J. Range Manage. 53: 550–555 September 2000 Seed production in sideoats grama populations with differ- ent grazing histories

STEVEN E. SMITH, REBECCA MOSHER, AND DEBRA FENDENHEIM

Authors are associate professor, School of Renewable Natural Resources, University of Arizona, Tucson, Ariz. 85721; graduate student, University Program in Genetics, Duke University, Durham, N.C. 27706; and research specialist, School of Renewable Natural Resources, University of Arizona, Tucson, Ariz. 85721.

Abstract Resumen

Frequent and intense defoliation of grasses has been associated La frecuente e intensa defoliación de los zacates ha sido asocia- with the evolution of “grazing morphotypes” that exhibit a vari- da con la evolución de "morfotipos de apacentamiento" que pre- ety of vegetative traits correlated with improved grazing resis- sentan una variedad de características vegetativas correla- tance. While recovery from a seed bank is not considered an cionadas con una mejor resistencia al apacentamiento. Mientras important grazing resistance mechanism, relatively little is actu- que la recuperación a partir del banco de semillas no se consid- ally known regarding seed (caryopsis) production in grazing era un mecanismo importante de resistencia al apacentamiento, morphotypes of caespitose grasses. The goal of this research was actualmente poco se sabe respecto a la producción de semilla to compare components of seed production in 2 populations of (cariópside) de los "morfotipos de apacentamiento" de zacates sideoats grama (Bouteloua curtipendula var. caespitosa Gould & cespitosos. La meta de este estudio fue comparar los compo- Kapadia) from nearby sites with different histories of livestock nentes de la producción de semilla de dos poblaciones de grazing. This was done using vegetative propagules of genotypes "Sideoat grama" (Bouteloua curtipendula var. caespitosa Gould y from both populations in a greenhouse study. The study was con- Kapadia) de sitios cercanos con diferentes historiales de apacen- ducted in 2 flowering seasons under conditions considered favor- tamiento por ganado. Esto se realizó a través de en un estudio de able for seed production. The population exposed to livestock invernadero utilizando propágulos vegetativos de genotipos de grazing showed a genetically based decrease in seed production ambas poblaciones. El estudio se condujo durante 2 estaciones de relative to the ungrazed population. Lower seed production per floración bajo condiciones consideradas favorables para la pro- in the grazed population was at least partially due to ducción de semilla. La población expuesta al apacentamiento por reduced numbers of tillers and panicles per plant and spikes per ganado mostró una disminución (genéticamente basada) en la panicle that may be associated with selection for grazing toler- producción de semilla en relación a la población sin apacen- ance. The grazed population also exhibited lower average seed tamiento. La baja producción de semilla por planta de la production per spike indicating lower inherent floral fertility. población con apacentamiento se debió parcialmente a el reduci- Seed production was not closely correlated with vegetative traits do número de hijuelos y panículas por planta y espigas por associated with increased grazing tolerance, nor was there evi- panícula, lo que puede estar asociado con la selección por toler- dence of obvious physiological trade-offs related to decreased ancia al apacentamiento. La población apacentada también seed production in the grazed population. Lower seed production exhibió un promedio menor de producción de semilla por espiga, potential in populations of sideoats grama intensively grazed by indicando una menor fertilidad floral inherente. La producción livestock may lead to reduced potential for seedling colonization. de semilla no se correlaciono estrechamente con las característi- cas vegetativas asociadas con el aumento en la tolerancia al apacentamiento, ni hubo evidencia obvia de cambios fisiológicos Key Words: B o u t e l o u a, defoliation tolerance, evolution, grazing relacionados con la disminución de producción de semilla de la resistance población apacentada. La potencial baja producción de semilla de poblaciones de "Sideoat grama" apacentadas intensivamente puede conducir a una reducción potencial de colonización por Grasses that are exposed to herbivores often possess unique life plántulas. history, morphological and physiological traits (Carmen and Briske 1985, Briske and Richards 1995, Briske 1996). When defoliation has been frequent over long periods of time, these 1983, Aarssen and Turkington 1985, Carmen and Briske 1985, traits may be associated with the evolution of genetically based Painter et al. 1989, 1993, Smith 1998 ). Defoliation tolerance in defoliation tolerance and the emergence of “grazing morpho- caespitose grasses is commonly correlated with a series of vege- types” in the plant populations affected (e.g., Detling and Painter tative traits including reduced plant height, a less upright growth form, fewer and smaller leaves, and increased tiller number Research was supported by the University of Arizona Foundation and the (Briske 1996). Arizona Agricultural Experiment Station. Authors wish to thank Lyman Nyquist Previous research (Smith 1998) compared vegetative responses for assistance in statistical analysis, Virgil Mercer for access to collection sites and under varying defoliation regimes of genotypes from 2 popula- information on their history, and Sharon Biedenbender, Mitchel McClaran, Bruce Munda, Mark Pater, and David Williams for technical assistance. tions of sideoats grama (Bouteloua curtipendula var. caespitosa Manuscript accepted 1 Jan. 2000. Gould & Kapadia) with different livestock grazing histories. In

550 JOURNAL OF RANGE MANAGEMENT53(5), September 2000 these studies, a population with long-term Materials and Methods that population and were isolated from the exposure to cattle grazing (approximately other main plot in that replication or from 100 yr) exhibited traits associated with other replications by at least 80 cm. This research utilized genotypes from 2 improved defoliation tolerance (primarily Mean daily temperature in the green- populations of sideoats grama from the increased tiller survival and reduced tiller house during the study was 32.7°C (mean semidesert grassland biotic community in mass) compared to a population that was low of 27.7°C, mean high of 35.6°C). southeastern Arizona (Brown 1994). One not exposed to grazing by livestock. Fans were used to circulate pollen during population (“ungrazed”) occurs atop a Evidence of reduced genetic variation anthesis. Ramets were irrigated individual- steep-walled butte (32° 42' N, 110°, 29' W within the grazed population was also ly when leaf curling and color indicated 1,655 m asl) near Mammoth, Ariz. that noted for various vegetative traits associat- drought stress. Soil moisture content was probably has never been grazed by large ed with defoliation tolerance. While the ~ 20 to 30% of field capacity at this point. (>100 kg) herbivores, including most environments the 2 populations inhabited Ramets were allowed to grow from June domestic livestock (Hadley et al. 1991). differed in ways other than the presence of through October in 1997 and 1998, a peri- The other (“grazed”) population occurs on livestock, the response of genotypes from od exceeding the entire flowering season a site (1,338 m asl) that has been regularly both populations was consistent with the for sideoats grama in southern Arizona. grazed by cattle since before 1900 (V. evolution of increased grazing tolerance in Inflorescences in sideoats grama are pani- Mercer, Mammoth, Ariz., personal com- the population from the grazed site. cles with up to 50 short, pendulous spikes. munication) that is located 4.8 km from the Increased seed production and expan- As panicles matured, at least 3 were har- ungrazed population. Deer (O d o c o i l e u s sion of a soil seed bank may represent an vested individually from each ramet in spp.) and small mammalian herbivores are additional defoliation resistance mecha- each main plot. Spikes were counted for able to reach both sites and may graze on nism (Briske 1996), but the effects of live- each panicle and individual seeds per pan- s i d e o a t s grama (Krausman et al. 1997). stock grazing on the evolution of this icle determined following threshing. Mean Prairie dogs (C y n o m y s spp.) occurred in resistance mechanism are not well charac- seed mass was determined from 50-seed southeastern Arizona (Hoffmeister 1986) terized. Reduced production of reproduc- samples dried at room temperature. The and may have also grazed on both sites tive tillers has been observed in popula- mean number of spikelets per spike was until about the late 1930's. No description tions of perennial grasses derived from determined by dissecting 5 spikes from of fire history or precipitation is available sites exposed to contrasting grazing each ramet. Spikelets were assumed to for either site. Both sites are on 20–35% regimes (e.g., Trlica and Orodho 1989, contain 1 floret (Cronquist et al. 1977). slopes with numerous rock outcrops and Painter et al. 1989, 1993). Nevertheless, Tiller number was recorded for each ramet shallow Haplargid soils (D. Robinett, little information is available regarding 8 weeks after defoliation in a regrowth USDA-NRCS, Tucson, Ariz., personal any differences in seed (caryopsis) pro- cycle that began 5 June 1998. Tillers that communication). Sideoats grama occurs duction between such populations. Scott showed any sign of a mature spike were sporadically in the area between the 2 and Whalley (1984) found that the mean counted as reproductive. sites, most commonly in drainages on the number of seeds produced per inflores- Some of the genotypes used in this slopes of the butte, and this may permit cence was significantly lower in genotypes study were also included in a greenhouse gene flow between the populations. from populations of 2 species of study that evaluated vegetative response to Twenty randomly selected genotypes that had been exposed to heavy different defoliation frequencies (Smith (basal area >15 cm) were dug from along grazing by sheep compared to relatively 1998). Seed production per spike in ramets a 400-m transect at each site in 1994. less grazed populations. However, protec- from these genotypes (ungrazed: N = 10, Individual genotypes were separated by at tion from livestock grazing for more than grazed: N = 9) was rank correlated (Sokal least 3 m and were assumed to represent 50 years did not result in altered seed pro- and Rohlf 1995) with various traits associ- the products of separate established duction in Indian ricegrass (A c h n a t h e r u m ated with vegetative growth from the pre- seedlings. Genotypes from both popula- hymenoides (Roemer & J.A. Schultes) vious study where defoliation occurred tions are high polyploids with 2n = 75–92 Barkworth) genotypes taken from grazed every 14, 28, or 56 days over a 168-day (D. Showalter, University of Arizona, or ungrazed sites when these genotypes period. These traits were: plant height, unpublished data, Gould and Kapadia were grown in a common environment root dry weight, shoot dry weight, total 1964). These genotypes were grown in 4- (Orodho et al. 1998). A lack of additional plant dry weight, and tiller number and liter pots in a shadehouse in Tucson, Ariz. data on seed reproduction and the evolu- dry weight. for 6 mo before a minimum of 6 ramets tion of grazing morphotypes prevents a Mixed-model analysis of variance was more integrated understanding of demo- were established for each genotype from used to compare traits among genotypes of graphic processes that may be affected by single rooted tillers (Smith 1998). the 2 populations. Proc MIXED of SAS the presence of grazing animals. Individual ramets were grown in 4-liter (Littell et al. 1996) was used with replica- The primary objective of this research pots in an evaporatively cooled green- tions, years, and genotypes within popula- was to determine whether exposure to dif- house and treated uniformly until June tions considered random effects. For com- ferent livestock grazing regimes is associ- 1997 with defoliation 5 cm above the soil parisons of the components of seed pro- ated with genetically based changes in seed surface every 6 to 12 weeks. The resulting duction (Table 1), significance of differ- production in sideoats grama. This was ramets were placed on greenhouse bench- ences between populations were assessed done under conditions of minimal defolia- es using a modified split-plot arrangement using single-degree of freedom contrasts. tion stress using genotypes from 2 popula- with 4 replications with population as the Least-squares means are reported through- tions with contrasting livestock grazing main plot and all 20 genotypes from each out for these traits. Distribution of mean histories (Smith 1998). Secondary objec- population placed randomly as sub plots seed production per spike in the 2 popula- tives were to determine whether variation within the main plots. Ramets within each tions was evaluated for each year separate- in seed production was associated with main plot were grouped together within an ly using frequency histograms and two- other traits related to defoliation tolerance. area of 1 m2 to concentrate pollen from

JOURNAL OF RANGE MANAGEMENT53(5), September 2000 551 Table 1. Least squares means for components of seed production by structural unit in ungrazed and grazed populations of sideoats grama.

Structural unit Population Spike Panicle Plant1 (Seeds (Seeds (Spikelets (Seeds (Spikes (Tillers (% (Panicles (Spikes (Seeds spikelet-1) spike-1) spike-1) panicle-1) panicle-1) ramet-1) reproductive ramet-1) ramet-1) ramet-1 tillers) Ungrazed 0.050* 0.200* 3.37 6.3* 31.3 33.7* 40.9* 9.2* 284* 55.8* Grazed 0.013* 0.140* 3.75 4.1* 28.4 29.2* 37.6* 6.8* 197* 35.1* 1Estimated from tiller counts made in 1998. *Population means significantly different by single degree of freedom contrast (P £ 0.05). tailed Fisher’s exact tests. The same the average number of spikelets per spike, in the grazed population (23.6 + 4.8 mg). approach was used to compare the number the number of seeds per spike may be con- Still, the mass of seeds produced by each of spikes per panicle but data from both sidered as the primary component explain- ramet would represent less than 2% of the years were combined since means were ing differences in floral fertility between aboveground biomass productions of these not significantly different between years the populations. This resulted in signifi- ramets based on results for the 2 popula- for this trait. Relationships between com- cantly higher mean seed production per tions from previous research (Smith ponents of seed output could be evaluated panicle in the ungrazed population. Using 1998). using rank correlation coefficients which tiller count data, it was possible to esti- Over all genotypes in both populations, do not assume a linear relationship mate additional components of seed output seed production per spike was significant- between the 2 variables examined (Sokal on a whole-plant basis. This showed that ly correlated between years (rs = 0.66, P £ and Rohlf 1995). The numbers of seeds in the absence of significant defoliation, 0.05) and the genotype x year, and popula- produced per spike and spikes produced ramets from the ungrazed population tion x year interactions were not signifi- per panicle were also rank correlated with would generally be expected to produce cant. However, the mean number of seeds various vegetative traits associated with significantly more tillers overall and produced per spike was significantly response to defoliation using data from a reproductive tillers, and therefore more greater in 1998 (0.26 + 0.02) than in 1997 previous study (Smith 1998) for genotypes panicles and spikes than ramets from the (0.12 + 0.01) based on a Wilcoxon signed that were included in that study and the grazed population (Table 1). Combination rank test. Analysis of frequency his- present one. Conservative rank correlation of tiller data with that from individual tograms of seeds produced per spike over procedures were also used in this case, spikes and panicles further revealed that genotypes (Fig. 1) in each year also sug- again because linear relationships could significantly more seeds would be expect- gested differing distributions of this vari- not be assumed but also because data orig- ed per ramet under these conditions in an able between the populations. This led to inated from separate experiments for some entire flowering season within the separate analyses of the distribution of pairs of variables. ungrazed population than within the seed production for the 2 years. These dis- Genetic analysis was done using Box- grazed population. tributions differed significantly between Cox transformed values for the number of There was no significant difference the 2 populations in both years with seed seeds per spike (Sokal and Rohlf 1995) and between the populations in average seed production in the ungrazed population untransformed values for the number of mass (ungrazed: 0.62 + 0.05 mg [least- consistently skewed toward higher values spikes per panicle. Broad-sense heritability squares mean + SE], grazed: 0.70 + 0 . 0 8 (Fig. 1). In 1997, the median number of was estimated for both traits within each mg). Because ramets from the ungrazed seeds produced per spike was 0.125 in the population using a split-plot in time design population would produce more seeds, the ungrazed population compared to 0.064 after Nyquist (1991). When the estimates mean total mass of seeds produced per seeds per spike in the grazed population. were significantly greater than 0, standard ramet was significantly higher in the In 1998, medians for the 2 populations errors were calculated using the method of ungrazed population (34.2 + 5.7 mg) than were 0.320 and 0.091 seeds per spike. The Gordon (1979). Phenotypic correlations were utilized throughout since the array of genotypes and experimental design utilized did not permit accurate estimation of addi- tive genetic variance which would be required for calculation of genetic correla- tions (Lynch and Walsh 1998). Statistical significance was assigned at P £ 0.05 in all ca s e s .

Results

Ramets from the ungrazed population produced significantly more seeds per spikelet and spike overall than did those Fig. 1. Distribution of mean seed production per spike for genotypes from ungrazed and from the grazed population (Table 1). grazed populations of sideoats grama. Population distributions significantly different (P Since the 2 populations did not differ in 0.05) based on Fisher’s exact test for experiments conducted in 1997 and 1998.

552 JOURNAL OF RANGE MANAGEMENT 53(5), September 2000 mean number of spikes per panicle did not 35.8 + 11.3%). Broad- differ between years, nor was there a sig- sense heritability of the nificant population x year interaction for number of spikes per pan- this trait. The distributions of this trait also icle was less than 2% and did not differ significantly between popu- did not differ significantly lations (Fig. 2). Medians for the ungrazed from 0 in both popula- and grazed populations were 31.6 and 28.6 tions. spikes per panicle, respectively. The number of seeds per spike was sig- nificantly and negatively correlated with Discussion and the number of tillers per ramet in the Conclusions ungrazed population as measured in this study (Table 2). The number of spikes per As was the case with panicle was also positively correlated with vegetative traits (Smith tiller number in this population. Overall 1998), this research shows seed production per ramet was positively that 2 sideoats grama pop- associated with the number of seeds pro- ulations from nearby sites duced per spike in both populations. In the with contrasting livestock Fig. 2. Distribution of mean spike production of spikes per grazed population, the number of seeds grazing histories differed panicle for genotypes from ungrazed and grazed popula- per ramet was also positively correlated in reproductive character- tions of sideoats grama. Population distributions are not sig- with both the number of tillers and the istics that may be associ- nificantly different (P > 0.05) based on Fisher’s exact test. number of spikes produced per ramet. This ated with plant demogra- would be expected given the lower mean phy (Table 1, Fig. 1). While the grazed given that sideoats grama occurs in the seed production per spike in this popula- population displayed evidence of selection area between the sites, that many native tion. The average mass of an individual for vegetative traits associated with herbivores are able to reach both sites, and seed was not significantly correlated with increased grazing tolerance in previous that genotypes from the grazed population any other primary reproductive measures research, the current study indicates that exhibit vegetative characteristics that are in either population. this population also exhibits decreased consistent with selection for tolerance to Combining data from this study with average seed production potential per defoliation (Briske 1996), it appears rea- those from Smith (1998), the number of plant. Observation of significant broad- sonable to speculate that exposure to live- seeds produced per spike was significantly sense heritability for the number of seeds stock may have affected reproductive correlated only with mean tiller weight per spike in both populations demonstrates characteristics of the grazed population. with defoliation every 28 days (rs = 0.52, a genetic basis for the variation among As in this research, Scott and Whalley P £ 0.05). No other significant phenotypic genotypes within these populations. (1984) found that from populations correlations were observed between any Because the 2 populations occupy envi- of Danthonia linkii Kunth and D. richard - measures of reproductive output and vege- ronments that may differ in ways other sonii Cashmore that had been exposed to tative growth under different defoliation than their livestock grazing history, it is intense grazing by sheep produced fewer regimes for the 19 genotypes common to not possible to definitely conclude that seeds per panicle on average when grown both studies. differences in reproductive biology are without defoliation stress compared to Overall variability in the number of solely the result of exposure to livestock plants from populations from sites with seeds per spike appeared greater in the grazing in 1 population. For example, fire lower grazing stress. These results are also grazed population than in the ungrazed history and weather may differ at the 2 consistent with the observation that population (CV = 84 vs 121%). Broad- sites and could explain the differences in increased seed production to expand the sense heritability of the number of seeds vegetative traits (Smith 1998) and repro- seed bank is apparently not a common per spike over years was significantly ductive biology between the populations. mechanism of defoliation resistance in greater than 0 in both populations, but did The 2 populations may have also diverged perennial caespitose grasses (Pyke 1990). not differ significantly between the popu- genetically before livestock were intro- Reduced average seed production poten- lations (ungrazed: 24.5 + 6.0%, grazed: duced to the grazed site. Nevertheless, tial per plant in the grazed population is due in part to a combination of traits that Table 2. Rank correlation coefficients between components of seed production ungrazed (upper lead to a reduction in overall plant size. values and grazed (lower) populations of sideoats grama. These include a reduced number of tillers and panicles compared to the ungrazed Spikes Tillers % reproductive Spikes Seeds Seed -1 -1 -1 -1 population (Table 1). This morphology and panicle ramet tillers ramet ramet mass its effects on seed production is consistent Seeds spike — – 0.50*/0.40 — — 0.68*/0.93* — with findings from previous research on Spikes panicle 0.56*/0.04 — 0.48*/0.22 — – the evolution of grazing-adapted morpho- Tillers ramet 0.68*/0.81* 0.04/0.56* — types in caespitose grasses (e.g., Detling % reproductive and Painter 1983, Painter et al. 1989, tillers 0.85*/0.58* — — 1993). These morphological adaptations -1 Spikes ramet 0.31/0.55* — affecting seed production may therefore -1 Seeds ramet — represent simply secondary consequences *Significant at P £ 0.05, of strong selection for vegetative traits — = P > 0.05; Ungrazed population/Grazed population, N = 20 for both. associated with defoliation tolerance.

JOURNAL OF RANGE MANAGEMENT53(5), September 2000 553 The observation of reduced floral fertili- associated with grazing resistance in the Cheplick, G.P. 1995. Life history trade-offs in ty (number of seeds per spike) in the grazed population in the roughly 100 years Amphibromus scabrivalis ( P o a c e a e ) : grazed population suggests that selection that it has been exposed to livestock. Allocation to clonal growth, storage, and under grazing stress may have affected the Reduced seed production would have cleistogamous reproduction. Amer. J. Bot. production of seeds within an individual negative consequences most noticeably in 82:621–629. Cronquist, A., A.H. Homgren, N.H. spikelet. This could be driven by competi- reducing propagules available for colo- Holmgren, J.L. Reveal, and P.A. tive advantages afforded to genotypes that nization of open sites (Richards 1990). Holmgren. 1977. Intermountain Flora. allocate additional photosynthetic Reduced seed production as demonstrated Vascular plants of the Intermountain West, resources to vegetative growth and less to here could also affect genetic structure U.S.A. Columbia University Press, New seed production (Bazzaz et al. 1987, within the grazed population since the pro- York, N.Y. Ashman 1994). Physiological trade-offs of duction of fewer seeds would result in Detling, J.K. and E.L. Painter. 1983. this sort (Stearns 1992) would be expected reduced opportunities for sexual reproduc- Defoliation responses of western wheatgrass to lead to negative correlations between tion should it occur. Indeed, previous populations with diverse histories of prairie the life-history traits affected although genetic analyses of these 2 populations dog grazing. Oecologia 57: 65–71. Freter, L.E. and W.V. Brown. 1955. A cyto- these have been difficult to document in (Smith 1998) suggests that the grazed pop- taxonomic study of Bouteloua curtipendula perennial grasses (Reekie 1991, Cheplick ulation is less variable genetically in vege- and B. uniflora. Bull. Torr. Bot. Club 1995). While this research was not tative traits associated with defoliation tol- 82:121–130. designed to identify physiological trade- erance. The grazed population may there- Gordon, I.L. 1979. Standard errors of heri- offs, there is some evidence for such a fore be more susceptible to local extinc- tabilities based on perennial observations, trade-off in the ungrazed population where tion (O’Conner 1991) due to either the with application to Yorkshire fog grass. the number of tillers per plant is negative- direct demographic effects of reduced con- Euphytica 28:81–88. ly correlated with the number of caryposes tribution to the soil seed bank, or to the Gould, F.W. 1951. Notes on apomixis in per spike (Table 2). 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Cuany, and M.J. Trlica. ate genetically and therefore less respon- munities: southwestern United States and northwestern Mexico, University of Utah 1998. Previous grazing or clipping affects sive to selection than non-apomictic seed of Indian ricegrass. J. Range Manage. species. Canfield (1957) showed that indi- Press, Salt Lake City, Ut. Canfield, R.H. 1957. Reproduction and life 51:37–41. vidual sideoats grama plants did not live span of some perennial grasses of southern Overath, R.D. and M.A. Asumssen. 1998. longer than 3 years on sites grazed by cat- Arizona. J. Range Manage. 10:199–203. Genetic diversity at a single locus under via- tle in southern Arizona. Together, these Carmen, J.G. and D.D. Briske. 1985. bility selection and facultative apomixis: findings indicate that significant genetic Morphologic and allozymic variation Equilibrium structure and deviations from variation may have existed and sufficient between long-term grazed and non-grazed Hardy-Weinberg frequencies. 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554 JOURNAL OF RANGE MANAGEMENT53(5), September 2000 Painter, E.L., J.K. Detling, and D.A. Richards, A.J. 1986. Plant Breeding Systems. Smith, S.E. 1998. Variation in response to Steingraeber. 1989. Grazing history, defoli- Allen & Unwin, London. defoliation between populations of Bouteloua ation, and frequency-dependent competition: Richards, A.J. 1990. The implications of curtipendula var. caespitosa () with effects on two North American grasses. reproductive versatility for the structure of different livestock grazing histories. Amer. J. Amer. J. Bot. 76: 1368–1379. grass populations. p. 131-153. I n: G.P. Bot. 85:1266–1272. Painter, E.L., J.K. Detling, and D.A. Chapman (ed.), Reproductive versatility in Stearns, S.C. 1992. The evolution of life histo- Steingraeber. 1993. Plant morphology and the grasses. Cambridge Univ. Press, ries. Oxford Univ. Press, Oxford. grazing history: Relationships between native Cambridge. Trlica, M.J. and A.B. Orodho. 1989. Effects grasses and herbivores. Vegetatio 106:37–62. Scott, A.W. and R.D.B. Whalley. 1984. T h e of protection from grazing on morphological Pyke, D.A. 1990. Comparative demography of influence of intensive sheep grazing on geno- and chemical characteristics of Indian rice- co-occurring introduced and native tussock typic differentiation in Danthonia linkii, D . grass, O r y z o p s i s h y m e n o i d e s . O k i o s grasses: persistence and potential expansion. richardsonii and D. racemosa on the New 56:299–308. Oecologia 82:537–543. England tablelands. Aust. J. Ecol. Westoby, M., E. Jurado, and M. Leishman. Reekie, E.G. 1991. Cost of seed versus rhi- 9:419–429. 1 9 9 2 . Comparative evolutionary ecology of zome production in Agropyron repens. C a n . Sokal, R.R. and F.J. Rohlf. 1995. B i o m e t r y . seed size. Trends in Ecol. and Evol. J. Bot. 69:2678–2683. 3rd ed. W.H. Freeman, New York, N.Y. 7:368–372.

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