Table Of ContentSelbyana 13: 39-49
EFFECTS OF FIRE ON BROMELIADS IN SUBTROPICAL
HAMMOCKS OF EVERGLADES NATIONAL PARK, FLORIDA
KEVIN ROBERTSON AND WILLIAM J. PLATT
Department of Botany, Louisiana State University,
Baton Rouge, Louisiana U.S.A. 70803
ABSTRACT. Responses of five species of subcanopy bromeliads (Tillandsia balbisiana, T. fasciculata, T.
setacea, T. utriculata, and T. valenzuelana) to large-scale disruption by naturally-occurring, low-intensity
fire were inferred from comparisons of populations in burned and unburned subtropical hardwood ham
mocks one year after fire on Long Pine Key in Everglades National Park, Florida. In burned hammocks,
the soil humus was consumed. Many trees were defoliated and/or killed, which opened the canopy. Com
parisons of epiphyte populations in burned and unburned hammocks, which were very similar in pre-fire
characteristics, indicate that fire had few direct effects on these species. Nonetheless, by altering the envi
ronment, fire changed the basic demography (increased mortality, growth, and flowering) of bromeliad
popUlations. Field study of epiphyte seeds on different species of host trees also indicated that short-term
adherence on trunks differed between two species (T. fasciculata and T. utriculata), and between burned
and unburned hammocks. These results indicate that large-scale disturbances of the overstory can result
in rapid changes in population dynamics of epiphytes. Such changes can potentially influence long-term
composition and dynamics of populations of subcanopy epiphytes.
Efectos de incendio sobre bromelias de los bosques subtropicales de madera dura del Parque Nacional
Everglades, Florida.
RESUMEN. Las respuestas de cinco especies de bromelias (Tillandsia balbisiana, T.fasciculata, T. setacea,
T. utriculata, y T. venezuelana) de la canopia inferior del bosque subtropical de madera dura ("hammock")
a las perturbaciones de gran escala debidas a incendios de baja intensidad, fueron inferidas a partir de
comparaciones entre las poblaciones existentes en areas quemadas y no quemadas, observadas un ano
despues de un incendio, en Long Pine Key en el Parque Nacional Everglades, Florida. En las areas quemadas
se consumi6 el humus del suelo. Muchos arboles fueron desfoliados y/o murieron, 10 que determin6 la
apertura de la canopia. Las comparaciones entre las poblaciones de epifitas de areas de "hammock"
quemadas y no quemadas, muy similares antes del incendio, indican que el fuego tuvo poco efecto directo
sobre las especies estudiadas. Sin embargo, a traves del cambio del ambiente, el incendio determin6
modificaciones en la demografia (incremento la mortalidad, crecimiento y floracion) de las poblaciones de
bromelias. EI estudio de campo de la adherencia de semillas de epifitas sobre los troncos de arboles hue sped
de diferentes especies indico que, en el corto plazo, la adherencia difiere entre dos especies (T. fasciculata
y T. utriculata) y entre bosques quemados y no quemados. Estos resultados indican que los disturbios de
gran escala en la canopia pueden resultar en cambios rapidos en la dinamica poblacional de las epifitas.
Tales cambios pueden influir, potencialmente, en la composici6n y en las dinamica de largo plazo de las
poblaciones de epifitas de la canopia inferior.
INTRODUCTION of tolerance of adverse environmental condi
tions.
Studies of the ecology of vascular epiphytes
Disturbances are widely recognized as pro
have focused primarily on the harsh nature of
ducing important changes in the environments
the physical environment in the crowns of trees.
experienced by tropical plants (Brokaw, 1985;
Epiphyte morphology and physiology have been
Platt & Strong, 1989). Responses of plants to
related to specialization for photosynthesis, ab
these changes may be as important as tolerance
sorption of water, and uptake of nutrients in ae
of adverse conditions (Grime, 1977). Responses
rial environments that differ in light levels, mois
of epiphytes (particularly those that inhabit
ture availability, and mineral nutrients (Benzing,
closed-canopy forests) to changes in environ
1990). Epiphytes have been categorized by the
mental conditions produced by natural distur
relative tolerance of different light levels, desic bances have not been studied. In this study, we
cation, and by mechanisms for obtaining nutri explored changes in populations of subcanopy
ents (e.g., Pittendrigh, 1948; Benzing & Renfrow, bromeliads in subtropical hardwood hammocks
1971; Griffiths & Smith, 1983; Gentry & Dod on Long Pine Key in Everglades National Park
son, 1987). Species have been labeled as canopy following a naturally occurring fire. Populations
or subcanopy, based in large part on the degree in burned hammocks were censused one year
39
40 SELBYANA [Volume 13
after the fire and compared to populations in minimize pre-fire differences: 1) all hammocks
adjacent unburned hammocks. were in close proximity (about 1 km apart at the
west end of Long Pine Key); 2) all hammocks
had a similar fire history; prior to 1989, all six
METHODS
hammocks were last recorded as burned in 1940
Study Area (Olmsted et aI., 1983); 3) all hammocks were of
similar size, from 80-100 m diameter, and were
This study was conducted on Long Pine Key, more or less circular in shape. Burned hammocks
a habitat island of about 8,000 ha of oolitic lime 9, 10, and 119 were approximately 0.8 ha, 1.1
stone outcroppings in Everglades National Park, ha, and 0.5 ha, respectively, and unburned ham
Dade County, Florida. Long Pine Key, which has mocks 90, 101, and 113 were approximately 0.8
a maximum elevation of 5 m msl, contains ex ha, 0.5 ha, and 0.6 ha, respectively.
tensive pine savannas dominated by Pinus el
A transect was established through the center
liotti var. densa that interdigitate with and are
of each hammock and extended to the surround
surrounded by short hydroperiod prairies and
ing pinelands. Quadrats of 5 x 5 m were estab
sawgrass savannas dominated by Cladium ja
lished at 20 m intervals along the transects. Two
maicense and Muhlenbergia jilipes (Craighead,
out of three each of burned and unburned ham
1971; Olmsted et aI., 1983; Snyder et al., 1990).
mocks contained 5 quadrats; the third hammock
Hammocks imbedded within the pine savannas
of both types contained four quadrats. In each
contain an arborescent flora (more than 100 spe
hammock, quadrats 1 and 5 (4) were close to the
cies) that is predominantly tropical and West In
edge (but still inside the hammock) and 2-4 (3)
dian in origin (Robertson, 1953; Craighead, 1971;
were in the interior. A total of 700 m2 was sam
Alexander & Crook, 1973; Olmsted et aI., 1983).
pled for this study. Half oft his area was in burned
On Long Pine Key, hammocks typically have a
hammocks, and half in unburned hammocks.
closed canopy around 5 m, with scattered trees
In each quadrat, the species and the sizes of
emergent above the canopy (Olmsted et aI., 1980).
all trees ~2 cm dbh (diameter at breast height)
They are located in areas containing solution holes
were measured and recorded as alive or dead
filled with water. This produces a humid micro
above ground level. In each quadrat, each bro
environment, especially during the wet season
meliad located below 5 m (canopy height) was
from June-November (Phillips, 1940). Ham
surveyed and the following data recorded: spe
mock soils consist of peak deposits of 10-50 cm,
cies, size (measured as length oflargest leaf), lo
depending on elevation and recent fire history
cation (on ground, tree trunk, treefall, branchfall,
(Simpson, 1920; Alexander, 1967; Craighead,
or vine), and height above ground. If present on
1974; Olmsted et aI., 1983). Hammocks on Long
a tree, the size (dbh) and species of the host t~ee
Pine Key have been numbered and mapped
was recorded. Each plant was recorded as ahve
(Olmsted et al., 1983); that numbering system
or dead, and notes were taken on bolting, flow
was used in this study.
ering, and ovary/fruit development.
The impact of the Ingraham Fire on survival,
Survival, Growth and Flowering Following Fire growth, and flowering ofbromeliads was inferred
from differences between plants in burned and
In May 1989, a 50,000 ha lighting-initiated
unburned hammocks. Five of the ten species of
fire (the Ingraham Fire) burned pan of Long Pine
Tillandsia recorded for Everglades National Park
Key. Two hammocks, #8 and #119, were ob
(Avery & Loope, 1983) were included in this
served in detail during the fire. The fire was con
study: (T. balbisiana Schult., T. fasciculata Sw.,
fined to the peat layer; flame lengths rarely reached
T. setacea Sw., T. utriculata L., and T. valen
0.2 m above the substrate. Trees were top-killed
zuelana A. Rich.). Of these five species, T. se
and/or completely killed by girdling of trunks or
tacea and T. valenzuelana liave been designated
roots (but not by crown fires). Observations dur
subcanopy species with atmospheric and tank
ing and shortly after the fire indicated that epi
trichomes, respectively (Snyder et al., 1990). Til-
phytes were not directly harmed unless they were
landsia fasciculata and T. utriculata have been
located on the ground or less than about 1 m
described as canopy species with tank- and at
above ground.
mospheric-absorbing trichomes (Pittendrigh,
In the summer of 1990, approximately one
year after the fire, six small hammocks were se 1948; Griffiths & Smith, 1983). Tillandsia bal
lected for study. Three (#9, 10, and 119) had bisiana (which resembles T. jlexuosa and T. cir
burned and three (#90, 100, and 113) had not cinnata in its morphology) has an incompletely
burned during the fire. These hammocks were developed tank. Tillandsiafasciculata, T. utricu
selected on the basis of the following criteria to lata, and T. balbisiana exhibit positive CO fix-
2
1992] ROBERTSON & PLATT: FIRE AND EVERGLADES EPIPHYTES 41
TABLE 1. Density and mean diameter at breast height (dbh) of all trees and of canopy tree species in burned
and unburned hammocks.
Hammock Type
Burned Unburned
Density dbh Density dbh
(trees/ (trees/
Species hal (x± SE) Range N hal (x ± SE) Range N
Bumelia salicifolia 364 8.3 ± 1.0 3.8-17.0 13 448 5.1 ± 0.8 2.0-15.0 16
Bursera simarouba 84 5.3 ± 1.0 2.0-8.0 3 56 8.6 ± 0.2 5.5-6.1 2
Lysiloma bahamense 56 7.S ± 0.4 7.0-8.0 2 224 8.3 ± 1.8 2.0-18.0 8
Quercus virginiana 336 11.9 ± 1.8 4.5-28.0 12 448 10.9 ± 1.4 2.4-22.0 16
All trees 3,080 6.7 ± 0.5 2-28.0 110 4,172 5.6 ± 0.4 2.0-22.0 149
ation in the dark, which is indicative of CAM no significant differences, so data were not trans
(Crassulacean Acid Metabolism) (Medina, 1974). formed.
RESULTS
Seed Adherence
Comparison of Burned and Unburned
A study of the adherence and initial persistence Hammocks
of seeds on tree trunks was conducted to deter
mine if they were different in burned and un Data from the six hammocks indicated that
burned hammocks. Four hammocks differing in burned and unburned hammocks contained sim
fire history were used in this study. Two ham ilar densities of trees at the time of the fire (TABLE
mocks (#8 and # 119) had burned during the In O. The 25 m2 quadrats in burned and unburned
graham Fire of late May 1989. The other two hammocks contained 7.9 ± 1.7 (SE) and 10.3 ±
hammocks (#4 and #113) were of similar size 1.4 (SE) trees, respectively. These densities were
and were in the same vicinity as the burned ham not significantly different (P > 0.05; Fl .• = 2.157),
mocks, but had not burned within the past sev based on analysis of variance using differences
eral decades. In each hammock, five live canopy in tree density among hammocks as an error term.
trees each of Bursera simarouba, Pinus elliotti In addition, densities of trees of the four most
var. densa, and Quercus virginiana were random abundant overstory species also were similar in
ly selected. The location of each tree was mapped, the six hammocks at the time of the Ingraham
and dbh was measured. Fire (TABLE 1). Quercus virginiana and Bumelia
Seeds from mature seed pods of T. fascicu!ata salicifolia were most abundant in all six ham
and T. utriculata were placed in a horizontal row mocks; Bursera simarouba and Lysiloma baha
1. 5 m above ground by brushing seeds against mense were common in all six hammocks, but
the bark. The location of each seed was marked did not occur in all plots.
with a plastic map tack color-coded for each spe Sizes of trees were similar in burned and un
cies. Ten seeds of T. utriculata were placed on burned hammocks at the time of the fire (TABLE
each tree on 7 July 1990 and were censused 8, 1). The mean sizes oflive trees (all species com
17,23, and 41 days after placement. Ten seeds bined) in the quadrats were not significantly dif
of T. fasciculata were set out 15 July 1990 and ferent (tl•22 = 0.96; P > 0.05) in burned [6.7 ±
were censused on 9, 15, and 33 days afterward. 0.7 (SE) cm dbh] and unburned [5.9 ± 0.7 (SE)
During each census, the number of seeds re cm dbh] hammocks. The ranges of sizes of trees
maining on each tree was recorded. also were similar in the two sets of hammocks.
The number of seeds still attached at each cen Quercus virginiana was the largest tree in both
sus was used in an analysis of variance. The ex burned and unburned hammocks; scattered large
perimental design involved a hierarchical or split individuals were present in all hammocks and
plot design in which individual trees were the in some of the plots. Trees of Bumelia salicifolia,
experimental units in a randomized complete Bursera simarouba, and Lysiloma bahamense,
block design. Census dates were treated as split smaller than those of Q. virginiana in all ham
plots within each cell of the design. Data were mocks, were similar in size in burned and un
tested for homogeneity of variances using an F burned hammocks (TABLE 1).
test involving the largest and smallest variances The fire altered the vegetation structure in the
within the experimental units. Results indicated three burned hammocks. All quadrats in the un-
42 SELBYANA [Volume 13
60
C/) ~ T. balblslana _ T. fasclculata ~ ~
Q) !llillil T. utriculata EZa T. valenzuelana
·0
Q)
a.
CJ)
..c
0
rn
-w
0
.....
C/)
s:::::
rn
-
a.
0
.....
s:::::
Q)
0
~
aQ.)
Quadrat Number
FIGURE 1. The percentage of each species of bromeliad that occurred in the different quadrats in each
hammock. Quadrats I and 5 are located at the periphery of hammocks; quadrats 2-4 are located in the interior
of hammocks.
burned hammocks contained 100% crown cover, category (TABLE 2). All species tended to occur
while all quadrats in the burned hammocks con primarily in the interior of hammocks: periph
tained less than 25% crown cover. Dead trees eral quadrats (#1 and #5) contained many fewer
comprised more than twice as many of the trees epiphytes than did interior quadrats (FIGURE 1).
in the quadrats in burned (78%) as in unburned Species tended to be patchily distributed in the
(34%) hammocks. In addition, mean sizes ofd ead interior of hammocks, with no apparent species
trees in burned hammocks [6.7 ± 0.7 (SE) cm specific distribution. Three species, T. balbi
dbh] were significantly larger [tl,23 = 3.08; P < siana, T. Jasciculata, and T. setacea, were com
0.005] than the mean sizes of dead trees in un mon and occurred in six to eight of the fourteen
burned hammocks [4.6 ± 0.4 (SE) cm dbh]. Many quadrats in both burned and unburned ham
dead trees in burned hammocks were only top mocks. The other two species, T. utriculata and
kilied, and by one year after the fire, resprouting T. valenzuelana, were less common; each oc
had occurred from the trunks, bases, or roots. In curred in only three quadrats in unburned ham
addition, two species (Trema micranthum and mocks (TABLE 2). The three most abundant spe
Carica papaya) had sprouted from dormant seeds cies were located primarily at heights well below
and were present in most quadrats in burned the canopy. The density of T. balbisiana, T. Jas
hammocks as shrubs 1-2 m tall. Neither sprout ciculata, and T. setacea decreased as the height
ing nor germination of Trema and Carica oc above the ground increased (FIGURE 2). Tilland
curred in any of the quadrats in unburned ham sia valenzuelana, which occurred primarily in
mocks, which were characterized by sparse burned hammocks, also tended to be located at
vegetation near ground level. heights below the subcanopy; 59% of the plants
were located in the 0-1.0 m height category. Only
T. utriculata tended to be more abundant in the
Distribution of Bromeliads in Hammocks
canopy; there were large numbers of this species
All five species of bromeliads occurred in at above 5 m in some quadrats.
least two out of the three hammocks in each burn Size of host did not appear important in de-
1992] ROBERTSON & PLATT: FIRE AND EVERGLADES EPIPHYTES 43
termining bromeliad distribution. Shrubs such
as Myrica cerifera, Ardisia escallonoides and Psy o
chotria nervosa, as well as small trees less than ~o'0-C0i oo
I cm in dbh (e.g., Bumelia salicifolia), were com S +1
mon hosts of epiphytes. For each species, more fMi
than half of the individuals (T. balbisiana: 54%,
T. Jasciculata: 68%, T. setacea: 76%, T. utricu 00 \~0
0000
lata: 54%, T. valenzuelana: 63%) occurred on -"<I" +1
trees between 2 and 9 cm in diameter at breast N
fi
height (1.5 m). Since 60% of trees in the transects
were in this size range, the proportions of small
r-or~> o
and large trees with epiphytes were close to the N N ~"",:",o o
observed frequencies of these trees in the ham ~ +1
mocks. or)
M
M
Survival, Growth, and Flowering o""'o:<o""N'-i
Following Fire -~ -+1
~
The proportions of live bromeliads in the M
quadrats were lower in burned than unburned \0
hammocks (TABLE 2); 20.3% of the bromeliads oo<rr-l :Nd
were dead in the burned hammocks, but only ~ +1
2.3% were dead in the unburned hammocks. The o
-.0
percentages of dead plants in burned hammocks
o
differed among species, ranging from less than "<I'M
t-o\O
10% for T. Jasciculata and T. utriculata to over DO +1
50% for T. valenzuelana. The higher percentages r
oo
of dead plants in burned hammocks appears to
have resulted directly from fire and/or indirectly or>
or o
from cold damage following the fire. Although \000 o
dead plants in general did not appear to have S +1
been burned, there may have been some direct M
00
effects of fire on plants located on the ground or
00
very low on tree trunks. The dead plants were \00
00"""';""':
anboot vrea nthdeo mgrloyu dndis.t Irnib buuternde wdi htha mremspoecckts t,o t hhee pigehr t ~ -"-+"1
centages of dead plants of some species (e.g., T.
setacea, T. valenzuelana) increased as the ele
vation above the ground decreased (FIGURE 3).
For T. valenzuelana, the majority of dead plants
of this species below 1 m was charred. Mortality
of this species was probably high because its ver
tical distribution was strongly skewed towards
hammock floors.
For T. balbisiana, T. Jasciculata, and T. se
tacea, there was a decrease in the percentage of
plants in the 0-1.0 m height categories in burned
relative to unburned hammocks (FIGURE 2). Since
percentages of plants in this category included
those that had fallen to the ground, the decrease o
may have resulted from the consumption of such ~
~
plants by fire. This biased the fire-related mor
tality, because more plants were killed by the fire
than were recorded one year after fire.
Bromeliad mortality could have also resulted
indirectly from the fire. A severe freeze occurred
24-25 December 1989. At the research center of
Everglades National Park, located about 2 km
east of the burned hammocks, the temperature
44 SELBYANA [Volume 13
I I
~ burned _ unburned
Tillandsia balbisiana_
-
Tillandsia fasciculata
..c
0>
'(5
:::c
..c
o
C'CI 20
W
.-S
c:
Q)
~
aQ.)
Tillandsia setacea
0-1.00 1.01-2.00 2.01-3.00 3.01-4.00 4.01-5.00
Height Class (m)
FIGURE 2. The percentage of populations of each of three species of Tillandsia that occurred in one meter
height classes in burned (diagonal hatching) and unburned (solid) hammocks.
fell below O°C at 2300 hr and remained below tudes above 26°N along the Florida peninsula
freezing for 10 hours, reaching a minimum of (A. Herndon, pers. comm.).
- 3°C. These measurements, taken in the open, The sizes of plants varied between burned and
probably are close to those experienced in burned unburned hammocks. The mean maximum leaf
hammocks. The settling of cold air might have length was greater for plants in burned than un
killed plants closer to the ground (especially sen burned hammocks for three species: T. balbi
sitive species). There may have been a relation siana, T. Jasciculata, and T. setacea (TABLE 2).
ship between mortality and distribution; mor These lengths were compared using a two-sample
tality was lowest for those species (T. Jasciculata, t-test (Sokal & Rohlf, 1969). The maximum leaf
T. utriculata) with distributions that reach lati- sizes of T. setacea and T. Jasciculata were sig-
1992] ROBERTSON & PLATT: FIRE AND EVERGLADES EPIPHYTES 45
100.-----
80
60
Tillandsia
40 balbisiana
100
.;C:D: 80
«
60
Tillandsia
0>~ - 40 fasciculata
0)
CD 20
Cii
-
() 0
.J:: 100
.Q>
::CrD: 80
..c: 60
0 Tillandsia
CO 40 setacea
W
c 20
Ul 0
C
-aC.O. 100
-
0
Tillandsia
c utriculata
CD 40
0
C~D 20
0...
0
Tillandsia
valenzuelana
0-1.00 1.01-2.00 2.01-3.00 3.01-4.00 4.01-5.00
Distance From Ground (m)
FIGURE 3. Percentage of live subcanopy bromeliads of a given species in different height classes in 1990 in
hammocks burned (diagonal hatching) and not burned (solid) during the Ingraham Fire of 1989.
nificantly larger in burned than unburned ham Size frequency distributions of plants varied
mocks (t,,444' = 31.7; P < 0,0001 and (,,253 = between burned and unburned hammocks
2.36; P < 0,0 I, respectively). The maximum leaf (FIGURE 4). In the burned hammocks, there were
size of T. balbisiana was marginally significantly markedly fewer plants of T. setacea and T. bal
larger in burned than unburned hammocks (t,,159 bisiana in the smallest size category(ies) and
= 1.39; 0.10 > P > 0.05). higher percentages in larger categories. A similar
46 SELBYANA [Volume 13
80
60
Tillandsia
balbisiana
40
20
0
80
60
Tillandsia
fasciculata
(/) 40
(/)
CIS
0 20
~ B-
CD 0 (II l'\o~
N
Ci5
80
.s::
0
CIS 60
W
Tillandsia
.S: 40 setacea
....
(/)
c 20
CIS
0-: 0
.0.. . 80
c
CD 60
.0.. .. Tillandsia
0C.D. 40 utriculata
20
0
80
60
Tillandsia
40 valenzuelana
20
0
5 15 25 35 45 55 65 75 85 95
Length of Leaves (em)
FIGURE 4. Percentage of subcanopy bromeliads in different size categories (based on lengths oflongest leaves)
in hammocks burned (diagonal hatching) and not burned (solid) during the Ingraham Fire of 1989.
pattern characterized T. fasciculata, while insuf less than 10% of the former and less than 2% of
ficient data were obtained on T. utriculata and the latter species bolted in unburned hammocks.
T. valenzuelana to make comparisons of sizes in For the other three species, 6-10% of the plants
burned and unburned hammocks. bolted in burned hammocks, while no individ
A larger proportion of the plants bolted in uals of these species bolted in unburned ham
burned than unburned hammocks (TABLE 2). mocks. A similar proportion of the epiphytes
Over half the T. setacea and almost 30% of the that bolted died in burned (16%) and unburned
T. balbisiana bolted in burned hammocks, while (21 %) hammocks.
1992] ROBERTSON & PLATT: FIRE AND EVERGLADES EPIPHYTES 47
100 __- -----------------------------------------~
80
ctS
>
.~
::J
Cf) 60
'"0
Q)
Q)
Cf)
...... 40
c
Q)
.(..).
aQ..)
20
O+o- ----------1-0. ----------2~0r _---------3-0~ ----------~4 0
Days After Placement of Seeds
FIGURE 5. Percent survival of seeds of Tillandsia fasciculata (squares) and Tillandsia utriculata (circles) in
experimental cohorts placed on trunks of trees in burned (closed symbols) and unburned (open symbols) ham
mocks in Everglades National Park.
Seed Adherence 1y, but significantly (P < 0.05) higher in un
burned than burned hammocks (FIGURE 5). The
Persistence of seeds of both T. utriculata and
observed differences between burned and un
T. Jasciculata decreased most rapidly during the
burned hammocks resulted from differences in
initial time period after they were placed on trees
persistence prior to the second census. The spe
(FIGURE 5). Analysis of variance indicated that
cies of host tree did not influence persistence of
significant differences in numbers of seeds oc
seeds. There were no significant differences among
curred only during the first time interval (P < the other three species (Quercus virginiana, Pinus
0.001 for both species); subsequently, there were
elliotti, and Bursera simarouba) in persistence of
no significant differences in persistence for seeds seeds (P > 0.05). Two- and three-way interac
of either species in burned or unburned ham
tions among treatments were also not significant.
mocks or on trees of different species (P > 0.05
for all comparisons).
Persistence was greater for seeds of T. Jascicu
DISCUSSION
lata than for seeds of T. utriculata. Although
significant at all time intervals (P < 0.001), the Among the most pronounced differences in the
differences resulted from considerably lower per demography of epiphytes in burned and un
sistence of T. utriculata during the initial week burned Everglades hammocks were increases in
that seeds were present on the trees (FIGURE 5). mortality, flowering, and size of plants in burned
Subsequently, persistence of seeds of both spe hammocks. Because the hammocks selected for
cies was high. Thus, although seeds of T. utricu study were located in close proximity and were
lata were placed out in the field a week earlier similar in size, fire history, and tree species com
than those of T. Jasciculata (pods of the former position and size, the observed differences in epi
dehisce earlier during the growing season), the phyte demography were inferred to have resulted
differences in the lengths of time that seeds were from fire and/or changes in environmental con
present in the field did not cause the observed ditions occurring after fire. Although the Ingra
differences in persistence of seeds. ham Fire damaged some epiphytes, mortality re
Persistence of seeds of both species was slight- sulting from fire was restricted to plants closest
48 SELBYANA [Volume 13
to or on the ground. While these plants might sera simarouba) frequent sloughing of the bark
have flowered (if not killed), their seeds would may greatly lower the chances of survival on
not likely have colonized other tree trunks. Thus, those hosts (Benzing, 1981, 1990).
direct mortality from fire probably had minimal Reproduction, dispersal, and colonization, as
impact on the post-disturbance population dy well as rapid growth, occur in the intervals be
namics of epiphytes in the Everglades ham tween fires. The growth rates suggested by size
mocks. differences between epiphyte populations in
Changes in the Everglades hammock environ burned and unburned hammocks indicate that
ment following fire may have altered epiphyte at least some species may be capable of rapid
population dynamics. The loss of canopy cover growth under high light conditions that occur
and organic humus on the hammock floor ex following disturbances. Humidity may also re
posed the epiphytes in burned hammocks to more main fairly high in the interior of opened ham
extreme environmental conditions-drier and mocks, especially for subcanopy plants, because
colder (during the freeze of December 1989) of the dense ground cover of plants that have
than those in unburned hammocks. Differential resprouted or that have initiated growth shortly
reductions in population sizes ofbromeliads (by after the fire. Thus, survival and growth of ju
events such as a freeze) could potentially have veniles in high light environments of burned
an impact on the relative abundances of bro hammocks may be higher than hammock edges
meliad species in hammocks recovering from fire or in more exposed habitats such as cypress
as a result of reduced flowering and seed dis swamps (Benzing, 1978, 1981, 1990).
persal. During the transient period of high light con
Increased flowering of bromeliads in burned ditions, species that are normally characterized
hammocks occurred in both the canopy and in as canopy CAM species with tank and atmo
the subcanopy. Increased light levels following spheric trichomes (i.e., T. utriculata, T. Jascicu
removal of the canopy may have triggered bolt lata; Pittendrigh, 1948; Griffiths & Smith, 1983),
ing. Alternatively, increased flowering may be a as well as other known CAM species (T. balbi
result of fire. Abeles (1973) summarized the ef siana; Medina, 1974), may colonize and grow at
fects of ethylene (or ethylene analogs) produced low heights along trunks of suitable hosts in the
during the combustion of organic materials as Everglades hammocks. These species appear to
stimulatory to flowering in the Bromeliaceae. be capable of surviving intervening periods with
Long exposure to low concentrations of ethylene lower light levels and then responding again when
have been shown to have a considerable stim the forest overstory is reopened. As a result, dif
ulating effect on flowering; such conditions may ferences in vertical height distributions of dif
well have occurred in the Everglades hammocks, ferent species of epiphytes may be less apparent
many of which burned for several days and were in Everglades hammocks than in other forests
full of smoke (especially at night). Ethylene levels which are less likely to experience periodic dis
may also be increased by cold damage (Abeles ruptions of the canopy.
1973; Field 1985). Increased flowering in burned Our study of the effects of the Ingraham Fire
hammocks might also have resulted from cold on epiphyte populations in Everglades ham
damage during the December 1989 freeze. mocks suggests that periodic openings of ham
As suggested by our study and those of Benzing mocks are punctuations that interrupt longer in
(1978, 1981), colonization of bark surfaces is not tervening phases ofg reater stasis. We suggest that
a random process. First, there appear to be dif the ephemeral periods of opening by disturbance
ferences among epiphyte species in the adherence are likely to be times of rapid turnover that result
of seeds to the bark of trees. In our study, T. from changes in mortality, reproduction, dis
Jasciculata seeds were more likely to adhere to persal, and growth. As a consequence, the com
tree trunks than were T. utriculata seeds; pre position and dynamics of subcanopy epiphyte
sumably these differences reflect differences in populations can change. Hence, observed pat
coma morphology. Second, although dispersal is terns in the distribution and abundance of epi
undoubtedly enhanced by the opening of the for phytes may reflect the nature and pattern of dis
est by fire, it is likely to be a local phenomenon, turbances (and responses of populations to
with seeds having the greatest likelihoods ofim environmental conditions following distur
migrating onto nearby trees and at lower heights bances), more than the conditions experienced
than onto parent plants (Benzing, 1990). Third, in intervening years.
if seeds do land on hosts, there is a strong like
lihood that the seeds may be transported down
ACKNOWLEDGMENTS
the trunks by heavy rainfall, skewing the distri
butions of new recruits towards the forest floor. Monica Alvarez, Roger Kieffe, Greg Miller,
Fourth, although seeds readily become attached and Mark Miller assisted with field work. Alan
to some potential hosts (e.g., Pinus elliotti, Bur- Herndon, Harold Slater, and John Stenberg pro-
