Interpreting ancient swamp communities: Can we see the forest in the peat?

Review of Palaeobotany and Palynology, 52 (1987): 217-231 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

217

INTERPRETING ANCIENT SWAMP COMMUNITIES: CAN WE SEE THE FOREST IN THE PEAT? ANNE RAYMOND Department of Geology, Texas A & M University, College Station, TX 77843 (U.S.A.) (Received December 3, 1984; revised and accepted October 20, 1986)

Abstract Raymond, A., 1987. Interpreting ancient swamp communities: Can we see the forest in the peat? Rev. Palaeobot. Palynol., 52: 217-231. A comparison of the taxonomic composition of mangrove (salt-water) peats from the Everglades National Park and the swamp forests in which the peats accumulated shows that the rank abundance of roots belonging to a taxon in peat can be used to predict the rank abundance of that taxon in the swamp forest. Thus the rank abundance of taxa in permineralized peats can be used to reconstruct ancient swamp-forests. Factors that correlate with salinity, primarily the abundance of detritovores in mangrove swamps, appear to control the shoot-root ratios of the peats studied. Fresh-water swamp peats have few detritovores, low rates of decomposition, and relatively high shoot-root ratios (mean = 2.03). Mangrove peats have many detritovores, high rates of decomposition, and low shoot-root ratios (mean = 0.16). Variation in the shoot-root ratios of marsh peats do not correlate with salinity. However, shoot-root ratios may be useful in predicting the growth environment of ancient permineralized peats derived from ancient swamp forests. Paleotropical permineralized peats of Late Carboniferous age have extremely high shoot-root ratios. A model for peat accumulation based on the work of Janzen (1974) suggests that these high shoot-root ratios may be indicative of tropical blackwater swamps. Factors that are uncorrelated with salinity such as rainfall regim~ may determine the difference between Okefenokee shoot-root ratios (mean=2.03) and extremely high shoot-root ratios (> 100) of paleotropical permineralized peats from putative blackwater swamps.

Introduction I n w h a t w a y do f o s s i l a s s e m b l a g e s m i r r o r ancient communities? Even the most optimistic p a l e o n t o l o g i s t w o u l d h a v e to a n s w e r , " h a p hazardly, if at all". The relationship between death assemblages and living communities has been investigated by invertebrate and vertebrate paleontologists, and recently, by compress i o n - f l o r a p a l e o b o t a n i s t s (see B e h r e n s m e y e r , 1978, 1982; S c h o p f , 1978; D a m u t h , 1982; S c h e i h i n g a n d P f e f f e r k o r n , 1984, a m o n g o t h e r s ) . H o w e v e r , few p a l e o b o t a n i s t s h a v e c o n s i d e r e d the relationship between swamp forests and 0034-6667/87/$03.50

peat Because permineralized peat constitute s o m e o f t h e few p l a n t f o s s i l d e p o s i t s t h a t contain both trees and understory plants, the r e l a t i o n s h i p b e t w e e n s w a m p s a n d p e a t s is crucial to the study of terrestrial community paleoecology. Studies by workers such as Phill i p s e t al. (1977), P h i l l i p s a n d D i M i c h e l e (1981), a n d R a y m o n d a n d P h i l l i p s (1983) u s e d t h e relative abundance of plant debris in perminera l i z e d p e a t s f r o m U p p e r C a r b o n i f e r o u s c o a l s to reconstruct ancient swamp-communities. These workers assumed that the abundance of taxa in permineralized peat reflects their abundance in the swamp forest.

© 1987 Elsevier Science Publishers B.V.

218 In this paper, I demonstrate that the relative abundance of canopy debris in peats faithfully records the relative abundance of canopy genera and families within the living forest. I limited my investigation to swamp peats (peats that accumulate under forest communities) because most permineralized peats were produced by forest communities. One general attribute of peat, the ratio of aerial debris to root debris (referred to as shoot-root ratio) can be measured in both ancient and modern deposits. Low shoot-root ratios in peat have been tied to salinity, rapid decomposition rates, and tidal transportation of shoot debris out of the swamp (Cohen and Spackman, 1977) and to seasonality in rainfall (Phillips and Peppers, 1984). The available data suggest that factors which correlate with salinity control shoot-root ratios. Low shootroot ratios reflect rapid decomposition rates and transportation of shoot debris out of the swamp, both of which occur most often in tidally-influenced marine or brackish water swamps that have abundant detritovores. High shoot-root ratios reflect slow decomposition and negligible transportation of shoot debris out of the swamp, which occur most often in freshwater swamps with acid water and few detritovores. Previous literature

Taxonomic studies Cohen (1968) and Cohen and Spackman (1977) characterized the taxonomic composition of debris in peats in order to study the decomposition of peat constituents with depth. In the Florida Bay area, Cohen collected twenty cores from fresh-water and salt-water peats with a piston corer which yielded cores up to 4.5 m in length. He sampled these cores at 7.5 cm intervals and used a paraffin impregnation technique to prepare microtome thinsections of the peat. Each set of thin-sections sampled a peat surface approximately 1.0 cm 2. In ten cores collected along a fresh-watermangrove vegetational transect in the Joe

River area of the Everglades National Park, Cohen (1968) attempted to relate the taxonomic composition of a peat sample with a surface area less than 2.5 cm 2 collected 7.5 cm below the sediment surface, with the taxonomic composition of the community living above the sample site. Although he found that he could assign peat samples to particular swamp communities based on the presence of certain types of debris, he failed to find fluctuations in the dominance of tree species at each site reflected in the peat samples. The assets of Cohen's sampling method were the use of (1) a piston corer to obtain long cores which showed evidence of community replacement in response to changing environmental conditions; and (2) a paraffin impregnation technique which minimized disturbance of the peat matrix prior to thin-sectioning. This technique allowed Cohen to observe the formation of autochthonous pyrite in plant tissues and measure the ratio of framework constituents to matrix in the peats. Framework constituents are peat particles with one dimension greater than 10 /~m; peat matrix consists of particles with no dimension greater than 10 pm. However, Cohen's sampling method prevented him from investigating peat constituents larger than 2.5 cm 3, the maximum volume of peat which could be impregnated using his paraffin impregnation technique (Cohen, 1968). Because he trimmed samples to eliminate disturbance, the actual maximum thin-section size was closer to 1 cm 2 (Cohen, 1968). In addition, Cohen was unable to thin-section peats with an inorganic fraction higher than 50%; this figure was much lower when the inorganic fraction consisted of quartz or shell material (Cohen, 1968). His sample size of approximately I cm 2 was probably insufficient to demonstrate the relationship between the taxonomic composition of peats and the communities which form peat.

Taphonomic studies Cohen and Spackman (1977) contains an extensive survey of how habitat affects the

219

taphonomy of accumulating peats. They reviewed thirteen peat types deposited in both marshes and swamps, from both fresh-water and salt-water habitats, and considered the effect of taphonomic processes on the organ composition of the peat by measuring the ratio of shoot debris to root debris. Cohen and Spackman (1977) referred to this ratio as the N/S ratio, I refer to it as the shoot-root ratio in accordance with Phillips and Peppers (1984). Table I contains a summary of the shoot-root ratios of the swamp peats studied by Cohen (1968), Spackman et al. (1976) and Cohen and Spackman (1977) as well as a summary of the shoot-root ratios of swamp peats included in this study. Based on approximately 150 cm 2 of TABLE I Summary of shoot-root ratios observed in freshwater and mangrove swamp peats Peat type

Habitat

Shoot-root

Taxodium ~

freshwater warm temperate freshwater warm temperate freshwater warm temperate freshwater warm temperate freshwater warm temperate freshwater subtropical brackish to salt water subtropical brackish to salt water subtropical salt water subtropical salt water subtropical salt water subtropical

4.26

Taxodium b

Tree Island a Nyssa a Cyrilla b Myrica Persea-Salix c Conocarpus a

transitional Conocarpus c

transitional Rhizophora a Rhizophora ~

sedimentary Rhizophora c Acrostichum Mariscus Avicennia ~ A v i c e n n i a --~ Rhizophora

salt water subtropical salt water subtropical

1.50 3.00 1.20

mangrove peat, previous workers found low ratios of aerial to root debris (from 0.02 to 0.04) in all mangrove peats except the allochthonous Rhizophora peat which had an aerial to root debris ratio of 0.40. They found high ratios of aerial to root debris (1.86 to 1.50) in fresh-water swamp peats from the Okefenokee Swamp and one subtropical swamp in Florida. Methods

Measuring the relative abundance of tree species within the swamp In order to test whether the relative abundance of a tree species within a swamp determines the abundance of its debris in peat, I compared the importance of tree species in the swamp and peat in four sampling localities in mangrove-dominated areas of the Everglades National Park. Figure 1 shows these sampling localities. I measured species importance within the swamp in two ways: (1) the Bitterlich variable-angle method (Grosenbach, 1952); and (2) by counting the number of individuals of each species within a quadrant 1/40 ha or larger, or along a 100 m transect, and measuring their diameter at breast height (DBH) to obtain a combined DBH-stem count

1.86 1.86 0.30 0.04 0.22 0.40 0.02 J

0.37 0.07

Source: athis study; bSpackman et al., 1976; ¢Cohen and Spackman, 1977.

~'

FLORIDA 30

KM

Fig.1. Mangrove sampling sites. 1: Lumber Key; 2: Lopez River; 3: Joe River; 4: West Key.

220 importance measure (DBHS) for each species within the quadrant. In order to assess species changes in the four sampling localities, I tallied the importance of living and of living and dead individuals separately. I also counted the number of seedlings along a 50 m transect at each sampling locality. The two measures of species importance (Bitterlich and DBHS) tallied for both living and for living and dead individuals yielded four measures of the importance of each species present within a sampling locality. These were: L-Bitter (living Bitterlich), T-Bitter (living and dead Bitterlich), L-DBHS (living DBH-stem count), and T-DBHS (total DBH-stem count). The four sampling localities contained a total of four canopy species: Rhizophora mangle, Avicennia germinans, Languncularia racemosa and Conocarpus erecta. For convenience, I refer to each species by its generic name.

Measuring the relative abundance of tree species within the peat To measure species importance within the peat, I collected peat cores from each sampling locality using a piston corer approximately 9 cm x 35 cm made from PVC vinyl pipe with a closed-cell foam piston. I extruded the cores in the field into mesh bags and soaked them in formalin to inhibit decay. In the laboratory, I dried the cores and impregnated them with an epoxy-propylene oxide mixture in a ratio of two to three parts epoxy to one part propylene oxide. The propylene oxide-epoxy mixture required up to two weeks to harden completely. After the peat chips hardened, I made thinsections using standard petrographic techniques. The epoxy-impregnation method does not yield the fine-scale anatomical detail of Cohen's paraffin impregnation method. In addition, drying the peat completely before impregnation prevented measurement of the framework/matrix ratios in peat. However, with this method I could make sections of large pieces of peat, and peat with greater amounts

of inorganic sediment than with the paraffinimpregnation method. To obtain species-importance values, I covered the thin-sections with a transparent 0.25 cm 2 grid, and recorded the organ and species of the largest pieces of debris in each grid square. This method of measuring species importance within peat parallels the method used by Phillips et al. (1977) and others to measure species abundance in permineralized peats of Carboniferous age. I censused a total of 1.341 cm 2 of mangrove peat for this study. I compared species importance within the swamp and peat using Kruskal's Rank correlation (a non-parametric technique) (Campbell, 1974) and regression analysis (Simpson et al., 1960). Because the comparison of all four measures of swamp importance to peat importance yielded similar correlation values, I report only the comparison of living DBHS to peat importance, which was available for all fifteen cores. Results

Swamp importance Table II lists the species importance values obtained for the four mangrove species present in the Joe River quadrant. The DBH-stem count measures (L-DBHS and T-DBHS) represent a larger sample of the swamp forest (250 m 2) than the Bitterlich measures (L-Bitter and T-Bitter) which sampled the swamp forest immediately surrounding the core site (generally within 3 m of the core site and approximately 28m2). In most cases, although individual species importance values varied greatly, all the values reflected the same rank abundance of species. The Joe River samples contained all four species, two with very similar abundances, and represent a worst case for the swamp forest sampling techniques. Here, all of the importance values except T-DBHS yielded the same order of rank abundance: Rhizophora, Languncularia, Conocarpus, Avicennia. As previously stated, in comparisons of species importance in the swamp

221 TABLE II Comparison of swamp importance values obtained for Joe River Quadrant, Core A - - Bitterlich Survey Species

L-DBHS

T-DBHS

L-Bitter

T-Bitter

Rhizophora mangle Languncularia racemosa Conocarpus erecta Avicennia germinans

0.45 0.43 0.10 0.02

0.37 0.54 0.08 0.02

0.62 0.25 0.12 0.02

0.45 0.45 0.09 0.02

and peat, all of the species importance values yielded similar correlation values.

Shoot-root ratios Table III shows the average shoot-root ratio, and the range in shoot-root ratio values observed for individual core increments, for cores from each sample quadrant. The average shoot-root ratio values range from 0.06 to 0.30. The maximum value for an individual core increment was 0.66, the minimum was 0. Table IV shows the size distribution of aerial (shoot) debris in the six cores from Lumber Key which contained only Rhizophora aerial debris. In these cores, four pieces of debris contributed 80% of the aerial debris; 68 pieces contributed the remaining 20%.

Peat importance Table V shows the summary statistics for cores from each of the sampling localities. Values for each quadrant represent average of three cores. RH stands for Rhizophora; AV stands for Avicennia; CO stands for Combretaceae. Peat importance values of the two species belonging to the Combretaceae (Languncularia and Conocarpus) are lumped into one category because I could not distinguish the debris of these two species in the peat. Table VI shows the change in root importance with depth in peat cores from the Lumber Key transect. In all the cores, the percentage of Avicennia debris decreased with depth. In all but one of the ten cores containing Rhizophora and Avicennia, the importance of the latter decreased with depth.

TABLE III Shoot-root ratios of mangrove peats from Ten Thousand Islands area and the Florida Bay area of Everglades National Park based on measurements of 2.5-cm increments

Lumber Key sample plot Lumber Key transect Lopez River West Key Joe River

Peat depth (cm)

Average shoot-root ratio

Range in values

15

0.28

0.66-0

19 1 3 15

0.15 0.23 0.06 0.30

1.27-0 1.0 0 3.0 -0 1.0 -0

222 T A B L E IV Size f r e q u e n c y of aerial debris, e x c l u d i n g leaves, e n c o u n t e r e d in cores from L u m b e r K e y Size clase of debris ( a r e a m m 2)

<__0.05 0.05-0.20 0.20-0.80 0.80-1.80 1.80 3.10 3.10-7.10 7.10-12.6 12.6-19.6 19.6-28.3 28.3-38.5 38.5-50.3 50.3-63.6

Number of pieces

Total ( a r e a m m 2)

Total (pieces)

Total (area)

(%)

% < < < <

3 4 10 4 15 10 9 5 5 1 1 1

0.15 0.80 8.00 7.20 46.50 71.00 113.40 98.00 141.50 38.50 50.30 63.60

4 6 14 6 21 14 13 7 7 1 1 1

180 200 400 1,886

1 1 1 1

180.00 200.00 400.00 1,886.00

1 1 1 1

5 6 12 57

Total

72

3,305.00

99

98

Comparisons of swamp and peat importance Table VII lists the values of Kendall's rank correlation coefficient (T~) obtained in comparisons of peat importance to swamp importance (T-DBHS) for each taxon. In this analysis, I treated the peat importance of each taxon as an independent variable and the swamp importance of each taxon as a dependent variable. The values of Tc for the comparison of Rhizophora root importance and aerial debris importance in the total core to Rhizophora-T-DBHS values are significant. The value of T¢ for the comparison of Avicennia root importance in the total core and Avicennia-T-DBHS values is significant; that for the comparison of Avicennia aerial debris and T-DBHS values is not significant. Combretaceae debris occurred in too few cores to perform a Kendall's Rank Correlation Analysis. Table VIII lists the product-moment correlation coefficient (r) and the regression slope obtained in comparisons of root importance in the total core and T-DBHS for each taxon. All

1 1 1 1 1 2 3 3 4 1 2 2

the correlation coefficient values are significant. The slope values are near one; however the 95% confidence intervals are large, between +0.24 and __+0.34. Table IX lists the number of significant correlation coefficient values obtained in comparisons of peat importance measures to the four swamp importance measures for each taxon. The peat importance measures for each taxon were: (1) aerial debris importance in the total core; (2) root importance in the total core; (3) root importance in the top 2.5 cm of the core. The swamp importance measures were T-DBHS, L-DBHS, L-Bitter and T-Bitter. Comparisons of root importance in the total core to swamp importance for each taxon yielded significant r's for all measures of swamp importance. Comparisons of root importance in the top 2.5 cm of the core to swamp importance for each taxon yielded fewer significant r's. The comparison of aerial debris importance in the total core to swamp importance in the total core yielded only one significant r. These results suggest that root

223 TABLE V

TABLE VI

Summary of the peat importance values for each taxon in cores from the five sampling areas

Change in root importance with depth in peat cores from the Lumber Key transect

RH

AV

CO

(%)

(%)

(%)

Depth

Lumber Key Quadrant

Core 1

Core 2

Core 3

RH

AV

RH

AV

RH

AV

(%)

(%)

(%)

(%)

(%)

(%)

69 69 75 88 100 97

31 31 25 12 0 3

67 77 65 55 53 92 92 88

33 23 35 45 47 8 8 12

28 50 25 12 81 74 60 92 100

72 50 75 88 19 26 40 8 0

83

17

74

26

58

42

surface area measured: 268 cm 2 Root importance Total debris importance Aerial debris importance

91 92 100

8 7

72 82 100

28 18

34 28 36

66 72 64

0.0 2.5 2.5- 5.0 5.0 7.5 7.5-10.0 10.0 12.5 12.5 15.0 15.0-17.5 17.5--20.0 20.0-22.5

Lumber Key Transect surface area measured: 383 cm 2 Root importance Total debris importance Aerial debris importance

core average

Lopez River Quadrant surface area measured: 101 cm 2 Root importance Total debris importance Aerial debris importance

Joe River Quadrant surface area measured: 299 cm 2 Root importance Total debris importance Aerial debris importance

38 36 38

62 64 62

West Key Quadrant surface area measured: 291 cm 2 Root importance Total debris importance Aerial debris importance

20 19 28

importance in the total core predictor of swamp importance.

80 81 72

is

the

best

Discussion

T a p h o n o m y of mangrove peats This study yielded shoot-root ratios for mangrove peats that are similar but somewhat h i g h e r t h a n t h o s e r e p o r t e d b y C o h e n (1968) a n d C o h e n a n d S p a c k m a n (1977). C o h e n (1968) reported average shoot-root ratios between 0.02 a n d 0.04 f o r m a n g r o v e p e a t s ; a v e r a g e shoot-root values obtained in this study ranged

f r o m 0.06 t o 0.30. T h r e e f a c t o r s m a y c o n t r i b u t e to t h e d i s c r e p a n c y i n v a l u e s . F i r s t , C o h e n based his analysis on a very small peat sample, approximately 150cm/; the sample investigated in this study was considerably larger, 1 3 4 1 c m 2. I n a d d i t i o n , a e r i a l d e b r i s h a s a patchy distribution in the peat cores, and the earlier workers may not have sampled enough peat to obtain a representative sample of aerial debris. Second, aerial debris tended to occur in large pieces. In the Lumber transect cores, the four largest pieces of aerial debris contributed 80~o o f t h e t o t a l a e r i a l d e b r i s i n t h e c o r e s . C o h e n ' s s a m p l i n g t e c h n i q u e l i m i t e d t h e size o f t h e d e b r i s h e s a m p l e d to 1 c m 2 o r less. U s i n g this criterion, Cohen would have excluded those four pieces, and 80% of the aerial debris in the Lumber Key transect cores. T h i r d , C o h e n (1968) d i s t i n g u i s h e d Rhizophora r o o t p e a t f r o m R h i z o p h o r a s e d i m e n t a r y or allochthonous peat. Rhizophora sedimentary peat exhibited current alignment of root debris as well as a high shoot-root ratio when c o m p a r e d to o t h e r m a n g r o v e p e a t s . I c o u l d n o t discern alignment of root debris in the shootrich layers of mangrove peat, and thus could not distinguish between Rhizophora root and sedimentary peat. However, Cohen may have classed all Rhizophora shoot-rich mangrove

224

TABLE VII Kendall's Rank Correlation Analysis (To) of species importance values in the peat and swamp for the total core

Rhizophora Avicennia

Root importance

Aerial debris importance

Tc

Number of observations

Tc

Number of observations

0.80

15 11

0.83 not significant

12 5

0.87

TABLE VIII Regression analysis of species importance values in the peat and swamp Taxon

r

Slope

Rhizophora Avicennia

0.92 0.91 0.98

0.97 0.96 1.02

Combretacea

+ + _

Confidence interval

Observations

0.24 0.27 0.37

15 11 4

TABLE IX Number of significant product-moment correlation coefficient values (r's) obtained in comparisons of peat importance measures to swamp importance measures for each taxon Taxon

Rhizophora Avicennia Combretaceaea

Aerial debris in total core

Root importance in total core

Root importance in top 2.5 cm of core

no. significant r's (4 = total) range

no. significant r's (4 = total) range

no. significant r's (4 = total) range

1 0 -

4 4 4

4 2 -a

0.69 -

0.86-0.94 0.85-0.97 0.97-0.98

0.85-0.89 0.82 -

aNo Combretaceae debris of this type occurred in this portion of the cores. p e a t s w i t h Rhizophora s e d i m e n t a r y p e a t s , which w o u l d r e s u l t in a lower a v e r a g e shootr o o t r a t i o for m a n g r o v e r o o t p e a t s .

Predicting swamp importance values from peat important values C o m p a r i s o n s of p l a n t i m p o r t a n c e i n p e a t s and in the swamps using both parametric and non-parametric tests suggest that the root i m p o r t a n c e of a t a x o n i n t h e e n t i r e c o r e c a n be

u s e d to p r e d i c t t h e i m p o r t a n c e o f t h a t t a x o n i n t h e s w a m p . N o t o n l y is t h e r e a s i g n i f i c a n t a n d high correlation between increases in peat importance and increases in swamp import a n c e for t h e t a x a , b u t a l l t h e s l o p e v a l u e s for the r e g r e s s i o n lines are n e a r one, i n d i c a t i n g t h a t the root i m p o r t a n c e of t a x a in the e n t i r e c o r e m a y p r o v i d e a d i r e c t p r e d i c t i o n of s w a m p i m p o r t a n c e (see T a b l e s V I I a n d VIII). The root i m p o r t a n c e of a t a x o n in the top 2.5 c m of t h e p e a t is n o t a s u s e f u l i n p r e d i c t i n g

225 swamp importance (see Table IX), probably due to the differential rooting depths of Avicennia and Rhizophora. In most of the cores containing both Rhizophora and Avicennia debris, the percentage of Avicennia debris decreases with depth. This decrease seems to be controlled by the differential rooting depths of Rhizophora and Avicennia. Avicennia has a shallow rooting system, and produces most of its absorbing rootlets in the top 2-3 cm of the soil (Chapman, 1943). The root system of Rhizophora penetrates the substrate to a depth of I m (Gill and Tomlinson, 1977); Rhizophora produces most of its rootlets at a depth of approximately 20 cm (H. Wanless, pers. comm., 1979). The effect of rooting depth on peat importance values suggests that the peat importance values used to predict swamp importance should be based on 10 or more cm of peat. Of all the measures of peat importance investigated, the aerial debris importance of a taxon in the total core was the worst predictor of swamp importance. This is probably due to the scarcity of aerial debris in mangrove peats. Not only is aerial debris rare, the organ composition of aerial debris varies by taxonomic group. Rhizophora aerial debris consisted of twigs, bark, leaves, and occasional wood. Avicennia debris consisted predominantly of wood and leaves, but few twigs or bark pieces. Languncularia contributed no recognizable aerial debris to the peat examined. Conocarpus contributed twigs, leaves and bark. My inability to distinguish between Languncularia and Conocarpus roots illustrates a frustrating aspect of this study. Roots, one of the least diagnostic organs in the peat, provide the most accurate prediction of swamp abuno dance. In addition, if all mangrove (salt-water swamp) peats have low shoot-root ratios similar to those found by Cohen (1968) and in this study, extinct mangroves preserved as permineralized peats may be found primarily as root fossils which are difficult to distinguish from the roots of related, fresh-water forms. The aerial portions of such plants may occur

primarily as compression-impression fossils preserved in marine sediments. Scheihing and Pfefferkorn (1984) found accumulations of Rhizophora mangle leaves in the beach sands of the Orinoco delta.

A model for peat accumulation Vegetation layers and predicting swamp importance To a paleoecologist, in the ideal peat swamp the amount of debris contributed to the peat by each forest layer (canopy, shrub and herbaceous understory) would reflect the relative biomass and primary productivity of that layer. The amount of debris contributed by each species to the peat would reflect the relative biomass and primary productivity of that species in the living swamp. Finally, decomposition would be slow or non-existent, and all debris would decay at the same rate, so that the relative abundance of species in permineralized peat could be used to reconstruct the relative abundance of plants in the living swamp community. Swamps in which the layers of vegetation contribute debris to peat relative to their biomass probably do not exist outside of the paleoecologist's imagination. Data from Okefenokee swamp peats (Table X) suggest that the amount of debris contributed to peat by forest layers reflects their growth habit and growth rate as well as their relative biomass. Herbaceous understory plants, which account for very little of the living biomass of the swamp, contributed up to 25% of the peat biomass in one of the three sites sampled. However, the contribution of the shrub layer to peat may reflect the relative biomass of this layer more accurately. Shrubs contributed 32% of the debris to peat from the tree island site. This site has a luxuriant, nearly inpenetrable shrub layer, with individual shrubs from 2 to 3 m tall. Shrubs contributed only 2% of the peat underneath the Taxodium (cypress) forest samples, where the dense canopy limited the size of the shrubs to sparse plants less than one meter tall.

226

TABLE X Percentage of vascular plant debris contributed to the peat by each forest layer in three Okefenokee swamp sites Swamp forest type

Total vascular plant debris

(%)

Tree Island Taxodium swamp Nyssa swamp

Canopy

Shrub

37 73 61

32 2

The results presented here suggest that in mangrove swamps, ' which have only one vegetation layer, the root importance of a taxon in peat can be used to predict the importance of the taxon within the swamp community. Preliminary studies of the Okefenokee swamp (See Table in Note added in proof) suggest that the total (root and aerial) debris importance of tree species in peat predicts the importance of tree species in this fresh-water swamp. However, the lack of importance measures that can relate the relative importance of trees to that of shrubs and herbaceous plants hinders the complete analysis of the Okefenokee and other fresh-water peats.

The significance of shoot-root ratios In the ideal swamp, the shoot-root ratio of individual species would not vary with environmental conditions, but would reflect the ratio of shoot standing crop and productivity to root standing crop and productivity. Instead, swamp peats show a variety of shootroot ratio values (Table I). This compilation suggests that the shoot-root ratios of mangrove peats (mean=0.16, n=6) are significantly lower than those of fresh-water peats (mean = 2.03, n = 6). The value of Student's t for the comparison of the mean of these two samples (3.95) has less than a 2°//0 chance of resulting from a random distribution of values. A number of explanations have been advanced to explain shoot-root ratios in Recent and ancient swamp peats. For Recent peats, Cohen (1968) and Cohen and Spackman (1977)

Vine

Herbaceous understory

-

20

11

-

25

-

26

8

UNID

4

suggested that tidal currents, the stilt root growth habit of Rhizophora, the composition and anatomy of Avicennia debris, rapid surface degradation, low water tables and fire contributed to low shoot-root ratios. They invoked the root system morphology of the Persea-Myrica-Salix community, high water tables and rapid deposition to explain high shoot-root ratios in peats. For ancient peats, DiMichele et al. (1985) and Phillips and Peppers (1984) invoked low rainfall to explain low shoot-root ratios in tropical permineralized peats of Carboniferous age. The factors used to explain shoot-root ratios fall into three categories: (1) attributes of the physical environment such as salinity (which correlates to the presence of tides); water table (which in swamps correlates to the frequency and severity of fires); and rainfall regime; (2) attributes of the biological environment such as high rates of debris production and rapid degradation; (3) attributes of the plant community such as root system morphology or the anatomy and composition of taxa. Only if attributes of the physical or biological environment determine shoot-root ratios will they have interpretative value for ancientswamp communities. If attributes of individual taxa determine these ratios, the pattern revealed in Table I applies only to the swamps studied. For instance if stilt roots cause low shoot-root ratios, tropical fresh-water peats from communities with abundant stilt roots might resemble the low values of mangrove peats shown in Table I.

227

The influence of one attribute of swamp communities on the shoot-root ratios of peat can be tested. Mangrove communities have sparse herbaceous and shrub layers; freshwater swamp communities may have luxuries herbaceous and shrub layers. If shrubs and herbaceous plants contribute disproportionate amounts of shoot debris to peat, this could cause mangrove peats to be shoot-poor compared to fresh-water peats. Table XI shows the shoot-root ratios of fresh-water and mangrove tree genera in peat. The shoot-root ratios of fresh-water tree genera in fresh-water peats are high. The shoot-root ratios of mangrove genera and families in salt-water peats are low. Thus, the increased shoot-root ratio of freshwater peats are not due entirely to the abundance of shrubs and herbaceous plant in fresh-water communities. Any consideration of swamp-peat shoot-root values is necessarily incomplete until more is known about both cold temperate and tropical swamp peats. In addition, it is necessary to note that marsh peats do not fit the pattern observed for swamp peats. However the data available allow evaluation of all of the environmental attributes invoked to explain swamp shoot-root ratios except high rates of debris production.

Rainfall. Variations in rainfall cannot account for the observed variations in shoot-root ratios. All of the peats shown in TableI accumulated in swamps that experienced dry seasons, which should result in low shoot-root ratios. Yet their shoot-root ratios vary greatly. In the Okefenokee, the dry season lasts from October to J a n u a r y and the difference in rainfall between the driest and the wettest months is about 148 mm (Rykiel, 1977). In both the subtropical fresh-water locality of Cohen (1968) and the mangrove localities, the dry season lasts from April to November, and the rainfall difference between the driest and wettest months is about 178mm (Thomas, 1974). If rainfall controlled shoot-root ratios, both the fresh-water subtropical peat and the mangrove peats should show similar shoot-root ratios, and these ratios might be slightly lower

than those of the Okefenokee peats. Instead, all of the fresh-water samples show similar high values compared to mangrove samples.

Water table. The height of the water table also appears unable to account for variations in the shoot-root ratios. Among the fresh-water peats sampled, the sample with the highest shootroot ratio (Taxodium, 4.26) came from the driest site. This sample was collected from a water-logged rather than a flooded area; Sphagnum (peat moss) which does not grow in flooded areas was common in the peat; and Taxodium, the dominant tree at this site, requires a dry surface for seeds to start. The Nyssa swamp sample (shoot-root ratio=1.20) and the Tree Island swamp sample (3.0) came from flooded sites. If water table controlled shoot-root ratios, I would have expected the Nyssa and the Tree Island peats to have high shoot-root ratios, and the Taxodium peat to have lower shoot-root ratios. Instead, the Taxodium peat had a higher shoot-root ratio. Among the mangrove peats sampled, the presence of herbaceous and Salicornia (salt bush) at Joe River and West Key indicate a lower water table at these sites than at Lumber Key and Lopez River. Again, the sites with a T A B L E XI P e r c e n t a g e of aerial debris e x c l u d i n g r e p r o d u c t i v e o r g a n s in peat for e a c h t a x o n Taxon

Aerial debris

Habitat and growth habit

(%) Nyssa Ilex Acer Cyrilla Taxodium Rhizophora A vicennia Combretaceae a

68 96 100 63 57 97 83 81 12 9 9

f r e s h - w a t e r tree f r e s h - w a t e r tree f r e s h - w a t e r tree fresh-water shrub f r e s h - w a t e r tree mangrove mangrove 1 species: m a n g r o v e 1 species: b r a c k i s h a n d f r e s h - w a t e r tree

aAll of t h e C o m b r e t a c e a e s a m p l e d c a m e from b r a c k i s h w a t e r or m a n g r o v e s w a m p s .

228 low water table had slightly higher shoot-root ratios (average 0.22) than the sites with high water tables (average 0.18), although this difference is not statistically significant. Finally, although it is difficult to compare watertable height at the fresh-water sites to watertable height at the mangrove sites, fresh-water (Okefenokee) peats from both high water and low water sites had higher shoot-root ratios than the mangrove peats.

Salinity. In the swamps studied, salinity appears to control the shoot-root ratio. Within swamp environments salinity correlates not only with the presence of tides, but also with biological factors such as the presence of abundant detritovores, that cause rapid decomposition rates. Tides cause low shoot-root ratios because they transport detached aerial debris out of the swamp. The result of tides is such that in mangrove swamps flooded at high tides, the peat consists of attached root debris. In sites that are water-logged but not flooded at high tide, tidal flusing can remove only decomposed debris. In order to explain low shoot-root ratios in these peats, one must consider the action of detritovores. Detritovores, which are common in mangrove swamps, speed the decomposition of plant debris by reducing particle size (Mann, 1976). Odum (1971) reported that mangrove leaf litter placed in mesh bags on the swamp floor decomposed more rapidly in salt-water than fresh-water. In the salt-water sites only 9°//0 of the leaf material remained after four months. In fresh-water sites, 54% of the leaf material remained after four months. Odum attributed this result to the presence of more detritovores in salt-water habitats. In all mangrove peats, detritovores break down aerial and dead root debris, and tides flush the fine particles from the peat. Thus aerial debris disappears from mangrove swamps that remain unflooded at high tide in the form of fine particulate matter. As a result of the dual action of tides and detritovores, the mangrove peats studied consisted mostly of living and recently dead root debris. The absence of aerial debris reflects a

relatively high rate of decomposition as suggested by Cohen (1968) and Cohen and Spackman (1977). Because this high rate of decomposition also affects root debris, mangrove peats probably have low rates of accumulation compared to fresh-water peats, given equal rates of debris production. Little is known about the abundance of detritovores in the Okefenokee swamp. However Swift et al. (1979) documented decreased numbers of detritovores in cold temperate peat bogs when compared to adjacent mineral soils, possibly due to low nutrient availability and acid waters. Janzen (1974) noted the absence of fish and insect larvae in tropical backwater swamps, due to acid water conditions. These studies and the work of Odum (1971) suggest that fresh-water swamps have fewer detritovores than mangrove swamps. As a result, fresh-water peats have lower rates of decomposition and higher shoot-root ratios than mangrove peats.

The significance of framework to matrix ratios Cohen and Spackman (1977) and Spackman et al. (1976) linked both low framework/matrix ratios (F/M) and low shoot-root ratios to rapid rates of decomposition, although they noted that tidal flushing caused extremely high F/M ratios (9.0) in tidally-flushed mangrove peats (Cohen and Spackman, 1977). In general, the F/M ratios of mangrove peats are 1.0 or higher while those of fresh-water swamp peats are 1.0 or lower (range= 1.0 to 0.2). Thus fresh-water swamp peats have high shoot/root ratios indicating slow decomposition and low F/M ratios, indicating rapid decomposition. This contradiction between shoot-root and F/M ratios can be resolved if F/M ratios reflect export of matrix material out of the peat rather than decomposition rates. If so, mangrove peats all of which are flushed regularly by tides, would be expected to have high F/M ratios and low shoot-root ratios. High shoot-root ratios and low F/M ratios would result in fresh-water peats with slow decomposition and little opportunity for tidal export of matrix material.

229

The Janzen model for peat accumulation The work of Janzen (1974) on tropical blackwater swamps suggests that these ecosystems approach the paleoecologist's ideal. Organic acids leached into the water of these swamps trigger peat accumulation by poisoning decomposer organisms. There is limited recycling of nutrients from the peat to the forest due to the lack of decomposer organisms and detritovores. The plants that have chemical defenses against herbivores and that keep as much of their biomass as long as possible are the best competitors in such an environment. This selection pressure in favor of plants with strong chemical defenses increases the organic acid content of the water and further enhances peat accumulation rates. The average rate of peat accumulation in one tropical blackwater swamp (2.2 mm yr ~) is about three times that of the Okefenokee swamp (0.7mm yr -~) (Anderson, 1964; Spackman et al., 1976). Although the shoot-root ratios of peats from modern tropical blackwater swamps are not known, these swamps may be so saturated in organic acids, that almost no decomposition occurs. The high rates of peat accumulation in these swamps support this idea. Thus these environments could be modelled as extreme versions of the Okefenokee swamp. If so, their peats should have higher shoot-root ratios than warm temperate fresh-water peats such as those from the Okefenokee swamp.

Interpretation of ancient peats The results of this study suggest that the shoot-root ratios of ancient peats indicate the paleosalinity of ancient swamps. Raymond (1983) used the low shoot-root ratios of permineralized peats dominated by Cordaites to suggest that some of these trees grew in salt water. Although the shoot-root values of ancient peats overlap values observed in modern peats, some ancient peats have extremely high shoot-root values. Figure 2 shows histograms of the shoot-root values of permineralized peat samples (coal balls) from two Upper Carboniferous coals. Salt-water and freshwater Cordaites trees dominated the deposit

shown in the top histogram. This deposit came from the Urbandale Coal seam in Dallas Co., Iowa; according to Phillips (1981) the age of the Upper Carboniferous Iowa coal seams is late Westphalian B or early Westphalian C. The peat samples with shoot-root ratios between 0 and 0.11 probably accumulated in salt-water swamps. However, the large number of coal balls with shoot-root ratios above 0.25 indicates that most of the peat in this deposit accumulated in fresh-water swamps. Arborescent lycopods, which probably grew in fresh water, dominated the deposit shown in the bottom histogram. This deposit came from the Herrin coal in Illinois; its age is Westphalian C (Phillips, 1981). Both the fresh-water portion of the Cordaites histogram and the arborescent lycopod histogram show a much wider range of high shoot-root ratios (maximum value: all shoot, average value for the fresh-water Herrin coal-ball peat: 1.43) than those observed for modern peats (range: 1.17 to 4.26, average value: 2.03). The depositional environment of the Upper Carboniferous coals may account for their high shoot-root values. Both of these coals accumulated near the paleoequator and pre-

24t --(A/

n-96

(E~)

20"

~

1

12-

.2 ;

-!!!ili o o

o o o o o o o

percentages of aerial debris in each coal ball

percentages of aerial debris in each coal ball

Fig.2. Histogram of shoot-root ratios of Urbandale (Iowa) and Herrin (Illinois) coal balls. The percentage of coal balls from each sample within each shoot percentage class represents the percentage of total surface area contributed by coal balls with that shoot percentageor shoot-rootratio. Coal balls from the Urbandale Swamp show a peak in the 0 10% shoot percentage class (0 0.11 shoot-root ratio). These coal-ball peats probably accumulated in salt-water swamps. Coal ball peats with shoot-percentageabove 20% (shoot-root ratio, 0.25) in the Urbandale and Herrin Coals probably accumulated in fresh-water swamps.

230

sumably in a wet tropical climate. The Okefenokee Swamp and the mangrove swamps of southern Florida may be an extremely poor modern analogue for ancient tropical swamps. Modern tropical blackwater swamps such as those described by Anderson (1964) and Janzen (1974) are probably a much better environmental analogue for the Upper Carboniferous coals of the mid-continent. Finally, although physical factors such as rainfall regime, water table height and temperature seasonality do not appear to control the variation in shoot-root ratios for the mangrove and fresh-water swamps studied, these factors may account for the differences between "low" fresh-water shoot-root ratios (0.66-1.5) and the ~'high" fresh-water shoot-root ratios (1.5 to all shoot) observed in permineralized peats of the Upper Carboniferous. Establishing the relationship between these factors and peat accumulation requires the study of both tropical blackwater swamp peats and cold temperate fresh-water swamp peats.

Conclusions The results of this study suggest that the taxonomic composition of peat reflects the taxonomic composition of the swamp forest. Thus the rank abundance of debris derived from canopy genera in ancient peats can be used to reconstruct ancient swamp forests. Analysis of peats from modern warm temperate and subtropical swamps suggests that salinity and factors that correlate with salinity such as detritovore abundance and tidal flushing control the shoot-root ratios of these peats. However these results apply only to swamp forest peats; the shoot-root ratios of marsh peats cannot be explained by factors correlated with salinity and matrix export rates. Permineralized peats from Pennsylvanianaged tropical swamps show extremely high shoot-root ratios. These peats accumulated in tropical swamps, and their extremely high shoot-root ratios may be similar to those of modern tropical blackwater swamps. Although factors that correlate with salinity

account for peats with very low shoot-root ratios (0-0.3), additional factors such as rainfall regime, water table height and temperature seasonality may account for the variation between the shoot-root ratios observed in the warm temperate Okefenokee peats and tropical swamp peats of Pennsylvanian age.

Acknowledgements I would like to acknowledge the enthusiasm and persistence of the late T.J.M. Schopf, who encouraged me to undertake this project. I developed many of the ideas presented here as the result of discussions with W.C. Parker, R.C. Aller, T.L. Phillips, W. DiMichele, and P. Domenico. I would like to thank my field assistants C. Kazmir, W. Ullman, and R. Humphreville, and also the National Park Service for allowing me to sample peat in the Okefenokee and Everglades National Parks. Finally, I would like to thank the Harvard University Paleobotanical Herbarium and the University of Illinois Paleoherbarium for lending me coal-ball specimens.

References Anderson, J.A.R., 1964. The structure and development of the peat swamps of Sarawak and Brunei. J. Trop. Geogr., 18: 7-16. Behrensmeyer, A.K., 1978. Taphonomic and ecologic information from bone weathering. Paleobiology, 4: 150-162. Behrensmeyer, A.K., 1982. Time resolution in fluvial vertebrate assemblages. Paleobiology, 8: 211-227. Campbell, R.C., 1974. Statics for Biologists. Cambridge University Press, Cambridge, England, 2nd ed., 385 pp. Chapman, V.J., 1943. Cambridge University Expedition to Jamaica, Part 2. The morphology of Avidennia nitida Jacq. and the function of its pneumatophores. J. Linn. Soc. Bot., 52: 487-533. Cohen, A.D., 1968. The petrology of some peats of southern Florida (with special reference to the origin of coal). Thesis. Pennsylvania State Univ. Cohen, A.D. and Spackman, W., 1977. Phytogenic organic sediments and sedimentary environments in the Everglades-Mangrove Complex, Part II. The origin, description and classification of the peats of southern Florida. Paleontographica, 162B: 71-114. Damuth, J., 1982. Analysis of the preservation of community structure in assemblages of fossil mammals. Paleobiology, 8: 434-446.

231 DiMichele, W.A., Phillips, T.L. and Peppers, R.A., 1985. The influence of climate and depositional environment on the distribution and evolution of Pennsylvanian coal swamp plants. In: B. Tiffney (Editor), Geologic Factors in the Evolution of Plants. Yale University Press, New Haven, Conn., pp. 223-256. Gill, A.M. and Tomlinson, P.B., 1977. Studies on the growth of red mangrove (Rhizophorea mangle L.) 4. The adult root system. Biotropica, 9: 145-155. Grosenbach, L.R., 1952. Plotless timber estimates - - new, fast, easy. J. For., 50: 32-37. Janzen, D.H., 1974. Tropical blackwater rivers, animals, and mast fruiting by the Dipterocarpaceae. Biotropica, 6:69 103. Mann, K.H., 1976. Decomposition of marine macrophytes. In: J.M. Anderson and A. Macfadyen (Editors), The Role of Terrestrial and Aquatic Organisms in Decomposition Processes. Blackwell, London, pp.247-267. Odum, W.E., 1971. Pathways of energy flow in a south Florida estuary. Univ. Miami Sea Grant Tech. Bull., 7, 162 pp. Phillips, T.L., 1981. Stratigraphic and geographic occurrences of permineralized coal-swamp plants Upper Carboniferous of North America and Europe. In: D.L. Dilcher and T.N. Taylor (Editors), Biostratigraphy of Fossil Plants. Dowden, Hutchinson and Ross, Stroudsburg, Pa., pp.25-92. Phillips, T.L. and DiMichele, W.A., 1981. Paleoecology of Middle Pennsylvanian age coal swamps in southern Illinois/Herrin Coal Member at Sahara Mine No.6. In: K.J. Niklas (Editor), Paleobotany, Paleoecology, and Evolution. Vol.I. Praeger, New York. N.Y., pp.231 284. Phillips, T.L. and Peppers, R.A., 1984. Changing patterns of Pennsylvanian coal-swamp vegetation and implications of climatic control on coal occurrence. Int. J. Coal Geol., 3:205 255. Phillips, T.L., Kuntz, A.B. and Mickish, D.J., 1977. Paleobotany of permineralized peat (coal balls) from the Herrin (No.6) Coal Member of the Illinois Basin. In: P.N.

Note added in proof Comparison of peat and swamp importance of canopy trees in Okefenokee swamp peats Peat type and tree genera

Swamp importance of tree genera

Peat importance of tree genera

(%)

(%)

Tree Island IUex Taxodium Magnolia

59 35 6

59 39 2

Taxodium Taxodium

100

96

Nyssa Nyssa 74 Illex 13 Acer 6 unknown (Acer?) 6

75 16 7

Givens and A.D. Cohen (Editors), Interdisciplinary Studies of Peat and Coal Origins. Geol. Soc. Microform Publ., 7:18 49. Raymond, A., 1983. Gradient analysis of Iowa coal ball floras. Geol. Soc. Am. Abstr. with Programs, 12: 506. Raymond, A. and Phillips, T.L., 1983. Evidence of an Upper Carboniferous mangrove community. In: H. Teas (Editor), Second Int. Symp. Biology and Management of Mangroves. Junk, The Hague, pp. 19 30. Rykiel Jr., E.J., 1977. Toward simulation and systems analysis of nutrient cycling of the Okefenokee Swamp, Georgia. Univ. Georgia, Okefenokee Ecosystems Invest. Rep., 1, Athens, Ga., 139 pp. Scheihing, M.H. and Pfefferkorn, H.W., 1984. The taphonomy of land plants in the Orinoco Delta: a model for the incorporation of plant parts in clastic sediments of Late Carboniferous age of Euramerica. Rev. Palaeobot. Palynol., 41:205 240. Schopf, T.J.M., 1978. Fossilization potential of an intertidal fauna: Friday Harbor, Washington. Paleobiology, 4: 261 270. Simpson, G.G., Roe, A. and Lewontin, R.C., 1960. Quantitative Zoology. Harcourt, Brace and World, New York, N.Y., 440 pp Spackman, W., Cohen, A.D., Given, P.H. and Casagrande, D.J., 1976. Environments of Coal Formation. A Comparative Study of the Okefenokee Swamp and the Everglades-mangrove Swamp-Marsh complex of southern Florida. Coal. Res. Stn., P. State. Univ., College Park, Pa., 403 pp. Swift, M.J., Heal, O.W. and Anderson, J.M., 1979. Studies in Ecology, Vol.5, Decomposition in Terrestrial Ecosystems. Univ. California Press, Berkeley, Calif., 372 pp. Thomas, T.M., 1974. A detailed analysis of climatological and hydrological records of south Florida with reference to man's influence upon ecosystem evolution. In: P.J. Gleason (Editor), Environments of South Florida: Present and Past. Miami Geol. Soc. Mem., 2: 82-122.