a 2oo2 by ch";;l*?:g:i;".11';ff H'*" Alien Plants and Fire in Desert Tortoise (Gopherus agassizii) Habitat of the Mojave and Colorado Deserts Mlrrnnw L. Bnoorsr uu Tonn C. Eseun' tUnited States Department of the Interior, I|nited States Geological Suney, Western Ecological Research Center, lns Vegas Field Station, 160 N. Stephanie, Henderson, Nevada 89074 USA I E-mail : matt _brooks @ us I s. gov ; F ax : 702 - 5 64 -4600 ]
Ansrru,cr. - Alien plants and fire have recently been recognized as significant land management problems in the Mojave and Colorado deserts. Annual species dominate the alien flora, although only Bromus rubens, Schismus spp., and Erodiam cicutarium are currently widespread and abun- dant. These species can compete with native plants, and B. rubens in particular has contributed to significant increases in fire frequency since the 1970s. Native desert plants are often poorly adapted to fire, and recurrent fire has converted native shrubland to alien annual grassland in some areas. Changes in plant communities caused by alien plants and recurrentfire may negatively affectnative animals such as the desert tortoise by altering habitat structure and the species composition of their food plants. The dominance of alien annual plants and the frequency of fire may increase in the future due to increased levels of urbanization and atmospheric nitrogen and carbon dioxide. Increases or decreases in rainfall could also cause changes in alien plant dominance and fire frequency. Land managers should focus on early detection and eradication ofnew alien species, especially those that pose significant fire threats, and on law enforcement to minimize the frequency of ignitions by humans. Additional information on the ecology and effects of invasive plants and fire in the Mojave and Colorado deserts are needed to develop effective management plans.
Knv Wonrs. - Reptilia; Testudines; Testudinidae l Gopherus agassizii., Bromus; Schismus;conserva- tionl managementl disturbance; fire; exotic species; invasive plants; plant invasionsl Mojave Desert; Colorado Desert; USA
Threats from alien species currently rank second to 2002). These conclusions are based on limited data,but they habitat destruction in causing species endangerment (Wilcove indicate that fire may have mostly negative effects in this et al., 1998), and about 427o of federally threatened or region. endangered species are listed because of threats from alien The Mojave population of the desert tortoise (Gopherus species (The Nature Conservancy, 1996; Babbitt, 1998). agassizii) is a Federally Threatened species that is listed Approximately 5000 alien plant species have become estab- partly due to the negative effects of alien plants and fire. The lished in natural habitats outside of agricultural areas in the Mojave population occupies critical habitat in the Califor- United States, and many of these species are believed to have nia, Nevada, and Utah parts of the Mojave and Colorado displaced native plants and altered ecosystem integrity (Morse deserts located north and west of the Colorado River (U.S. et al., 1995). Alien plants invade approximarely 700,000 Fish and Wildlife Service ,1994). Members of the multiagency hectares of wildlife habitat per year (Babbitt, 1998). Accord- Desert Tortoise Management Oversight Group ranked alien ingly, the control of alien plants is now a top priority for land species and fire second only to disease as major threats to the management agencies in the United States. desert tortoise (annual meeting,29 March 2000). Bioregional The Federal wildland Fire Policy defines fire as a management plans such as the West Mojave Coordinated critical natural process that should be reintroduced for the Management Plan (California), Clark County Habitat Con- benefit of ecosysrem inregriry (Glickman and B abbitr, I 995). servation Plan (Nevada), and Washington County Habitat It also recognizes fire hazards that can result as fuels accu- Conservation Plan (Utah) have been structured primarily mulate where fire has been previously suppressed and rec- around conservation of the desert tortoise, and have accord- ommends that fire be used to reduce these high fuel loads. ingly placed the management of alien plants and fire high on However, in some ecosystems fire may not be a natural their list of management priorities. However, limited infor- ecosystem component or an appropriate tool to reduce fuel mation about methods for control hinders the implementa- loads. Such ecosystems include the Mojave and Colorado tion of comprehensive management actions. deserts, where fire appears to be historically rare (Humphrey, In this paper we present information that can be inte- 197 4: R. Minnich, unpubl. data). Plant communities in rhese grated into land management plans and used to establish deserts are generally slow to recover from fires (O'Leary and priorities for future studies in desert tortoise habitat. We Minnich, 198 1;Brown and Minnich, 1986), and fires often evaluate the current status of alien plants and fire, describe increase landscape flammability due to postfire dominance their effects, discuss future trends, propose management of alien annual grasses (Brooks, 1999a; Esque and Schwalbe, actions, and suggest research priorities. Bnoors AND Eseug - Alien Plants and Fire 331
ALIEN PLANTS annual plant biomass (Brooks, 1998 , 1999b) and are present in at least 50Vo of wildland desert sites (K. Thomas, unpubl. Alien plants comprise a relatively small portion of desert data). Thus, alien annuals are the most widespread and floras worldwide (Lonsdale, 1999), and the Mojave and abundant alien taxa in the Mojave and Colorado deserts, and Colorado deserts are no exception. By the end of the 1970s we focus on the ecology and management of alien annual the California portion of the Mojave Desert contained 1836 species in this paper. vascular plant species, of which only 156 (9Vo) were alien species (Rowlands et al.,1982). In 1998 it was estimated that Dominant Alien Taxa I3Vo of all plant species were alien at the Lake Mead National Recreation Area (J. Haley, pers. conlnx.). These percentages Although relatively few alien plant species have invaded compare to an average of I 6Vo ahen plant species among 184 this region, and even fewer have invaded natural habitats sites located in various ecosystems worldwide, and an average (Kemp and Brooks, 1998), many are either locally abundant of 327o on island ecosystems (Lonsdale, 1997, 1999). As a or widespread and abundant and pose various ecological result, most research on the ecology and effects of alien plants problems for land managers (Table 1). Some species are has focused on island ecosystems, and very little research has recogn rzedby states as noxious agricultural weeds, but none been done in the Mojave and Colorado deserts. are federal noxious weeds. More are recognized as alien Annual plant species currently dominate the alien flora plants of significant ecological concern in wildland areas in wildland areas of the Mojave and Colorado deserts (Table (California Exotic Pest Plant Council, 1999). 1). Although the most extensively studied alien species in Poaceae and Asteraceae dominate alien floras worldwide this region are the perennials Tamarix spp. and Arundo (Mack et al., 2000), and along with Brassicaceae and donax, the riparian habitats that they occupy comprise less Chenopodiaceae dominate the alien flora of the Mojave and than 37o of the entire Mojave Desert (Rowlands et al ., 1982). Colorado deserts (Table 1). The most widespread and abun- The remaining upland area is occupied almost exclusively dant species are the annual s Brontus madritensis ssp. rubens by alien annuals, which can comprise 66-917o of the total (hereafter calle d B rontus rubens), Schismzs spp., and Erodium
Table 1. Alien plant species in critical desert tortoise habitat ofthe Mojave and Colorado deserts. This list only includes species that are currently found in or threaten to invade wildand areas. ## Compiled from information in Hickman (1993), CalFlora Database (2000), California Exotic Pest Plant Council (1999), Southwest Exotic Plant Mapping Program (2000). The Nature Conservancy (2000), Fire Effects Information System (2000), and information cited in the text. Life Form: AG = annual grass; AF = annual forb; BH = biennial herbaceous; PG = perennial grass; PH = perennial herbaceous; PW = perennial woody shrub or tree. Weed Ranking: CaIEPPC = California Exotic Pest Plant Council l{l, A2, B, RA (red alert), NI (need more information), AG (annual grasses)l; CA = California Department of Food and Agriculture [A, B, C]; AZ= ArizonaDepartment of Agriculture (restricted), Regul (regulated), Prohib (prohibited)]; UT [Restr * = Utah Department of Agriculture [NW (noxious weed)]; NV Nevada Department of Agricuture [NW (noxious weed)] abundance: ** x** = **** = individual or scattered occurances; = locally rare, = locally abundant; - widespread and abundant. Effects: A = allelopathic; C = competition with other plants; F = promotes fire; T - toxic to livestock. (Table I contirutes on next page).
Family Scientific Name Common Name Life Weed Ranking Abundance Effects Form
Amaranthaceae Annrcuttltus albus prostrate pigweed AF * Apiaceae Apiulrt graveolens celery AF >1. Apocynaceae Nefitutt olecutder South American oleander PW ;tr :!; Asteraceae Ce ntcutre u nte I it e n s i s tocalote AF CaIEPPC-B C Centatu'e(r repens Russian knapweed AF CA-B. AZ-Restr. UT-NW. >I: C, A, T :r Centaure a s o I stitictl i s yellow stafthistle AF CaIEPPC-A1. CA-C, AZ-Restr, UT-NW C,T C i rsi wrt. och roc ert nun yellow-spined thistle PH CA-A >ir Conv:,a bonariensis South American conyza AF .t, Di nto rpltothecu s irtuctct Alrican daisy AF AZ-Regul G rindel ia squorroso v ar. se rntlata curly cup gumweed BH *T :l: H ete rotlte c 0 p s anmtophila camphor weed AF ktc'tuca serriolcr prickly lettuce AF rl: Sonchtts anensis perennial sow thistle PH CA-A, AZ-Prohib, NV-NW *c Sont'hus lsper ssp. csper prickly sow thistle AF Soncluts oleraceus sow thistle AF * Tarcaactun fficir'tale corrrmon dandelion AF Tragopogort dubius goat's beard AF :!; Brassicaceae Brassicct.jturcea Indian mustard AF :F Brassiccr tountefofiii mustard AF CdEPPC-A2 .1..t 4. rt, C ap s el I a bursa- pasto ri s shepard's purse AF Chorispora tenellct blue mustard AF CA-B :F Descuraria soplict tansy mustard AF CaIEPPC-M En'simmn repcu'tdunt wallflower AF ,n Hirscltfeldia incana Meditenanean mustard BH CaIEPPC-M :i<*:i: Le p i d i r,t nt p e rfo I i a nutt shield cress BH I-obularia nmitinn sweet alyssum PH * Malcol.ntia ctf icanrt African mustard AF 4r .t, \t, .L S i q:ntb ri um al iti s s i nttu rt tumble mustard AF Sisyntbrir,tm irio london rocket AF S i s-r"n tb ri w n o fi e nt a I e Oriental mustard AF Thlaspe nrevense fan weed AF )t ,t, Chenopodiaceae Atrip lex s enibac c cttct Ausffalian saltbush PW CaIEPPC-A2 Bassia h,;ssopifolict bassia AF CaIEPPC-B X:>F>fi T 332 CHEr-oNrAN CoNSERVATToN AND BroLocv, Volume 4, Number 2 - 2002
Table t, continued
Clrcnopodiwn albunt lamb's quarters AF {< Chenopodium murale pigweed AF * * C o ri s pe nnam hy s s op ifo I i unt Corispermum AF * C), c I o I o nm at r ip I ic ifo I ttun winged pigweed AF Halogeton glomeratus halogeton AF CaIEPPC-RA, CA-A, AZ-Restr * T Kochia scopafia Kochia AF >F Salsola paulsenii barbwire Russian thistle AF Salsolo tragus Russian thistle AF calEPPC_NI, CA_C *** C Convolvulaceae Convolvttlus aruensis bindweed PH CA-C, AZ-Regul, UT-NW, * C, T i< Elaeagnaceae El ae a gnrts an g u s tifol i u s Russian olive PW Euphorbiaceae Clmrnaesyce nutculata spotted spurge AF Fabaceae Alhagi pseudallugai camel thorn PW CaIEPPC-RA, CA-A, AZ-Restr, NV-NW * Medicctgo lupulinn black medick AF Medicago sativa alfalfa PH Melilons albct white sweetclover AF Melilorus inclica sourclover AF Melilonts fficiltalis yellow sweetclover BH Pa*insonia aculeata Mexican palo verde PW * TriJblh,un repens white clover PH Gerenaceae Erodiwtt cicntariwn red-stemmed filaree AF **{
cicutarium.These species invaded this region during the late spp., and the annuals Brassica tournefortii, B. tectorum, 1880s and early 1900s or earlier, and became widespread or Bromus trinii,, Centaurea melitensis,, C. repens, Descurania
locally dominant after the 1950s (Parish,I913, 1920; Jack- s ophia, Hirschfeldia incana, P ennis eturn setaceum, Salsola son, 1985; Heady, 1988; Brooks,2000a,b; Brooks and spp. , Sis ,-'mbrium altissimltnx, and S. irio (Table 1). Many of Esque, 2000; Minnich and Sanders, 2000). these species cause significant ecological problems in other Bromus rubens is classified within section Geneaof the ecosystems (California Exotic Pest Plant Council, 1999), genus Bromus (Pavlick, 1995), along with its close relative and should be high priorities for monitoring and control B. tectorum. Bromus rubens became common in most parts efforts in the Mojave and Colorado deserts. of the Mojave Desert by 1950 (Hunter, 1991), and dramati- Brassica tournefortii is one of the locally abundant cally increased in dominance since the 1970s (Beatley, species that is of particular concern for land managers 1966; Hunter,l99l). Bromus rubens is considered an inva- because of its rapid rate of colonrzatton and profuse produc- sive weed in its Mediterranean home range (Jackson, 1985), tion of biomass. This species first appeared in California in and is reco gnrzed as a potential wildland pest in the Mojave 1927 , became dominant in coastal southern California dur- and Colorado deserts (California Exotic Pest Plant Council, ing the 1980s, and is currently spreading north and east in the 1999). Its dominance is positively correlated with rainfall, Mojave and Colorado deserts (Minnich and Sanders, 2000). soil nitrogen, fire frequency, and elevation (Brooks, 1998), It has been observed as far north as the southern Owens and negatively correlated with low winter temperatures Valley in California and in St. George, Urah (MLB and TCE, (Hulbert, 1955). It is typically most abundant under shrubs pers. obs.). It primarily spreads along roads, especially in where soil moisture and nitrogen levels are highest (Brooks, sandy soils, and at an eastern Mojave Desert site was 1999b). It appears to experience high seedling mortality observed to spread > 1.5 km away from a paved road in 6 during years of low rainfall and can subsequently become years (K. Berry, unpubl. data). locally rare (R. Minnich, pers. comm.), but actual mortality rates during drought have not been quantitied. We hypoth- Ecological Effects of Alien Annual Plants esized that Bromus rubenr can reproduce in relatively mesic refugia at high elevations, on north-facing slopes, and along Bromus tectorlulx can effectively compete with native roads even during the driest years, allowing it to persist in species for water in the Great Basin Desert (Eissenstat and this region (Esque et al., 2001). Caldwell, 1988; Melgoza etal., I 990;Melgoza andNowak, schismlls spp. is comprised of two closely related spe- l99l), and B. ntbens, Schisnxus spp., and E. cicutariunt cies, S. arabiczs and S. barbatus. It was first reported in appear to compete effectively with native annuals for soil California around 1935 (Hoover, 1936; Robbins, 1940), nitrogen in the Mojave Desert (Brooks, 1998; 2000c; in invaded the Mojave and Colorado deserts during the 1940s, press). Experimental thinning of alien annual seedlings and became abundant there by the 1950s (O. Clarke pers. reduced density, biomass, and species richness of native comm.). Schismus spp. is not considered an invasive weed in Mojave Desert annuals (Brooks, 2000c). Although the dis- its Middle East home range (Brooks, 2000 b), but is consid- placement of native species by alien species has not been ered a potential wildland pest in the Mojave and Colorado documented, a previously common native annual grass, deserts (California Exotic Pest Planr Council, 1999). Its Vulpia octoflora, appeared to become uncommon during the abundance is negatively correlated with that of B. rubens, 1940s after the invasion of the ecologically similar Schismus possibly due to competition between them (Brooks, 2000c). spp. (O. Clark,, pers. comm.). It benetits from increased levels of water and nitrogen, but Densely packed alien annual plant seedlings and accu- is also well adapted for arid, low-nutrient conditions (Brooks, mulated plant litter may inhibit the germination of native 1998, 2000b). It can germinate and reproduce even during annual plants. After germination, alien annual plants such as very dry years (Burgess et al.,l99I; Brooks, 1999b), and can E. cicutarium, Bromus spp., and Schismus spp. initiate proceed from germination to reproduction faster than many vegetative growth earlier than most native species in the other desert annual plants (Szarek et al. , 1982). Mojave Desert (Jennings, 2001). Established annual plants Erodium cicutarium is often not considered a weedy seedlings can inhibit the subsequent germination of annual plant, perhaps because it has been naturalizedin southwest- plants seeds (Inouye, 1980, I99I), although the specific ern North America for so long. It has been present in this effects of alien seedlings on germination of native seeds has region since the 1600s, in contrast to most other alien not been evaluated. Plant litter created by alien annual annuals that invaded in the late 1880s and early ro mid- 1900s grasses decomposes more slowly than that of native annuals (Heady, 1988). Nevertheless, this species is widespread and because shoot fiber content is higher in alien than in native appears to compete with native plants in the Mojave (Brooks, plants (DeFalco, 1995) and accumulates during successive 2000c) and Sonoran (Inouye et al., 1980) deserts. years. Thick plant litter may impede germination of plant seeds There are anumber of alien species that are only locally by shading the mineral soil, reducing the amount of water that abundant in specific habitats such as road verges, agricul- reaches the mineral soil, and suspending seeds above and out tural fields, livestock watering sites, off-highway vehicle of contact with the mineral soil (Facelli and Pickett, I99l). staging areas, riparian corridors, sand dunes, or at high Alien annual grasses alter the fuel structure and fire elevations. These species include the perennial Tamarix behavior in the Mojave and Colorado deserts, making them 334 CHEloNrnN CoNSERVATToN AND BroLoGv, Volurne 4, l',.lutttber 2 - 2002 more susceptible to frequent fires. Stems of alien annual Table 2. Ftre history data from the Mojave and Colorado desert portions of the Bureau of Land Management, California Desert grasses in the genera Bromus and Schisnlrl.r remain rooted District (BLM-CDD), 1980- 1995 (n -2103 fires). and upright into the summer fire season and can accllmulate over successive years, whereas those of most native forbs Re-gion Annual Mean (SD) Annual Range senes- and aliens such as E. c'icutariutrt crumble soon after Mojave Desert cence (Brooks, 1999a). High frequency and cover of stand- No. Fires 123 (s3) 42-223 Hectares 3s26 (3410) 58- l 2280 ing dead annual ._grass stems creates continuous fine tuel beds Hectarcs lfre 42 (70) t-292 that facilitate the spread of fires (Duck et al., 1997, Brooks, Li-ehtning 2t ( ls) I -70 1999a; Esque and Schwalbe, 2002; Esque et al., in review). Human 97 (4t ) 2t-t86 Colorado Desert No. Fires 8 (s) 2-t9 FIRE Hectares tM2 (zMt) < l-991I Hectares/fue 94 (149) < t-522 Changing fire regimes threaten tortoises worldwide. Of Lightnin.-e 2 (5) I Human 1 (2- l8) t-4 the 40 tortoise species listed by Swin,_eland and Klemens Total (1989),12 were listed as threatened by flre. Only one other No. Fires r 3l (s6) 4t -236 threat, general habitat destruction, was listed for more spe- Hectares 4s69 (4383) l8l-t4282 Hectares lflre 48 (t t1 2-219 cies (23 species). Of the l2 species threatened by fire, direct Li-qhtning 28 (ls) 7 -72 effects of fire threatened 3 and indirect effects, particularly Human 104 (50; 25-196 habitat loss from fire, threatened 9. For all but one species, increased rather than decreased fire fi'equency constituted the threat. Tortoise species may be poorly adapted to fire 237o) (Tabl e 2). This could be partly attributed to the larger because most evolved in arid or semi-arid habitats (Swingland size of the Mojave Desert, although much of this area is not and Klemens.' 1989) where fire is often historically rare. Fire managed by BLM and is not included in the BLM-CDD management is therefore an important factor for the conser- database. In contrast, the average fire size was 557o larger in vation of at least 307o of the world's tortoise species. the Colorado Desert, due to the higher proportion of lar.-ee especially those that live where rainfall is low. fires there compared to the Mojave (Fig. 1A). Lightning The desert tortoise is one species threatened by altered caused roughly 257o of all fires in both desert regions, with fire regimes due to alien plant invasions. Although fire was the remainder attributed to human activities (Table2). Most not included as a threat for this species in Swingland and fires occurred during the summer months of May throu-9h Klemens (1989), increased fire frequency due to annual September, although this summer peak in fire activity was grass invasions (Bronlus and Schisntrls spp. ) was one reason more pronounced in the Mojave than the Colorado Desert the Mojave population was listed as Threatened by the U.S. 1Fi._e. 1B). Fish and Wildlife Service ( 1990, 1994). The annual number of fires increased significantly between 1980 and 19951Fi-e .2A). The number of lightnin-e- Recent Fire History caused fires remained unchanged durin.-e this time interval (Fig . 2B), so increased fire frequency was partly due to an In this section we describe patterns of fire occurrence increased number of human-caused fires (Fig. 2C). Another since the 1970s in the Mojave and Colorado deserts. We rely possible reason for increased fire frequency between 1980 primarily on a BLM-CDD database of 2103 recorded fires and 1995 in the California deserts is related to the period of that burned 73,099 ha (180,490 acres) in the California above average rainfall that began at the end of the 1970s desert between 1980 and 1995 (DI- 1202 fire reports). This (Hereford and Webb, 200 l). Increased rainfall likely dataset does not include desert tortoise habitat in Arizona, increased fine fuel loads, especially those created by Nevada, and Utah. It also is an underestimate of the number alien annual grasses, contributing to increased fire fre- of fires that occuffed and the amount of area burned within quency. the California deserts, because it does not include unre- Most fires in the Colorado Desert occurred along the ported fires or fire records for large expanses of lands northwestern ecotone with the Mojave Desert near Joshua managed by the National Park Service, the Department of Tree National Park and the San Gorgonio pass (Fig. 3; fire Defense, and other local or privately owned lands. However, cluster l). Fires in this area typically burn in creosote bush the BLM-CDD dataset is the largest single collection of fire scrub, Joshua tree woodland, blackbrush, and piflon-juniper records for the Mojave and Colorado deserts, and we use it desert vegetation, but they also often spread up slope into this paper to evaluate the recent status and trend of fire across chaparral and coniferous forests along the eastern slope of a large portion of desert tortoise habitat. We supplement this the San Jacinto and San Bernardino mountains. dataset with oLlr personal knowledge of recent fire history in There are a number of regional fire clusters in the desert tortoise habitat outside of California. Mojave Desert, mostly along the southwest and northeast The percentage of fires and total area burned in the ecotones with more mesic neighboring ecoregions (Fig. 3). BLM-CDD from 1980 to 1995 was much higher in the Fire cluster 2 is in the Antelope Valley, at the ecotone with Mojave (947o and 777o) than the Colorado desert (67o and a region dominated by annual grasslands and blue oak Bnoors AND EseuE - Alien Plants and Fire 335
A. Fire Size Distribution A. All Fires o Mojave r Colorado 250
200 o 0) rso o f (D o c) iI €t 100 o J o z -o 50 E z 100 80 0 60 B. Lightning Caused Fires 40 20 80 0 70 4 (10) 40 (100) 405 (1000) 4050 (10000) Size - ha (ac.) $60 ir 50
:40o €o go B. Fire Season Distribution 220:f 10 400 0 300 C. Human Caused Fires 200 a 100 o i! 0 200 o 160 o o o E LL J 30 120 z o q) 20 -o 80 E zJ 10 40
0 0 Feb Mar Apr May Jun Jul Arg Sep Oct Nov Dec 80 81 87 88 89 Month Year
Figure 1. Frequency distributions of fires by (A) size and (B) Figure 2. Change in the (A) total number of fires, (B) fires caused seasonal occurrence in the Bureau of Land Management, Califor- by humans, and (C) fires caused by lightning between 1980 and 1995 nia Desert District. in the Bureau of Land Management, California Desert District.
woodlands. Fire clusters 3, 4, and 5 are located within the Effects of Fire on the Desert Tortoise Mojave Desert, in desert vegetation dominated by creosote and its Habitat bush. Fire cluster 6 is located in the Mojave National Preserve, in Joshua tree woodland, sagebrush, and piflon- Direct Efficts.-Desert tortoises have been found dead juniper vegetation. Fire cluster 7 is in the Spring Mountains or mortally injured immediately after fires (Woodbury and of Nev ada, in creosote bush scrub, blackbrush, sagebrush, Hardy, 1948; Duck et al. ,1997; Homer et al., 1998; Esque et and piflon-juniper, and fires there often spread upslope into a1.,2002; Esque et al., in review). Fires can kill animals by interior chaparral and coniferous forest. Fire cluster 8 is in incineration, elevating body temperature, poisoning by the Black Mountains of Arizona, within creosote bush scrub smoke, or asphyxiation (Whelan, 1995). Tortoises occupying and blackbrush vegetation. Fire cluster 9 extends from the underground cover sites such as burrows, caliche caves, rock Mormon Mountain and Tule desert region of southern Ne- shelters, and dens are less exposed to high temperature and vada to the Beaver Dam Slope of southwest Utah, and smoke during fires. In general, deep cover sites proviCe more includes creosote bush scrub, Joshua tree woodland, protection from fire than shallow ones. Because the desert blackbrush, piflon-juniper, and interior chaparral. Fire clus- tortoise cannot move quickly, its best chance to avoid the direct ter l0 extends from the Virgin Mountains to the Grand Wash effects of fire is to already be in an underground cover site. Cliffs of northwest Arizona, and includes creosote bush Early season fires are potentially more threatening than scrub, Joshua tree woodland, blackbrush, sagebrush, and summer fires to the desert tortoise. During the spring, piflon-juniper desert vegetation, with fires extending upslope tortoises are active aboveground and therefore vulnerable to into interior chaparral and coniferous forests. fire throughout their geographic range (Esque et &1., in Most of the smaller fires in the Mojave Desert are review). Although temperatures may be lower during early concentrated along major highways (Fig. 3). These small than late season fires (Brooks,2002), they still appear to be fires are mostly cause by human activity. In contrast, larger high enough to kill tortoises located on the surface. fires tend to be located in more remote wildland areas and are Years of high rainfall not only promote early season typically caused by lightning. fires by stimulating growth of fire fuels, but also high rates 336 CHEr-oNlnN CoNSERVATToN AND BIoLocv, Volume 4, Number 2 - 2002
\ \
Fire Size Fire Clusters 0 - 4 ha (10 ac,) 1. Joshua Tree/San Gorgonio Pass 2. Antelope Valley ., - 40 ha (100 ac.) 4 3. El Paso Mountains/Red Mountain O > 40 ha (100 ac.) 4. Opal Mountain 5. Stoddard Valley
N 6. Mojave National Preserve 7. Spring Mountains A 8. Black Mountains 9. Mormon Mountains/Beaver Dam Slope 0 50 100 F-trm 10. Virgin Mountains/Grand Wash Cliffs
Figure 3. Spatial distributions of fires between 1980 and 1995 in the Bureau of Land Management, California Desert District, and clusters offire activity since 1970s across the entire Mojave and Colorado deserts. Major roads are included as geographic reference points.
of desert tortoise reproduction (Turner et al., 1986). Small (Nagy et al. , 1 99 8 ) . The desert tortoise may selectiv eIy graze individuals such as hatchlings are more at risk from lethal native forbs and herbaceous perennials, but they have also heating than large ones, because they have a higher surface been shown to eat alien annual plants. In the western Mojave to volume ratio that allows heat to penetrate their vital organs Desert native plants comprised 95%o of the desert tortoise relatively quickly. In the highly variable environment where diet (Jennings, 1993), whereas in the northeastern Mojave desert torloises live, recruitment during years of high rainfall Desert alien annual grasses comprised a significant part of may be very important. Unfortunately, tortoises are faced with their diet (Hansen et al., l9l6; Esque,1994). a double-edged sword. High precipitation levels produce abun- Although single fires may not produce long-term reduc- dant food plants and drinking water, but also create abundant tions in the cover of perennial plants or biomass of native fuel loads that place tortoises at increased risk from fire. annual plants (O'Leary and Minnich, 1981), recurrent fire Indirect Effects.-Fire can negatively affect desert tor- can convert native desertscrub to alien annual grassland toises by reducing plant cover. Perennial plants, especially (Brown and Minnich, 1986; Duck et al.,1997; Esque et al., woody shrubs, provide protection for desert tortoises from in review). This new habitat is more prone to burning mortality due to predators or overheating from the sun because alien annual grasses often increase in dominance (Woodbury and Hardy, 1948; Burge,l9J 8; Mushinsky and after fire (D'Antonio and Vitousek, 1992).Alien annual grass- Gibson, 1991). Approximately 75Vo of all tortoise cover land has little shrub cover or native annual plant biomass, and sites are located beneath shrub canopies in creosotebush/ desert tortoises are very uncommon in such habitat (K. Bery bursage plant associations (Burge, 1977 , I97 8; Berry and and M. Brooks, pers. obs.). Areas of recurrent flre therefore Turner, 1986). appear to render habitat inhospitable for the desert tortoise. Reduced abundance and diversity of native annual plants caused by fire may deprive desert tortoises of impor- FUTURE TRENDS tant forage. Effects can include reduced availability of food plants, loss or reduction of available nutrients and trace Rainfall patterns during the 1900s in the Mojave Desert elements, and change in seasonal availability of food plants suggest that we may be entering a25-35 year drought period Bnoors AND EseuE - Alien Plants and Fire 337
(Hereford and Webb, 2001 ). If this is true, then alien species ciliare create even greater fire hazards in the Sonoran Desert that are sensitive to drought, such as B. rubens, may decrease of Arizona and northern Mexico respectively (Bfrquez et in dominance at lower elevations, thereby reducing fuel a1.,2002; Esque et a1.,2002).Itis unknown if these species loads and decreasing the frequency of fires. However, native can become widespread and dominant in desert tortoise shrub communities such as blackbrush and sagebrush, and habitat, but their effects in similar ecosystems elsewhere piflon-juniper woodland, may become more prone to burn- should be a cause for concern. ing as their live fuel moisture levels decline in response to Dominance of alien annual plants is often highest near drought. Alien species that are adapted for arid conditions, roads (Brooks, 1998; Kemp and Brooks, 1998), so min rmiz- such as Schisnxus spp., will likely remain abundant regard- ing the number of paved and dirt roads and maintaining non- less of rainfall fluctuations. roaded wilderness areas may reduce the dominance of aliens. Deposition of atmospheric nitrogen from air pollutants Other disturbances such as livestock grazing (Webb and may reduce the spatial and temporal heterogeneity of soil Stielstra,, I979) and off-highway vehicle use (Davidson and nitrogen in the Mojave Desert and alter competitive hierar- Fox, 197 4) also promote the dominance of alien annuals, but chies among native and alien species. Experimental addition this effect can be reduced by fenced protection from these of nitrogen (3.2 g NH*NO./mtlyr) significanrly increased forms of disturbance (Brooks 1995, 2000d). density and biomass of alien species and decreased that of Invasions of new species often occur along roads and native species (Brooks, 1998, in press). Species richness of washes in the deserts of California (Brooks, 1998, 1999b; aliens was unaffected, but richness of natives decreased. Kemp and Brooks, 1998), so efforts to monitor the arrival of Nitrogen deposition rates of 4.5 glmzlyr have been recorded new species and eradicate them should be focused in these in the Los Angeles basin (Bytnerowicz and Fenn, 1996), and areas. Early detection and eradication of potentially invasive are associated with high dominance of alien annual grasses species is the most cost-effective way of managing any plant and the loss of native shrub communities there (Allen et al., invasion (Mack et al., 2000). 1998). Although current deposition rates are undoubtedly The best ways to reduce fire frequency are to reduce the much lower in the Mojave Desert, future deposition rates number of fires started by humans and minimrze the in- will likely increase as human population and air pollution creases in fuel loads caused by alien plants. Reducing the levels rise. number of fires started by humans may be accomplished by changes in global climate may also encourage the education and enforcing regulations limiting human activi- invasions of alien annual species in the Mojave and Colo- ties that cause fires such as camping, target practice with rado deserts. Atmospheric concentrations of carbon dioxide firearms, and vehicle use. Unfortunately, increasing human have increased more than 25Vo since pre-industrial times, populations within desert tortoise habitat, especially in the and carbon dioxide concentrations in the atmosphere are western and eastern parts of its range, will make enforce- expected to double before the end of the 21st century ment more difficult and costly. A potentially more important (Gammon et al., 1985; Houghton et al., 1990). Increases in approach to reducing fire frequency is to minimize the atmospheric carbon dioxide characteristic of the past cen- biomass of the alien species that fuel desert fires, and prevent tury are known to enhance production of rapidly-growing, the invasion of new species that may further increase fuel cool season species such as alien annual grasses and forbs loads. Currently, methods used to reduce fuel loads (Poorter et al ., 1996), and make communities generally more include prescribed fire, mechanical removal, livestock invasible (Dukes and Mooney, 1999). An increase in alien grazrng, and chemical treatments (Esque and Schwalbe, annual grass production may contribute to increased fre- 2002), but large scale use of any of these methods is not quency and severity of wildfires in western North America currently consistent with competing management needs (Mayeaux et al., 1994). in the Mojave Desert. Urbani zatronwill bring more invasive alien species into Desert fires should be suppressed in most cases, be- the region and increase ignition sources for fires. Increased cause disturbances caused by fires are generally greater than amounts of landscaping with drought tolerant alien plants those caused by appropriately planned fire suppression that may spread into wildlands, automobile travel within the activities (Duck et al., 1997). Most large fires and areas of region, locally produced air pollution, and recreational use recuffent fire occur within mountainous areas designated as of wildland areas will all increase the threat of plant inva- wilderness. The lack of motorized vehicle access in these sions and fire. areas can hinder fire suppression efforts, and managers should consider allowing motorized access by fire crews to MANAGEMENT RECOMMENDATIONS control fires. Wilderness areas that are dominated by B. rubens and those experiencing recurrent fire and type- Management of alien plants and fire should be closely conversions to annual grassland would be prime candidates. integrated, because alien plants can create fuel conditions In some rare cases natural or prescribed fire may be a that promote fire in otherwise fire resistant landscapes that useful management tool. For example, fire early in the predominate in the Mojave and Colorado deserts. Although season may be used to temporarily reduce the dominance of B. ruberzs is currently the primary cause of fire in this region, alien annuals prior to revegetation of native plants. This other alien species such as Pennisetunt setaceunt and p. would only be appropriate in habitat that is already type- 338 Cssr-oNrnN CoNSERVATToN AND BtoLocv, Volume 4, Number 2 - 2002 converted to alien annual grassland, where native plants and LITERATURE CITED animals are relatively scarce, and the chance of recurrent fire is high. The few native plants and animals that may die in this ALLeN, E.B., PaocErr, P.E., BvrNenowICZ, A., RNo MtNNtcH, R.A. sage vegetation of depaup arate habitat are potentially worth the possibility of 1998. Nitrogen deposition effects on coastal southern California. U.S. Departmentof Agriculture, Pacific South- restoring more diverse and less flammable native desertscrub. west Research Station, General Technical Report No. 164. However, if alien annual grasses are abundant in these Bnserrr, B. 1998. Statement by Secretary of the Interior on invasive communities, fires should not be used because of the signifi- alien species. Proceedings, National Weed Symposium, April 8- the native shrubland be replaced by cant chance that will I 0, I 998. http://www-a.blm.gov/weeds/sympos98. alien annual grassland. Benrr-Ey, J.C. 1966. Ecological status of introduced brome grasses (Brome spp.) in desert vegetation of southern Nevada. Ecology Research Needs 47:548-554 BERRy, K.H. ANo TuRNEn, F.B. 1986. Spring activities and habits of Much additional information is needed to effectively juvenile desert tortoises ,, Gopherus agassizii, in California. Copeia 1996:1010- 1012. manage alien plants in the Mojave and Colorado deserts. We Bnoors, M.L. 1995. Benefits of protective fencing to plant and rodent provide a list of species known to occur in this region and a corrrmunities of the western Mojave Desert, California. Environ- preliminary assessment of their potential ecological threats mental Management 19:65-7 4. (Table 1). Species on this list need to be prioritized for Bnoors, M.L. 1998. Ecology of a biological invasion: alien annual control using a method similar to the one developed by plants in the Mojave Desert. Ph.D. Thesis, University of Califor- Hiebert (1991). This method includes two major compo- nia, Riverside. nents: 1) determining the significance of impact, requiring Bnoors , M.L. 1999a. Alien annual grasses and fire in the Mojave knowledge of both current and potential ecological effects; Desert. Madrofl o 46:1 3- I 9. and 2) determining the feasibility of control and manage- Bnoors, M.L 1999b. Habitat invasibility and dominance by alien Mojave Desert. Biological Invasions ment, requiring information on abundance and geographic annual plants in the western I:325-337. range, ease of control, side effects of control measures, Bnoors, M.L. 2000a. Bromus madritensis subsp. rubens (L.) Husnot effects of land uses, and biological control agents. Such a I B. rubens L.f, Foxtail Chess (Red Brome). In: Bossard, C., method is currently under development for use in California, Hoshovsky,M., and Randall, J. @ds.). Noxious Wildland Weeds of Arizona, and Nevada by the California Exotic Pest Plant California. Berkeley: University of California Press, pp.72'76. Council (www.caleppc.org) and the Southwest Vegetation Bnoors, M.L. 2000b. Schismu.s spp., Schismus arabicus Nees and Management Association (www.swvma.org). Most of the Schismus barbatas (L.) Thell., Meditenanean Grass (Split Grass). information needed to rank individual alien species for In: Bossard, C., Hoshovsky, M., and Randall, J. (Eds.). Noxious control is currently unknown for the Mojave and Colorado Wildland Weeds of California. Berkeley: University of California -291. deserts, So future alien plant research should focus on Press, pp.287 Bnoors, M.L. 2000c. Competition between alien annual grasses and providing this information. native annual plants in the Mojave Desert. American Midland Very little is known about historical fire regimes or the Naturalist I 14:92-108. in desert tortoise habitat. Conditions current effects of fire B noorcs, M. L. 2000d. Does protection of desert tortoi se habitat generate before, during, and after fires need to be documented to other ecological benefits in the Mojave Desert? In: Cole, D. and develop predictive models. The mechanisms by which fire McCool, S. (Eds.). 2000 Proceedings, Wilderness Science in a Time promotes dominance by alien annual grasses also need to be of Change. Proc. RMRS-P-000. Ogden UT, U.S. Deparftnent of Agri- understood to develop methods to mintmtze their postfire cultue, Forest Service, Rocky Mountain Research Station, pp. 68-73. dominance. Research is needed to understand the effects of Bnoors, M.L. 2002. Peak fire temperatures and effects on annual plants in the Mojave Desert. Ecological Applications 12:1088-1102. fire on soils, plants, and animals. The quality of fire manage- Bnoors, M.L. In press. Effects of increased soil nitrogen on the ment planning will be limited until this information becomes dominance of alien annual plants in the Mojave Desert. Journal of available. Applied Ecology. Bnoors, M.L. AND Esqur, T.C. 2000. Alien grasses in the Mojave and AcxnowLEDGMENTS Sonoran deserts. Proceedings of the 1999 California Exotic Pest Plant Council Symposium 6:39-M. We thank personnel at the following land management BRowN, D.E. AND MtulucH, R.A. 1986. Fire and creosote bush scrub units for providing information on the alien species found of the western Sonoran Desert, California. American Midland within their jurisdictions: Red Rock Canyon State Park, Naturalist I 16:4Il-422. 1977. Movements and behavior of the desert tortoise, California; Joshua Tree National Park, Lake Mead National BuRce, B.L. agassieii. M.S. Thesis. University of Nevada, Las Vegas. Recreation Area, Death Valley National Park; and the Gopherus Bunce, B.L. IglS.Physical characteristics and patterns of utilization Ridgecrest, Barstow, Needles, and Palm Springs field of- of cover sites used by Gopherus agassizii in southern Nevada. In: fices of the Bureau of Land Management. We also thank the Proceedings of the Desert Tortoise Council Symposium, pp. 80- l I I . California Desert District Office of the Bureau of Land Buncnss, T.L., Bowsns, J.E., aNoTunr.mn, R.M. I99L Exotic plants atthe Management for providing fire records. Anders Rhodin and desert laboratory, Tucson, Arizona. Madroflo 38:96-114. two anonymous reviewers provided helpful suggestions for Buneunz-Moxruo, A., MnLen, M.E., AND M,tnrnvnz-Ymznn, A. improving this manuscript. 2002. Mexican grasslands, thornscrub, and the transformation of Bnoors AND EseuE - Alien Plants and Fire 339
the Sonoran Desertby invasive exotic buffelgrass. In: Van Devender, Avail able onlin e: http : I lwww. fs. fed. us/l and/wdfi re. htm. T.R. (Ed.). The Sonoran Desert Tortoise: Natural History, Biol- HnNsEx, R.M., Jour.rsol, M.K., AND VnN DEveNnEn, T.R. l976.Foods ogy, and Conservation. Tusconk, Arizona: Arizona-Sonora Desert of the desert tortoise, Gopherus egassi:i, in Arizona and Utah.
Museum and The University of Arizona Press. pp. 126-146. Herpetologica 32(3):247 -25 | . BvrNEnowrcz, A. AND FrNN, M.E. 1996. Nitrogen deposition in HEADv. H.F. 1988. Valley grassland. In: Barbour, M.G. and Major, J. California forests: a review. Environmental Pollution 92:l2l -146. (Eds.). Terrestrial Vegetation of California. Davis, CA: California CnlFr-onn DnTABASE. 2000. Ann Dennis, Ginger Ogle, and Tony Native Plant Society, pp. 491-514.
Morosco. http : //elib. cs. berkeley. edu/c gi/c alfl ora. HEnepoRo, R. AND Wess, R.H. 2001 . Climate variation since 1900 in CnrmonNtn Exolc PEsr PlnNr CouNcrr- . 1999. Exotic Pest Plant the Mojave Desefi region affects geomorphic processes and raises Species of Greatest Ecological Concern. http://www.caleppc.org/ issues for land management. In: Reynolds, R.E. (Ed.). The Chang- info/plantlist.html. ing Face of the East Mojave Desert. Fullerton, CA: California State D'ANroruro, C.M. AND Vnouspr, P.M. 1992. Biological invasions by University, Desert Studies Consortium, pp. 54-55. exotic grasses, the grass/fire cycle, and global change. Annual HrcrvnN J.C. (Ed.) 1993. The Jepson Manual: Higher Plants of Review of Ecology and Systematics 3:63-87 . California. Berkeley, CA: University of California Press. DnvrpsoN, E. AND Fox, M. 1974. Effects of off-road motorcycle HnsERr, R.D. 1997 . Prioritizing invasive plants and planning for manage- activity on Mojave Desert vegetation and soil. Madrofio22:381-412. ment. ln: Luken, J.O. and Thieret, J.W. Eds.). Assessment and Manage- DEFnI-co, L.A. 1995. Influence of cryptobiotic soil crusts on winter ment of Plant lnvasions. Springer-Verlag, New York, pp. 195-214 annuals and foraging movements of the desert tortoise (Gopherus HovtER, 8.L., BeRRy, K.H., BRowN, M.8., ELLIS, G., RNo JRconsoru, agassizii) in the northeast Mojave Desert. M. S. Thesis. Colorado E.R. 1998. Pathology of diseases in wild desert tortoises from State University, Ft. Collins, CO. 48pp. California. Journal of Wildlife Diseases 34:508-523. Ducr, T.A., EsquE, T.c., AND HucHEs, T.J. 1997. Fighting wildfires Hooven, R.F. 1936. Notes on California grasses. Madrofro3:227-230. in desert tortoise habitat, considerations for land managers. Pro- HoucHrox, J.T., JpNruNs, G.J., AND ETHRRUvE, J.J. (Eds.). 1990. ceedings for Symposium on: Fire Effects on Rare and Endangered Climate Change: The IPCC Scientitic Assessment. Cambridg., Species Habitats Conference. November 13-16, 1995. Coeur United Kingdom: Cambridge University Press, 365 pp. D'Alene, ID. International Wildland Fire Association. Hurnpnr, L.C. 1955. Ecological studies of Brontus tectorum and Dures, J.S. nNo MoowEv, H.A. 1999. Does global change increase the other bromegrasses. Ecological Monographs 25:18l-213. success of biological invaders? Trends in Ecology and Evolution HuHapsnEv, R.R. 1974. Fire in the deserts and desert grassland of l4:135-139. North America. In: Kozlowski, T.T. and Ahlgren, C.E. (Eds.). Fire ErsseNsrnr, D.M. AND Cnlowplr-, M.M.. 1988. Competitive ability is and Ecosystems. New York: Academic Press, pp. 365-400.
linked to rates of water extraction. OecologtaT5:l-7 . HuNren, R. I 991. Bromus invasions on the Nevada Test Site: present EsquE, T.C. 1994. Diet and diet selection of the desert tortoise status of B. rubens and B. tectorwrz with notes on their relationship (Gopherus agassizii) in the Northeast Mojave Desert. M.S. Thesis. to disturbance and altitude. Great Basin Naturalist 51:176-182. Colorado State University. INouvE, R.S. 1980. Density-dependent germination response by seeds EsquE, T.C. AND Scrwnlen, C.R. 2002. Alien annual plants and their of desert annuals. Oecolo gia 46:235-238. relationships to fire and biotic change in Sonoran Desertscrub. In: INouye, R.S. 199I. Population biology of desert annual plants. In: Tellman, B. (Ed.). Invasive exotic species in the Sonoran region. Polis, G.A. (Ed.). The Ecology of Deseft Communities. Tucson, Tucson, Arizon a: Anzona-Sonora Desert Museum and University Arizona: The University of Arizona Press, pp.27-54. of Arizona Press,pp. 165-194. INouye, R.S., Byens, G.S., AND BRowN, J.H. 1980. Effects of predation EsquE, T.C., DEFar-co, L.A., MEorcR, P.A., Bnoors, M.L., ANDWenn, and competition on survivorship, fecundity, and corrrmunity struc- R.H. 2001. Predicted changes in Mojave fire frequency related to ture of desert annuals. Ecology 6l:l34y''-1351. exotic annuals. Symposium on predicting hydrologic, geologic, JecrsoN, L.E. 1985. Ecological origins of California's Mediterranean and biologic responses to a drier and warmer climate in the desert grasses. Journal of Biogeography 12:349-361. southwest. U.S. Geological Survey, Tucson, Arizona. JeNNn rcs, W.B. 1993 .Foraging ecology ofthe desert tortoise (Gophe rus EsquE, T.C., BunquezM.,A., ScuwALBE, C.R., VnN DEvENneR, T.R., agassizii) in the western Mojave Desert. M.S. Thesis, University of NuHurs, M.J.M., AND ANNTNc ,P.2002. Fire ecology of the Sonoran Texas at Arlington. desert tortoise. In: Van Devender, T.R. (Ed.). The Sonoran Desert JpxNtNcs, W.B .2001. Comparative flowering phenology of plants in Tortoise: Natural History, Biology, and Conservation. Arizona- the western Mojave Desert. Madrofro 48: 162-171. Sonora Desert Museum and The University of Arizona Press, Ktue, P. AND Bnoorcs, M.L. 1998. Exotic Species of California Tucson, pp. 3 12-333. Deserts. Fremontia 26:30-34. Esqur, T.C., ScHwnr-eE, C.R., DEFaLCo, L.A., HucHEs, T.J., AND LoNsnele, W.M. 1997 . Global patterns of plant invasions. Bulletin of DuNcnN, R.B. In review. Effects of wildfire on small desert the Ecological Society of AmencaTS:2l. vertebrates, especially desert tortoises (Gopherus agassizii). The LoNsnru-p, W.M . 1999. Global patterns of plant invasions and the Southwestern Naturalist. concept of invasibility. Ecology 80:l 522-1536. FecEllr, J.M. AND Prcxsrr, S.T.A. lggL Plant litter: its dynamics and MRCr, R.N., SntgpRLoFF, D., Lolsnnt-g, W.M., EvRNS, H.,CLout, M., effects on plant community stucture. The Botanical Review 5l:l-32. AND Brzzez,F. 2000. Biotic invasions: causes, epidemiology, global Frns Eppecrs INronuerloN Svsrev. 1996. Prescribed Fire and Fire consequences, and control. Issues in Ecology Number 5, 20 pp. Effects Research Work Unit, Rocky Mountain Research Station MnyEnux, H.S., JoHNSoN, H.B., AND PoLLEy, H.W. 1994. Potential (producer). www.fs.fed. us/database/feis. interactions between global change and intermountain annual GnHauoN, R.G., SuNoeursr, E., RNn FnnsEn, P.J. 1985. History of grasslands. In: Monsen, S.B. and Kitchen, S.G. (Eds.). Proceed- carbon dioxide in the atmosphere. In: Trabalka, J.R. (Ed.). Atmo- ings of Ecology and Management of Annual Rangelands. ogden, spheric Carbon Dioxide and the Global Carbon Cycle. Washing- UT: Intermountain Research Station, pp. 95-l 10. ton, DC:U.S. Department of Energy. MprcozA, G. AND Nowar, R.S. 1991. Competition between cheatgrass GrtcrvRN, D. AND Baeetrr, B. 1995. Federal Wildland Fire Policy. and two native species after fire: implications from observations 340 CHEr-oNnN CoNSERVATToN AND BroLocv, Volume 4, Number 2 - 2002
and measurements of root distribution. Journal of Range Manage- Mojave Desert. In: Bender, G.L. (Ed.). Reference Handbook of ment 44:27 -33. North American Deserts. Connecticut Greenwood Press, pp. 1 03 - 1 59. MElcozR, G., Nownr, R.S., AND Tauscu, R.J. 1990. Soil water Sournwnsr Exouc PmNr Mnpprxc Pnocnev. 2000. Southwest Exotic exploitation after fire: competition between Bromus tectorum Plant Mapping Program. http ://www. us g s. nau. edu/swemp. (Poaceae) and two native species. Oecologia 83:7-13. SwrNcmNn, I.R. AND Kt-p,ueNs, M.W. Eds. 1989. The Conservation Mn nucH, R.A. AND SnNosns, A.C. 2000. Brassicatourneforrii (Gouan.) Biology of Tortoises. Gland, Switzerland: Occasional Papers of Sahara mustard. In: Bossard, C., Hoshovsky, M., and Randall, J. the IUCN Species Survival Commission (SSC) No. 5, 204 pp. (Eds.). Noxious Wildland Weeds of California. Berkeley: Univer- SzRREr, S.R., StvtttH, S.D., AND RYRN, R.D. 1982. Moisture stress sity of California Press, pp. 68-72. effects on biomass partitioning in two Sonoran Desert annuals. MoRsE, L.E., KanrEsz, J.T., AND KurxEn, L.S. l995.Native vascular American Midland Naturalist 108:338 -345. plants. In: LaRoe, E.T., Puckett, G.S., Doran, P.D., and Mac, M.J. THs Nnrunn, CoNssnvANCy. 1996. America's Least Wanted: Alien (Eds.). Our Living Resources, a Report to the Nation on the Species Invasions of U.S. Ecosystems. Arlington, VA: The Nature Distribution, Abundance, and Health of U.S. Plants, Animals, and Conservancy. Ecosystems. Washington, DC: U.S. Department of the Interior, The Nature Conservancy. 2000. The Nature Conservanc), Wildland National Biological Service, pp. 205 -209. Invasive Species Program, Management Library. Online, http:l I MusnnvsKy, H.R. AND GmsoN, D.J. 1991. The influence of fire tncweeds.ucdavis.edu periodicity on habitat structure. In: Bell, S.S., McCoy, E.D., and Tunxnn, F.B., HAvoEN, P., Bunce, 8.L., AND RosnnsoN, J.B.. 1986. Mushinsky, H.R. (Eds.). Habitat Structure, the physical arrange- Egg production by the desert tortoise (Gopherus agassizii) in ment of objects in space. New York Chapman and Hall, pp. 237 -259. California. Herpetologic a 42:93 -l 04. Nrcv, K.A., HENEN, B.T., AND Vyns, D.B. 1998. Nutritional quality U.S. FrsH AND Wnoup'E Ssnvrcn. 1990. Endangered and threatened of native and introduced food plants of wild desert tortoises. wildlife and plants; determination of threatened status for the Mojave
Journal of Herpetology 32:260-267 . population of ttre desert tortoise. Federal Register 55:12178-1219I. O'Lsnny, J. F. AND MnrNrcH, R.A. 1981. Postfire recovery of creosote U.S. FrsH AND Wrr-olrrs SsnvrcE. 1994. Desert Tortoise (Mojave bush scrub vegetation in the Western Colorado Desert. Madroflo Population) Recovery Plan. United States Fish and Wildlife Ser- 28:61-66. vice, Portland, Oregon, 73 pp.. PRnrsu, S.B. 1913. Note on some introduced plants of southern WEnn, R.H. AND Srm,lsrna, S.S. 1979. Sheep grazing effects on California. Muhlenbergia 5: 109- I 15, l2I-128. Mojave Desert vegetation and soils. Environmental Management PerusH, S.B. 1920. The immigrant plants of southern California. 3:517 -529. Bulletin of the Southern California Academy of Sciences 19:3-30. WrmuN, R.J. 1995. The Ecology of Fire. Cambridge,United King- Pnvr-rcr. L. 1995. Bromus L. of North America. Victoria, British dom: Cambridge University Press, 346 pp. Columbia: Royal British Columbia Museuffi, 160 pp. Wu-covE, D.S., RorgsreN, D., Dunow, J., PnuFS, A., AND Losos, E. Poonrnn, H., Rorirrrer, C., AND CaMpeEI-I-, B.D. 1996. Interspecific 1998. Quantifying threats to imperiled species in the United States. variation in the growth response of plants to elevated COr: a search Bioscience 48 :607 -615 . for functional types. In: Korner, C. and Bazzaz, F.A. (Eds.). WoooeuRy, A.M. AND HRRoy, R. 1948. Studies of the desert tortoise, Carbon Dioxide, Populations, and Communities. San Diego: Aca- Gopherus agassizii. Ecological Monographs I 8: 145-200. demic Press, pp. 37 5-412. Received: 3 June 2000 RoeerNs W.W. 1940. Alien plants growing without cultivation in 2002 California. University of California Agricultural Experimental Reviewed: 8 August Revised and Accepted: 12 September 2002 Station Bulletin No. 637. RowmNos, P.G., JoHNsoN, H.B., Rlrtrn, E., RNn ENoo, A. 1982. The