Saharan Mustard Invasion History and Patterns of Spread

IInnvvaassiionon HHiissttoorryy aanndd PPaatttteerrnnss ooff SSpprreadead bbyy SSaahaharraa MMuussttaarrdd ((BrBraassssiicaca ttouourrneneffoorrttiiii)) iinn SSououtthhwweesstteerrnn NNoorrtthh AmAmeerriicaca

Matt Brooks U.S. Geological Survey, Western Region Western Ecological Research Center Las Vegas Field Station www.werc.usgs.gov Invasion History • Sahara mustard first appeared in the Coachella Valley of the northwestern Colorado Desert in the 1920’s • Subsequently spread: – East • through the Sonoran Desert of CA and AZ (during the mid-1900s?) • and on into NM and TX (when?) – West • into the interior cismontane valleys of southern CA (by mid-century, but only recently displaced other species as the dominant mustard in some places during the 1980s-1990s) – North • through the Mojave Desert of CA, NV, AZ, and UT (probably began during the early 1980s, but became dominant in some places by the late 1990s) • on into the Colorado Plateau of UT (around 2000) • and the southwestern Great Basin in Owens Valley of CA (probably by the late 1990s) – South • into Mexico (when did it get there?) Patterns of Spread •• SSahaaharraa mmuusstatarrdd isis ccuurrrrententllyy ababsseenntt ffrroomm mmoosstt wwililddllandand aarreaeass,, aallthoughthough ththiiss peperrsspepeccttivivee cchahanngedged sisigngniifficicantantllyy dudurriingng sspprriingng 2005.2005. •• WWheherree iitt doedoess ooccccuurr iitt isis oftenoften oonnlyly pprreessentent aallongong rroadoadsisidedess,, wwhhicichh hahass beenbeen iittss pprriimmaarryy pathpathwwaayy ofof sspprreeaad.d. MMaajjoorr PPooiinntt #1#1

•• SSahaaharraa mmuussttaarrdd ffiirsrstt aarrrriivveded iinn NNoorrtthh AmAmeerriiccaa dudurriinngg tthehe eaearrllyy 19190000s.s. •• ItItss bebeccaammee aa ssiigngniiffiiccaanntt mmananaagegemmenentt ccononcceerrnn dduurriingng tthehe llaattee 19801980ss,, andand eesspepecciiaallllyy dduurriingng tthehe eaearrllyy 20200000s.s. •• RRoadoadssiidedess aarree tthehe pprriimmaarryy papatthhwwaayy ooff sspprread.ead. SSahaaharraa MMuusstatarrdd InInvvaasisionon ofof PPaavveedd RoadRoadsisidesdes

• A study to compare stem densities of Sahara mustard at various distances up to 1 mile from the edge of a paved arterial road. • This species was not

600 previously detected at this Roadside Distribution in ChemehueviValley site during plant surveys in 500 )

t Spring 1999 c

1977, 1979, 1982, 1988, or e s (Berry and Brook in prep)

n 400 a r t /

1992. s R2=0.93 m e

t 300 s • The 7-year period between d a e d ( 1992 and 1999 included two 200 y t i s n

years of very high rainfall e

D 100 (1995 and 1998) which 0 probably facilitated spread 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 away from the roadside. Distance from paved arterial road (miles) MMaajjoorr PPooiinntt #2#2

•• SSahaaharraa mmuussttaarrdd ccanan eessttabablliisshh aallongong papavveded rrooadadssiiddeess andand sspprreeadad upup ttoo 11 mmililee iinnttoo tthehe opopenen dedesseerrtt wwiitthhiinn 77 yyeeaars.rs. SaSahhaarraa MMuusstatarrdd IInvnvaassiionon ooff PaPavveedd RRooaaddssiiddeses

A study to compare plant density, plant biomass, seed density, and seed biomass of Sahara mustard in beneath- canopy and interspace microhabitats along roadside berms compared to 20m off-road of major paved arterial roads.

2 sites with silty/rockysoils

1 site with sandy soils StStududyy SSiitteses

Needles and Coachella sites Mohawk dunes site Silty/rocky soils Sandy soils Atypical of invaded areas Typical of invaded areas PPllantant PPrroduoduccttiionon ReRessponponssee toto PPaavveded RoadsRoads

25 Plant Density interspace beneath-canopy

15 2 m 5 2 . 0

# 10

0 Offroad Roadside Offroad Roadside Offroad Roadside Coachella Valley Mohawk Dunes Needles

Overall, plant density did not display any consistent trends between roadside and offroad areas PPllantant PPrroduoduccttiionon ReRessponponssee toto PPaavveded RoadsRoads

120 Plant Biomass

100

80 2 m 5 2

. 60 0

0 Offroad Roadside Offroad Roadside Offroad Roadside Coachella Valley Mohawk Dunes Needles

In contrast, plant biomass was significantly higher in roadsides than offroad areas, except at the sandy-soiled Mohawk dune site. MMaajjoorr PPooiinntt #3#3

•• PPaavveded rrooaaddss mmaayy hhaavvee ttheheiirr ggrreaeatteesstt eeffffeeccttss wwheherree ssooililss aarree ssiilltyty//rroocckkyy,, ccondondiittiiononss wwheherree SSahaaharraa mmuussttaarrdd mmaayy ootthheerrwwiissee hahavvee ddiiffffiiccuullttyy eessttabablilisshhiinng.g. PPllantant PPrroduoduccttiionon ReRessponponssee toto PPaavveded RoadsRoads

Plant biomass was especially high in roadside interspaces. SSeedeed PPrroduoduccttiionon ReRessponponssee toto PPaavveded RoadsRoads

18000 Seed Density Seed Biomass interspace 25 16000 beneath-canopy 14000 20 12000 2

m 10000 2

5 15 2 m . 5 0 / 8000 2 . # 0 / g 6000 10

4000 5 2000

0 0 Offroad Roadside Offroad Roadside Offroad Roadside Offroad Roadside Offroad Roadside Offroad Roadside Coachella Valley Mohawk Dunes Needles Coachella Valley Mohawk Dunes Needles

Seed density and biomass were both higher in roadside than offroad interspaces at the two sites with silty/rocky soils, due to their strong positive relationships with plant biomass (R2~0.90). Seed Density and biomass did not significantly differ between roadside and offroad areas at the site with sandy soils. MMaajjoorr PPooiinntt #4#4

•• PPaavveded rrooaaddss mmaayy ffaacciililittaattee tthhee sspprreadead ooff SSahaaharraa mmuussttaarrdd bbyy pprroommoottiingng ppllaanntt bbiioommaassss aandnd sseedeed pprrooduduccttiionon iinn iinntteersrspapaccees.s. •• IInntteersrspapacceess ccoommpprriissee aapppprrooxxiimmaatteellyy 8585--9900%% ooff mmoosstt crcreoeossoottebuebushsh scrscrubub llandandssccapeapess wwhhiicchh mmaayy oottheherrwwiissee bebe mmuucchh lleessss iinnvvaassiibblele.. SaSahhaarraa MMuusstatarrdd IInvnvaassiionon ooff DiDirrtt RRooaaddssiiddeses

To evaluate the pattern of spread along dirt roads I measured plant density and biomass of Sahara mustard at various distances from roads with and without berms in areas of silty/rocky compared to areas of sandy soils.

silty/rocky soil sandy soil Density and biomass of Sahara mustard along dirt roads with berms (dark grey bars) and without berms (light grey bars)

silty/rocky soil sandy soil 20 20

15 15 ) 2 m / # (

y 10 10 t i s n e D

5 5

800 800 0 5 10 20 0 5 10 20

60 60 ) 2 m / g (

s 40 40 s a m o i B

20 20

0 0 0 5 10 20 0 5 10 20 Distance from Roadside (m) Distance from Roadside (m) distance from berm(m) 20 10 5 0 Sandy Soil

Silty/Rocky Soil MMaajjoorr PPooiinntt #5#5

•• BBeermsrms aalloonngg ddiirrtt rroadoadss appappeeaarr ttoo hahavvee ttheheiirr ggrreaeatteesstt eeffffeeccttss onon ssililtyty//rroocckkyy ssooililss wwhheerree SSaahaharraa mmuussttaarrdd mmaayy oottheherrwwiissee hahavvee ddiiffffiiccuullttyy eessttabablilisshhiing.ng. •• TThehe deepedeeperr lloooosseerr ssooililss ttyyppiiccaall ooff rroadoadssiidede beberrmsms crcreaeattee llooccaalliizzeded ssooililss ccondondiittiiononss tthahatt SSahaaharraa mmuussttaarrdd pprreeffeers.rs. BBoorrrrooww PitPitss UUsseedd ffoorr RRooaadbdbeedd MMateaterrialialss CCaann bbee HaHavveennss ffoorr SaSahhaarraa MMuusstatardrd • These areas are often in close proximity to major paved roads which Sahara mustard can use as pathways to spread into new areas. • The roadbed materials collected from these sites inevitably contains Sahara mustard seeds which innoculate new sites where the materials are used. OtOthheerr DiDissttuurrbbeedd AArreaeass CCaann AlAlssoo BBee DDoommiinnateatedd bbyy SaSahhaarraa MMuusstatardrd

OHV Open Areas

Postfire Landscapes

Preferentially establishing in motorcycle tracks EEphpheemmeerralal WWaasshheess FacilitaFacilitattee SSpprreeaadd AwAwaayy ffrroomm RRooaaddssiiddeess aanndd OtOthheerr DiDissttuurrbbeedd AArreaseas

• Washes have deeper and sandier soil than the surrounding landscape, conditions that Sahara mustard prefers. MMaajjoorr PPooiinntt #6#6

•• MMononiittoorriingng ttoo dedetteecctt nneeww popupopullaattiioonnss ooff SSahaaharraa mmuuststaarrdd sshhoouulldd ffooccuuss onon rroaoaddssiiddeess,, wwaasshheess,, andand oottheherr ddiissttuurrbbeedd aarreeaas.s. RReceecenntt SSaahhaarraa mmuusstatarrdd DDoommiinnaannccee onon SShhallalloow/w/RRoocckkyy SSooililss onon HillHillssllopopeses

• During spring 2005 Sahara mustard was found in significant stands on mid-slopes and mountaintops where it had not been previously observed.

River Mountains, NV Calico Mountains, CA • These newly discovered populations occured in areas that could not be any more different than the deep sandy alluvial soils that Sahara mustard most frequently dominates. • Extremely high rainfall during winter 2004-2005 clearly led to the high biomass of these stands which must have established and were previously overlooked in previous years when rainfall was much lower. • Many of the hillslope patches are discontinuous with patches on alluvial soils in the valleys below. • It is currently unknown how these disjunct patches became established. • Extensive flocks of granivorous birds were observed foraging in these stands, and it may be that birds were the primary dispersal vectors. • It is unlikely that rodents were the dispersal vector. • Wind is another plausible dispersal vector, as plants often break off at the base and tumble across the landscape, disarticulating and spreading seeds in the process. MMaajjoorr PPooiinntt #7#7

•• SSuuiittababllee habhabiittaatt ffoorr SSahaaharraa mmuussttaarrdd mmaayy bebe mmoorree wwiiddeesspprreadead tthanhan pprreevviioouussllyy tthoughhought.t. TaTakkee HHoommee MMeessssaagesges •• SSahaaharraa mmuussttaarrdd hahass bebeeenn iinn ssououtthhwweesstteerrnn NNoorrtthh AmAmeerriiccaa ffoorr abouaboutt 8080 yyeaears.rs. •• ItIt hahass rreeccenenttllyy iinncrcreaeasseedd iittss rrangeange aanndd dodommiinnananccee ssiinnccee tthehe 19801980s.s. •• ItIt wwillill vveerryy lliikkeellyy eexxppandand iittss rraangenge aanndd dodommiinnananccee wwiitthhiinn tthehe rreeggiiononss iitt ccuurrrrenenttllyy ooccccupupiiees.s. •• ItIt iiss ununkknnoowwnn hhooww ffaarr iitt wwiillll sspprreadead ououttssiidede ooff tthehe hohott dedesseerrtt rregegiionons.s.