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Teosinte in Europe – Searching for the Origin of a Novel Weed

A novel weed has recently emerged, causing serious agronomic damage in one of the most important maize-growing regions of Western Europe, the Northern Provinces of Spain. The weed has morphological similarities to a wild relative of maize and has generally been referred to as teosinte. However, the identity, origin or genetic composition of ‘Spanish teosinte’ was unknown. Here, we present a genome-wide analysis of single-nucleotide polymorphism (SNP) data for Spanish teosinte, sympatric populations of cultivated maize and samples of reference teosinte taxa. Our data are complemented with previously published SNP datasets of cultivated maize and two Mexican teosinte subspecies. Our analyses reveal that Spanish teosinte does not group with any of the currently recognized teosinte taxa. Based on Bayesian clustering analysis and hybridization simulations, we infer that Spanish teosinte is of admixed origin, most likely involving Zea mays ssp. mexicana as one parental taxon, and an unidentified cultivated maize variety as the other. Analyses of plants grown from seeds collected in Spanish maize fields and experimental crosses under controlled conditions reveal that hybridization does occur between Spanish teosinte and cultivated maize in Spain, and that current hybridization is asymmetric, favouring the introgression of Spanish teosinte into cultivated maize, rather than vice versa.

Introduction

Increasing human transfer of plants across geographical regions leads to frequent introductions of non-native plant species around the world. If these introduced taxa meet close relatives, hybridization and introgression can occur 1 . The inadvertent introduction of species closely related to major crop plants may lead to hybridization and the formation of weedy lineages 2 . Alternatively, weedy taxa can also evolve directly from a domesticated ancestor 3 , as in the case of some weedy rice populations 4, 5 . Weedy lineages can incur massive yield losses and cause major costs 6 .

Maize is the third most important crop plant in Spain with production reaching almost 4.7 million tonnes 7 . In 2009, farmers in Northern Spain (Aragon) began to observe plants in their maize fields that resembled cultivated maize before the onset of flowering but then developed highly branching phenotypes with small cobs and shattering seeds 8 . These traits are typical for teosinte, wild relatives of cultivated maize 9 . Until 2014, this so-called ‘Spanish teosinte’ has spread in Aragon and has also been reported from a neighbouring region in Catalonia 10, 11 . About 750 ha of maize cultivation have been affected so far, mostly in Aragon 11 . Due to maize monocropping, density of Spanish teosinte can become high on affected fields and may cause severe maize yield losses and high economic costs 12 . In some regions of Aragon, this weed has become the prime agronomic problem for maize farmers 10 .

Maize (Zea mays mays) was domesticated from its wild relative teosinte about 9,000 years ago in southern Mexico 13 . Domesticated maize, including high-yielding hybrid maize varieties grown in Europe, is strictly monopodic, with non-shattering kernels that remain tightly attached to the cob, in contrast to the shattering kernels of the cob-less, and highly branched, teosinte. Although maize cobs, or parts of them, can remain in the field and do germinate and grow feral, it is considered unlikely that they successfully establish feral populations beyond arable fields and without human support 9 . Hence, cultivated maize is generally considered to have little to no risk of causing concerns as a volunteer or feral weed 14 .

Teosinte is the common name of a group of wild grasses (Poaceae) and includes highly variable species and subspecies that occur in scattered populations in many areas across Mexico and Central America (Mesoamerica) 15 . In many areas of Mesoamerica, teosinte populations have come under serious threat with the expansion of ranching and farming and are facing a massive decline in abundance to the point of extinction of some species 16 , which forced the Mexican government to install conservation measures for their protection 17 . Thus, in their centre of origin, many teosinte populations are endangered and require protection measures, although occasionally they can also act as local weeds 18 .

Teosinte and maize belong to the same genus, Zea, which consists of five species: 1) perennial diploid (2n = 20) Z. diploperennis, 2) perennial tetraploid (2n = 40) Z. perennis, 3) annual diploid (2n = 20) Z. luxurians, 4) annual diploid (2n = 20) Z. nicaraguensis, and 5) the annual species Zea mays. The latter encompasses four annual diploid (2n = 20) subspecies: (i) ssp. mays, the domesticated maize, (ii) ssp. mexicana, (iii) ssp. parviglumis, and (iv) ssp. huehuetenangensis 19 . Z.m. ssp. mexicana and Z.m. ssp. parviglumis are most closely related to domesticated maize, the latter subspecies being called ‘Balsas’ teosinte and considered the ancestor of cultivated maize 9 . All teosintes are believed to be endemic to Mesoamerica 20 where cultivated maize and teosintes often grow in geographic proximity and flower synchronously. Overall, Z.m. ssp. mexicana grows in cooler, drier central highlands, mostly above 1800 m, while Z.m. ssp. parviglumis grows in warmer, wetter lower elevations in the river valleys of southern and western Mexico, mostly below 1800 m 15 .

Although it is known that all teosintes can hybridize with maize, this typically occurs at low rates even when teosinte is abundant 21 . Hybridization appears to be most common between domesticated maize and Z.m. ssp. parviglumis 15, 22 but gene flow does not occur reciprocally, which may explain why teosintes continue to coexist even when growing in close vicinity to much larger maize populations 21 . When teosinte pollen is applied to maize silks, resulting hybrids are vigorous and highly fertile 23 . However, when teosinte is pollinated by maize pollen, plants of Z.m. ssp. mexicana set seed very inconsistently or not at all 22 . Similarly, Hufford et al. found evidence of adaptive introgression of Z.m. ssp. mexicana alleles into maize during its expansion to the highlands of central Mexico, but observed very little evidence for adaptive introgression in the other direction, from cultivated maize into Z.m. ssp. mexicana 24 .

This asymmetrical pollination success is under the control of a gene called the ‘Teosinte crossing barrier’ (Tcb) 23 which may reproductively isolate at least some teosinte species from maize. Aylor et al. postulated that gene flow and subsequent introgression of maize alleles into teosinte populations most likely occur when teosinte first pollinates maize 20 . The resultant hybrids would then backcross with teosinte which could lead to the introgression of maize alleles into the teosinte background. They speculated that the pollination from teosinte to maize most likely represents the rate-limiting step in the introgression of maize alleles into teosinte 20 . However, this has not been investigated yet to any conclusive extent, even though teosinte and maize have intensively been studied from a population genetics perspective, including studies utilizing DNA samples recovered from archaeological specimens 13, 24,25,26,27 .

So far, the Spanish authorities have speculated that the introduced teosinte is Z.m. ssp. mexicana 11 . Spanish teosinte can produce long-lasting seed banks, and its control either by mechanical means, crop rotation or herbicide treatment has proven difficult 10, 11 . Knowing the origin of the novel weed in Spain may help to prevent introductions of further seed material, to monitor the spread of this weed, and to take targeted control measures in the future. The taxonomic identity of the Spanish teosinte, its introduction history, as well as its ecology and potential risks for local and neighbouring European farming systems remain largely unknown. To develop effective measures for monitoring, control and prevention, data on these aspects are fundamental. Here, we addressed the following research questions: (i) to what taxon can teosinte-like plants collected in Spain be assigned?, (ii) what is the potential origin of these Spanish teosinte lineages?, (iii) is there evidence for on-going hybridization between Spanish teosinte and commercially cultivated maize in Spain? To answer these questions, we collected Spanish teosinte and hybrid-like seeds (autumn 2014 and 2015) and leaf samples of Spanish teosinte and cultivated maize (summer 2015) in the region of Aragon, Spain, and genotyped these together with teosinte reference plants using the MaizeSNP50 BeadChip, a widely used resource for high-density genotyping of maize and its wild relatives 28,29,30 .

Results

Using PCA, the SNP data allowed a clear separation between commercial maize varieties and all teosinte taxa included in this study (Fig. 1). Samples from Spanish maize varieties collected in the field or grown from seeds grouped with other commercial maize varieties previously analysed by Olukolu et al. 29 . Similarly, reference samples of Z.m. ssp. mexicana and ssp. parviglumis obtained from USDA grouped with samples of these species previously investigated by Pyhäjärvi et al. 28 (Fig. 2). All other teosinte species obtained from CIMMYT and USDA formed separate groups (Figs 1 and 2). Teosinte from Brazil grouped with reference samples of Z. luxurians, confirming the results of Silva et al. 31 .

PCA of maize, teosinte and hybrid samples collected in Spain. Samples were collected from plants growing in the field (black labels), from seeds collected in the field and subsequently grown in a climate chamber (grey labels) or from reference material (grey labels). Labelled symbols represent own data that were compared to other publically available data (without labels) (27′476 SNPs and 662 individuals).

PCA of teosinte samples grown from the seeds obtained as a reference material. Labelled symbols represent own data that were compared to other publically available data (without labels) (27′476 SNPs and 280 individuals).

Samples collected in maize fields in Spain that did not represent commercial maize varieties formed two groups. One group, containing all individuals initially scored as Spanish teosinte plants, was clearly separated from commercial maize varieties but also from all currently recognized Mexican and Nicaraguan teosintes (Fig. 1). The second group of plants from Spain were intermediate between Spanish teosinte plants and commercial maize varieties (Fig. 1). Plants in this group were grown from hybrid-like seeds collected in the field that shared phenotypic features of maize and Spanish teosinte (represented by light green circles in Fig. 1). In addition, this group also encompassed our own experimental F1 hybrids between Spanish commercial maize and Spanish teosinte plants in which the teosinte served as pollen donors.

STRUCTURE analysis of samples from Spain, together with Z.m. ssp. mexicana and ssp. parviglumis, also clearly separated Spanish teosinte from the two subspecies of Z. mays and further revealed that Spanish teosinte shares some alleles with Z.m. ssp. mexicana. Further, individuals grown from the seeds that were collected in the field in Spain and that were intermediate between Spanish teosinte plants and Spanish commercial maize (represented by light green circles in Fig. 1) were found to be early generation hybrids between maize cultivated in Spain and Spanish teosinte (Fig. 3).

STRUCTURE analysis of Spanish maize varieties, hybrids, Spanish teosinte, Z.m. ssp. mexicana and ssp. parviglumis. Bar plot of assignment proportions at K = 6 combining own SNP data and other publically available data (27′476 SNPs and 360 individuals).

Because Spanish teosinte plants did not group with any of the known Mexican and Nicaraguan teosinte taxa, we evaluated a possible admixed origin by simulating hybridization between Spanish commercial maize genotypes and different teosinte species. Results suggest that commercial maize varieties currently planted in Spain were not involved in the formation of Spanish teosinte (Fig. 4), which is also consistent with the outcome of the STRUCTURE analysis (Fig. 3).

PCA of simulated hybridization between Spanish maize varieties and Z.m. ssp. mexicana. Coloured symbols represent empirical data (27′476 SNPs and 360 individuals) and black symbols represent simulated data.

Discussion

The novel weed found in maize fields in Spain has been tentatively identified as a wild relative of maize, teosinte, but no systematic effort to establish its identity or ancestry has been undertaken until now. Knowing the identity of this weed is not only useful for monitoring and control, but fundamental to trace its origin and to assess its potential for interbreeding and future evolution towards a possibly even more damaging weed. We, therefore, genotyped the Spanish teosinte plants together with commercially grown maize and all currently recognized teosinte taxa from Mexico and Nicaragua. Surprisingly, the Spanish teosinte was not only clearly separated from cultivated maize but also from all teosinte species tested (Fig. 1). These results are not compatible with the published suggestion that teosinte plants observed in Spain represent Z.m. ssp. mexicana 11 .

In one attempt at testing the hypothesis that there could be some relation between Spanish teosinte and expatriated populations of teosintes sensu stricto, we included material from a Brazilian population of teosinte that was introduced there as a forage species. Recently, it was reported that this teosinte from southern Brazil belonged in the species Z. luxurians 31 , based on morphological traits and the location of chromosomal knobs. However, no genetic analysis had been performed to date. Our results confirmed that this Brazilian teosinte is Z. luxurians, as suggested by the study of Silva et al. 31 , but it did not associate with Spanish teosinte.

The fact that Spanish teosinte did not group with any currently recognized teosinte taxa from Mexico and Nicaragua, or with known populations of expatriated teosintes, led us to hypothesize that it might be of admixed origin. Indeed, our STRUCTURE analysis revealed that Spanish teosinte shares alleles with Z.m. ssp. mexicana. Very likely, Spanish teosinte could originate from hybridization and subsequent backcrossing between cultivated maize and Z.m. ssp. mexicana. Such a process would seem necessary to explain our data, given the current state of knowledge on the asymmetric crossing behaviour between maize and teosintes in general 20,21,22,23 . Simulated hybridization suggests that the hybridization leading to Spanish teosinte likely did not occur in very recent years. Whether such a weed had originally formed in Europe or elsewhere is open to further research.

The key hybridization event(s) leading to the emergence of Spanish teosinte could have taken place inadvertently in any of the Mesoamerican regions where native Z.m. ssp. mexicana and domesticated maize coexist, or indeed through human intervention elsewhere, including Europe, as Z.m. ssp. mexicana has been used in various breeding programs in order to improve agronomic traits of cultivated maize 32 or to evaluate the suitability of the hybrids as a forage grass 33, 34 . Against such a single-introduction hypothesis, the origin of Spanish teosinte could also be complex, resembling the case of weedy rice in the USA, where some populations seem to have arisen via hybridization between cultivated rice and weedy rice 35 , whereas other seem to have evolved directly from cultivated rice 5, 36 .

The risk of further hybridization events between commercial maize and Spanish teosinte is worth considering in light of the possibility of further development of even more invasive weed than the population currently found in Northern Spain. The plants grown from hybrid-like seeds, which we collected in the field in Spain, proved indeed to be intermediate between cultivated maize and teosinte, having elongated lateral branches and producing bunches of ears, often combined with tassels. These samples grouped together (light green circles in Fig. 1) with our experimental F1 hybrids derived from crosses in which Spanish commercial maize plants acted as pollen recipients and Spanish teosinte as pollen donors. Thus, we suggest that further hybridization is not only possible, but is already happening in the field, albeit so far we have only encountered teosinte-to-maize hybrid seed, and not maize-to-teosinte seed.

Surprisingly, we have not detected any hybrids among the plants that we sampled directly in the field. A plausible explanation is that when maize is pollinated by teosinte, the seeds do not easily shatter, with many of them being harvested together with the commercially grown maize. Pollination occurring in the other direction, from cultivated maize to teosinte, is known to be significantly less viable, due to the control of Teosinte crossing barrier (Tcb) 23 . Lu et al. observed that in silk carrying the Tcb, pollen tubes had clustered callose plugs and their growth was slower in comparison to pollen tubes of compatible crosses 37 . Such crossing incompatibility may also exist between commercial maize and Spanish teosinte. From our exploratory crossing experiments, we can confirm that hand-pollination of Spanish teosinte with maize pollen results only rarely in viable seeds (unpublished results). Thus, while the crossing barrier may reduce the frequency with which such hybrids are formed in the field, it is important to note that it does not preclude their formation, even though at low rates. Therefore, we expect that more intensive sampling, over a longer period of time, may likely reveal the existence of maize-to-teosinte introgressants, especially if on-going hybridization between teosinte and maize leads to the formation of hybrids that are more compatible with a teosinte mother plant, as postulated by Aylor et al. 20 .

Further studies are needed to fully understand the evolutionary origin and demographic processes involved in the formation of Spanish teosinte. In particular, much more intensive sampling, both spatially and temporally, could reveal the amount of genetic and phenotypic diversity, as well as the extent to which the population of Spanish teosinte is changing, including through hybridization with cultivated maize. However, we now have a basis of understanding that enables necessary practical measures to confront the challenge of this serious, possibly invasive, weed. With this understanding, it should be possible to establish monitoring and mapping efforts to track, and hopefully contain, the spread of the weed. Whether this weed remains a serious problem only in the Spanish region where it is now confined, or whether it expands to other maize-growing areas in Europe and beyond may be determined by the degree with which these questions are pursued.

Methods

Plant material

Teosinte and hybrid-like seeds were collected from two different sites in the region of Zaragoza, Spain in autumn 2014 and 2015. In summer 2015 we surveyed the maize producing region around Zaragoza and collected leaf samples from three fallow and five standing maize fields. We sampled 3–20 plants per field (Supplementary Table S1). The leaf samples were dried and stored on silica gel for further molecular analysis.

Seeds collected in the field were germinated and grown under controlled conditions (20–25 °C, 50–65% rh, 16/8 h L/D) in climate chambers at ETH Zurich. Once matured, leaf samples were collected from 34 plants (Supplementary Table S1), dried and stored on silica gel for further molecular analysis.

Reference material of the different teosinte taxa was obtained from CIMMYT and USDA (Supplementary Table S1). Seeds of teosinte grown in Brazil were obtained from local markets in the state of Santa Catarina, Brazil. All teosinte seeds were shelled and germinated on filter paper under controlled conditions in the climate chamber (20–25 °C, 50–65% rh, 16/8 h L/D). Also, the seeds of the following maize varieties grown in Spain were germinated on filter paper: LG30490YG, ES TORQUAZ, ROJO, PR33D48, DKC66-66. The first leaves were sampled, lyophilized and stored in silica gel for further molecular analysis.

Experimental crosses were performed with six Spanish insect resistant Bt maize (LG30490YG) and 14 teosinte plants grown from the seeds collected in Spain under controlled conditions in a climate chamber (20–25 °C, 50–65% rh, 12/12 h L/D). The adventitious roots were sampled, lyophilized and stored on silica gel for further molecular analysis. Three seeds from one cob from a maize plant (mother) pollinated by teosinte were germinated and sampled in the same way as the reference material.

SNP genotyping

DNA was extracted from dried leaf and root material with CTAB buffer, following a slightly modified protocol described in Doyle and Doyle 38 , and quantified using NanoDrop (Thermo Science). For the experimental crosses, DNA for the mother plant (Bt maize) was pooled from 6 different plant samples using the same concentration levels, similarly, for the father plant (teosinte) DNA from 14 different plant samples was pooled together. Genotyping was conducted at the UC Davis Genome Center using the MaizeSNP50 BeadChip and Infinium HD Assay (Illumina, San Diego, CA, USA).

Data analysis

SNPs were called using GenomeStudio V2009.1 (Illumina). Gen Train score had to be larger than 0.7 to retain a SNP leading to 41,784 SNPs. SNP data were deposited in the Dryad repository (http://dx.doi.org/10.5061/dryad.60210). To the dataset we added published and publically available data on Z.m. ssp. mexicana and ssp. parviglumis individuals 28 and commercial maize varieties 29 (Supplementary Table S1).

Genotype data was then imported into R 39 and individuals and loci with more than 12% and 5% missing data, respectively, were excluded from the analysis, leading to 27′476 SNPs and 662 individuals. From that dataset we used subsets of data for the different analyses. The packages adegenet 40 and FactoMineR 41 were used to conduct principal component analysis (PCA). Our samples from Spain were assigned into three different categories: Spanish maize, Spanish teosinte, and hybrid, based on the morphological traits shown in Fig. 5.

Basic morphological differences between Spanish commercial maize, hybrid and Spanish teosinte plants. Plants were grown in a climate chamber from seeds collected in the field in Spain.

STRUCTURE analysis 42 was conducted with different K’s and 10 replications using the admixture model with correlated allele frequencies. The analysis was performed separately for all Spanish maize varieties, hybrid, Spanish teosinte genotypes, Z.m. ssp. mexicana and ssp. parviglumis (Fig. 3) and for all commercial maize varieties, Spanish teosinte, Z.m. ssp. mexicana, ssp. parviglumis, Z. luxurians, Z. diploperennis, Z. perennis and Z. nicaraguensis (Supplementary Fig. S1). The best K was then selected using STRUCTURE harvester 43 .

To simulate the putative hybrids between the Spanish maize and Z.m. ssp. mexicana we use the hybrid function of adegenet 40 with 10 individuals each.

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Acknowledgements

We thank Stiftung GEKKO, Stiftung Rübel, Zukunftsstiftung Landwirtschaft and Software AG Stiftung for financial support.

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Affiliations

ETH Zurich, Institute of Integrative Biology (IBZ), Universitätstrasse 16, 8092, Zurich, Switzerland

Miluse Trtikova, Andre Lohn, Bernadette Oehen, Alex Widmer & Angelika Hilbeck

Agroecology and Food Systems Chair, Universitat de Vic – Universitat Central de Catalunya, c/de la Laura 13, 08500, Vic, Spain

University of California Berkeley, Department of Environmental Science, Policy and Management, 108 Hilgard Hall, 94720, Berkeley, USA

ETH Zurich, Genetic Diversity Centre (GDC), Universitätstrasse 16, 8092, Zurich, Switzerland

  1. Miluse Trtikova

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Contributions

A.H., A.W., I.C. and M.T. conceived and designed the study. A.H., R.B., B.O. and M.T. conducted the field work. A.L. and M.T. performed the experiments and the laboratory work. N.Z. analysed the data and produced the figures. M.T., A.W. and A.H. wrote the main manuscript text. All authors reviewed the manuscript.

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Trtikova, M., Lohn, A., Binimelis, R. et al. Teosinte in Europe – Searching for the Origin of a Novel Weed. Sci Rep 7, 1560 (2017). https://doi.org/10.1038/s41598-017-01478-w

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Seed weed in spanish

Table of Contents

INTRODUCTORY

AUTHORSHIP AND CITATION:
Zouhar, Kris. 2005. Spartium junceum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.fed.us/database/feis/plants/shrub/spajun/all.html [].

TAXONOMY:
The currently accepted name for Spanish broom is Spartium junceum L. (Fabaceae) [27,30,50].

OTHER STATUS:
Spanish broom is classified as a noxious weed in Hawaii and Oregon, and as a "Category A" nonnative species in Washington [54]. See the Invaders database for more information.

DISTRIBUTION AND OCCURRENCE

Spanish broom was introduced into the California ornamental trade in 1848 in San Francisco. Beginning in the late 1930s, it was planted along mountain highways in southern California. By 1949, Spanish broom had escaped cultivation and established populations in Marin County ([38], and references therein). It now occurs in the north coast counties of California, the San Francisco Bay region, the Sacramento Valley, through the south coast counties into northern Baja California [27,37], in the western Transverse Ranges, and the Channel Islands [38]. It also occurs on dry slopes in the eastern half of the Santa Monica Mountains [15]. Of the invasive brooms in California, Spanish broom is less widespread and is considered less of a problem than Scotch broom (Cytisus scoparius) and French broom (Genista monspessulana) [28]. There is no information in the literature on distribution of Spanish broom in Oregon, Washington, or Hawaii. Plants database provides a state distribution map of Spanish broom.

Spanish broom is 1 of 4 nonnative invasive broom species that occur in North America. Spanish broom, Scotch broom, Portuguese broom (C. striatus), and French broom occur in similar habitats. Common gorse (Ulex europaeus) is another leguminous shrub that occurs in similar habitats.

The following lists include vegetation types in which Spanish broom is known to be or thought to be potentially invasive, based on reported occurrence and biological tolerances to site conditions from studies of Spanish broom in California. There is no information about Spanish broom distribution or site tolerances outside California; therefore, these lists are somewhat speculative and may be imprecise.

ECOSYSTEMS [22]:
FRES17 Elm-ash-cottonwood
FRES20 Douglas-fir
FRES21 Ponderosa pine
FRES27 Redwood
FRES28 Western hardwoods
FRES34 Chaparral-mountain shrub
FRES37 Mountain meadows
FRES41 Wet grasslands
FRES42 Annual grasslands

STATES/PROVINCES: (key to state/province abbreviations)
UNITED STATES

CA HI OR WA

MEXICO

B.C.N.

BLM PHYSIOGRAPHIC REGIONS [4]:
1 Northern Pacific Border
2 Cascade Mountains
3 Southern Pacific Border
4 Sierra Mountains
5 Columbia Plateau
6 Upper Basin and Range
7 Lower Basin and Range

KUCHLER [31] PLANT ASSOCIATIONS:
K002 Cedar-hemlock-Douglas-fir forest
K005 Mixed conifer forest
K006 Redwood forest
K009 Pine-cypress forest
K010 Ponderosa shrub forest
K025 Alder-ash forest
K026 Oregon oakwoods
K028 Mosaic of K002 and K026
K029 California mixed evergreen forest
K030 California oakwoods
K033 Chaparral
K034 Montane chaparral
K035 Coastal sagebrush
K036 Mosaic of K030 and K035
K048 California steppe

SAF COVER TYPES [19]:
217 Aspen
221 Red alder
222 Black cottonwood-willow
229 Pacific Douglas-fir
230 Douglas-fir-western hemlock
232 Redwood
233 Oregon white oak
234 Douglas-fir-tanoak-Pacific madrone
237 Interior ponderosa pine
243 Sierra Nevada mixed conifer
244 Pacific ponderosa pine-Douglas-fir
245 Pacific ponderosa pine
246 California black oak
247 Jeffrey pine
248 Knobcone pine
249 Canyon live oak
250 Blue oak-foothills pine
255 California coast live oak

SRM (RANGELAND) COVER TYPES [45]:
109 Ponderosa pine shrubland
110 Ponderosa pine-grassland
201 Blue oak woodland
202 Coast live oak woodland
203 Riparian woodland
204 North coastal shrub
205 Coastal sage shrub
206 Chamise chaparral
207 Scrub oak mixed chaparral
208 Ceanothus mixed chaparral
209 Montane shrubland
214 Coastal prairie
215 Valley grassland
216 Montane meadows
217 Wetlands
409 Tall forb
411 Aspen woodland

HABITAT TYPES AND PLANT COMMUNITIES:
The following description of habitat types and plant communities in which Spanish broom occurs is taken from the few examples found in the literature on Spanish broom occurrence in California. Spanish broom is probably not restricted to these types within these areas. There is very little information in the literature on vegetation types in which Spanish broom occurs.

Spanish broom seems to be most common in disturbed areas, especially along roadsides [15,35,38], where it was seeded in the early 1900s [38]. In 1958, Hellmers and Ashby [26] stated that Spanish broom has been planted along roads for 20 years, where it survives and grows well, but has not been able to invade the adjoining stands of chaparral. It has since become invasive in chaparral in southern California [10], where it was seeded for revegetation after fires in the early 1900s [3].

According to the California Invasive Plant Council [8], Spanish broom occurs in coastal scrub, grassland, wetlands, and oak (Quercus spp.) woodland throughout California, and forests in the northwestern part of the state. Spanish broom is associated with coyote bush (Baccharis pilularis) in the interior Santa Cruz Mountains, with a large monospecific stand of French broom located downslope [36]. Spanish broom also occurs in redwood (Sequoia sempervirens) forests [8,43].

There is no information in the literature on habitat types or plant communities in which Spanish broom occurs in Oregon, Washington, or Hawaii.

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

GENERAL BOTANICAL CHARACTERISTICS:
Spanish broom is 1 of 4 nonnative invasive broom species that occur in North America. All are perennial, leguminous shrubs. DiTomaso [16] provides a table of characteristics to distinguish among broom species and common gorse. Spanish broom, French broom, Scotch broom and Portuguese broom have some similar morphological characteristics, while common gorse is morphologically distinct from the brooms. Spanish broom is not as widely distributed nor as common as Scotch and French brooms (see Distribution and Occurrence), and less is known about its biology and ecology. According to DiTomaso [16] Spanish broom may have ecological characteristics similar to Scotch broom.

The following description of Spanish broom provides a summary of the range of characteristics described in reviews [16,38] and florae [15,27]. It provides characteristics that may be relevant to fire ecology, and is not meant for identification. A key for identification is available in Hickman [27].

Spanish broom is a tall shrub to small tree, up to 10 to 15 feet (3-5 m) tall. Its long, slender stems are erect with few branches. Stems are cylindrical, rush-like, and green when young, maturing into woody branches with bark. Mature plants have 1 to several trunks. Spanish broom leaves are small, 0.5 to 1 inch (2-2.5 cm) long, oval, and smooth-margined. Leaves are ephemeral, remaining on the plant for 4 months or less. The inflorescence is an open terminal raceme with several flowers located on current-year shoots. Flowers are large, pea-like, up to 1 inch long, and grow on short stalks on both sides of the main stem. Fruit is a linear, dehiscent legume, 2 to 4 inches (5-10 cm) long and 5 mm wide, with 10 to 15 seeds.

At the time of this writing (2005), no descriptions of Spanish broom root structure or morphology are available for plants growing in North America. Chiatante and others [9] describe root morphology of Spanish broom growing in 3 different rooting environments in Italy: terrace, plane, and 40 ° slope. The authors observed that the overall architecture of the root system was modified on a slope by an increase in the length and number of root apices of 1st-, 2nd-, and 3rd-order lateral roots. This suggests that Spanish broom reinforces its anchorage strain by changing the organization of its root system, particularly in the up-slope direction [9].

Although the leaves of both Spanish broom [36] and Scotch broom [5,36] are ephemeral, their canopies have a constant positive carbon balance due to stem photosynthesis. Both species have positive stem assimilation resulting in approximately 200 mmol per m ² per day carbon dioxide assimilation on study sites in California. Although these species grow in different habitats with different vapor pressure and temperature, assimilation response to vapor pressure is similar between species. Water-use efficiency is higher and intercellular carbon dioxide is lower for Spanish broom compared to Scotch broom. The constant carbon gain throughout the year, from stem assimilation, may enhance the growth capacity of both species in disturbed habitats [36].

Botanical traits of Spanish broom vary somewhat between cloned individuals and those grown from seed. In a greenhouse study, several mean growth traits were significantly (P<0.05) different between ramet and genet populations, and the variance in these traits tended to be higher in genet populations. Seedlings had consistently greater whole plant dry mass per shoot length, a higher percentage of total biomass in leaves, and more roots compared with cloned individuals. In contrast, few mean physiological traits differed between ramet and genet populations, and variance was similar between the 2 population types. Environmental variance accounts for a large proportion of the variance in physiological traits, and about 33% of the variance in growth traits [37].

Growth form and stand structure: According to a review by DiTomaso [16], dense broom infestations produce substantial dry matter that can create a serious fire hazard. While this is particularly true for gorse and French broom [16], Nilsen [38] also suggests that mature Spanish broom stands should be considered a fire hazard during the dry season, because patches can be dense and may contain a large amount of dead wood.

REGENERATION PROCESSES:
Most of the information on regeneration in Spanish broom comes from a review by Nilsen [38], and no indication of the source of the information is given in the review.

Spanish broom spreads by producing abundant seeds. No research has been conducted on Spanish broom seed banks, germination, or seedling recruitment [38].

Breeding system: Spanish broom plants are monoecious and outcrossed [37].

Pollination: Spanish broom flowers are pollinated by bees [38].

Seed production: Seed production begins when Spanish broom plants are 2 to 3 years old. Each inflorescence produces 10 to 15 pods containing approximately 15 seeds each. One plant can produce 7,000 to 10,000 seeds in one season [38].

Seed dispersal: Spanish broom seeds fall near the parent plant and are subsequently moved by erosion, rain wash, and possibly ants [38].

Seed banking: According to Nilsen [38], Spanish broom seeds remain viable for at least 5 years, suggesting that a large seed bank may be present in Spanish broom stands. The source of this information is not given, nor is there any additional information in the literature about seed banking in Spanish broom. More research is needed in this area.

Germination: Spanish broom seeds, collected in the Santa Cruz Mountains of California and germinated in the greenhouse, had 100% germination rates [37]. Similarly, Spanish broom seeds from Israel that were used in an experiment in California "germinated readily with no pretreatment" [26]. However, scarification is said to result in "greater" germination rates (Cabral 1954, as cited by [28]). More research is needed on germination and seed bed requirements of Spanish broom.

Seedling establishment/growth: Results from an experiment in California indicate that ambient temperature affected Spanish broom growth form. Spanish broom plants grown for 24 weeks at day/night temperatures of 73/79 °F (23/26 ° C), 86/39 °F (30/4 ° C), and 86/63 °F (30/17 ° C) were weak and did not stand erect; whereas Spanish broom plants grown at cooler temperatures ( 63/39 °F (17/4 ° C), 63/63 °F (17/17 ° C), and 73/39 °F (23/4 ° C)) had shorter and thicker stems that were able to support their own weight. Leaves were retained on Spanish broom plants grown at cooler temperatures. At higher temperatures the leaves dropped soon after they were formed. All Spanish broom plants had green stems, were branched, and had a very bushy appearance, especially at temperatures higher than the 17/4 temperature condition. The roots were nodulated and branched, and permeated the entire medium in a 1-gallon can at the end of the growth period [26].

Asexual regeneration: According to Nilsen [38] Spanish broom is "an effective stem sprouter," suggesting that Spanish broom may sprout from stumps or root crowns following damage or destruction of aboveground biomass.

SITE CHARACTERISTICS:
Spanish broom occurs along the west coast of North America, in scattered populations in upland areas on interior sites of the coastal mountains and in the foothills of the Sierra Nevada [18,36]. It rarely grows in coastal sites in California [36]. Spanish broom invades disturbed riparian sites in the Sierra Nevada [18]. No information is available on Spanish broom site characteristics outside California, other than that it occurs on unstable river islands in coastal Oregon [37].

Spanish broom was planted along roadsides [10,26] and seeded in chaparral sites for revegetation after fires in California in the early 1900s [3]. Populations of Spanish broom have persisted and spread along roads [8,15,35] and in other disturbed areas such as eroding slopes, riverbanks, and abandoned or disturbed lands [27,35,38]. Earlier accounts indicate that Spanish broom is not invasive in native habitats [26], (McClintock 1985, as cited by [47]). It escaped cultivation and invaded chaparral in southern California, particularly after fire [10,38].

According to Conrad [10], Spanish broom was planted along roadsides below 6,900 feet (2,100 m), while Hickman [27] states that it occurs below 2,000 feet (600 m). Spanish broom commonly occurs on steep slopes [38].

The invasive brooms are successful in high irradiance, disturbed habitats, most likely due to their photosynthetic stems, rapid growth, and ability to fix nitrogen. Drought stress severely inhibits photosynthesis of brooms. Photosynthetic stems allow them to utilize a deciduous leaf phenology (to avoid water stress) and still maintain a large photosynthetic surface area in summer and fall after leaves have abscised. On interior mountain sites where Spanish broom occurs, there is a large difference between summer and winter climate compared with coastal sites [36]. Although the leaves have twice the photosynthetic rate of stems [36], photosynthesis in stems provides most to the whole plant carbon gain because of their longer life span and larger surface area (Nilsen and Bao 1990, cited in [38]).

SUCCESSIONAL STATUS:
Research by Williams [59] suggests that broom stands are early successional and can be replaced by later seral vegetation if left undisturbed. No other information is available on this topic. Research is needed to characterize Spanish broom’s invasiveness and impacts in native plant communities of various seral stages.

Shade tolerance: Seedlings of Spanish broom had greatest survival (

97%) in moderate shade (30% full sunlight),

70% survival in 100% full sunlight, and

10% survival in deep shade (3% full sunlight). Rates of net photosynthesis were somewhat (although not significantly) higher in full sun versus moderate shade, and dark respiration was significantly (P<0.005) higher in full sun than in moderate shade. Spanish broom was tentatively classified by the authors as a shade avoider, being neither highly tolerant nor intolerant of shade, although further tests are needed for this to be definitive [55].

SEASONAL DEVELOPMENT:
In California, Spanish broom shoots initiate growth in late winter and early spring, and most rapid growth occurs in May. Shoots elongate quickly and produce leaves with long internodes in March. Leaf longevity is 4 months or less. The shoots harden off in late spring (June), and leaves drop [36,38]. Stem photosynthesis occurs all year [36,38]. At one site, daily carbon gain decreased from spring to fall in Spanish broom due to a decrease in shoot water potential [36].

According to Nilsen [38] Spanish broom flowers in late March to early April in California, while other authors [10,15,35] indicate flowering in Spanish broom occurs from April to June in the Santa Monica Mountains in southern California. Spanish broom flowers remain when flowers of most associated native species have faded and folded [10,15,35]. Spanish broom pods mature from late May through early July, depending on location, after leaf abscission [36,38].

No information is available on seasonal development in other areas of North America where Spanish broom occurs.

Spano and others [46] gathered phenological data, derived threshold temperatures for the computation of degree-days, and evaluated the sensitivity to weather variations of 9 plant species, including Spanish broom, at an experimental garden on the Mediterranean Coast in Italy. Results were as follows [46]:

Phenological stage Mean date Earliest Latest Mean calculated cumulative degree-day values
Bud break 06 April 23 Mar. 1994 22 Apr. 1993 1021*(96)**
Flowering 27 April 13 Apr. 1994 21 May 1992 1296(235)
Full ripe fruit 20 July 10 July 1995 30 July 1991 2963(143)

*Cumulative degree-day values are calculated from 1 January
**Standard deviations calculated using a 0 ° C temperature threshold

Spanish broom phenology showed little sensitivity to weather variations at this Mediterranean site within its native range [46].

FIRE ECOLOGY

Fire regimes: No information is available on fire regimes in plant communities where Spanish broom evolved.

It is unclear how the presence of Spanish broom might affect fire regimes in invaded communities. In general, in ecosystems where it replaces plants similar to itself (in terms of fuel characteristics), it may alter fire intensity or slightly modify an existing fire regime. However, if Spanish broom invasion introduces novel fuel properties to the invaded ecosystem, it has the potential to alter fire behavior and potentially alter the fire regime (sensu [6,13]). Given this perspective, it seems unlikely that Spanish broom will alter fire regimes where it is invasive in California chaparral communities. It is unclear which other plant communities, and to what extent, Spanish broom invades see Habitat types and plant communities). Some examples of potential fire regime changes brought about by Scotch broom and French broom are reviewed in FEIS.

The following list provides fire return intervals for plant communities and ecosystems where Spanish broom is important. It may not be inclusive. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

POSTFIRE REGENERATION STRATEGY [48]:
Tall shrub, adventitious bud/root crown
Ground residual colonizer (on-site, initial community)

FIRE EFFECTS

See FEIS reviews on French broom and Scotch broom for more information on fire effects on these species.

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
No additional information is available on this topic.

PLANT RESPONSE TO FIRE:
As of this writing (2005) no information is available on the response of Spanish broom to fire. A review by Nilsen [38] suggests that Spanish broom is likely to sprout vigorously from trunk bases and stem meristems following low-severity fire. However, a severe fire that kills all aboveground stems and burns hot and close to the ground will completely kill standing individuals and most likely remove some of the seed bank. Seeds of Spanish broom are similar in structure to those of Scotch broom. In heterogeneous or low-temperature fires Scotch broom seed banks are not effectively reduced. Under similar fire conditions it is unlikely that fire will effectively reduce seed bank regeneration of Spanish broom [38].

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
No additional information is available on this topic.

FIRE MANAGEMENT CONSIDERATIONS:
Postfire colonization potential: According to Nilsen [38], Spanish broom is especially invasive in southern California chaparral after fire. No other information on postfire colonization potential of Spanish broom is available.

Preventing postfire establishment and spread: The USDA Forest Service’s "Guide to Noxious Weed Prevention Practices" [52] provides several fire management considerations for weed prevention in general that may apply to Spanish broom.

Preventing invasive plants from establishing in weed-free burned areas is the most effective and least costly control method. This can be accomplished through careful monitoring, early detection and eradication, and limiting invasive plant seed dispersal into burned areas by [23,52]:

  • re-establishing vegetation on bare ground as soon after fire as possible,
  • using only certified weed-free seed mixes when revegetation is necessary,
  • cleaning equipment and vehicles prior to entering burned areas,
  • regulating or preventing human and livestock entry into burned areas until desirable site vegetation has recovered sufficiently to resist invasion by undesirable vegetation,
  • detecting weeds early and eradicating before vegetative spread and/or seed dispersal, and
  • eradicating small patches and containing or controlling large infestations within or adjacent to the burned area.

In general, early detection is critical for preventing establishment of large populations of invasive plants. Monitoring in spring, summer, and fall is imperative. Managers should eradicate established Spanish broom plants and small patches adjacent to burned areas to prevent or limit postfire dispersal and/or spread onto the site [23,52].

The need for revegetation after fire can be based on the degree of desirable vegetation displaced by invasive plants prior to burning, and on postfire survival of desirable vegetation. Revegetation necessity can also be related to invasive plant survival as viable seeds or root crowns [23].

Managers can enhance the success of revegetation (natural or artificial) by excluding livestock until vegetation is well established (at least 2 growing seasons) [23]. See Integrated Noxious Weed Management after Wildfires for more information.

When planning a prescribed burn, managers should preinventory the project area and evaluate cover and phenology of any Spanish broom and other invasive plants present on or adjacent to the site, and avoid ignition and burning in areas at high risk for Spanish broom establishment or spread due to fire effects. Managers should also avoid creating soil conditions that promote weed germination and establishment. Weed status and risks must be discussed in burn rehabilitation plans. Also, wildfire managers might consider including weed prevention education and providing weed identification aids during fire training; avoiding known weed infestations when locating fire lines; monitoring camps, staging areas, helibases, etc., to be sure they are kept weed free; taking care that equipment is weed free; incorporating weed prevention into fire rehabilitation plans; and acquiring restoration funding. Additional guidelines and specific recommendations and requirements are available [52].

Fire as a control agent: While prescribed fire is sometimes used in management of French broom and Scotch broom, no information is available on using fire to control Spanish broom.

Fire hazard potential: According to Nilsen [38], Spanish broom can grow in tall, dense patches and form a tangle containing a large amount of dead wood and, therefore, mature stands should be considered a fire hazard during the dry season. DiTomaso [16] also suggests that dense broom infestations produce substantial dry matter that can create a serious fire hazard.

MANAGEMENT CONSIDERATIONS

IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Spanish broom provides poor forage for native wildlife [38], and presumably poor forage for livestock as well. Domestic goats may eat young Spanish broom plants [28].

Palatability/nutritional value: No information is available on this topic.

Cover value: No information is available on this topic.

OTHER USES:
Spanish broom flowers are used in the ornamental trade and are also used for yellow dye. Stems are used for fibers, which accounts for one of its common names, weaver’s broom [27,38]. Aqueous extracts of Spanish broom have been shown to have antiulcerogenic activity [60].

IMPACTS AND CONTROL:
Impacts: Spanish broom rapidly colonizes disturbed habitats and develops thick shrub communities that prevent colonization by native chaparral species. It may be a fire hazard during the dry season [38]. However, it is listed by the California Invasive Plant Council as a "wildland pest plant of lesser invasiveness" [8].

As a nitrogen-fixing plant, Spanish broom may enrich soil nitrogen levels in invaded communities. Although nitrogen fixation has not been studied in Spanish broom, Scotch broom is capable of fixing nitrogen throughout the year in regions with mild winters [58]. The ability of the brooms to fix nitrogen increases the total amount of nitrogen and the way in which nitrogen cycles in invaded communities [25]. Nitrogen enrichment is unlikely to benefit native plants and may reduce species diversity [14], except in ecosystems dominated by nitrogen-fixers. This may have implications for restoration and rehabilitation efforts [25].

Control: There is little information on controlling Spanish broom. Nilsen [38] presents a summary of possible control approaches based on the biology of the plant, rather than on information derived from controlled experiments [38]. See FEIS reviews on Scotch broom and French broom for information on controlling these similar species.

It is likely that the success of any control method will vary with site characteristics (topography, soils, climate), age and density of plants in the stand, and the availability of human and technical resources. Since a large and persistent seed bank is predicted for this species, it is likely that seedlings will establish rapidly following fire or mechanical removal of aboveground biomass [38].

A comprehensive monitoring of control effectiveness is critical because there is no scientifically based knowledge about control of Spanish broom. Experimental manipulations should be monitored at least annually. Each monitoring visit should determine the number of new plants and the size or age distribution of the recovering populations. Attention should be placed on the proportion of new individuals coming from the seed bank or sprouting from old plants. Monitoring should continue for at least 5 years after control treatment [38].

Prevention: The most effective method for managing invasive species is to prevent their establishment and spread. Some methods of prevention include limiting seed dispersal, containing local infestations, minimizing soil disturbances, detecting and eradicating weed introductions early, and establishing and encouraging desirable competitive plants [44]. One way to help prevent continued introductions of Spanish broom into wildlands is to prevent its sale as a horticultural species.

Integrated management: A particularly effective control combination for Spanish broom may be saw cutting followed by application of herbicide to the cut stem to kill adult plants. Spanish broom seedlings are likely to establish from the soil seed bank so monitoring and follow-up treatments of new seedlings is necessary for several years [38].

Physical/mechanical: In general, physical and mechanical control methods are likely to be effective only when Spanish broom is young [38]. The Nature Conservancy’s Element Stewardship Abstract on Spanish broom provides a general overview of physical and mechanical control methods that may be effective for controlling infestations [28].

Pulling with weed wrenches is effective for small broom infestations or in areas where an inexpensive, long-duration labor source is dedicated to broom removal [51]. Hand-pulling Spanish broom plants may be most practical and effective when the stand is 1 to 4 years old, and plants are small enough, as long as roots are removed and follow-up treatment of seedlings is done. The optimal season for pulling may be July to September when plants are experiencing water stress [36]. When plants have matured to small tree size, they cannot easily be removed with hand tools [38].

Nilsen [38] suggests that machines such as brush hogs are probably impractical for Spanish broom removal, since it commonly occurs on steep slopes, and because the trunks of Spanish broom grow rapidly to a size outside the range of effectiveness for this technology. Saws can be used to cut plants with larger stems; however, Spanish broom has a great facility for sprouting from a saw cut even when the cut is close to the ground. When brush hogs or saws are used to cut Spanish broom stems, sprouting should be expected. Among all the mechanical methods, saw cutting is least likely to be effective in preventing sprouting [38].

Fire: See the Fire Management Considerations section of this summary.

Biological: Biological control of invasive species has a long history, and there are many important considerations before the implementing a biological control program. Tu and others [51] provide general information and considerations for biological control of invasive species in their Weed control methods handbook. Additionally, Cornell University, Texas A & M University, and NAPIS websites offer information on biological control.

As of this writing (2005) there are no USDA approved biological control agents for Spanish broom. In greenhouse situations plants are susceptible to mealy bugs and show evidence of viral depression of growth [38]. An insect purposely introduced for control of Scotch broom, the Scotch broom bruchid (Bruchidius villosus) [11], also attacks Portuguese broom, Spanish broom, and French broom. See Coombs and others [12] for more information on this insect, its distribution, and effects.

Domestic goats are said to be effective at controlling reestablishment of broom [28].

Chemical: Herbicides are effective in gaining initial control of a new invasion (of small size) or a severe infestation, but are rarely a complete or long-term solution to invasive species management, as they do not change conditions that allow infestations to occur [7]. Herbicides are more effective on large infestations when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations. See the Weed control methods handbook [51] for considerations on the use of herbicides in natural areas and detailed information on specific chemicals and adjuvants.

Spanish broom is sensitive to applied pesticides. In greenhouse situations only mild pesticides can be used without detrimentally affecting the plants. Therefore, it is highly likely that application of chemicals such as glyphosate or triclopyr will drastically reduce population size. The ramifications of applying herbicides to a plant community must be carefully considered, because effects on nontarget species are likely, especially when foliage spray methods are used [38]. Rusmore and Butler [42] compared the efficacy of basal bark applications of varying rates of triclopyr on different size Spanish broom shrubs, at 3 phenological stages, under different moisture and shade conditions on a California riparian site. Small differences were observed among treatments, although results were not statistically significant. The kill rate averaged over 90% across all treatments [42].

See The Nature Conservancy’s Element Stewardship Abstract on Spanish broom for a more detailed review of chemical control [28].

Cultural: Research by Williams [59] suggests that broom stands are early successional and can be replaced by later seral vegetation if left undisturbed; however, tests of this assumption are not reported in the literature. A review by Hoshovsky [28] suggests that planting of tall growing shrubs or trees in or near broom stands may aid in reducing photosynthesis in broom plants and possibly lead to their demise.