58 Rangelands 10(2), April 1988

Big galleta grass—a warm-season bunchgrass in the Sono- ran and Mojave Deserts Ronald Robberecht

TheSonoran and Mojave Deserts of the American south- west presentformidable environmentsforthe establishment and survIval of species. in these hot, arid envi- ronmentshave evolved many adaptations to the high tempera- tures and limited soil water supply, as exemplified by the numerous species of succulents in these areas. A dominant warm-seasongrass species in the hot deserts of southeast- ern , southern Nevadaand , westernArizona, and parts ofBaja California is big galleta grass ( rigida (Thurb.) Benth. ex Scribn). This perennial plant has a bunchgrassgrowth form and exhibits some interesting phy- siological adaptations to hot, arid environments. In addition tothe stress Imposedon plantsby these desert habitats, this bunchgrass species may be subject to a high degree of utilization by native bighorn sheep and domestic livestock (Hughes 1982).Despite its potential importance asa forage Smallsingle-species stands ofbig galleta grasscan occur, partic- species in these hot deserts, little is known about its life ularlyin areas with uniformly sandy to sandy loam soils. Competi- history, ecology, and responseto grazing. tion for water may be intensified among these bunchgrasses because of similarities in their growth pattern and physiological LIfe History requirementsfor soil water. Big galleta grass ranges in height from 15to 35 inches and to reach maximum size at about 200 to 300 culms. Theroot system ofbig galleta grass is typicalof hot desert appears roots of These culms typicallybranch up to three times, with each plants. Highly suberized relatively large diameter or more and branch having fiveto 10 leaf blades. Taxonomic descriptions canextend one yard horizontally fromthe plant, to about one-yard depth in the soil. The establishment of these bunchgrassesseems to be favored on sandy tosandy loam soils, though they can be found in small sandy soil pockets on rockyslopes. Under favorable soil conditions, it is not uncommon for big galleta grass to bethe only peren- nial grass and. the dominant species in the community.

Big galleta grass-dominatedcommunity in the Sonorandesert of southeasternCalifornia. Thisspecies can comprise upto 32% ofthe vegetationcover, which typicallyis about 50%of the ground surface. indicate that this species is highly rhizomatous, with short and rather woody from which new plants com- monlyarise (Cronquist etal. 1977). Plant establishmentfrom The highly suberizedwoody roofs or Dig galleta grass. seed appearsto be rare, and is probably relatedto low seed- ling survival in these deserts where high temperatures and AdaptatIonsto Desert Environments drought prevail for most of the year. Big galleta grass has severalphysiological adaptations to cope with the stress ofhigh temperaturesand drought corn- The author isin the Departmentof Range Resources, Universityof Idaho, Moscow 83843.Contribution of the Forest, Wildlife and Range Experiment Station, No. 373. Rangelands10(2), AprIl 1988 59 mon in the Sonoran and Mojave deserts. Since this warm- season bunchgrass species has the C4 photosynthetic pathway, very high levels of photosynthesis are possible at high ambient air temperatures. This species has one of the highest photosynthetic rates known (Nobel 1980). Further- more, these photosynthetic ratesare achievedwith relatively low water loss by the plant. The capacity forhigh photosyn- thesis and low transpiration combine to make this bunch- grass a very efficientuser of water.This can be a considera- ble advantage in desert environments where air tempera- tures can exceed 85° F for three months of the year and where the rainfall is unpredictable and as low as 5 inches annually. The phonology of big galleta grass closely follows the occurrence of rainfall. Although two major growth periods generally occur, coinciding with thebi-modal annualrainfall Considerableover/a p in the rootsystems of bunch- pattern in the of California, new leaf blades neighboring grass plants may exist. This overlap results in significant cornpeti- can beinitiated in responseto small isolated rainfall events. tion for soil wateramong theseplants. Vegetativegrowth occurs in the spring and fall, interrupted by adormant period when temperatureand drought extremes water. This appears to be the case for big galleta grass. In exist. habitatswhere sandy to sandy loam soilsoccur, biggalleta is Competition for soilwater may bea significant factor in the often the dominant species or may be the only species. In ecology and distribution of plants in hot desert environ- these situations significant competition among individual ments. This interaction may be particularly intense in situa- bunchgrassesof big galleta can occur, particularly during tions where membersof the same species compete for soil the transition from the wet to dry periods (Robberecht and Nobel 1983).

Responseto Grazing Although big galleta grassis notparticularly palatable,the presenceof a semipalatable grass among spinycacti, agaves, and shrubs presents pleasing forage for herbivores such as the desert bighorn sheep and domestic livestock. In some situations, utilization of big galleta grass by domestic live- stock is relatively high (Hughes 1985). The studies by Hughes (1985 and personal communication) indicated that utilization ofthis warm-seasongrass ranged from 10to 60% and averaged32% over a ten-year period. Although the data were not extensive, the abundance of big galleta grass generallydecreased during this period in responseto domes- tic livestock grazing. There areno detailed experimental studies onthe response of this bunchgrass to grazing. Hughes (personal communi- cation) suggeststhat big galleta grass is relatively resistant toheavy grazing by domestic livestock becauseof its coarse and rigid cuims. However,when the stress of light to heavy utilization occur simultaneouslywith water and competition stress, growth and survival of this grass species may be significantly affected.This combination of factors may have caused the decreased abundance in big galleta grass after grazing was suggested by the studies of Hughes (1985). Although the physiological and growth characteristics ofbig galleta grass appearto be adaptive for survival in the hotand dry American southwest deserts, these traits may not be effective for toleranceto intense grazing by domestic herbi- vores. CIted and Suggested LIterature Conzattl, C. 1946. Flora taxonomica Mexicana.Tomo I. Soc. Mexi- cana de Historia Nat. ifW too ywmsor vig gal/eta grass can extend over one yard Conrqulst,A., A.H. Holmgr.n,N.H. Holmgr.n, J.L Reveal, and P.K. from the plant and to about oneyard depth in the soil. Rootlength Holmgren.1977. Intermountainflora. Vol.6 Columbia Univ. Press, and distribution may be restrictedin more rockysoils. New York. Hughes,L.E. 1985. Agrazing systemin the MohaveDesert. Range- lands 4:256-257. 60 - Rangelands10(2), AprIl 1988

Nobel, P.8.1981.Spacing andtranspiration ofvarious sized clumps Robberscht, R., and P.S. Nobel. 1983. Experimental removal of of a desert grass, . J. Ecology. 69:735-742. intraspecif,ccompetitors—effects on water relations and produc- Nobel, P.S. 1980. Water vapor conductance and C02 uptake for tivity ofa desert bunchgrass,Hilaria rigida. Oecologla 60:21-24. levelsof a C4 desert grass, Hilaria rigida. Ecology61:252-258. WIggIns, l.L 1980. Flora of Baja California. Stanford Univ. Press, Calif.

Comments on Skovlin's Article "Southern Africa's experience with intensive short duration grazing" Published in Rangelands 9(4):162-67. Don Hedrick

My first reaction to this article was one of relief, i.e., we nent grass species is likely to be the dominant influence. finally have an objective review whereas many have been These authorsstressed that we are notsuggesting that com- largely emotional! Accordingly, I would like to congratulate petition, patchinessand grass growth responsesto grazing Jon on his contribution. do not contribute to sward dynamics, but rather that such While residing in Zimbabweand aftera trip to SouthAfrica factors are insignificant comparedto the effects of rainfall. in which I visited with ecologists inthe Botany Departmentat These authors continue: "Such a suggestion in turn the University of the Witwatersrand in Johannesburg, I'm implies that managementbased on a constant strategy of inclined to think some of the theoretical explanations for manipulation ofgrazing (andhence grassgrowth responses these apparentdiscrepancies in results have been left out of and inter-tuft competition), regardlessof rainfall character- his review. This is not surprising since only one source has istics, is bound to fall short ofobjectives, or at best produce recently been published and twoare thesesfrom the Univer- disappointing results. The results of the two studies imply sity of theWitwatersrand. that a mixed sward is best managed by the application of One by Walker, Mathews and Dye (1986) has been pub- managementactivities at certain critical times, coinciding lished in the South AfricanJournal of Science under News with certain rainfall characteristics. For example,there may and Views and it containsthe referenceto two theses: one by becertain times withinseasons or evenin certainyears when Mathews, D.A. (1984) Grass dynamics in a southern African either grazing or lack of grazing will have a significant effect grass-herbivoresystem. MSc thesis, Universityof the Witwa- on the dynamics of particular component species. It is vital tersrand, Johannesburg;and thesecond by Dye, P.V. (1983) that we learnto recognizethese times and conditions, since Prediction ofgrass growth in asemi-arid inducedgrassland. managementefforts independent of these are likely to be PhD thesis, University of theWitwatersrand, Johannesburg. ineffectual or even deleterious." According toWalker et al. (1986) thesetwo studies (theses Many of us over theyears have noted similar effects when by Mathewsand Dye)suggest that, although thedynamics of fixed grazing regimes are applied rigidly in the US and a mixed sward are a function of numerous biological and elsewhere. environmental factors, variable rainfall (intra and inter- seasonally)with its consequencesfor the individual compo- Literature Cited Walker, B.H., D.A. Mathews, and P.J. of The author is Visiting Professor, FuibrightScholar, Departmentof Animal Dye (1986).Management Science,University of Zambabwe, Herare, Zimbabwe. grazing systems—existingversus an event—orientedapproach. South African Journal of ScienceVol 82:172.