, butnot taller, plants. Itwould appear The surface of the mounds appeared darker in that aerial dynamics ofthe crown ofblack sage color, and the reddish tinge to the clay surfac'e brush plants inflnence mound height. soils of the interspac'e was not apparent. If the With thesenescent age/size cIa-iS. adivergence I , ; , , •, 1992] BLACK SAGEBRUSH DEMOGRAPHY 319 TADlE 5. Mound characteristics in relation to age/size classes of Artemisia now. Illustrates that mound characteristics change with age/size classes ofshrubs. Mound litter Black sa~rush Diameter Perennial Cheatgrass growth c ses Height Max Min Cove, Depth Weight cover Number (em) (em) (em) (%) (em) (g) f,"'"permoundensiJJ) (%) Samples SeeclUn~ 0 0 0 0 0 0 0 0 I Young p ant 2-S 60 30 5-10 0.5 40 0 2 6 Mature 5-15 80 40 40-«> 1-1.5 480 2.0 15 76 Patriarch 10-15 100 60 80 2-3 690 2.8 12 21 Senescent 10-15 100 60 80 2-3 720 21 60 15 Dead 10-15 100 60 80 2.5-5 970 6.4 5 6 TABLE 6. Mean nitrate level (mWkg) ofsoil at the stem, ~nopy edge, and outside the canopy of black sagebrush plants in relation to maturity classes. BuckskIn Mountains, Nevada.u Locationb Age/size class Stem Canopy Outside AgelSw, (ppm) (ppm) (ppm) class meanl; YOtmg plant 4.7h 4.3 hi 4.1 hi 4.4d Mature 66~ 5.5~ 4.0j S.4c Patriarch 10.5 12.0 8.0e 10.20. Senescent 13.2. 11.3e 7.0f 10.5. Dead 8.4c 7.0f 7.lf 7.5 b Mean locationh 8.7 a 8.0b 6.0c ~e>ln$ followed by tile :;a,ne letter llre not .s:ignific:mtlydiErerent.t tbe .Olleve1 of prubAAllily ll$ Jclertnrued by Dunauo', Multiple Rotnge rest. "Meanf of1oaltlon followed by the same lettll'r ue not signili.antly different Itt the m~ ofprob-Willl)' ~ d~rmtned by DW'IClln'~ Multiple HMge te5t. "MeaJUI of age/si~ cl.~ followed by the same letter are not siglllficandydifferent!.lt the .OIIe"E:! ofprobability u detcrmin«! by Duncan's Multiple nm)b~ test. in herbaceous species composition on the beneath the canopies ofdesertshrubs, has been mounds occurs (Table 5). Some mounds documented by the research ofN. E. West and become densely covered with cheatgrass as co-workers (Charley and West 1977, West and black sagebrush plants become senescent and Skujins 1977, West 1979). Nitrate levels ofsur others support colonies ofsquirreltail. face soils dropped Significantly (P < .01) once Afte< the black sagebrush plants die, litter the black sagebrush plants died. Nitrate levels weight continues to increase and litte' changes in surface soils at the edge of shrub mounds in appearance. Litter under dead plants is com increased with apparent increasing age ofblack posed ofstringy bark fragments, and individual sagebrush plants and mounds. These areas cor black sagebrush leaves cannot be distinguished respond to tbe micro-topoedaphic situation in the litter. described as coppice benches by Eckert et al. (1989) for shrub mounds In big sagebrush com Soil Nitrate Levels munities. Apparently the increase in soil nitrate results from leaching from the mounds. Once Surface soil nitrate levels we'e bigher black sagebrush plants are dead and grasses beneath shrub canopies tban in the interspace (Table 6). Levels were highest next to shrub dominate the mound, soil nitrate levels decrease. stems. Nitratelevels beneath the canopy rose as Mounds and Black Sagebrush age/size classes of black sagebrush indicated Community Structure older plants and mounds. Tills is in itself an indication that age/size classes actually do We did not frod grass-dominated mounds or reflect increasing age. The development ofver grass-dominated mounds with black sagebrush tical and horizontal patterns in soil nitrogen, seedlings. We did note the remains of mounds attributed to the localization of litter fall that appeared to be eroding away. Apparently, 320 GREAT BASIN NATURALIST [Volume 52 mounds are dynamically formed and eroded in dominated semi-desert ecosystems of Utah. Soil Biol relation to theestablishmentandeventual death ogy and Biochemistry 9: 357-365. ECKEH'f, R. E., JR., F. F. PF.TF.HSON, M. K. WAHD, W H. of black sagebrush plants. The failure to find BUCKBUHN, AND J. L. STEPHENS. 1989. The role of grass-dominated mounds may be a function of soil-surface morphology in the function of semiarid herbivory by domestic livestock [sbeep, feral rnngelands. Nevada Agricultural Eqleriment Station borses (Equus caballus), and black-tailed jack Bulletin TB-89-01. University of Nevada, Reno. EVAN1i, R. A .• AND R. M. loVE. 1957. The step-point rabbits (Lepus califomictts)J. Grass-dominated method ofsampling. Apractical tool in range research. mounds may fail to persist since grasses cannot Journal afRange Management 10: W8--212. maintain mounds because ofleaffall and canopy HUDSON, D. M.,AND W. M. OHIF.L, 1979. Geologic map of structure differences compared with black sage the Buckskins Range, Nevada. Map 64, Nevada Bureau of Mines and Geology, Mackay School of Mines, Uni brush phmts. The only patchy vegetation versity ofNevada, Reno. encountered in the communities was groups of LuGASKJ. T. P., AND J. A. YOUNG. 1988. Utilization of rabbitbrusb plants. Perhaps rabbitbrush LANDSAT thermatic mapper data and aerial photog increases after relatively short-lived squirreltail raphy for mapping the geology, soils and vegetation assemblages of the Buckskin Mountain Ranges, plants die or are reduced by grazing. In an Nevada. American Congress on 5Ulveying and Map adjacent big sagebrush community we pre ping and Americ..'1o Society for Photogr , ,• , , ,, · ,•