Great laurel has been described as "fire-intolerant" [14] and as having intermediate to high fire tolerance [75].
Stems less than 2 inches (5 cm) DBH are more susceptible to fire than larger stems. A North Carolina prescribed burn resulted in 70% of stems less than 1.0 inch top-killed, but only 5% of stems over 3.0 inches died [56].
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g., [44,92,108]).
Great laurel is an evergreen shrub or small tree [24,92,105,109,121] that grows up to 40 feet (12 m) tall [15,18,44,63,72,84,90,99,105,108,109,121,127] and 25 feet (7.5 m) wide [72]. Mature height depends on climate and varies from 3 feet (1 m) in the colder northeast to 40 feet in the southeast [66,106]. Growth form often exhibits a crooked trunk [15] reaching 1 foot (0.3 m) in diameter [18,90,99,108,121] and heavy, contorted branches [15]. Multiple stems grow from a large root crown, and individual stems may reach 100 years old [90].
Great laurel's thick, leathery leaves are oblanceolate to narrowly elliptic, 3 to 14 inches (8-35 cm) long and 0.8 to 3 inches (2-8 cm) wide [15,18,44,72,86,92,108,109]. In cold temperatures or during droughts, great laurel leaves roll up around the axis of the leaf [15,109]. Leaves are retained for up to 8 years, and once shed, are slow to decompose [18]. Great laurel leaf area has been positively correlated to the likelihood of flowering, with the probability of flowering greatest for younger shoots (≤ 2 years) [23]. The rose to white colored flowers of great laurel are numerous, 1.5 inches (3.5 cm) wide, and borne in an umbel-like inflorescence [15,44,66]. Great laurel fruits are oblong capsules that split along the sides soon after ripening to release large numbers of seed (approximately 400 seeds per capsule) [18,97]. Seeds are 1.5 to 2 mm long [92].
Stand Structure: The predominant growth form of great laurel is a dense thicket that excludes nearly all other plants under its closed crown [26,57,62,63,77,90]. Here great laurel can achieve greater than 80% coverage [90]. These thickets grow 10 to 23 feet (3-7 m) tall [10,84,108], and may form an almost unbroken shrub stratum. In one study, great laurel attained an average height of 8 to 10 feet (2.5-3 m) with nearly impenetrable, low-branching woody growth [2].
Great laurel occurs throughout much of eastern and southeastern North America, from Nova Scotia and southern Ontario south to Georgia and west to Ohio, Kentucky, Tennessee, and Alabama [11,15,44,71,92,105,108,126]. It is especially common in the eastern mountains of the United States [11,44,84,86]. It is rare in Maine [31,73]. In Massachusetts great laurel is "local", occurring in very few places though populations may be large where they do occur, and is vulnerable to depletion due to collection and/or habitat destruction [21].
Plants Database provides a distributional map of great laurel.
Fire adaptations: Great laurel typically survives fire by sprouting from the root crown or stump [56,75,98].
FIRE REGIMES: Woodland and forest communities in which great laurel occurs have a range of FIRE REGIMES including replacement, mixed-severity, and surface fires (see the fire regime table below). Most fires in these regimes are of low or mixed-severity, particularly in the Southern Appalachian and Southeastern regions. Fire intervals are wide-ranging, with low-severity fires averaging 4-year intervals in some communities to 100 or 1,000 years in other communities. Fires of mixed-severity and stand-replacing fires are much less frequent. Great laurel is well-adapted to these predominantly low-severity FIRE REGIMES, with its ability to survive fire through sprouting [56,75,98] and spread into canopy openings through stem layering and suppression of other species (for more discussion of these characteristics, see Successional Status and Other Management Considerations). In the southern Appalachians, where great laurel is likely to form dense, pure understories, fires are neither severe enough or frequent enough to eliminate great laurel. Fire exclusion in the southern Appalachians has contributed to the spread of great laurel [96]. Though occasional fires serve to increase the vigor of great laurel populations, annual intense burning over extended periods, as was historically common in many Appalachian forests, resulted in the eventual elimination of great laurel from many areas [90]. Baker and Van Lear [4] speculate that historic fires top-killed great laurel, allowing other species to grow ahead of its sprouting, and frequent fires during the growing season could have completely killed individual stems.
Great laurel is also well-adapted to FIRE REGIMES with a high proportion of stand-replacing fires. If present in a community, great laurel is effective at colonizing canopy gaps and maintaining an understory presence. Following large-scale stand-replacement events, great laurel often sprouts and forms dense thickets where reestablishment of overstory species is substantially reduced. Great laurel often dominates xeric coniferous heathlands in the southern Appalachians, a vegetation type that is maintained in part by fire [77]. Heath balds likely represent secondary successional communities following disturbance, especially fire in extreme exposure sites [77]. Once established, heath balds can also be considered topographic climaxes, with exposure producing microclimate and edaphic conditions to maintain them [77,120].
The following table provides fire regime information that may be relevant to great laurel. 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".
Fire regime information on vegetation communities in which great laurel may occur. For each community, fire regime characteristics are taken from the LANDFIRE Rapid Assessment Vegetation Models [65]. These vegetation models were developed by local experts using available literature, local data, and/or expert opinion as documented in the PDF file linked from each Potential Natural Vegetation Group listed below. Cells are blank where information is not available in the Rapid Assessment Vegetation Model. Great Lakes Northeast Southern Appalachians Southeast Great Lakes Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics Percent of fires Mean intervalGreat laurel has increased in the Appalachian Mountains due to fire exclusion [75,95]. Historically, great laurel occurred primarily in riparian zones, but fire exclusion and the cessation of fire use as a management tool in the 20th century, combined with other factors, allowed great laurel to expand into other areas under some conditions. For more information on the expansion of great laurel, see Other Management Considerations. Baker and Van Lear [4] speculate that, historically, fires in forests of the southern Appalachians may have top-killed great laurel and allowed other species to grow ahead of its sprouting, while frequent fire could have completely killed individual stems.
Great laurel has been described as a "serious woody weed" in the southeastern United States [37]. Prescribed fire has been used to control great laurel with limited success [18]. A fall prescribed burn to control great laurel on the Bent Creek Experimental Forest in North Carolina found that some shoots survived fire and vigorous sprouting occurred the following spring. The sprouts were "very susceptible" to herbicide applications in the second postfire growing season, with nearly 100% top-kill achieved [98].
A spring prescribed burn conducted in a dry, mixed-oak (Quercus spp.) community type at Wine Spring Creek Watershed, North Carolina reduced frequency of great laurel but had little effect on density (see table below) [36]. More information on this prescribed fire and postfire response of plant community species can be found in the Research Project Summary Early postfire effects of a prescribed fire in the southern Appalachians of North Carolina.
Great laurel abundance before and after prescribed fire (April 1995) at Wine Spring Creek watershed, North Carolina [36]
Pre-burn (July 1994) Postfire year 1 (July 1995) Frequency (%) 22 11 Density (stems/ha) 1,605 1,358 Average basal area (m²/ha) 7.231 7.789If fire is used to control great laurel, desired tree seedlings should be planted immediately in patches with complete top-kill to overcome vigorous great laurel sprouting [90].
Great laurel occurs beneath a variety of canopy types, both evergreen and deciduous, predominantly along stream complexes and adjacent slopes and coves [84,86]. Great laurel occupies a variety of mesic sites [26,127], including lakeshores [5], stream banks [72,92,126], cool acid swamps [63,109], and moist or wet wooded areas [44,72,88,92,103,108,121,126]. It is found in Appalachian bogs, montane floodplains, and high elevation (>3000 ft) seepage swamps [28,39]. Great laurel is a facultative species, strongly associated with groundwater [48]. Preferred habitat is a sheltered mountain cove, along a stream to insure an abundance of both soil moisture and atmospheric humidity, though good aeration is also necessary [90,99]. Though historically confined to riparian areas and other mesic sites, great laurel takes advantage of disturbed areas where it is present to advance onto submesic, subxeric, or even xeric sites and rock outcrops [18,90,120,121]. It grows in valley bottoms and on north-facing slopes [33,70,105], expanding along watercourses and the peripheries of existing great laurel patches [33,105].
Great laurel is shade tolerant, generally occurring on sites with medium to dense shade receiving little direct sunlight [90,105]. Optimum site condition is medium shade produced by a high canopy [90]. Great laurel growth is inhibited both by high irradiance and by low water availability [69,70,105]. These factors limit growth of great laurel on southwestern slopes [69]. Twig elongation in low, moist, sunny sites is 25% to 30% greater than on dry, exposed sites at higher elevations. The mesic sites produce 50% to 100% more leaves and the mean weights of these leaves are 25% to 100% greater than on xeric sites [78]. In a southern Appalachian study at Coweeta Hydrological Laboratory, North Carolina, great laurel understory with >70% coverage was common in valley bottoms and north-facing slopes, while a mix of great laurel and mountain-laurel was common on south-facing slopes and along mid-elevation slopes. Mountain-laurel occurrred in greater numbers with increasing distance from streams [33].
Soils: Great laurel prefers deep, well-drained, acidic soils (pH 3.55 to 5.6) [18,26,30,72,77,90,97,105,105,123] high in organic matter, where the litter produces a thick, peat-like humus [18,99]. In xeric heathlands, it is also found on nutrient poor soils [77]. Ectomycorrhizal associations improve great laurel fitness in nutrient-poor, acidic soils where availability of soluble inorganic nutrients is likely to be low. In particular, these associations improve phosphorus uptake [32]. Great laurel is generally unable to colonize high pH soils; establishment of seedlings and layering of branches is potentially inhibited where pH is >6.0 [12].
Great laurel leaves and roots are rich in phenolics and other organic compounds. According to an eastern Kentucky study of single-tree influence on soil properties, the release of these compounds through decomposition "results in intensified leaching and acidification of soil, and may interfere with mineral soil retention of basic cations liberated from decomposing organic matter". By reducing pH on sites where they are established, great laurel thickets prevent herbaceous growth and establish stable boundaries over time [12].
Elevation and Aspect: Great laurel occurs from sea level to 6,000 feet (1,830 m) [4,40,66,72,92,99,120]. Slope aspect is generally more important in great laurel success than topography [99], with better development found on north-facing slopes [33,70,105]. Great laurel is also present, however, on east, west, and dry southern exposures [127].
Climate: Great laurel generally prefers cool, humid climates with annual precipitation >79 inches (2,000 mm) [122]. Great laurel thickets maintain a favorable microclimate by reducing summer temperatures, excluding desiccating winter winds, moderating cold temperatures, and increasing relative humidity [90]. Arora and others [3] report that great laurel is generally cold-hardy to -62 °F (-52 °C). Sakai and others [101] offer the following limits for specific plant tissues: flower buds are cold hardy to -16.6 °F (-27 °C); xylem to -31 °F (-35 °C); and leaf, vegetative bud, and cortex to -76 °F (-60 °C).
Great laurel is browsed by white-tailed deer [22,42,51,74,99,109], primarily in fall and winter [60,97,99]. Ruffed grouse [97,99], wild turkeys [47,99], and sharp-tailed grouse eat the buds and leaves [74,99]. Beavers browse great laurel twigs [74]; small mammals, including the white-footed mouse and Allegheny wood rat, eat the leaves [74,97]; and rabbits eat the bark, young wood, leaves, and buds [99].
Great laurel is poisonous to horses, causing labored breathing, nausea, constipation, diarrhea, and death [38].
Palatability/nutritional value: The following tables describe great laurel nutrient levels. The first is based on plant structure; the second table is based on leaf age.
Great laurel nutrient levels (kg/ha), Southern Appalachian hardwood forest [27,76]
Structure N P K Ca Mg Bark --- --- 1.22 3.71 0.13 Wood --- --- 8.38 4.11 0.47 Twigs 0.24 0.01 0.37 0.24 0.04 Leaves 14.47 1.51 6.02 13.92 2.21Great laurel leaf nutrient levels (% dry weight), Coweeta Hydrologic Laboratory, North Carolina [78]
Leaf age N P K Ca Mg 1 month 1.60 0.17 1.34 0.36 0.17 6 months 1.05 0.16 0.87 1.00 0.29 2 years 0.93 0.13 0.41 1.20 0.20 3 years 0.86 0.12 0.35 1.30 0.21 4 years 0.88 0.12 0.34 1.40 0.16 5 years 0.99 0.13 0.47 1.40 0.14 >5 years 0.95 0.14 0.54 1.10 0.17 Fresh litter 0.48 0.10 0.19 1.50 0.19Cover value: Great laurel provides valuable winter and escape cover for white-tailed deer, eastern cottontail, black bear, snowshoe hare, ruffed grouse, wild turkey, and many songbirds [18,90,97]. Dense thickets of great laurel provide den sites, daybeds, and escape cover for black bears [118].
Great laurel is an important species in several eastern plant communities, most
notably southern Appalachian forest communities where it occurs as an understory
dominant on millions of acres [18]. It often occurs as a dominant shrub in Table
Mountain pine (Pinus pungens) and pitch pine (P. rigida) forests
[29,77], xeric coniferous heathlands [77], mixed heath balds [77,122,127], chestnut oak
(Quercus prinus) forests and heaths [77,122], lower elevations of the
southern Appalachian spruce zone (red spruce (Picea rubens)) [1,62], and scarlet
oak-red maple forests (Q. coccinea-Acer rubrum) [77]. It is also a dominant
shrub in mixed mesophytic forests [15], particularly eastern hemlock (Tsuga
canadensis) [57,77,88] and oak-hickory (Quercus spp.-Carya
spp.) forests (formerly oak-American chestnut (Castanea dentata))
[15,77,83,127]. Great laurel is common in northern red oak-white oak
forests (Q. rubra-Q. alba) [16] and northern red oak-yellow-poplar
(Liriodendron tulipifera)-chestnut oak forests [77]. Great laurel
is commonly associated with mountain-laurel (Kalmia latifolia) [26,127];
together these are the 2 most typical and frequent shrubs of the southern Appalachian
forest [26].
Though great laurel often occurs as a component in various mixed shrub
associations and in the understory of Appalachian forest types, the predominant
growth form is that of a dense thicket in which crown closure has occurred and
nearly all other herbs, shrubs, and trees have been excluded [26,57,62,63,77,90].
Here great laurel can achieve greater than 80% coverage [90]. For more information
on great laurel thickets see Stand Structure.
Plant communities where great laurel is a dominant species are
described by state as follows:
North Carolina:
Tennessee:
Virginia:
West Virginia:
Great laurel has been described as a "serious woody weed" in the southeastern
United States [37]. As great laurel increases its range and understory abundance in southern
Appalachian forests, it may reduce species richness, limit advanced regeneration
of overstory tree species, and change successional patterns [4,18,52,81,85].
Several studies in the southern Appalachian region have documented that
great laurel inhibits recruitment of canopy tree seedlings and may reduce the growth
of mature canopy trees [7,9,10,17,81,83]. Studies at Coweeta Hydrologic Laboratory
found great laurel thickets significantly decreased (P=0.051) seedling
survival of yellow-poplar, red maple, and northern red oak [7]. Red maple seedlings
beneath great laurel were 0.41 times less likely to survive as seedlings in areas
without the shrub [9], and seedling survival of northern red oak was reduced 40% in the
presence of a great laurel thicket compared to forest without thickets [81]. Great laurel can
substantially or completely eliminate midstory and overstory species in riparian forests [4]. A
riparian ecosystem study of the Wine Spring Creek watershed, North Carolina found that
species richness and percent cover in the regeneration layer were inversely
related to great laurel thicket density. On average, 6 species were found on
plots with high great laurel stem density (7,950-17,400 stems/ha) compared to 26
species present on plots with low great laurel stem density (0-2,600 stems/ha).
The regeneration layer was dominated by great laurel with only a few trees
present (eastern hemlock, red maple, yellow birch, northern red oak, American
beech). Both woody and herbaceous species regenerated poorly under the thicket
canopy [4]. A study of southern Appalachian riparian areas previously dominated by American
chestnut found that vegetative diversity and eastern hemlock regeneration were greatly reduced
on sites where great laurel was present [115].
Great laurel's inhibition of canopy tree regeneration may cause incomplete or slowed
succession [86]. Possible sources of inhibition include allelopathy, competition for
resources including light, physical and chemical attributes of the forest floor and soil,
and interactions between some or all sources [13,18,81,85,111]. Seedling mortality of
transplanted first-year oak seedlings (northern red oak, chestnut oak) at Coweeta was
positively correlated with great laurel basal area, soil aluminum, and leaf herbivory.
Oak seedling biomass and tissue C:N ratios were negatively correlated with great
laurel basal area. The results indicate tree seedling establishment and growth
are limited by shade beneath great laurel, and suggest the potential importance of
herbivory and aluminum toxicity in great laurel thickets [10]. Another Coweeta
investigation found that great laurel had no significant effect on canopy tree species
(red maple, yellow-poplar, northern red oak, chestnut oak, hickory) seed reaching the
forest floor. Germination of tree seeds was also not reduced by leaves and substrates
in the great laurel thicket. Seedling mortality, however, was 5 times higher in great
laurel thickets than outside the thickets. Tree seedlings under great laurel were not
nitrogen limited, and lack of tree seedling success was therefore attributed to light
limitation, herbivory, and litter fall [68]. The effects of light limitation and great
laurel litter characteristics are discussed in more detail below.
Light attenuation: Other studies have also demonstrated the effects of
great laurel on species richness, species diversity, and seedling regeneration. A study
of canopy gaps at Coweeta found that the presence of great laurel affected these variables
more than gaps did, reducing species richness and diversity and seedling establishment.
Weak recruitment was attributed to both low seed availability and competition with the
great laurel shrub understory, largely due to the attenuation of light. Outside the
great laurel understory, gap formation increased light levels 2-fold; beneath great
laurel light levels did not increase following gap formation [8]. Most seedling inhibition
by great laurel occurs after emergence and is attributed to substantially reduced
light (50-80%) where great laurel thickets are present, compared to hardwood forest
understory without shrubs [83]. Clinton [17] studied great laurel subcanopies at Coweeta
Hydrologic Laboratory and found light levels were 77% lower during the growing season
and 70% lower in the dormant season in areas with great laurel compared to areas without.
During the growing season, light levels beneath great laurel were <2% of full sun.
These low-light environments are extremely limiting to regeneration of hardwood species
[17]. Another North Carolina study found that the presence of a great laurel thicket
reduced the availability of light by 80%, the frequency and duration of sunflecks
(sudden, short-term increases in light intensity) by 96%, the availability of
water by 20%, and the availability of several soil nutrients by variable amounts
[81]. Additional studies have demonstrated that while great laurel reduces light
sufficiently to prevent the regeneration of tree seedlings such as northern red oak
and black cherry (Prunus serotina), there is enough heterogeneity in the
light to suggest that the deep shade only partially explains the complete
inhibition of regenerating canopy trees under great laurel [67].
Litter characteristics: Inhibition of canopy tree seedlings is
exacerbated by competition for soil water, nitrogen, and phosphorus [83],
thick accumulations of litter, potential loss of mycorrhizal benefits, and
inhibition of soil fauna.
Great laurel litter is slow to decompose [13,111], and the thick litter may
inhibit tree seedling establishment [8]. One study of plant litter at Coweeta Hydrologic
Laboratory compared great laurel litter to that of yellow-poplar and chestnut oak and found
significantly more (α=0.05) great laurel litter mass retained after 1 year [58]. When
newly germinated tree seedlings were found in a Southern Appalachian great laurel understory,
they preferentially occurred in microsites with mineral soil, suggesting that great laurel
litter suppresses seedling recruitment [8]. However, other studies have found that leachates
from great laurel leaves, litter, humus, or throughfall had no ecologically significant
toxicity [83], and alleopathic effect is not an important inhibitor of black cherry or
northern red oak seedling survival in great laurel thickets [85]. Allelochemicals in great
laurel litter can depress growth of some ectomycorrhizal species and reduce soil nutrient
availability, therefore indirectly reducing canopy tree seedling survival [85].
A Coweeta-based study found great laurel on study plots reduced mycorrhizal colonization of
eastern hemlock seedlings after the first year; results showed 62% colonization on
plots without great laurel compared to 19% on plots with great laurel present [117].
Hoover and Crossley [58] demonstrated that microarthropod abundance was significantly
lower (α=0.05) in great laurel litter than in yellow-poplar and chestnut oak litter.
Great laurel litter also depresses earthworm activity [13,111]. A study done in Kentucky
found significantly fewer (P<0.01) earthworms in the mineral soil under both eastern
hemlock and yellow-poplar trees with a great laurel understory than under the same tree species
without a great laurel understory [13]. The authors hypothesized that the lower earthworm
density is likely due to phenolics or other chemicals leached from great laurel litter,
foliage, and roots [13]. Slow decomposition and reduced soil fauna facilitate the
development of a thick forest floor which may give competitive advantage to great laurel
seedlings over other understory vegetation [13,111].
Due to great laurel's preference for low pH soils, efforts at control have included
the addition of of limestone to increase soil pH. However, over 5 years, limestone applications
did not change pH below 1-inch (2.5 cm) depths and had no effect on great laurel growth
[30]. Sprouts may be suppressed with 2,4-D [35,104,128], and the use of surfactants may
improve foliar herbicide uptake and efficiency [37]. Picloram may also be useful in
controlling great laurel [80].
Great laurel produces 1 annual leaf cohort [84] in a growth period that begins in early May [76,76,78]. The flowering period of great laurel varies from late May in the southern parts of its range to August in the northern areas [18,44,66,72,86,92,97,103,106,108,123,126], beginning when approximately 60% of current twig growth is complete [76,78]. Twig growth continues until late summer, when the enlargement of flower buds in August terminates the growing season [76,78]. Fruit develops in September and October [92], with seed fall in November [99].
Leaf mortality occurs during late August and September [84]. Abscission begins in late August, but senescent leaves may remain on great laurel through November [78,86]. In an Appalachian study of great laurel, leaf mortality was primarily due to senescence (as opposed to herbivory or pathogens) [84]. Leaves can remain on great laurel for 7 or 8 years [18,27,76,78,86] under a tree canopy; however, at Virginia study sites where the overstory was absent, leaf survivorship decreased to zero after 3 years. Leaf survivorship is greater in low light environments [84,86] because great laurel chloroplasts are rapidly damaged in high light environments [82,84,86]. Decreased irradiance also results in decreased shoot growth, earlier initiation and cessation of annual growth, and reduced woody biomass in relationship to leaf area [86].
Great laurel reproduces primarily through layering, root sprouting, or stump sprouting, and occasionally by seed when conditions are conducive [96,109]. Despite its shade tolerance, any great laurel regeneration, whether by seed or vegetative growth, requires openings in the canopy before thickets can form or increase in size [91]. Site requirements under which great laurel can reproduce are often quite restrictive; reproduction and colonization by seed often occurs in very scattered patches [90]. An abundance of seedlings has been linked to drier great laurel thickets with more open canopies, while thickets with wetter conditions and denser canopies produce very few seedlings but exhibit profuse layering [44,91,103].
Pollination: Great laurel is insect-pollinated [99].
Breeding system: Romancier [98] reports some evidence of self-pollination in great laurel, implying it is at least sometimes monoecious.
Seed production: Great laurel seeds are produced in capsules, each containing 300 to 400 seeds 1/32 inch long [109]. One study found that a great laurel thicket produced an average of 300 seeds per square foot [99].
Some authors [76,78,123] have observed that only twigs receiving full sunlight flower.
Seed dispersal: Great laurel seeds are dispersed passively or by wind [55,97,109].
Seed banking: Reports of great laurel seed banks are inconclusive. Romancier [99] states that great laurel "seed viability may persist for several years under normal forest conditions". A study of low- to mid-elevation slopes at Coweeta Hydrologic Laboratory in North Carolina found some evidence of great laurel seed banking, though a persistent seed bank could not be unequivocally determined [55]. A study of riparian forest (dominated by black cherry (Prunus serotina), red maple, sugar maple (A. sacharum), and American beech) seed banking on the Allegheny Plateau, Pennsylvania found no evidence of great laurel seed banks [49].
Germination: Great laurel requires moist, partially shaded areas for germination and growth [97,109]. However, because some light is required for germination, the dense shade and thick litter layer that typically develops beneath mature great laurel is an effective physical barrier to seedling establishment [90,99]. No stratification is required for great laurel germination; seeds can germinate as soon as they are released from the capsule [99]. Germination rates range from 75% to 90% [96].
Seedling establishment/growth: Great laurel seedlings may be highly specific in their establishment and growth requirements, including relatively high humidity, moderate light exposure (e.g., lightly wooded forest), and soil pH 3.5 to 5.0 [18,96,105]. Seedlings grow best in partial shade and can survive in deep shade, but do poorly in fully exposed sites [99]. In extremely dense shade, great laurel growth is straggly and crooked, with leaves confined almost entirely to new growth at stem tips. Branches grow only 1 to 2 inches (2.5-5 cm) per growing season, and the number of blooms produced per season is relatively small [105]. In less dense shade, plants are more upright with thick compact stems, leaves more or less all along the stems, branches exhibiting 6 to 8 inches (15-20 cm) terminal growth per season, and many blooms [105]. Plocher [90], in his study of great laurel population dynamics in West Virginia, found no seedlings within a live great laurel thicket. The seedlings that did establish were always in light to medium shade adjacent to thickets (up to 100 feet distant). In general, great laurel exposed to high sunlight (such as with overstory removal) display reduced vigor [109].
Great laurel seedlings grow well on bare mineral soil but may be susceptible to frost heaving [99]. Once established, great laurel has a "tremendous" capacity for avoiding cavitation during freeze-thaw cycles [18].
Vegetative regeneration: Great laurel reproduces clonally [10]. It benefits from overstory disturbance, forming even-aged thickets via rapid vegetative spread [87]. Once a thicket is formed, little outward spread occurs unless the thicket is disturbed [90]. Great laurel sprouts vigorously following disturbance [86,90] and stems layer readily to fill openings in thickets [90]. Layering is one of the principal means of vegetative regeneration in natural stands [78,109]. Branches that bend to the ground, especially on the perimeter of the stand, form a root system where the branch contacts the ground. Shoots and sprouts develop from the root system to contribute to the spread of a stand, and suckers grow profusely when stems are cut [109,128]. A study of Appalachian oak forests found the density of great laurel was significantly greater (P<0.05) in gaps caused by ice storm damage than in understory plots [79].
Great laurel is often found in climax forest associations [26]. It is suited for low light conditions and flourishes in shaded areas [106]. Great laurel is also quick to occupy open sites (often forest gaps) [95,96], and may aggressively invade cleared or disturbed land in areas of high moisture and humidity [90]. Great laurel may not be initially competitive at low density with vigorous overstory trees. However, once established, great laurel is capable of preventing other species from regenerating and competing under its dense canopy [4,53,54,89,95,112].
Where extensive mortality has eliminated most of the overstory, great laurel forms a thick and continuous subcanopy known locally as "laurel slicks" or "laurel hells" [18]. In southern Appalachian forests, great laurel thicket development has been linked to limited livestock grazing [20], fire exclusion [20,43], and the widespread loss of American chestnut trees from chestnut blight [4,127]. The canopy openings resulting from disturbance [4,127], combined with the loss of great laurel inhibition by allelopathic properties of American chestnut litter [114], likely permit the establishment of great laurel thickets, which in turn reduce or even eliminate the recruitment of overstory trees [4,32,37]. The process of gap formation and recolonization plays an important role in the structure and composition of southern Appalachian forests [95,96]. Where great laurel is present, however, it plays a greater role in determining species richness and diversity than gaps or gap size, and alters forest structure. Great laurel dominates the midstory, adversely affecting mid- and understory development and diversity, and limiting advance regeneration of overstory tree species [95,96,112]. Where great laurel was present, Rivers and others [95] found significantly lower (P<0.05) species richness and density in both the herbaceous understory and midstory components of gap regeneration in a Blue Ridge Mountain study of riparian areas. Shade-intolerant midstory species were almost completely eliminated, and shade-tolerant species were severely reduced, preventing recruitment into the overstory [95]. A study at Coweeta Hydrologic Laboratory also found that tree species establishment in canopy gaps of southern Appalachian forests was a function of gap area, age, topographic position, and cover of great laurel. Gaps containing over 50% great laurel cover had significantly lower (P=0.01) tree seedling density (primarily northern red oak) than all other gaps, including those with >50% mountain-laurel cover [19]. Another study at Coweeta, simulating open to closed canopy conditions combined with the presence of great laurel, also suggested that in addition to light limitation associated with great laurel, low soil moisture and allelopathic compounds under great laurel thickets may inhibit red maple success [20]. For more information and discussion regarding the effects of great laurel on forest structure and canopy tree species, see Other Management Considerations.
Heath balds, which are often dominated or codominated by great laurel, are found in both disturbed and undisturbed areas. They have been considered both as pioneer successional communities on rocky and shallow-soiled ridges and as stable "end points" of succession. Some heath balds have developed from forest-heath communities after canopy disturbance [77,119]. Following the loss of the tree layer, deep leaf litter, acidic soils, and dense shade may prevent reestablishment of tree species. Many heath balds, whether the result of disturbance or not, become stable shrub communities [119], with exposure producing microclimate and edaphic conditions to maintain them [77,120].
Great laurel thickets effectively stabilize soil on slopes and stream banks [18,90,96].
Seeding, layering, and cutting are the major means of propagating great laurel [97]. Great laurel propagates well from stem cuttings [59,124,125]; however, based on a series of greenhouse experiments in Poland, increased age of stock plants may reduce rooting of stem cuttings [25]. Hormone treatments are useful in stimulating root development of great laurel cuttings. For further details on hormone treatments and rooting success, see Williams and Bilderbeck [124,125]. Under cultivation, great laurel usually reaches 13 to 16 feet (4-5 m) tall [106].
Mature great laurel seeds germinate with no pretreatment [46] but require light immediately after imbibing for germination [6,11,46]. If imbibed seeds are not exposed to light or if exposure is delayed, secondary seed dormancy may be induced, preventing germination [46]. Romancier [99] found germination rates in a field study ranged from 25% to 71%. A greenhouse study of great laurel seeds and watering regimes found average germination of 79%. The highest germination rate (88%) was reached with intermittent mist irrigation, compared to 77% with mat irrigation or 69% with hand-sprinkling irrigation [34]. Gibberellic acid treatment for 36 hours at 0, 50, 200, and 1000 ppm did not affect germination [34]. In a laboratory study, great laurel germination rates reached 92% to 97% after 21 days. Experimental conditions included photoperiods of >4 hours and alternating temperatures of 77/59 °F (25/15 °C) [11]. Baskin and Baskin [6] report optimum germination of great laurel at 79/70 °F (26/21 °C). Great laurel seed may be kept at room temperature if it will be used within 2 months [97], but seed viability can be maintained for longer periods with cold storage. In a laboratory study, viability was relatively unchanged after 5 years storage at constant 0 °F (-18 °C) and at 39 °F (4 °C), with total germination of 88%. Storage at 73 °F (23 °C) for 4 years, however, essentially reduced viability to zero [45].
Great laurel seedling growth was evaluated under laboratory conditions that included 9 hour days at 64, 72, 79, or 86 °F (18, 22, 26, 30 °C) combined with 15 hour nights at 57 (14 °C), 64, 72, or 79 °F. Total dry matter produced was greatest with 79 °F days and lowest with 64 °F days. The optimum day/night cycle for dry matter production was 79/72 °F. Leaf area was optimized with 64 °F nights, while 72 °F nights maximized total plant, leaf, and stem dry weights [106].
Rhododendron maximum — its common names include great laurel,[2] great rhododendron, rosebay rhododendron, American rhododendron and big rhododendron — is a species of Rhododendron native to the Appalachians of eastern North America, from Alabama north to coastal Nova Scotia.
Rhododendron maximum is an evergreen shrub growing to 4 m (13 ft), rarely 10 m (33 ft), tall. The leaves are 9–19 cm (3.5–7.5 in) long and 2–4 cm (0.79–1.57 in) broad.[3] The flowers are 2.5–3 cm (0.98–1.18 in) diameter, white, pink or pale purple, often with small greenish-yellow spots. The fruit is a dry capsule 15–20 mm (0.59–0.79 in) long, containing numerous small seeds. The leaves can be poisonous. Leaves are sclerophyllous, simple, alternate, and oblong (10 to 30 cm long, 5 to 8 cm wide). It retains its waxy, deep-green leaves for up to 8 years, but once shed are slow to decompose. It produces large, showy, white to purple flowers each June and July.[3]
Rosebay rhododendron is the most frequently occurring and dominant species of Rhododendron in the southern Appalachian region,[4] and occurs occasionally on mesic hill-slopes throughout the upper Piedmont Crescent of the Southeastern United States.
Approximately 12,000 square miles in the southern Appalachians are occupied by this species[5] where it dominates the understory. This species has historically been confined to riparian areas and other mesic sites but takes advantage of disturbed areas where it is present to advance onto sub-mesic sites. It prefers deep well-drained acid soils high in organic matter where it produces a thick, peat-like humus. It prefers low to medium light conditions for optimum carbon gain, and has a tremendous capacity for avoiding cavitation during freeze-thaw cycles.[6] Where extensive overstory mortality has eliminated most of the overstory, this species forms a thick and continuous subcanopy known locally as ‘laurel slicks’ or ‘laurel hells’. Rosebay rhododendron is an important structural and functional component of southern Appalachian forest ecosystems. What isn't clear is whether or not we are in a period of advancement or retreat for this species. For example, on poorly drained sites on ridge or upper slope positions, large areas of rosebay rhododendron, particularly at the high elevations, have recently died out presumably due to the Phytophthora fungus, or due to recent prolonged periods of below-average precipitation. Yet, rosebay rhododendron now occupies sites that historically were free of evergreen understory. There are still important questions to be answered regarding this species to completely understand its role in forest understories.
In North Carolina, USA, Pestalotiopsis which is a genus of ascomycete fungi causes grey-brown spots on the living leaves.[7]
Rosebay rhododendron is clonal. It is capable, however, of reproducing both vegetatively and sexually. It reproduces vegetatively through a process called ‘layering’ where it produces roots from above ground woody parts when in contact with the forest floor. The fruit is produced from showy flowers from March to August. The fruit is an oblong capsule that ripens in the fall, and splits along the sides soon after ripening to release large numbers of minute seed (approx. 400 per capsule).[8] Microsite requirements for seed germination are relatively specific (e.g., high in organic matter such as rotting logs); hence, the majority of reproduction is vegetative resulting in a clonal distribution.
Seeds from rosebay rhododendron are minute and it is estimated that approximately 11 million are contained in 1 kg. Commercial seed production is generally from cultivated hybrids. Seeds from wild sources are not commonly sold commercially. Rosebay rhododendron is a slow-growing shrub and has a very high sprout potential. If mechanical removal is attempted in the case of forest management, extremely high densities are attained by this species in a matter of a few years. Prescribed fire has also been used to control this species but with limited success.[9]
Rosebay rhododendron is a striking and aesthetically pleasing feature of mesic southern Appalachian forests. It is one of the largest and hardiest rhododendrons grown commercially. Several cultivars with white to purple flowers have been selected for the horticultural trade.[10] Where it occurs naturally, it produces a showy, white, pink, or light purple flower primarily in June, but occurs from March into August. Rosebay rhododendron maintains deep-green foliage year round. This species affords protection to steep watersheds and shelter for wildlife. The wood is very hard and is occasionally used for specialty wood products.
For all its prized qualities as a naturally occurring component of the landscape or as plantings in residential and commercial landscaping, rosebay rhododendron can have an inhibitory effect on regeneration of other plant species. There is some evidence to suggest that due to fire suppression and the absence of other cultural activities (i.e., mountain-land grazing), this species has advanced beyond the mesic forest sites into sub-mesic understories.[11] The significance of this movement onto previously unoccupied sites centers around the impacts of rosebay rhododendron on plant succession[12] and resource availability.[13] Rosebay rhododendron is associated with reduced woody and herbaceous seedling abundance throughout its range, and hence poses a serious impediment to the production of wood products. The mechanism(s) by which rosebay rhododendron reduces seedling survival has been the subject of much debate. Possible sources of inhibition include allelopathy, competition for resources including light, physical and chemical attributes of the forest floor and soil, and interactions between some or all sources.[14][13]
R. maximum has also been called:
Rhododendron maximum is the state flower of the U.S. state of West Virginia.
Rhododendron maximum — its common names include great laurel, great rhododendron, rosebay rhododendron, American rhododendron and big rhododendron — is a species of Rhododendron native to the Appalachians of eastern North America, from Alabama north to coastal Nova Scotia.
