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Patterns of Primary Succession on Granite Outcrop Surfaces Donald J. Shure; Harvey L. Ragsdale Ecology, Vol. 58, No. 5. (Sep., 1977), pp. 993-1006. Stable URL: http://links.jstor.org/sici?sici=0012-9658%28197709%2958%3A5%3C993%3APOPSOG%3E2.0.CO%3B2-J Ecology is currently published by Ecological Society of America. Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/about/terms.html. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/journals/esa.html. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is an independent not-for-profit organization dedicated to and preserving a digital archive of scholarly journals. For more information regarding JSTOR, please contact [email protected]. http://www.jstor.org Mon Jun 11 16:08:37 2007 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. LIT-F.RAT-UK~ CIT~D Auclair. A . N..and F. G. Goff. 1971. Diversity relation5 of forests i n the treat ~~k~~ area, ~ ~ 105:499-528. t . I.,ite Suninier 1977 PRILIARY SUCCESSION ON GRANITE OUTCROPS Bard. G . 1952. 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