Download Patterns of primary succession on granite outcrop surfaces

Patterns of Primary Succession on Granite Outcrop Surfaces
Donald J. Shure; Harvey L. Ragsdale
Ecology, Vol. 58, No. 5. (Sep., 1977), pp. 993-1006.
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Ecolo,yv ( 1977) 58: pp. 993-1006
P A T T E R N S O F PRIMARY S U C C E S S I O N ON G R A N I T E OUTCROP SURFACES1 Ah.strtrct. The patterns of primary succession were 5t~1diedin soil-idand c o m m ~ ~ n i t i eon
s a granite
in area and depth over time and the soil becomes
outcrop in Georgia. The island c o m m ~ ~ n i t i eincrease
s
more organic. The \trong moisture and temperature fluctuation, that occur in shallow pioneer soils are
significantl) reduced in the later ,rages. Plant biomass and vertical 5tratification increase t h r o ~ ~ g h o u t
succession a5 larger plant specie, inbade the deeper communities. A small hinter annual is the
dominant pioneer species. Lichens. annuals. and eventually perennial species Invade as succe\,ion
progres\es. Inter5pecific competition for moisture and nutrient, regulates plant species composition at
in den\it). bionias\, and dibersit) throughout
succe\sibe stages. Macroarthropod p o p ~ ~ l a t i o nincrease
s
,~~cces,ion.The few \oil microarthropod species that occur in the pioneer stages often exhibit rapid
density oscillations in the shalloh substrates. The deeper and more environmentally constant s u b t r a t e
of later stage, contains a greater variet) of niicroarthropods.
Biotic dibersity general11 increase, d ~ ~ r i npriniarl
g
succession on the outcrops. Plant dicer,ity
peaks at intermediate stages while microarthropod and macroarthropod diversity increa\e from
pioneer through later 5tages. The strong ph),ical factors on the outcrops determine the rate and extent
of comniunit1 development in particular soil-islands. Howeber. as the many soil-islands undergo
succes\ion the) conberge in comniunit1 characteristics \uch as total densit). biomass. and diver5ity.
IN
l R O D U t I ION
Successional studies over the past f e u decades have
of ecosystem
greatly contributed to our ~~nderstanding
functioning. Houever. most successional s t ~ ~ d i have
es
concerned a single taxonomic group (Keever 1950.
Bard 1952. Pearson 1959. Chaduick and Dalke 1965.
Bazzaz 1968. Reiners et al. 1971. Kricher 1973.
Nicholson and Monk 1974) o r an intensive study of the
structural and fi~nctional relationships at particular
stages (Golley 1960, 3965. Odum 1969. Odum et al.
1962. Wiegert et al. 1967. Menhinick 1967. Shure
1973). Ecosystem changes over most of a sere have
generally remained undocumented (Olson 1958).
Therefore, holistic approaches are needed to more
fully determine the processes of ecosystem development (Margalef 1968. Odum 1969. McNaughton and
Wolf 1973).
The present study u a s conducted to gain further
understanding of the patterns of primary succession on
a southeastern granite outcrop. The granite outcrops
of the Southeast possess numerous plant communities
undergoing primary succession. Considerable information has accrued concerning the composition and
successional relationships of outcrop plant communities (Whitehouse 1933, Oosting and Anderson
1939. McVaugh 1943. Keever et al. 1951. Winterringer
and Vestal 1956. Burbanck and Platt 1964. Cumming
1969). These communities occur in granite depressions
uhich result from exfoliation and ueathering of the
rock surface (Hopson 1958). Soil building in granite
' blanuscript received 3 Februar) 1976; accepted 14 April
1977.
depressions is considered a more significant process in
plant succession than mat formation and soil development on exposed rock surfaces (Burbanck and Platt
1964. Snyder 1971). Recent outcrop st~ldieshave concerned bioenergetics (Lugo 1969) and material cycling
(Meyer et al. 1975. Hay 1973) at particular stages and
plant responses to competition and limiting factors
(Cumming 1969. Mellinger 1972. Sharitz and McCormick 1973. McCormick et al. 1974).
Burbanck and Platt (1964) described and classified
plant communities on several outcrops around Atlanta, Georgia. They divided the "island communities"
into diamorpha (D), lichen-annual herb (L,4). annualperennial herb (,4P). and herb-shrub (HS) stages.
Community separation u a s based on correlations in
maximum soil depth and characteristic flora.
Diamorpha communities originate as mineral soil accumulates in exfoliation depressions. Diur?ror-phu
c~?ino.su(syn. Srtlrrt?~st?~crllii.s ee Sharitz and McCormick 1973). a uinter annual, is the dominant plant
species in these pioneer communities (Wiggs and Platt
1962). Lichens (Clncloirin) are uashed or bloun in and
develop as the lichen-annual stages are initiated. The
lichen cover traps debris and the gradual increase in
soil depth leads to greater soil moisture and more
soil-organic matter (Burbanck and Platt 1964). These
substrate changes favor the successfi~lgermination of
several annual herbaceous species uhich compete for
available moisture and soil nutrients (McCormick et
al. 1974). Perennial mosses, herbs. and grasses occur
as soil depth, soil moisture. and soil-organic matter
increase in annual-perennial stages.
Woody plants occasionally become established in
994
DONALD J . S H U R E A N D HARVEY L. RAGSDALE
Ecology, Vol. 58, No. 5
FIG. 1. Map of Panola Mountain indicating approximate location, size, and configuration of communities studied. The
exposed outcrop surface (clear) is bordered by forests which are continuous in the surrounding area (stipling incomplete).
Unmarked contour lines represent 15.2-m increments.
the deeper soil islands. Small shrub-tree communities
even occur in a few depressions on level outcrop surfaces (Rogers 1971). However, soil moisture stresses
can produce high tree or shrub mortality during very
dry summers (Burbanck and Platt 1964. Rogers 197 1)
and thus restrict the development of woody vegetation
climaxes. Herb-shrub or earlier communities may be
terminal stages in many cases.
Community size and maximum soil depth increase
at each stage in succession (Burbanck and Platt 1964).
Soil depth also tends to increase from the periphery to
the center of soil-island communities, which promotes
the development of concentric rings or zones of earlier
seral stages (McCormick et al. 1974). However, depressions may deepen irregularly and the development of
these zones is varied o r absent in man! island communities (Burbanck and Platt 1964).
The basic objective of our stud! was to determine
the seral changes in plant and animal communities on a
particular outcrop and to establish the probable impor-
tance of abiotic or biotic factors in community
changes. Vegetation studies in outcrop communities
have generally been descriptive or experimental.
Quantitative assessments of seral changes have been
lacking. The animal communities present at each stage
have remained unstudied. Soil and aboveground arthropod populations were thus analyzed along with the
plant communities at each stage to provide a more
complete understanding of successional changes. Several keq microenvironmental factors were also
analyzed at each stage to evaluate their possible influence on successional changes in biotic communities.
We were particularly interested in determining if microenvironmental conditions become more constant as
succession progresses. Thus. the use of an holistic approach enabled us to compare whether different biotic
components exhibit similar successional changes in
population characteristics such as species diversity,
and to gain some idea of the probable causal mechanisms for diversit! o r other patterns.
Late Summer 1977
PRIMARY SUCCESSION ON GRANITE OUTCROPS
We conducted the study at Panola Mountain. a large
domed outcrop 2 2 4 km southeast of Atlanta, Georgia
(Fig. 1). The outcrop rises =66 m above the surrounding terrain and occupies 40 ha of the 188-ha State Conservation Park. Panola Mountain is similar geologically to other southeastern granite outcrops (Holland
1954). Matthews (1941) and Bostick (1971) provided
taxonomic descriptions of the flora. Rogers (1971)
studied the small forested stands on the crest of the
outcrop, and Ragsdale and Harue11 (1969) mapped the
soil-island communities of Panola Mountain.
Ten communities of 3 seral stages (diamorpha,
lichen-annual, and annual-perennial) were studied.
W e s e l e c t e d 5 n o r t h e a s t - and 5 n o r t h w e s t - o r
southuest-facing communities to account for slope
differences in environmental conditions. Small areas
were sampled in each communit! since we felt the
continued removal of organisms and materials could
affect temporal responses in the island communities.
Ten communities u e r e thus used to estimate changes
at each stage, instead of the usual intracommunity
sampling replication employed for larger communities.
However, most plant comn~unitieson Panola Mountain lack the concentric zonation of seral stages u hich
sometimes occurs on other outcrops (Madeline Bur: 0 . J . Shure and H .
banck. p ~ r ~ o t ~(.ot)l/~l~/t~i(.crtiot~
iil
L . Ragsdale. pc.r.c.otlcr1 oh.c.rr,.crriotl.s). The vegetation
type was relatively homogeneous throughout each
community.
A rainfall gauge was established in a clearing near
the base of the mountain for continuous monitoring of
precipitation.
The total area and average soil depth u e r e determined for each comrnunit!. We mapped each community and plotted the results on overlay paper for unitarea determinations. Soil depths u e r e measured everq
' I , , , of the transect distance through the maximum
length ( ~ ~ p s l o ptoe downslope) and uidth of each
community. Mean soil depth was calculated from
these 18 measurements.
We collected soil samples from each community for
pH. cation exchange capacity (CEC). soil moisture.
and bulk-density (gramsicubic centimeter) determinations. Film cassettes (35 cm3 each) were used to obtain
duplicate soil cores from all 30 communities in November 1970 and November 1971. The samples u e r e
air-dried and passed through a 2-mm sieve. Soil pH was
determined with a pH meter. Cation exchange capacity
was measured (Jackson 1958) at normal soil pH. The
soil was pretreated with IN NaOAc.
Soil moisture detel-minations u e r e conducted in
conjunction with soil microarthropod extractions.
T u o 35-cm%oil cores were obtained monthly in each
995
community. The soil samples u e r e neighed prior to
Tullgren funnel extraction of soil microarthropods. We
reweighed the soil after arthropod extraction and determined dry weight, percent soil moisture, and bulkdensity. Bulk-density was estimated at 6-mo intervals.
The diurnal fluctuations in aboveground and soil
temperatures were measured in each community on a
typical summer day in August 1970. Thermistor probes
were mounted on meter sticks at 2, 4, 8, 16, 32, 64,
and 96 cm. Each thermistor was shielded from direct
solar radiation. Additional probes measured temperature at the soil surface. 2-cm depth, and soil-rock interface at the bottom of the community. All probes u ere
attached to scanning telethermometers. We obtained
vertical temperature profiles at the center of each
community and from the adjacent rock surface. Temperatures were monitored everq 2 or 3 h from 0800 h
on 7 August until 0800 h 8 August.
Quadrat sampling enabled vegetation density.
phenology . and diversity estimates in each community. Five fixed quadrats were located along transects
through the maximum length and width of each community. We centered metal quadrat frames at 25. 50.
and 75% intervals of the length and at 25 and 75%
intervals along the uidth transect. The quadrat size
varied in each community tqpe ( B - 5 x 5 cm. LA10 X 10 cm. AP-I5 X 15 cm) because of differences
in communit? size and vegetation composition. H o u ever. all vegetation data were converted to 0.25 m'
before attempting diversity or other comparisons. The
vegetation was sampled approximately monthly from
July through November 1970 and from February
through October 1971.
Vegetation and litter biomass u a s estimated at the
peak standing crop for each community type. We
sampled the lichen-annual and annual-perennial communities on 13-14 September 197 1 and the diamorpha
communities in mid-April 1972. A single 15- x 15-cm
sample was obtained from a representative location
within each community. All samples were ovendried
at 100°C for 24 h.
Vacuum sampling permitted quantitative arthropod
collections. A portable vacuum cleaner attached to a
12-V battery was used to collect the foliage and litterdwelling arthropods from a hollou polqethylene cylinder (0.17 m' in area). Arthropods vacuumed from the
vegetation and litter were contained in cheesecloth
bags fitted uithin the vacuum cleaner. The cheesecloth bags were removed after each sample and sealed
in plastic bags containing carbon tetrachloride.
Thirt! vacuum samples were obtained each month
from Jul! 1970 until October 1971. One sample was
collected randomly from each cornmunit!. Each sample was sorted into species groups and ovendried at
Ecology. Vol. 5 8 . No. 5
DONALD J . S H U R E A N D HARVEY 1..RAGSDALE
996
COMMUNITY SIZE
BULK D E N S I T Y
N-50
.8 o o -6 0 .4 1.0
69
SOIL DEPTH
T I'
T
\
.2
T
J.
-
D
LA
AP
D
FIG.7. Successional changes in cornmunit) size and selected soil parameters. Means ( N
LA
=
AP
10) and 95% confidence intervals
are presented. Bulk-density results are pooled from 6 sample periods.
6WC for 48 h. Sampling was limited in each community
since we felt replicate sampling or more frequent sampling intervals could remove most arthropods from the
smaller ( < 1.5 m2) island communities.
Soil microarthropod populations were sampled
monthly from July 1970 through February 1972. Two
35-em3 soil cores were obtained from each community.
composited, and placed in modified high-gradient
Tullgren funnels (MacFadyen 1953) for soil arthropod
extraction. The 30 composite samples were placed in
polyethylene containers (4.3 cm diameter x 7.2 cm
high) for 4 days. Microarthropods were collected in
vials containing picric acid.
The Shannon-Weaver (1949) information theory
formula H' = -ZP,log,P, was used to determine
plant and animal diversity (heterogeneity. Peet 1974).
The data were treated as sampling estimates of diversity from hithin each parent community (Pielou 1966).
The Shannon formula behaves mainly as an equitability measure (Whittaker 1972). Species counts per sam-
ple were used as an independent measure of species
r~chness.
The variance associated with single samples from 10
communities was sometimes large and nonhomogeneous. Analysis of variance was considered inappropriate in making many successional comparisons. Instead,
confidence intervals (95%) were used to establish
significant differences. Standard errors were often
used to reflect the seasonal or successional changes in
variance components.
Soil p r ~ p c ~ ~ . t i c ~ s
The soil-islands increased significantly in average
size (2.7 to 16.9 m') and mean soil depth (2.7 to 21.4
em) between diamorpha and annual-perennial stages
(Fig. 2). Soil bulk-density decreased significantly as
the soil deepened and became more organic. Cation
l.ate Summer 1977
1
PRIMARY SUCCESSION ON GRANITE OUTCROPS
997
magnitude of die1 fluctuations. Lichen-annual soils
varied 6 C and annual-perennial soils only 2°C over 23
0 - . 4 LA
R\
h. Soil temperatures at each stage u e r e significantly
/
0--2 D
/
\
-n AIR
higher than ambient air because of the high heat absorptive capacity of surrounding granite. The abovep LSI
ground temperature profiles were generally similar
above each comti~unitytype and over exposed rock
surfaces. The only effect of the island communities
u,as an afternoon heat buildup above the soil surface
P
--m----a ((klc m).
/
Soil moisture in the island communities was generally higher and more constant during hinter when rainfall u,as greater and evapotranspiration h a s minimal
(Fig. 4). In summer. sporadic rainfall and high evapoAUG 7
AUG 8
transpiration losses produced widely-fluctuating soil
F I G . 3. llaily soil temperature profiles in outcrop cornmoisture levels. Moisture levels h e r e especially critimunities during summer. Means (!V = 10) ;ire presented for cal during the lou rainfall in September and October.
each community t)pe and for ambient air temperatures (AIR)
Soil moisture levels increased significantly and beat 96 cm above the expoied rock surface. The lea\t significant
came more conbtant as the soil deepened throughout
interval ( L S I ) h a \ derived from anallsis of variance and
applie\ to all means for each community t l p e . This interval
succession. The shallou diarnorpha soils often dried
reprejents a graphic extension ( 2 L.sd.2) of the lea\t \ignifiout or flooded (saturation =30%) during summer. Wet
cant difference (Steel and Torrie 1960) and mas computed at
conditions usually persisted in the winter. Lichenthe .O?-level.
annual communities also exhibited large moisture fluctuations. although flooding (saturation =4%45?) and
exchange capacity increased nearlq 4 fold and so11pH desiccation u,ere infrequent. Annual-perennial combecame dightlq less acidic as the substrate changed munities never dried out (saturation >IOU%) o r
during succession.
flooded.
The die1 fluctuations in summer soil temperatures
u,ere reduced significant11 during succession (Fig. 3 ) .
The shallou diamorpha substrate varied = 10'C over a
Peak standing crop (40-1.132 g1ni2)and litter (16-600
24-h period. Ambient air temperature shou,ed a similar glni') biomass increased significantly during succesS O I L TEMPERATURE
1970
\
4
SOIL M O I S T U R E
FIG.4. Semimonthly rainfall distribution and soil moisture changes in outcrop communities on Panola Mountain. Means
(N = 10) and I standard error are indicated for C/r soil moisture.
Ecology, Vol. 58, No. 5
DONALD J . S H U R E AND HARVEY L . RAGSDALE
MACROARTHROPODS
300-
240
120-
so-
1
'
"
'
1
'
'
"
1
t
.A P
-.A
LA
* - - O D
F I G . 5 , Total standing crop and litter biomass (dry u t ) at
the peak \tanding crop for each community t l p e . Means
(A' = 10) and 95% confidence intervals are presented.
sion (Fig. 5 ) . Standing crop to litter biomass ratios
h e r e similar in diamorpha (2.50) and lichen-annual
(2.87) communities and dropped by annual-perennial
stages ( 1.87).
Plant density shoued no consistent pattern of
change during succession. Density u a s highest in
diamorpha communities because of the many small
Llicitlzorpllc~ 1.y1110.sc1 (Table I ) . Plant density dropped
significantly as larger plant species invaded the
lichen-annual stages. Lichens (Cladotzirr spp.) and annual herbs such as Hrilho.sty1i.s i~crpi1ltiri.s.C'roto~zop.si.s
c~lli~~tic~ci
. Vigrric,rci portrri , and Hypc,ric,~rtr~,qetztirrtzoitlc,~ u e r e relatively abundant in lichen-annual
communities during spring and summer. Diciir~oipllci
remained fairly numerous in a f e u communities and
Arr,trcirici hrcr,ifi~lici(syn. .Cli~zorrrtici)a nd A y r o ~ t i srlliotriti~zci were abundant in the spring. Plant density
H
A
*.*LA
P
D--0
1970
1971
FIG.7. Macroarthropod density and biomass changes in
successional stages. Means (A' = 10) and standard errors are
indicated.
increased significantly as perennial mosses ( P o l y t ric,llrr~lzc~ot,zttnrtlc2and an unidentified species) invaded
and Vigrrirrrr increased in abundance by annualperennial stages. Most lichen-annual species u e r e rare
or absent in annual-perennial communities.
Plant diversity was quite low in diamorpha communities (Fig. 6). Species richness and evenness remained lou during hinter as a result of the many
The increase in plant dioveruintering Llici~,~olpllri.
versity during summer 1971 resulted from the germina~ ~ . C'rototzol~.si.c
tion of a f e u Vigrtirrci. H y l ~ c , r i c . i r and
following high precipitation in early summer. These
species were absent from dianiot-pha communities in
1970.
E
;.
a 60-
4
.
7970
.
?Oil
FIG.6 . Plant diversity ( H ' ) changes in successional stages.
Means (.V
=
10) and standard errors are indicated.
Plant diversity increased significantly in lichenannual communities and then decreased slightly but
not statistically by annual-perennial stages. The relatively high diversity in lichen-annual communities resulted from high evenness among those species populations present. Species richness continued to increase
in annual-perennial stages. However. the high densities of rnosses and Vigrrieru reduced plant equitability
and diversit),.
Late Summer 1977
PRIMARY SUCCESSION ON GRANITE OUTCROPS
999
TABLE1. Plant species densities (,dm2, N = 10) in each community type during 1970 and 1971. Species represented exceeded 40 plants" per m' on at least 1 sample date
1970
7i9
1971
7/28
8/18
9114
10119
415
619
718
8i5
9113
0
0
<1
0
0
<I
0
0
<I
0
0
<1
6.232
59
6.291
10.800
59
10,871
0
0
II
0
0
11
0
0
11
0
0
11
0
0
56
0
254
156
84
568
0
0
72
0
258
130
72
544
0
0
76
0
218
134
78
512
0
0
72
0
220
132
78
488
2.972
0
6
0
38
I2
12
3.060
1,111
955
0
0
145
302
144
2,985
0
0
130
10
68
48
26
284
0
0
352
0
90
47
28
532
0
0
278
0
84
47
28
437
0
0
242
0
87
47
25
408
3 19
2.251
1.097
31
0
27
0
3.738
428
2.240
970
64
0
II
0
3.729
311
2.176
71 1
65
0
50
0
3.326
1.164
2.183
803
70
0
86
0
4.319
622
2.396
0
4
0
2
0
3.102
1,462
1,613
511
II
116
22
63
4,007
1,401
1.636
490
6
0
37
0
3,604
1.733
1,778
418
179
0
36
0
4,157
5.822
1.346
427
7
0
41
0
7.736
5.667
1,090
345
7
0
39
0
7,224
Diamorpha
Diamorpha
Arenaria"
Totalb
Lichen-Annual
Diatnorpha
Arenaria
B~ilhostylis
Agrostis
Croronopsis
Viglrieru
Hyperic.lim
Total
Annual-Perennial
Moss sp.
Polytric.hlrttr
Viglrieru
Crototrop.tic
Agrosti.,
Ut~iol~
Litrnriri
Total
a
I
.Moss densities represent number of aboveground shoots. Lichen abundances have been omitted.
Total densities are summed over all species present.
Skn. lMi~~lrr~rtiu.
see Sharitz and McCormick (1973).
Macroarthropod density increased significantl;
throughout succession (Fig. 7). Ver; feu macroarthropods occurred in the diamorpha cornmunities. An
endemic. soft-winged flower beetle (Co1lop.c s p . ,
Melyridae) u a s the major component (Fig. 8). Jumping
spiders (Salticidae), wolf spiders (Lycosidae), and the
endemic rock grasshopper (Trit?zc~rorrop/li.sctrstrri1i.s)
h e r e also sampled.
Macroarthropod densities increased in lichenannual stages. Flea beetles (Chr),somelidae) were relativel; abundant herbivores throughout the summer.
Two srnall dipteran species exhibited localized aggregations in fall and spring and leafhoppers (Hornopterans) reached peak densities late in the growing
season. Jumping spiders were the major arthropod
predators in lichen-annual communities.
Man; arthropod groups were faid!, abundant in
annual-perennial communities (Fig. 8). Homopterans
such as leafhoppers. treehoppers (Mernbracidae). and
planthoppers (Fulgoridae) were the most abundant
herbivores. Grasshoppers. crickets. dipterans. and a
f e u hymenopterans u e r e also present. Spiders remained abundant predators throughout the grouing
season. Most insect groups. including nectar- or
polle~r-feeding species such as h;nienopterans and
th;sanopterans (thrips). reached peak densities n h e n
Vigriic,rrr flowered in Septernber. Predatory species
such as floner bugs (Anthrocoridae) and parasitic
hymenopterans also increased in September n h e n
their pre; or host species b e r e readil!, available. Macroarthropod densities decreased after Vigrric~rtr had
flonered.
Mac1,oarthropod biomass (Fig. 7) increased only
slightl; betueen diamorpha and lichen-annual stages.
Houever. biomass levels were much higher in
annual-perennial stages, particularl; u h e n Vigciic,rtr
flouered.
Macroarthropod diversity ( H ' )also increased significantly during prirnary succession (Fig. 9). Species
richness and evenness remained Ion in diamorpha
communities. Diversit), rose as new species occupied
the lichen-annual cornmunities. Macroarthropod diversit) continued to increase significantl; betneen
lichen-annual and annual-perennial stages. The annual-perennial cornmunities contained many macroarthropod species that u e r e even]; represented.
Houever. the addition of man) thrips in September
1970 reduced diversit), despite the increase in species
richness as Vigrrirrtr flonered. Diversit) peaked in
September 1971 when species entering the annualperennial communities u e r e more equitabl; represented than in 1970.
Soil t ~ ~ i c ~ r o t r r f / ~ r o p o ( I ~
Microarthropod density n a s relatively high in
pioneer communities (Fig. 10). Mites u e r e important
in diamorpha soils since few springtails (collembo-
1000
DONALD J . SHURE AND HARVEY L. RAGSDALE
AP
Ecolog), Vol. 58. No. 5
- COLEOPTERA
-- DIPTERA
r,
i\
-*-
-.-
-..-A-
-.-
HEMIPTERA
HOMOPTERA
HYMENOPTERA
LEPIDOPTERA
ORTHOPTERA
THYSANOPTERA
ARACHNIDA
P.
.'."...
/
'.
+
/----
FIG.8 Mean (,V
=
-4
\~-$-
10) den4ties of major macroarthropod components during 1970 and 1971.
lans) or other soil forms were present (Fig. 1 1). Mite
species generally exhibited rapid density oscillations
and spurious distribution patterns in diamorpha communities. Population fluctuations were particularly severe during late summer and early fall when moisture
stresses (Fig. 4) were synchronal with mite population
crashes.
Soil microarthropod densities dropped but were
somewhat more constant in lichen-annual stages.
Mites were still the major component except for a midsummer increase in springtails. Microarthropod densities in lichen-annual communities tilso dropped during
the late summer-early fall dry period.
Microarthropod density was ~lsually highest in
annual-perennial communities. Species present in the
lichen-annual communities generally increased in density in annual-perennial stages. Mite species exhibited
similar seasonal changes in both stages (Fig. 1 I ) except that annual-perennial populations remained relatively constant during dry periods. Springtails were
also quite abundant in annual-perennial communities
particularly in early winter and early summer. A f e u
Symphyla. Diplura. Pseudoscorpionida and other taxa
were also present.
Microarthropod diversity increased significantly
throughout succession (Fig. I?). Species diversity was
particularly low and variable in diamorpha communities during spring and summer. A single mite
species was the major component at this time and its
population fluctuations influenced diversity through
Late Summer 1977
PRIMARY SUCCESSION O N GRANITE OUTCROPS
MACROARTHROPOOS
-
MITES
k
I00 l
AP
-.-LA
0-
- - -0 D
FIG.9 . Llacroarthropoci dikersity ( H ' )and species richness
=' 10) and standard
change, in succe\sional stages. Mean\ (,I
errors (H' data onl)) are prewnteci.
F I G . I I . hlite anci 5pringtail (collembolan) population fluctuations in successional stages. Mean (.\' = 10) numbers per
70 cm%f soil are presented.
changes in overall equitabilitq. Several mite species
n'ere present and equitablq repreaented in diarnol-pha
communities from fall until spring. More species Mere
present and in fairly even numbers in lichen-annual
communities. Thus. diversity incl-eased and remained
fairly constant except for particularlq dry periods (Fig.
4). Microarthropod diversity continued to increase as
neu species Mere established in the annual-perennial
communities. Diversity remained q ~ ~ i t ceonstant
throughout the year in the later stages.
i
SOlL ARTHROPODS
1
FIG. 10. Soil arthropod density fluctuations in successional
stages. Data reflect mean number per 70 cm3 of soil (N = 10)
uith standard errors included.
DISC[JSSION
The island communities on Panola Mountain and
other southeastern outcrops exhibit definite trends in
substrate development. The substrate remains quite
sandy (>85%) through annual-perennial stages (Meyer
et al. 1975). Organic matte; content increases =3-fold
1
SOlL ARTHROPODS
FIG.12. Estimated soil arthropod diversity (H') changes in
successional stages. Means (N = 10) and standard errors are
presented.
1002
Ecolog), Vol. 58. No. 5
DONALD J . SHURE AND HARVEY L. RAGSDALE
and cation exchange capacitq 3-5 times betueen
dianiorpha and annual-perennial stages (Burbanck and
Platt 1964. Meqer et al. 1975. Braun 1969). The increase in soil organic niatter promotes greater cation
exchange capacity and ma! also contribute to the
slight increase in soil pH during succession (Burbanck
and Platt 1964). These trends continue if soil depth
increases beyond annual-perennial stages. The small
= 40-70 crn
forested stands on Panola Mountain (i
deep) have higher soil pH. cation exchange capacitq.
and soil organic matter (Rogers 1971) than annualperennial stages.
Primarq succession on the outcrops occurs as a result of the reciprocal interactions betueen biota and
experience large
substrate. Dianiorpha conini~~nities
nutrient and material fluxes during periodic flooding
(Haq 1973). Hay. hou ever. concluded that materials
accumulate at a verq slou rate in these early stages.
The small lIicit~~orp/z(i
plants trap some materials and
the plants themselves represent an annual source of
organic production. Other herbaceous species such as
Vigriic,rtr germinate during favorable years such as
1971 (Mellinger 1972) and add organic rnatter to the
dianiorpha conini~~nities.
The plant cornrn~~nities
shift over time a s soil depth
graduallq increases in the island depressions. Lichens
and other invading plants trap debris and add organic
niatter in the lichen-annual stages. The increased plant
cover and deeper soil reduce moisture and temperature fluctuations within the substrate. These changes
continue until flooding. desiccation, and teniperature
extremes are minimized or absent by annual-perennial
stages. Lugo (1969) also found reduced die1 teniperature fluctuations in annual-perennial soils and Mellinger (1972) reported that moisture levels u e r e higher
and more constant in deeper outcrop substrates.
Plant cover is thus essential for the buildup of organic rnatter uithin the soil islancls. The resulting increase in conini~~nit!
depth favors further vegetation
developnient. Deeper and slightly convex annualperennial communities replace the shallou pioneer
stages. Soil depth increases above the rim of the exfoliation depression in many annual-perennial coniniunities ( H . L . Ragsdale. per.sotlrrl obser\~rrriotl).The
slight convex structure elevates most biotic components above flooding levels and minimizes nutrient and
organic matter losses during rainfall. Nutrient recqcling is thus important uithin annual-perennial coniniunities (Meqer et al. 1975) in contrast to the large
nutrient fluxes in diamorpha conini~~nitiesd uring
flooding.
Soil depth and nioisture levels influence interspeand thereby
cific competition among plant pop~~lations
determine vegetation changes in the island depressions
(Sharitz and hlcCorniick 1973. McCormick et al.
1974). The higher soil nioisture in deeper soil favor\
the survival of larger and more competitive species
(hlellinger 1972. McCorniick et al. 1974). Soil nutrients
Estimated number and percentage of macroarthropod species in different trophic levels of each community type. Certain h~menopteranand dipteran species
were placed into most probable trophic levels
T A B L E2.
LA
D
Herbivore
Predator
Omnivore-saprovore
Parasite
Specie\
4
11
5
1
C/r
Species
19.0
52.4
23.8
4.8
65
41
24
9
4P
%
Specieb
'Z
46.8
29.5
17.3
6.4
102
91
46
28
38.2
34.1
17.2
10.5
such as nitrogen often act as secondary regulator!, factors on plant distribution during favorable moisture
periods (McCorniick et al. 1974. Meqer et al. 1975).
Increased soil nioisture and nitrogen availabilitq over
time promote the successive dominance of plant
species uith greater nioisture and nitrogen requirements. Only 1 plant species generally occurs in
pioneer stages u h e r e moisture extremes and lou nL1trients limit species occurrence. More species invade
but at relat~vel\,lou densities and in varied distribution patterns in lichen-annual stages. Slight topographical variations in the lichen-annual communities select
for different species associations through interspecific
competition (Sharitz and McCormick 1973). Zedler
and Zedler (1969) reported a similar topographic effect
on plant successional patterns in drained marshes in
Wisconsin. C'iglriertr and a f e u other species are favored over most invaders as soil depth continues to
increase in the annual-perennial stages (Cuniniing
1969. Mellinger 1972).
on Panola Mountain generThe island conini~~nities
all!, have higher soil organic niatter and soil moisture
levels than other outcrops (Burbanck and Platt 1964.
Braun 1969. Mellinger 1972. Meyer et al. 1975). Plant
biomass is also higher (Lugo 1969) and concentric
plant zonation is general11 lacking in the annualon Panola Mountain. Species
perennial comni~~nities
such as Vigriic,rci porrc~rioccur throughout the annualperennial cnmm~lnitieson Panola Mountain. Moisture
and nutrient stresses ma!, be less severe in these comniunities than in the "zonal" annual-perennial communities on other outcrops
Consumer pop~llationson the outcrops respond to
vegetation or substrate changes. Macroarthropod density. biomass, and diversitq all generally increase during succession as does plant and litter biomass. The
niacroarthropod changes shoued less correlation uith
plant densit! and diversitq patterns. Macroarthropod
p o p ~ ~ l a t i o napparently
s
respond to the successional increase in net primary production and community
stratification. The nianq macroarthropod species in
annual-perenni'tl cornmunit~essugge4ts that food u e b
complexit! increases in later stages. .A larger nuniber
of species were present in each trophic level at successive seral stages (Table 2 )
Late Summer 1977
PRIMARY SUCCESSION O N GRANITE OUTCROPS
Soil niicroarthropod changes during primar!, succession are related to increased food availabilit! and
reduced environmental stresses. Mic~,oarthropndcomniunities are relativelq simple and exhibit rapid densitq
oscillations in the shallow, infertile, and environmentall!,-stressed diamorpha substrate. However.
plant litter biomass increases over time and provides
a greater substrate for fungal populations. Bostick ( 1968) reported that fungal diversit) increased during primar! succession on Panola Mountain. Therefore, the successional increase in litter. fungi. and
soil-organic matter promotes higher soil microarthropod diversit). The reduction in soil microenvironniental fluctuations over time also favors the invasion of certain microarthropods such as springtails.
A diverse microarthropod coniniunit) develops by
annual-perennial stages.
Biotic diversit! increases during primary succession
on the outcrops. although the patterns are different for
plant and animal components. Plant diversity changes
resembled the patterns observed for earl!, primarq
succession on .Alaskan glacial c h r o n o s e q ~ ~ e n c(Reines
ers et al. 1971). Reiners et al. also found that plant
diversity reached asqniptotic levels fairlq earl! in succession and diversity fluctuations u e r e closel>' correlated ~ i t hequitability changes. Margalef (1968).
Loucks ( 1970). .Auclair and Goff (1971). Shafi and Yarranton (1973). and Nicholson and Monk (1974) have
also reported an earl! or midsuccessional peak in plant
diversit!,. Horn (1974) generally concluded that diversity should be higher at some intermediate stage u here
a mixture of earl! and late successional species are
present.
Few successional studies have considered animal
diversity changes and conclusions on consumer diversit! patterns are someuhat tentative. Macroarthropod
and niicroarthropod diversity increase throughout
primark s ~ ~ c c e s s i oon
n the outcrops. In other studies.
bird diversit) increased at a decreasing rate (Karr
1968. Kricher 1973) and small-mammal diversitq
peaked earl! and subsequently declined during secondarq succession (data of Wetzel 19.58 and Pearson
1959 converted to ff').
The existing studies suggest that each major
taxonomic group responds differentlq during succession and that diversit) generalizations encompassing
all ecosqsteni components are unrealistic. Plant diversit! changes during succession are often related to
competition and available moisture (Pielou 1966.
Monk 1967. Auclair and Goff 1971). On the outcrops.
plant diversit! patterns are a result of competitive interactions for moisture or nutrients.
Animal diversit! is generally dependent on priniar)
production (Connell and Orias 1964). plant structural
complexity (Mac.Arthur 1965, Recher 1969, Murdoch
et al. 1972). and spatial heterogeneit) (Roth 1976).
Karr (1968) reported a high correlation betueen bird
diversit) and foliage-height diversit) in successional
1003
sqstenis. Sniall-niamnial diversity appears closelq related to successional changes in vegetational
heterogeneit! at the herb-shrub laqer (Pearson 1959).
Macroarthropod diversitq on the outcrops increased
as a result of greater primary production and community stratification. In contrast. microarthropod diversity increased and remained more constant as the
soil became more fertile and environmental extremes
such as drought or flooding became less frequent. So.
although biotic diversity ma! generall!, increase
during succession. the rate and degree of change is
dependent on the response of each taxonomic component to different causal mechanisms.
The type of species occupqing soil-island communities shifts considerabl!, during succession. F e u
species are present in the environnientally-stressed
diamorpha communities. Single species dominate each
taxonomic component and the) exhibit rapid density
oscillations (i.e.. mites). a large annual production of
small individuals (i.e.. Dia/norp/zcr), or opportunistic
food habits (i.e.. Co1lop.s sp. C. T . Hackney. par.cot~rrl
c . o / , ~ / , ~ r r t l i c ~ r r r i o t lThese
).
species undergo a n n ~ ~ ac!,l
cles, and densitq-independent factors such as heat or
moisture usuall!. control densitq levels. .A niilch different species composition occupies the niore
environnientally-constant. annual-perennial stages.
Plant species pop~llationsare larger in size. man)
species are perennial. and densit!,-dependent factors
such as competition (McCorniick et al. 1974) act as
intense ~.egulators. Consumer pop~llationsare relativel!' diverse uhich indicates the possibility of greater
specialization in food habits. The successional
changes in species composition suggest a shift from rto A'-adapted species (Pianka 1970). Further outcrop
s t ~ ~ d iare
e s needed to test the 'ict~lalfit to ther-A' selection continuum proposed for successional s)'stems
(Odum 1969).
The specific location and configuration of each granite outcrop exfoliation depl-ession can influence the
extent of successional development. Coules ( 1899)
and later Olson (19.58) indicated primary succession
ma) proceed in different directions and at different
rates depending on site conditions. The) shoued that
localized edaphic conditions can produce divergence
rather than convergence in later stages. Succession
proceeds more as a "variable approaching a variable"
(Coules 1901) rather than as a variable approaching a
constant. The small and fragile soil islands on granite
surfaces are highl!' susceptible to severe drought.
wind, or ice storms. The frequencq and severit! of
these events can affect the rate or degree of coniniunit), modification such that each successional stage
could conceivably be terminal. Small soil islands in
exposed or perched locations mould be particularl!,
limited in their development. Extreme storms could
remove some or all of the existing substrate from these
depressions and retard or prevent soil-building processes. Conini~~nitiesin niore favorable locations
1004
DONALD J . SHURE AND HARVEY L. RAGSDALE
T A B L E3. Coefficients of variation (C.V.
=
.\if)
in plant and animal population parameters at successional stages
-
Plant
Macroarthropods
Microarthropod\
Ecologq. Vol. 58, No. 5
--
D
LA
.4P
D
LA
.4P
D
L.4
.4P
Densitv
Biomass
0.927 -+ 0.540"
0.868 t 0.081
0.767 -+ 0.115
1.045 ? 0.209
0.837 t 0.142
0.494 2 0. I50
1.260 2 0.167
0.898 ? 0. 158
0.541 t 0.100
0.728
0.474
0.477
1.723 2 0.415
1.198 2 0.230
0.959 2 0.176
Diversitv
...
...
...
Each datum input represents the overall mean coefficient of variation (one C.V. determination on each sample date)
and its 95% confidence interval.
would ~ ~ n d e r g omore rapid succession through
annual-perennial stages. H o u e v e r , shrub or xerophytic tree species are ~~ltirnatelyrestricted to a feu
deeper depressions u h e r e limiting factors are less critical (Rogers 197 1 ) .
Coefficients of variation ( C . V . = s i x ) were determined for the 10 replicate communities at each successional stage to test for possible convergence in
cornmunit! characteristics (Table 3). All population
paranieters decreased in variance from diamorpha to
annual-perennial stages and many differences h e r e
significant. Consunier populations attained greater
similarity than plant populations. The temporal variance in these parameters (95% confidence intervals)
also generally decreased betueen dianiorpha and
annual-perennial stages. So. although primarq succession may proceed at different rates and to different
degrees. the soil islands generally converge in coniniunity properties such as densitq. biomass. and diversity at successive seral stages. The close similarity
among annual-perennial communities suggests near
equilibrium conditions uith respect to these parameters. Annual-perennial communities are also apparentlq approaching steady-state conditions in nutrient
cycling. carbon budgets. and accumulation of soil organic matter (Lugo 1969. Meyer et al. 1975).
f e u \oil arthropod species in dianiorpha conimunities
exhibit u i d e population oscillation4 in the shallou.
environmentallq-stressed substrates. Mites preclominate in pioneer stages and >oil moisture conditions
strongly influence their densit!, levels. The niicroarthropod community changes almost conipletelq as
springtails. different mite species, and other fornis invade the niore environnientallq-constant substrate of
lichen-annual and annual-perennial stages. Microarthropod density fluctuations are niore rhythmic in the
later stages.
Biotic diversitq general11 increases throughout primary succession on the outcrops. The diversitq patterns are varied since each taxonomic group increases
at different rates and to different degrees depending on
particular causal factors. The kind of species also
changes during succession. The feu species that occupq the earliest seral stages are adapted to persist
under strong abiotic stresses. Interspecific interactions become more important in determining conimunity structure as the abiotic stresses are reduced in
later seral stages. The strong ph!,sical factors on the
outcrops may limit the rate and extent of successional
development in particular soil-islands. Houever. as
the many soil-islands undergo succession theq converge in their density. biomass. and diversit! values.
As the man! soil-islands ~lndergoprimary succession the!, gradually deepen and the soil becomes more
organic. The shallou pioneer stages are periodicallq
subjected to severe abiotic stresses. Houever. the increase in depth and moisture-holding capacity
of the
soil significantlq reduces the temperature and moisture
fluct~lationsin later stages.
Plant biomass and vertical stratification increase
over time as larger plant species invade the deeper
communities. Interspecific competition for moist~lre
and nutrients regulates plant species composition
as succession progresses. Macroarthropod populations increase in density, biomass. and diversity
throughout succession as a result of the increase in
producer biomass and community stratification. The
We thank the many students and colleagues who assisted in
different phases of the studv, particularly Allen Sisk, Roger
s i s k . ~~b~~~~
~
i ~ ~~ ~ ~~~ ~~ , ~ ~ ~h ~ ~~ h dl ~kd l.~ l
John Ha). and James Ruttenber. Dr. P. E. Bostick assisted
uith much of the vegetation studies and Dr. D. A. Crossley
and several students from the University of Georgia participated in our nocturnal studies on Panola Mountain. Dr. R. B.
Platt and Mark Harwell Drovided suggestions or criticisms
concerning the manuscript and Dr. JIF. McCormich and a
second revieuer uere especially helpful during the review
Process The research uas supported in its entiret) by contract number AT-(40-1)-2412betueen Emory Universit) and
the Energy Research and Development Agency.
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