Dalea purpurea



INTRODUCTORY

SPECIES: Dalea purpurea
 
 

Thomas G. Barnes @ USDA-NRCS PLANTS Database

           Larry Allain @ USDA-NRCS PLANTS Database  



AUTHORSHIP AND CITATION:
League, Kevin R. 2004. Dalea purpurea. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].

FEIS ABBREVIATION:
DALPUR

SYNONYMS:
Petalostemum purpureum (Vent.) Rydb. [6,42,53]

NRCS PLANT CODE [106]:
DAPU5
DAPUA
DAPUP

COMMON NAMES:
purple prairie clover
violet prairie clover

TAXONOMY:
The currently accepted scientific name of purple prairie clover is Dalea purpurea Vent. (Fabaceae). Recognized varieties are as follows [63,112]:

D. purpurea var. arenicola (Wemple) Barneby
D. purpurea var. purpurea

LIFE FORM:
Forb

FEDERAL LEGAL STATUS:
No special status

OTHER STATUS:
Purple prairie clover is globally ranked as G5: demonstrably secure [74]. State rankings are as follows [106]:

Location Rank
Kentucky Special Concern
Michigan Probably Extirpated
Ohio Presumed Extirpated
Tennessee Endangered

DISTRIBUTION AND OCCURRENCE

SPECIES: Dalea purpurea
GENERAL DISTRIBUTION:
Purple prairie clover is indigenous through a large portion of North America. It is distributed  from eastern British Columbia eastward through Manitoba and south to western Alabama and west to Arizona. Frequent to infrequent populations exist in its extreme western extent in eastern Colorado, Wyoming, and Montana and in its extreme eastern extent in Ontario southward to New York and Tennessee [48,63,100,112]. There is considerable overlap of distributions between Dalea purpurea var. purpurea and D. p. var. arenicola, although D. p. var. arenicola's distribution is more restricted to the western Great Plains [49]. Plants database provides a distributional map of purple prairie clover.

ECOSYSTEMS [44]:
FRES10 White-red-jack pine
FRES14 Oak-pine
FRES15 Oak-hickory
FRES17 Elm-ash-cottonwood
FRES19 Aspen-birch
FRES21 Ponderosa pine
FRES29 Sagebrush
FRES31 Shinnery
FRES32 Texas savanna
FRES33 Southwestern shrubsteppe
FRES36 Mountain grasslands
FRES38 Plains grasslands
FRES39 Prairie

STATES/PROVINCES: (key to state/province abbreviations)
UNITED STATES
AL AZ AR CO IL IN
IA KS KY LA MI MN
MS MO MT NE NM NY
ND OH OK SD TN TX
WI WY

CANADA
AB BC MB ON SK

BLM PHYSIOGRAPHIC REGIONS [11]:
8 Northern Rocky Mountains
9 Middle Rocky Mountains
10 Wyoming Basin
11 Southern Rocky Mountains
13 Rocky Mountain Piedmont
14 Great Plains
15 Black Hills Uplift
16 Upper Missouri Basin and Broken Lands

KUCHLER [68] PLANT ASSOCIATIONS:
K016 Eastern ponderosa forest
K017 Black Hills pine forest
K018 Pine-Douglas-fir forest
K019 Arizona pine forest
K023 Juniper-pinyon woodland
K053 Grama-galleta steppe
K056 Wheatgrass-needlegrass shrubsteppe
K058 Grama-tobosa shrubsteppe
K059 Trans-Pecos shrub savanna
K060 Mesquite savanna
K061 Mesquite-acacia savanna
K062 Mesquite-live oak savanna
K063 Foothills prairie
K064 Grama-needlegrass-wheatgrass
K065 Grama-buffalo grass
K066 Wheatgrass-needlegrass
K067 Wheatgrass-bluestem-needlegrass
K068 Wheatgrass-grama-buffalo grass
K069 Bluestem-grama prairie
K070 Sandsage-bluestem prairie
K071 Shinnery
K073 Northern cordgrass prairie
K074 Bluestem prairie
K075 Nebraska Sandhills prairie
K076 Blackland prairie
K081 Oak savanna
K082 Mosaic of K074 and K100
K083 Cedar glades
K084 Cross Timbers
K085 Mesquite-buffalo grass
K086 Juniper-oak savanna
K087 Mesquite-oak savanna
K088 Fayette prairie
K089 Black Belt
K095 Great Lakes pine forest
K098 Northern floodplain forest
K099 Maple-basswood forest
K100 Oak-hickory forest
K101 Elm-ash forest
K102 Beech-maple forest
K111 Oak-hickory-pine

SAF COVER TYPES [41]:
14 Northern pin oak
15 Red pine
16 Aspen
20 White pine-northern red oak-red maple
25 Sugar maple-beech-yellow birch
26 Sugar maple-basswood
27 Sugar maple
28 Black cherry-maple
39 Black ash-American elm-red maple
40 Post oak-blackjack oak
42 Bur oak
46 Eastern redcedar
51 White pine-chestnut oak
52 White oak-black oak-northern red oak
53 White oak
55 Northern red oak
57 Yellow-poplar
58 Yellow-poplar-eastern hemlock
59 Yellow-poplar-white oak-northern red oak
61 River birch-sycamore
62 Silver maple-American elm
63 Cottonwood
64 Sassafras-persimmon
65 Pin oak-sweetgum
66 Ashe juniper-redberry (Pinchot) juniper
67 Mohrs (shin) oak
68 Mesquite
73 Southern redcedar
87 Sweetgum-yellow-poplar
88 Willow oak-water oak-diamondleaf (laurel) oak
89 Live oak
91 Swamp chestnut oak-cherrybark oak
92 Sweetgum-willow oak
93 Sugarberry-American elm-green ash
94 Sycamore-sweetgum-American elm
108 Red maple
109 Hawthorn
110 Black oak
235 Cottonwood-willow
236 Bur oak
237 Interior ponderosa pine
241 Western live oak
242 Mesquite

SRM (RANGELAND) COVER TYPES [94]:
301 Bluebunch wheatgrass-blue grama
303 Bluebunch wheatgrass-western wheatgrass
304 Idaho fescue-bluebunch wheatgrass
305 Idaho fescue-Richardson needlegrass
309 Idaho fescue-western wheatgrass
310 Needle-and-thread-blue grama
311 Rough fescue-bluebunch wheatgrass
322 Curlleaf mountain-mahogany-bluebunch wheatgrass
323 Shrubby cinquefoil-rough fescue
502 Grama-galleta
505 Grama-tobosa shrub
601 Bluestem prairie
602 Bluestem-prairie sandreed
603 Prairie sandreed-needlegrass
604 Bluestem-grama prairie
605 Sandsage prairie
606 Wheatgrass-bluestem-needlegrass
607 Wheatgrass-needlegrass
608 Wheatgrass-grama-needlegrass
609 Wheatgrass-grama
610 Wheatgrass
611 Blue grama-buffalo grass
612 Sagebrush-grass
613 Fescue grassland
614 Crested wheatgrass
615 Wheatgrass-saltgrass-grama
701 Alkali sacaton-tobosagrass
702 Black grama-alkali sacaton
703 Black grama-sideoats grama
704 Blue grama-western wheatgrass
705 Blue grama-galleta
706 Blue grama-sideoats grama
707 Blue grama-sideoats grama-black grama
708 Bluestem-dropseed
709 Bluestem-grama
710 Bluestem prairie
711 Bluestem-sacahuista prairie
712 Galleta-alkali sacaton
713 Grama-muhly-threeawn
714 Grama-bluestem
715 Grama-buffalo grass
716 Grama-feathergrass
717 Little bluestem-Indiangrass-Texas wintergrass
718 Mesquite-grama
719 Mesquite-liveoak-seacoast bluestem
720 Sand bluestem-little bluestem (dunes)
721 Sand bluestem-little bluestem (plains)
722 Sand sagebrush-mixed prairie
724 Sideoats grama-New Mexico feathergrass-winterfat
725 Vine mesquite-alkali sacaton
727 Mesquite-buffalo grass
728 Mesquite-granjeno-acacia
729 Mesquite
730 Sand shinnery oak
731 Cross timbers-Oklahoma
732 Cross timbers-Texas (little bluestem-post oak)
733 Juniper-oak
734 Mesquite-oak
735 Sideoats grama-sumac-juniper
801 Savanna
802 Missouri prairie
803 Missouri glades
804 Tall fescue
805 Riparian
809 Mixed hardwood and pine

HABITAT TYPES AND PLANT COMMUNITIES:
Purple prairie clover is an important component of Great Plains grassland communities. It is considered "common" in most grassland habitat types of the midwestern United States and southern Canada. Graminoids dominate these regions, comprising 80% to 90% to the total plant population. Purple prairie clover and other forbs generally make up 10% or less of total plant population in the Midwest [119]. Purple prairie clover is found in tallgrass, shortgrass, or mixed-grass prairies. Common associates in tallgrass prairies include little bluestem (Schizachyrium scoparium), big bluestem (Andropogon gerardii), prairie Junegrass (Koeleria macrantha), prairie dropseed (Sporobolus heterolepis), lead plant (Amorpha canescens), and silky aster (Aster sericeus). Associates in mid-grass prairie include silver bluestem (Bothriochloa saccharoides), purple threeawn (Aristida purpurea), sideoats grama (Bouteloua curtipendula), and sand dropseed (Sporobolus cryptandrus). Grass associates in shortgrass prairie include blue grama (Bouteloua gracilis), hairy grama (B. hirsuta), and buffalo grass (Buchloe dactyloides). Forb associates include wavyleaf thistle (Cirsium undulatum), gayfeather (Liatris punctata), and scarlet globemallow (Sphaeralcea coccinea). Forbs may be interspersed with several shrubs including American hazelnut (Corylus americana), smooth sumac (Rhus glabra), creeping juniper (Juniperus horizontalis) and/or trees including green ash (Fraxinus pennsylvanica), eastern cottonwood (Populus deltoides), eastern redcedar (Juniperus virginiana), white oak (Quercus alba), bur oak (Q. macrocarpa), and shagbark hickory (Carya ovata). Woody associates are especially common near riparian areas and on grassland/forest ecotones in the northern and eastern fringes of purple prairie clover's distribution [7,8,17,18,19,32,62].

Purple prairie clover also occurs in Nebraska sandhill prairie [7], cedar glade, limestone glade, dolomite glade [8], dry-mesic savanna, dry-mesic prairie, wet-mesic alluvial floodplain [19], quaking aspen (Populus tremuloides)-prairie ecotone [18], and mesic bur oak (Quercus macrocarpa), black oak (Quercus velutina), white oak (Quercus alba), and northern pin oak (Quercus ellipsoidalis) savanna communities [17]. In Illinois, it occurs in dolomite-hill prairie and "barren" communities [2,19].

In northern Arizona's Grand Canyon National Park, purple prairie clover is considered an "exotic species," and is found in riparian areas of the canyon [95].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Dalea purpurea
GENERAL BOTANICAL CHARACTERISTICS:
The following description provides general characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g. [6,43,48,49,100,108,112]).

Purple prairie clover is a perennial forb, 8 to 35 inches (20-90 cm) tall, with a woody stem. The numerous leaves are 0.4-1.6 inches (1-4 cm) long, with 3 to 7 leaflets. The inflorescence is a 0.4- to 2.6-inch (1-7 cm) spike located at the ends of the branches. Branches are numerous, usually 3 per stem, but sometimes as many as 10 to 12. The mature purple prairie clover has a coarse, nonfibrous root system with a strong woody taproot that is 5.5 to 6.5 feet (1.7-2.0 m) deep. The taproot gives rise to several minutely branched lateral roots. The fruit is a 1- to-2-seeded pod enclosed in bracts [6,48,62,100,112].

RAUNKIAER [86] LIFE FORM:
Hemicryptophyte

REGENERATION PROCESSES:
Purple prairie clover reproduces by seed [16].

Breeding system: Purple prairie clover is cross pollinated [16]. Mating system is primarily xenogamous, but self-pollination also occurs. In a Wisconsin prairie study, 45% of hand-pollinated, outcrossed flowers produced large, viable seeds, and 19% of selfed flowers produced seeds. Native bees and honeybees were pollinators [27].

Pollination is insect-mediated [16,27].

Seed production is highest with favorable soil moisture and nutrient conditions. A survey of native plant horticulturists in Minnesota indicated that purple prairie clover frequently produces low seed yields [15]. Another Minnesota study compared the phenological development of purple prairie clover in cultivated fields to noncultivated managed prairie. Cultivated fields produced 3 times as much seed as noncultivated prairie. Seed and inflorescence production on cultivated and noncultivated native prairie were [16]:

Item Cultivated Fields Noncultivated Prairie
Number of inflorescences initiated/plant 35.0 29.1
Weight/inflorescence (g) 0.36 0.25
Seed weight/inflorescence 0.043 0.015
Number of seeds/inflorescence 33.5 11.5
Seed weight/plant (g) 0.49 0.22
Number of seeds/plant 379.8 173.4

Cultivated fields were devoid of any other competing plants, fertilized, and only contained evenly spaced transplanted purple prairie clover plants from native prairie lands. The noncultivated, native prairie had a variety of other forb and grass species. The noncultivated prairie was under a regimen of prescribed fire every 2 to 3 years. Season of burning was not described [16]. Stevens [96] found that a single purple prairie clover plant may produce 368 seeds per plant (many of which may not mature), with seeds weighing 1.5 g/1,000 seeds.

Seed dispersal: Neither fruits nor seeds have specialized means of dispersal; thus, most seed falls near the parent plant [110].  A seed dispersal study using purple prairie clover and other seed in cattle feed showed that following ingestion, cattle were inefficient vectors for dispersing viable purple prairie clover seed [37].

Seed banking: Purple prairie clover has soil-stored seed [110], but further studies are needed on the relative importance of seed banking to purple prairie clover regeneration. A study on native Kansas prairie found low numbers of buried viable purple prairie clover seed [1].

Germination: Purple prairie clover germinates at soil temperatures ranging from 59 to 86 F (15-30 C) [9] while temperatures as low as 41 F (5 C) have broken dormancy [14].  A survey of native plant horticulture in Minnesota indicated low rates of germination of purple prairie clover [16].  Germination of purple prairie clover is enhanced by scarification, disturbing the litter and duff layers to expose soil, and stratification [14,100].

Seedling establishment/growth: Bjugstand and Whitman [14] used several varieties of forbs for reclamation of strip-mined land and found that purple prairie clover showed "excellent" germination and subsequent "vigorous" growth in the greenhouse. Purple prairie clover transplanted to reclamation areas continued to show excellent vigor and growth [14].

Asexual regeneration: The ability of purple prairie clover to regenerate vegetatively is unclear. Meier and Weaver [72] state that purple prairie clover does not reproduce asexually. However, Towne and Knapp [104] suggest that purple prairie clover sprouts from the root crown following top-kill by fire. Further research is needed on the ability of purple prairie clover to regenerate asexually.

SITE CHARACTERISTICS:
Purple prairie clover grows on a variety of sites throughout the Great Plains including dry plains, prairies, hillsides, open woodlands, shaded ravines, sandhills, and roadsides. It occurs on mesic and xeric sites in mixed-grass and tallgrass prairies of the northern and central Great Plains and the shortgrass prairie of the southern Great Plains [24,49,80]. It is most common on marginal sites where soil is exposed and grasses have not formed dense stands [82]. Mean annual precipitation for regions where purple prairie clover subsists ranges from 11 inches (280 mm) in southeast Alberta [32] to 32 inches (810 mm) in Kansas [105] and Oklahoma [13] to 53 inches (1,350 mm) in Mississippi [115].

Soils: Purple prairie clover can be found in most soil types throughout the Great Plains [31]. In the northern Great Plains purple prairie clover is found in sandy to silty loams [17,19]. Some specific soil characteristics have been identified with purple prairie clover in the Nebraska Sandhill prairie region. Here purple prairie clover occurs most frequently in sandy soils that contain medium to coarse grains. It is thought that the coarse sands intercept precipitation with minimal runoff, allowing most of the moisture to reach far below the surface. Due to its taproot morphology, purple prairie clover is able to access moisture from deep in the soil profile and thus is able to persist in areas where other shallow-rooted species cannot [7]. Soils in Minnesota where purple prairie clover is present were found to have pH values of 6.4 to 6.7 [16], with soil depths ranging from 6 to 24 inches (15-6.1 cm) [22].

Elevation: In eastern Colorado purple prairie clover occurs at elevations from 3,500 to 7,500 feet (1,067-2,286 m) [53].

SUCCESSIONAL STATUS:
Generally purple prairie clover is considered a mid- to late successional species [102]. Purple prairie clover can also fill a pioneer role, as seen in roadsides and disturbed locations [91]. The following is a general description of the successional pathways on prairie lands. Many details of succession in these associations remain unknown.

On the mixed-grass prairies of the southern Great Plains, purple prairie clover is part of a group of forbs found in late successional seres. A common pattern of succession in disturbed prairie regions begins with the dominance of native prairie annuals, nonnative annual weeds, and ragweeds (Ambrosia spp.), which may persist for 1 to 3 years. Soon following, a collection of nonnative and native grasses and perennial forbs, including purple prairie clover, create a mosaic of species that may take 15 to 40 years to develop, depending on environmental conditions and "competitive" factors. Common species that coexist with purple prairie clover in the tallgrass prairie during the later stages of succession include lead plant and prairie dropseed [91].

In its eastern range in forest openings where fires and other natural disturbances are suppressed, purple prairie clover can be shaded out by encroaching woody species [55]. Purple prairie clover is thought to be an indicator of prairie in its later successional sere and may be an indicator of pristine prairie ecosystems [91].

SEASONAL DEVELOPMENT:
Purple prairie clover is a warm-season forb that generally germinates during spring [84]. In May to August (depending on climate and geographic location), purple prairie clover produces several to many inflorescences. Average flowering dates for purple prairie clover from 5 years of observation in North Dakota were [99]:

Earliest 1st bloom

Latest 1st bloom

Median date of full flowering

Median date when 95% of flowering complete

Flowering period (days)

June 17th July 13th July 15th August 15th 35

Flowering dates for purple prairie clover are influenced more by temperature than precipitation. Warmer temperatures seem to promote earlier flowering, while warm temperatures and ample moisture in summer increase the duration of blooming [26]. Purple prairie clover seed matures in most locations from August to September [9]. In Minnesota seed development from anthesis to seed maturity took 15 weeks [16].

FIRE ECOLOGY

SPECIES: Dalea purpurea
FIRE ECOLOGY OR ADAPTATIONS:
Fire adaptations: Purple prairie clover establishes from soil-stored seed after fire. While not specifically documented, it is implied that purple prairie clover root crowns may survive burning that consumes the aerial portions of the plant, allowing postfire sprouting from the root crowns [104]. The large woody taproot allows for photosynthate and nutrient storage that can support postfire root crown sprouting. Additionally, fire creates favorable conditions (disturbed soil, decreased levels of mulch, reduced interference from forbs) that are favorable for purple prairie clover seedling establishment and growth [12,13,39,64].

Fire regimes: Historically fire has been an important natural component of grassland communities where purple prairie clover occurs [35]. Frequent, stand-replacement surface fires in plains grasslands and prairies affect species composition and vegetation dynamics [79]. Across the Great Plains, lightning-caused and human-caused fires may have occurred as frequently as every 1 to 10 years for thousands of years prior to European settlement [67,117]. The implications of the cessation of historical fire regimes in the last century on purple prairie clover are unknown. Purple prairie clover has responded favorably to burning in several prescribed fire studies [12,13,39,64] using various annual intervals and seasons (see Plant Response to Fire).

In some habitats fire is necessary to maintain purple prairie clover. For example, along woodland-grassland ecotones in purple prairie clover's eastern range, the cessation of fire has caused encroachment of woody species that shade out purple prairie clover and reduce its abundance [45].

The following list provides fire return intervals for plant communities and ecosystems where purple prairie clover is important. It may not be inclusive. For further information see the FEIS reviews on the dominant species listed below.

Community or Ecosystem Dominant Species Fire Return Interval Range (years)
sugar maple-basswood Acer saccharum-Tilia americana > 1,000 [109]
bluestem prairie Andropogon gerardii var. gerardii-Schizachyrium scoparium < 10 [67,79]
Nebraska sandhills prairie A. gerardii var. paucipilus-S. scoparium < 10
bluestem-Sacahuista prairie A. littoralis-Spartina spartinae < 10 [79]
silver sagebrush steppe Artemisia cana 5-45 [56,85,117]
sagebrush steppe A. tridentata/Pseudoroegneria spicata 20-70 [79]
basin big sagebrush A. tridentata var. tridentata 12-43 [89]
mountain big sagebrush A. tridentata var. vaseyana 15-40 [5,23,75]
Wyoming big sagebrush A. tridentata var. wyomingensis 10-70 (40**) [107,118]
plains grasslands Bouteloua spp. < 35 [79,117]
blue grama-needle-and-thread grass-western wheatgrass B. gracilis-Hesperostipa comata-Pascopyrum smithii < 35 [79,88,117]
blue grama-buffalo grass B. gracilis-Buchloe dactyloides < 35 [79,117]
blue grama-tobosa prairie Bouteloua gracilis-Pleuraphis mutica < 35 to < 100 [79]
cheatgrass Bromus tectorum < 10 [81,114]
sugarberry-America elm-green ash Celtis laevigata-Ulmus americana-Fraxinus pennsylvanica < 35 to 200 [109]
northern cordgrass prairie Distichlis spicata-Spartina spp. 1-3 [79]
black ash Fraxinus nigra < 35 to 200 [109]
Ashe juniper Juniperus ashei < 35 [79]
cedar glades J. virginiana 3-22 [52,79]
yellow-poplar Liriodendron tulipifera < 35 [109]
wheatgrass plains grasslands Pascopyrum smithii < 5-47+ [79,85,117]
pine-cypress forest Pinus-Cupressus spp. < 35 to 200 [4]
sycamore-sweetgum-American elm Platanus occidentalis-Liquidambar styraciflua-Ulmus americana < 35 to 200 [109]
eastern cottonwood Populus deltoides < 35 to 200 [79]
aspen-birch P. tremuloides-Betula papyrifera 35-200 [38,109]
quaking aspen (west of the Great Plains) P. tremuloides 7-120 [4,50,73]
mesquite Prosopis glandulosa < 35 to < 100 [71,79]
Texas savanna P. glandulosa var. glandulosa < 10 [79]
black cherry-sugar maple Prunus serotina-Acer saccharum > 1,000 [109]
mountain grasslands Pseudoroegneria spicata 3-40 (10**) [3,4]
bur oak Quercus macrocarpa < 10 [109]
oak savanna Q. macrocarpa/Andropogon gerardii-S. scoparium 2-14 [79,109]
shinnery Q. mohriana < 35 [79]
Fayette prairie S. scoparium-Buchloe dactyloides < 10 [109]
little bluestem-grama prairie S. scoparium-Bouteloua spp. < 35 [79]
elm-ash-cottonwood Ulmus-Fraxinus-Populus spp. < 35 to 200 [38,109]
**mean

POSTFIRE REGENERATION STRATEGY [97]:
Caudex/herbaceous root crown, growing points in soil
Ground residual colonizer (on-site, initial community)

FIRE EFFECTS

SPECIES: Dalea purpurea
IMMEDIATE FIRE EFFECT ON PLANT:
Purple prairie clover is top-killed by fire [104].

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

PLANT RESPONSE TO FIRE:
Plant growth: Purple prairie clover may recover from fire by establishing from soil-stored seed and sprouting from the root crown. Purple prairie clover has responded favorably to prescribed fire in various studies [12,13,39,64]. Due to the hard seedcoat of legumes like purple prairie clover, these species' germination rates maybe enhanced by burning [70].

Towne and Knapp [104] noted that purple prairie clover that was top-killed by fire showed great capacity to sprout after fire. There is no specific information available on how quickly purple prairie clover recovers after burning. Further research is needed on this topic.

Productivity: Several studies have focused on how the frequency of burning relates to productivity of purple prairie clover. Generally, annual burning favors annual grasses and reduces the abundance of perennial forbs including purple prairie clover [66]. Studies conducted in Minnesota in 1984 [101] and Missouri in 1964 [66] suggest that, compared to annual burning, biennial burning increases frequency and basal areas of legume species [66] including purple prairie clover [101]. In Wisconsin prairie restoration projects where purple prairie clover has prospered, managers recommend a 5-year burning interval [21].

Burning can enhance flower productivity in several prairie forbs including purple prairie clover. Purple prairie clover produced a greater abundance of inflorescences after a single spring burn on a Minnesota prairie than prior to burning [80]. The effects of this burn are attributed to the removal of litter and standing dead stems by the fire. Removal of litter allows for increased light intensities near the soil and thus higher soil temperatures, which enhance plant productivity. Litter reduces the presence and productivity of many forbs including purple prairie clover [39]. For more on the effects of litter on purple prairie clover, see Management Considerations.

The Great Plains region where purple prairie clover commonly occurs is typical of prairie and savanna ecosystems that require fire to maintain historical ranges of species composition and species richness [30]. Most of the available information has been based on short-term research [58]. Long-term effects (beyond the scope of current research; >20 years) of various fire regimes are not well known.

Season of burning: Interactions between season of burn and purple prairie clover phenology are not well known [60]. While spring burning generally decreases the immediate abundance of forb species that are actively growing [2], legume species including purple prairie clover in Kansas have shown increased growth and vigor 3 years following spring burns, nearly doubling stem biomass on upland sites and quadrupling stem biomass on lowland sites [104].

While most studies find that forb production is compromised after late spring burning, Bidwell and others [12,13] found that late spring backburning increased the productivity of purple prairie clover and other forb species (see Fire Management Considerations). Testing seasonal differences in annual prescribed annual burning on a Kansas prairie for 8 years, Towne and Kemp [103] found that legume species including purple prairie clover increased in cover in response to burning at any season. Greatest increases occurred 6 years after fire treatments, on autumn and winter prescribed burn plots. Others have found that most forbs including purple prairie clover decrease in abundance after being top-killed by late spring burning, while purple prairie clover increases after autumn and early spring burning [46,87].

The effects of  mid-summer burning are not available in current literature (2005). For the purposes of restoration ecology, dormant-season fires probably do not resemble historical disturbance regimes found before European settlement. It is suspected that varied burn seasons, and intervals brought by natural ignitions from lightning prior to the European settlement era, produced greater levels of biodiversity and species assemblages than any single management method for native prairie lands [60]. Unfortunately, information on purple prairie clover frequency and abundance prior to European settlement is not available.

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
Annual burning of prairie lands reduces available soil nitrogen and increases competition among plants limited by nitrogen availability [78]. Legumes including purple prairie clover have the ability to fix atmospheric nitrogen, and may have an advantage over other forbs and some grasses in nitrogen-stressed environments [69].

FIRE MANAGEMENT CONSIDERATIONS:
Fire severity affects purple prairie clover survivorship and postfire productivity. Bidwell and others [12,13] noted the effects of using a backing fire versus a headfire. In postfire year 1, abundance of forbs (including purple prairie clover) was 26% greater on backfired plots compared to headfired plots. This may be due to a reduction in interference from tallgrasses, such as prairie Junegrass and little bluestem, which are negatively affected by late spring burning. Also, backfires often create a mosaic of burned and unburned patches that may provide favorable microsites where purple prairie clover can escape lethal fire temperatures [13].

MANAGEMENT CONSIDERATIONS

SPECIES: Dalea purpurea
IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Purple prairie clover produces excellent forage for livestock and wildlife. When abundant on pasture lands it may be an important component in hay [62,100]. Purple prairie clover is recommended for use in restoration seed mixtures. It produces forage with high yields, extended grazing periods, and increased nutritional values [84,90]. Pronghorn graze purple prairie clover on summer ranges of Montana [113]. A 2-year study in Minnesota found that white-tailed deer did  not browse purple prairie clover [40], although this does not imply that deer and other ungulates never graze this species.

Palatability/nutritional value:
In North Dakota, crude protein levels of purple prairie clover ranged from 12% in June to 8% in August [84]. Due to its high palatability and high concentrations of nutritional protein, purple prairie clover is generally considered one of the most important legumes in native grasslands on the Great Plains, although some rare instances of bloat have been reported in livestock [62,100]. Crude and digestible protein content of purple prairie clover are as follows [76]:

Crude Protein 14.1%
Digestible Protein
   cattle 9.9 %
   domestic goats 9.7 %
   domestic rabbits 9.6 %
   domestic sheep 10.1 %
   horses 9.5 %

Cover value: No information is available on this topic.

VALUE FOR REHABILITATION OF DISTURBED SITES:
Purple prairie clover is commonly found in seed mixtures recommended for revegetation, reclamation [34,93], and native prairie restoration projects [65]. Gustafson and others [51] found that by using several local seed sources of purple prairie clover for restoration projects, local gene pools were maintained and regional genetic diversity was enhanced, promoting persistence and vigor in restored purple prairie clover populations. Purple prairie clover is frequently used in seed mixes for erosion control due to its ability to establish on disturbed sites and its capability to condition soil with nitrogen [92]. Legumes such as purple prairie clover fix atmospheric nitrogen in the soil. They may have an advantage over forbs and some grasses in degraded prairie or pasture sites [24,62].

Propagation: Purple prairie clover germination is enhanced in scarified soils [100]. Purple prairie clover in its natural habitat is often found in disturbed locations such as black-tailed prairie dog towns [98] and on dug mounds created by American badgers [83]. Stratification [100] and inoculation with rhizobium [65] have increased germination success of purple prairie clover in the laboratory. Purple prairie clover has been successfully used in several roadside vegetation projects throughout the Great Plains [28,33]. In a strip-mine reclamation project, purple prairie clover demonstrated excellent success as a colonizer, exhibiting high rates of germination and subsequent vigorous seedling growth in the greenhouse and afterwards during transplanting [14].

Purple prairie clover is highly dependent upon mycorrhizal fungi. A mycorrhizal inoculation study found prairie species uptake and transport of soil nutrients such as phosphorus and zinc was enhanced by mycorrhizae, but the study did not show any substantial effects on purple prairie clover seedling emergence  [54]. Conversely, a study that used a benomyl (a fungicide specifically for the removal of mycorrhizae in soils) considerably lowered survivorship of purple prairie clover [116].

Purple prairie clover is susceptible to interference from with exotic species during establishment due to its relatively slow rate of seedling growth compared to that of nonnative invasive species. In North Dakota some populations of purple prairie clover have been completely eliminated by infestations of leafy spurge (Euphorbia esula) [25]. Reducing weed interference using herbicide applications (imazethapyr + imazapic) has been successful in improving establishment of purple prairie clover in Nebraska [10].

OTHER USES:
Native Americans boiled purple prairie clover leaves for food. The Ponca tribe chewed the roots for their pleasant flavor and made tea from leaves. The Pawnee used the stems to make brooms.  Native American used boiled leaves to make a poultice for dressing on wounds [47]. Laboratory studies have found antibacterial and fungicidal compounds in purple prairie clover [61].

OTHER MANAGEMENT CONSIDERATIONS:
Low- to moderate grazing pressure may enhance purple prairie clover production by removing vegetative cover [18], but overgrazing can decrease coverage and frequency of purple prairie clover [49]. In Illinois, populations of purple prairie clover have recovered in areas where they had been removed under high grazing pressure [19]. In Iowa, purple prairie clover was 1 species in a large group of native forbs that decreased or disappeared under unspecified grazing pressure [111].

In prairies near forest lands, encroachment of forest species into grasslands changes vegetation structure and composition. In eastern Nebraska, eastern redcedar encroachment into prairies has been linked to the decline of many prairie species, including purple prairie clover, due to shading [45].

The accumulation of litter on prairies affects purple prairie clover populations. In Kansas, purple prairie clover decreased during a 50-year study on tallgrass prairie that has seen a shift from summer haying to spring biannual burning. The author speculates that these decreases resulted from the cessation of mid-summer mowing. In this study mowing was thought to be responsible for the removal of biomass during the summer months, altering microhabitat conditions that had supported purple prairie clover. The increase of mulch thickness may explain decreases in purple prairie clover and other native forbs [36]. In a study of the effects of cessation of mowing and introduction of prescribed fire, forbs including purple prairie clover increased in abundance due to the reduction of mulch [29]. For more on the effects of litter on purple prairie clover, see Fire Effects

Experimental research on effects of small rodent herbivory on native forb populations found that herbivory by meadow voles reduced  purple prairie clover density [59,77]. Whether or not these findings in laboratory communities were applicable to prairie communities was unclear.

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