2008 SOUTHEASTERN NATURALIST 7(4):571–580
Host-tree Selection by an Epiphytic Orchid,
Epidendrum magnoliae Muhl. (Green Fly Orchid), in an
Inland Hardwood Hammock in Georgia
Bradley J. Bergstrom1,* and Richard Carter1
Abstract - We characterized the tree community of a mesic hardwood hammock
in south-central Georgia as an oak-pine-hickory forest, with Liquidambar styracifl
ua (Sweetgum), Magnolia grandifl ora (Southern Magnolia), and Ilex opaca Ait.
(American Holly) as subdominants. We surveyed this forest for colonies of the
most northerly distributed epiphytic orchid in the Western Hemisphere, Epidendrum
magnoliae (Green Fly Orchid), and recorded the species and trunk diameter of 112
host trees (phorophytes) as well as the height and size of each orchid colony. We
calculated a selectivity index (SI) to compare phorophyte frequency with availability,
based on a point-transect survey. Green Fly Orchid occurred on 8 species of
hardwood trees, but had a strong preference for Southern Magnolia as a host and a
moderately strong preference for Quercus virginiana (Live Oak). Host trees were
much larger (presumably older) than the average of available trees, and that effect
was strongest for the most preferred host. Orchid colonies also occupied significantly
greater areas on individual Southern Magnolia than on other phorophytes. It is likely
that old-growth Southern Magnolia and Live Oak trees are critical to the viability of
this population of Green Fly Orchid, which is rare in inland forests in Georgia. In
addition to being the most persistent epiphyte substrates in this environment, their
broadleaf evergreen canopies—which would be especially true of Southern Magnolia
—may provide the most favorable microclimates in terms of shade, humidity, and
It has been estimated that epiphytic vascular plants comprise 10% of all
vascular plant species (Madison 1977) and 70% of all orchid species (Gentry
and Dodson 1987). Further, 60% of all epiphyte species are members of
the Orchidaceae (Kress 1986). Vascular epiphytes in general and epiphytic
orchids in particular attain their peak species diversities in tropical forests,
especially in the Neotropics (Gentry and Dodson 1987), but they also occur
in subtropical forests, with many species found in southern Florida (Luer
1972). There are 7 species of Epidendrum found in the continental US, all
of which are epiphytic; 6 of these are limited to subtropical hammocks of
peninsular Florida (Hágsater 2002). Epidendrum magnoliae Muhl. (= E.
conopseum W.T. Aiton) (Green Fly Orchid) is found in widely scattered
patches of humid coastal plain forest in 7 southeastern states from North
Carolina southward through central penisular Florida and westward into
southern Louisiana. Populations in eastern Mexico in the states of Nuevo
1Department of Biology, Valdosta State University, Valdosta, GA 31698. Corresponding
author - email@example.com.
572 Southeastern Naturalist Vol. 7, No. 4
León, San Luis Potosí, and Tamaulipas have been treated as E. conopseum
var. mexicana L.O. Williams (Hágsater 2002, Luer 1972). Green Fly Orchid
is the only species of epiphytic orchid found in the continental US outside
of Florida and has the northernmost distribution of any epiphytic orchid in
the Western Hemisphere (Correll 1950).
In some studies in Neotropical forests (e.g., Frei 1973), certain epiphyte
species showed marked preferences for host tree (phorophyte) species,
whereas in other studies little or no host specificity was seen (Trapnell
and Hamrick 2006, Zimmerman and Olmstead 1992). Generally, epiphytes
occur on a number of different phorophytes, but with variable frequency
(Benzing 1990). Possible mechanisms for host-tree or phorophyte specificity
in epiphytic orchids involve microclimate (see Callaway et al. 2002),
propensity for exfoliation (bark sloughing), presence of certain bark chemicals
(Frei and Dodson 1972), other bark characteristics (Benzing 1981),
and distribution of mycorrhizal fungal symbionts. Epiphytic orchids have
mycotrophic nutrition (carbon, other nutrients, and possibly water are supplied
to the plant by mycorrhizal fungi) and have been shown to require a
mycorrhizal symbiont for seed germination (McKendrick et al. 2000, Otero
et al. 2005).
Green Fly Orchid shares its geographic range in the southeastern coastal
plain with the “atmospheric” epiphyte Tillandsia usneoides (L.) L. (Spanish
Moss) and the rooted, epiphytic fern Pleopeltis polypodioides (L.) E.G.
Andrews and Windham (Resurrection Fern), although the latter two are far
more common within that range. These species also commonly attach to
branches, whereas Green Fly Orchid frequently grows on the main trunk
of its host. Outside of Florida, Green Fly Orchid is limited to near climax
hardwood forests and swamp forests within the most humid microclimates
available, which means hammocks primarily found along the coastal strip
and rarely found inland (Wharton 1989).
Correll (1950) reported that Epidendrum magnoliae (as E. conopseum)
grew primarily on Magnolia grandiflora L. (Southern Magnolia), and
Quercus virginiana Mill. (Live Oak), but that it had also been collected on
Acer rubrum L. (Red Maple), Carpinus caroliniana Walter (Hornbeam),
Fagus grandifolia Ehrh. (American Beech), Juniperus virginiana L. (Eastern
Red Cedar), Liquidambar styraciflua L. (Sweetgum), Nyssa spp. (Black
Gum or Tupelo), and Taxodium distichum (L.) Rich (Baldcypress). We have
also observed Green Fly Orchid on Tilia americana L. (Basswood) in a
bluff forest community along the Withlacoochee River in western Lowndes
County, GA, and epilithic on sandstone cliff faces (“Altamaha Grit” formation)
at “Broxton Rocks” in Coffee County, GA (Patrick et al. 1995; R.
Carter, unpubl. data).
In this study, we attempt to characterize the tree community of a rare inland
hardwood hammock in Georgia (Wharton 1989), which hosts a sizable
population of Green Fly Orchid, and examine the host-tree distribution and
specificity of this epiphytic orchid within this community. We are not aware
of any other similar studies of host-tree selection for this species.
2008 B.J. Bergstrom and R. Carter 573
Dudley’s Hammock, owned by Moody Air Force Base, is a relatively
undisturbed, elevated area, ca. 61 ha in size, within the Grand Bay wetland
complex, which comprises ca. 7000 ha of shallow Carolina bays or pocosins
and headwater streams in northeastern Lowndes and southwestern
Lanier counties in extreme south-central Georgia. It is located 17.6 km
NE of Valdosta at 30°57'02"N, 83°09'49"W (NAD27). The hammock rises
2–3 m above the surrounding cypress-gum swamps and pine fl atwoods and
is noted as a rare inland example in Georgia of undisturbed mesic hardwood
hammock, which is also characterized as lowland broadleaf evergreen forest
(Quarterman and Keever 1962, Wharton 1989). In the classification scheme
of The Nature Conservancy, the vegetation of Dudley’s Hammock appears
to be most closely related to the Southern Coastal Plain Oak Dome and
Hammock (CES203.494), with characteristics of the Southern Coastal Plain
Hydric Hammock (CES203.501), but lacking Sabal palmetto (Walter) Lodd.
ex Schult. & Schult. f. (Cabbage Palm; cf. NatureServe 2008).
Dudley’s Hammock is roughly bisected by an east–west 2-track access
road, and there has been some recent and historic disturbance (partial
clearing, burning) resulting from military activities on the northern half
(Bergstrom et al. 1994). Therefore, we limited our survey to the portion of
the hammock south of the road, which is relatively undisturbed and where
Green Fly Orchids had been observed.
Dominant trees in this less disturbed portion of the hammock include
Southern Magnolia, Live Oak, Q. nigra L. (Water Oak), Q. alba L. (White
Oak), Q. michauxii Nutt. (Swamp Chestnut Oak), Ilex opaca Ait. (American
Holly), Carya glabra (Mill.) Sweet (Pignut Hickory), Sweetgum, Nyssa
sylvatica Marshall (Black Gum), Pinus glabra Walter. (Spruce Pine) and
P. taeda L. (Loblolly Pine). Owing either to the small size and isolation of
Dudley’s Hammock, or to soil properties, American Beech is not found at
this site (Bergstrom et al. 1994), whereas it is a dominant tree of similar
hammocks in northern Florida (Monk 1968).
Arboreal vascular epiphytes commonly found in Dudley’s Hammock
include Spanish Moss, Resurrection Fern, and Green Fly Orchid. Green Fly
Orchid is protected in Georgia; its legal status is Unusual and its rank is S3
among Special Concern Plant Species (Georgia Department of Natural Resources
2007, Patrick et al. 1995).
In August 1994, working in a three-person team, we intensively surveyed
trees in the southern portion of the hammock for presence of the
epiphyte along a series of north–south overlapping compass transects. One
team member monitored the compass bearing, while the other two members
scanned trees using Pentax® 7×50 6.2o binoculars. The presence of Green Fly
574 Southeastern Naturalist Vol. 7, No. 4
Orchid was confirmed by two team members, and species and diameter at
breast height (DBH; cm) of phorophyte plus estimates of area of phorophyte
surface colonized by Epidendrum and mean height above ground of epiphyte
colony were recorded. At the time of the survey, fronds of Resurrection Fern
were in a relatively dehydrated state, which increased the visibility of Green
Fly Orchid plants.
In August 2006, we censused tree-species composition of the southern
portion of the hammock by point-quarter sampling every 20 m along three
180-m transects, yielding 10 sampling stations per transect for a total of 120
quadrants (and point-quarter trees). The transects were placed by a stratified
random method designed to traverse the area where Green Fly Orchid was
most commonly found, and they were at oblique angles to each other (compass
bearings 20º, 140º, and 240º). All observations were independent. The
nearest tree (up to 15 m) to each point in each of 4 quadrants (NW, NE, SE,
SW) that was at least 5 cm in DBH was chosen as the point-quarter tree, and
its species and DBH were recorded.
Basal area for each tree was determined by the formula πr2 where r =
DBH/2. Tree community profiles were constructed both by relative stem
frequencies and by species importance values (ln basal area per species).
A selectivity index (SI) modified from Ivlev (1961) was used to determine
host-species selectivity of the orchid, based on that host-tree’s availability
in the habitat, as follows:
SI = (Hs - As) / (Hs + As),
where Hs was the relative frequency of the host species among the sample
of actual host trees, and As was the relative frequency of that tree species
among the 119 trees from the point-quarter survey. For the pool of available
host-tree species for this index and to calculate As, we did not include pines
(which are not known to be hosts of Epidendrum), and we included only
species belonging to genera which actually were recorded as host trees in
this study. We did a separate calculation of SI using relative basal areas of
trees, by species, that were actual host trees (Hs) and relative basal areas,
by species, of trees from the point-quarter transect (As), again including or
not including species in the latter pool as per the above criteria. We present
these two indices for each species as SIS for stems and SIB for basal area.
This SI index can range from -1.0 for perfect avoidance to 1.0 for perfect
selectivity, or total preference. An SI of 0.0 indicates the tree species serves
as a host tree in the exact proportion that it is available in the habitat with
neither preference nor avoidance.
In only one of the 120 point-transect quadrants did we fail to identify a
point-quarter tree (because none of sufficient size occurred within 15 m of
the point); therefore our random sample of trees to estimate host-tree availability
and to characterize species composition of the hammock consisted of
2008 B.J. Bergstrom and R. Carter 575
119. Twelve species of trees were included among these, which accounts for
nearly all of the tree-sized woody species that occur on the hammock, except
for Black Gum. Of these 119 trees, 19 were pines and 85 belonged to genera
that were found to be host trees in this study. The latter formed the pool of
available hosts and, by the criteria for inclusion, included 1 tree —of a species
(Q. hemisphaerica Bartr. ex Willd. [Darlington Oak]) that did not serve
as a host tree. Aside from the 2 pines, American Holly (n = 14; mean DBH
= 10.1 cm) was the only other species that had substantial representation in
the point transects, but was not included in the pool of available hosts.
By stem count, Water Oak was the most abundant tree along the survey
Figure 1. a (top): Most abundant tree species on Dudley’s Hammock by stem count,
based on 119 point-transect trees. b (bottom): Species importance plot for Dudley’s
Hammock trees based on ln basal area of 119 point-transect trees. See Methods for
576 Southeastern Naturalist Vol. 7, No. 4
transects, followed by Pignut Hickory (Fig. 1a). Live Oak and Southern
Magnolia were 4th and 5th most abundant, respectively. By ln basal area, a
top tier of dominant species was apparent, including both pine species, three
oaks (Live Oak, Water Oak, and White Oak), and Pignut Hickory (Fig. 1b).
Sweetgum, Southern Magnolia, and American Holly formed a second tier of
subdominant species (Fig. 1b).
The orchid survey identified 112 host trees of 8 species; 60 (54%) of
these were Southern Magnolia, 35 (31%) were Live Oak, 7 (6.2%) were
Sweetgum, 3 each (2.7%) were Pignut Hickory and White Oak, 2 (1.8%)
were Swamp Chestnut Oak, and 1 each (0.9%) was Water Oak and Black
Gum. Three of the host trees had recently died (2 Southern Magnolia, 1
Swamp Chestnut Oak).
Both SI indices indicated that Green Fly Orchid showed a strong preference
for Southern Magnolia as a host and a moderately strong preference
for Live Oak, but the difference between these two preferred hosts and
the strength of selectivity for Southern Magnolia were greater for the SIB
(Table 1). SIS indicated that Sweetgum and Swamp Chestnut Oak were nearly
random with respect to selection by the epiphyte and that the remaining 4
species were strongly avoided (Black Gum is not included here, because 1
tree served as a host, but 0 trees were found on the point-quarter survey). A
similar pattern was shown for these 4 less-preferred host trees by the SIB,
except none was as close to random (all were avoided to some degree).
Host trees were much larger than available trees, being nearly twice the
DBH for the entire sample, three times the DBH for Southern Magnolia,
and 67% larger for Live Oak; there was no size difference between host
and available trees for Sweetgum (Table 2). Among the 3 most common
host trees, Green Fly Orchid covered a significantly larger area per host
tree on Southern Magnolia (mean = 11.61 cm2, F2,99 = 8.71, P < 0.0001)
than on the other two hosts. The range of mean heights above ground
where orchid colonies grew was also significantly greater (mean = 5.52 m,
F2,99 = 13.74, P < 0.001), and the minimum mean-height was significantly
lower (mean = 4.37 m, F2,99 = 9.30, P < 0.001) for Southern Magnolia than
for the other two hosts.
Table 1. Selectivity indices (SI) based on relative frequencies of occurrence (SIS) and relative
basal areas (SIB) for the 8 Dudley’s Hammock tree species that hosted Epidendrum magnoliae
(Green Fly Orchid). SI ranges from -1.0 for perfect avoidance to 1.0 for perfect selection, with
SI = 0.0 signifying neutral or random selection. Note: N. sylvatica was not encountered as a
potentially available host tree on the point-transect survey. See Methods for more details.
Host-tree species n SIS SIB
Magnolia grandifl ora (Southern Magnolia) 60 0.583 0.863
Quercus virginiana (Live Oak) 35 0.378 0.275
Liquidambar styracifl ua (Sweetgum) 7 -0.061 -0.678
Carya glabra (Pignut Hickory) 3 -0.786 -0.942
Quercus alba (White Oak) 3 -0.557 -0.961
Quercus michauxii (Swamp Chesnut Oak) 2 -0.136 -0.232
Quercus nigra (Water Oak) 1 -0.941 -0.883
Nyssa sylvatica (Black Gum) 1 n/a n/a
2008 B.J. Bergstrom and R. Carter 577
From our survey, Dudley’s Hammock can be characterized as a pine-oakhickory
dominated forest, with Southern Magnolia, Sweetgum and American
Holly as subdominants. Excepting American Beech, most of the elements
of the climax forest overstory of the southern mixed hardwood forest (sensu
Quarterman and Keever 1962) were present, but the co-dominance of Loblolly
Pine and Water Oak may indicate some recent disturbance, which means
the hammock is in a subclimax state at present. Nevertheless, it is a densely
shaded and humid microclimate with abundant growth of epiphytes and as
such provides one of the few habitats in the region for Green Fly Orchid.
Although occurring on 8 different hardwood tree species in Dudley’s
Hammock, Green Fly Orchid had a very strong preference for Southern
Magnolia as a host and a moderately strong preference for Live Oak. The
stong host preferences yet lack of strict phorophyte specificity of Green Fly
Orchid observed at Dudley’s Hammock is not surprising given earlier reports
of this species (Correll 1950) and other epiphytic orchids (Zimmerman
and Olmsted 1992) occurring on a range of host species. Laube and Zotz
(2006) showed the distribution of 103 vascular epiphyte species in a lowland
tropical forest to be neither host-specific nor random.
At Dudley’s Hammock, both Loblolly Pine and Spruce Pine have high
importance values (Fig. 2) and bark with markedly different physical characteristics.
The bark of Spruce Pine is distinctively ridged and furrowed
Table 2. Comparison of mean tree sizes (DBH in cm) of tree species hosting Epidendrum magnoliae
(Green Fly Orchid) (n ≥ 3) and the pool of “available” trees from the point transect. See
Methods for more details.
Species n DBH S.D. t P
All Hosts 112 39.8 13.0
All Available 85 20.3 18.8 8.59 <0.0001
Magnolia grandifl ora (Southern Magnolia)
Host 60 37.7 9.8
Available 12 10.6 4.0 15.84 <0.0001
Quercus virginiana (Live Oak)
Host 35 53.9 22.1
Available 12 32.2 16.0 3.65 0.0012
Liquidambar styracifl ua (Sweetgum)
Host 7 13.6 3.8
Available 6 14.5 6.7 -0.32 0.82
Carya glabra (Pignut Hickory)
Host 3 17.2 0.72
Available 19 16.8 8.82 0.23 0.82
Quercus alba (White Oak)
Host 3 17.0 6.7
Available 8 33.7 16.4 -2.39 0.044
578 Southeastern Naturalist Vol. 7, No. 4
and perhaps structurally more similar to Live Oak than to its congener
Loblolly Pine. However, Green Fly Orchid is absent from both species.
Presumably, chemical incompatibility between epiphyte and phorophyte
accounts for the complete absence of Green Fly Orchid from Loblolly Pine
and Spruce Pine. Laboratory studies have shown chemical attributes of
bark may affect germination and early development by epiphytic orchids
(Frei and Dodson 1972).
Southern Magnolia and Live Oak have very different bark characteristics,
growth habits, and patterns of branching and leaf abscission.
The low, broad crown of Live Oak with its massive spreading branches
presents a greater horizontal (or near-horizontal) surface for colonization
by epiphytes than Southern Magnolia with its more upright habit, more
cylindrical form, and absence of massive spreading branches. The bark of
Live Oak is thick and rough with prominent ridges and furrows, whereas
that of Southern Magnolia is smooth and relatively thin. It is presumed
that bark development in Southern Magnolia is slower than in Live Oak
and that diminished exfoliation would result in reduced shedding and thus
greater persistence of epiphytes. The predominance of Green Fly Orchid
on phorophytes with such markedly different physical bark characteristics
suggests other factors more strongly influence host selection. Unlike the
other, less-preferred phorophyte species observed, both Southern Magnolia
and Live Oak have a dense evergreen canopy that would provide deep
shade and decrease evaporative water loss year-round, including winter
when ambient humidity is lower.
Southern Magnolia and Live Oak differ in their patterns of leaf abscission.
Southern Magnolia is distinctly evergreen, and Live Oak is barely evergreen
with its leaves gradually falling during late winter, especially just prior to
the initiation of new growth in early spring. The absence of full-canopy
protection in Live Oak could make Green Fly Orchid more vulnerable to
desiccation and frost effects during late winter and early spring. This lack
of canopy protection may be partly compensated, as we observed, by orchid
colonies often growing under the horizontal limbs of large live oaks. Inland
populations of Green Fly Orchid near the northern limit of its range are
presumably all the more vulnerable to freezing temperatures, most likely
making frost protection an even more critical factor at Dudley’s Hammock.
Other studies have shown a positive correlation between the occurrence
of vascular epiphyte species and large host-tree size, presumably resulting
from greater available surface area and longer time for colonization provided
by larger, older phorophytes (Catling and Lefkovitch 1989, Clement et al.
2001, Dunn 2000, Migenis and Ackerman 1993, Muñoz et al. 2003). Given
that no host trees were encountered among the 119 randomly chosen pointquarter
trees and that host trees were much larger than the average for those
randomly encountered, it was also apparent that Green Fly Orchid generally
selected (and/or persisted on) only the largest host trees. Thus, the largest
and oldest Southern Magnolia and Live Oak trees are vital to this popula2008
B.J. Bergstrom and R. Carter 579
tion of Green Fly Orchid. A study of diversity and host-tree preference in a
temperate rainforest in southern Chile suggests combinations of particular
tree species and sizes promote epiphyte diversity (Muñoz et al. 2003). While
the vascular epiphyte diversity, actual and potential, for Dudley’s Hammock
is much lower than reported by Muñoz et al. (2003), the results of our study
nevertheless suggest habitat with a mixture of mature trees of Southern
Magnolia and Live Oak is essential for the conservation of large, viable
populations of Green Fly Orchid.
James Lusk and Carlton Wilson assisted with fieldwork in 1994. Financial support
for the initial epiphyte survey was provided by the Department of Defense, US Air
Force, through The Nature Conservancy of Georgia (Contract No. M6700491D0010-
5W01). Gregory Lee and Michael Burton provided access to the study site in 2006.
The Valdosta State University Faculty Research Fund paid publication costs. Two
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