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root ratios of peats
Review of Palaeobotany and Palynology, 58 (1989): 85 94 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
85
TAXONOMIC UNIFORMITARIANISM: THE PROBLEM WITH SHOOT/ROOT RATIOS OF PEATS DANIEL COVINGTON 1 and ANNE RAYMOND 2 1Department of Biology, Texas A & M University, College Station, TX 77843 (U.S.A.) 2Department of Geology, Texas A & M University, College Station, TX 77843 (U.S.A.) (Received December 10, 1987; revised and accepted February 26, 1988)
Abstract Covington, D. and Raymond, A., 1989. Taxonomic uniformitarianism: the problem with shoot/root ratios of peats. Rev. Palaeobot. Palynol., 58: 85-94. The ratio of shoot to root debris is readily measured in both ancient and modern peats. Several ecological causes of shoot-poor peats have been suggested, but these are based on comparisons between taxonomically dissimilar peatforming swamps. Here we evaluate the ecological significance of shoot/root ratios in taxonomically similar, yet ecologically variable communities: mangrove swamps. Our preliminary data indicate that all mangrove peats are shoot-poor regardless of environment of deposition. The shoot-poor nature of mangrove peats may be the result of some factor which is universally present in mangrove swamps and does not vary with local environment. We discuss the possibility that the growth habit of mangroves species themselves may account for the scarcity of aerial debris in mangrove peats. For example, mangrove species tend to have a larger portion of their living biomass as roots when compared to non-mangrove tree species. Further, mangrove species tend to form root mats which would serve to exclude aerial debris from sub-surface layers until aerobic degradation is complete. Neutral pH, which occurred in both the freshwater and saline mangrove localities studied, may also contribute to the production of shoot-poor peats. Since freshwater, shoot-rich peats do occur, we conclude that processes of peat formation vary. Universal statements correlating environments of deposition to peat characteristics should be applied with caution.
Introduction Permineralized peats from the Upper Carboniferous have been used to reconstruct p a l e o c o m m u n i t i e s ( P h i l l 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; R a y m o n d a n d P h i l l i p s , 1983). T h e s e s t u d i e s h a v e a s s u m e d t h a t plant frequencies in the community are faithfully reflected in the peat while studies of m o d e r n p e a t - f o r m i n g s y s t e m s ( C o h e n , 1968; C o h e n a n d S p a c k m a n , 1977; R a y m o n d , 1987) indicate that a taphonomic filter exists between a plant community and the peat it produces. If ancient peats are to be considered reliable repositories of paleoecological infor0034-6667/89/$03.50
mation, then taphonomic processes resulting in preservational bias have to be considered. Reconstructions of communities and environments of deposition depend on an imperfect record: only if the imperfections have systematic variation and understood causes will reconstructions have validity. We assess the reliability of the ratio of aerial to subterranean organs (shoot/root ratio)preserved in mangrove peats as an indicator of d e p o s i t i o n a l e n v i r o n m e n t . I f c o r r e l a t i o n s between ecological factors and differential preservation can be discerned in modern mangrove swamps, the consideration of such correlations may prove useful for reconstruction from
© 1989 Elsevier Science Publishers B.V.
86 ancient peats. Shoot/root ratios are readily determined and vary considerably in both ancient and modern peats. Ancient peats composed of arborescent lycopods, ferns, and seed ferns generally have high shoot/root ratios (1.1-1.9)(Phillips and DiMichele, 1981), which may be indicative of fresh water swamps. In contrast, ancient peats with high frequencies of the Cordaitalean seed plants have much lower shoot/root ratios (0-0.1) which may be characteristic of brackish-tomarine swamps (Raymond, 1987). Shoot/root ratios of modern peats have been investigated by Cohen (1968), Cohen and Spackman (1977), and by Raymond (1987). These studies cornpared peats formed in a variety of environments in the Okefenokee Swamp and the Everglades. In all cases, peats formed by freshwater communities (Okefenokee and inland Everglades) have higher shoot/root ratios than those formed by saltwater communities (mangrove of coastal Everglades). Causes of variable shoot/root ratios have been suggested for both ancient and modern peats. Cohen and Spackman (1977) suggested that root-rich modern saline peats result from rapid decomposition and tidal flushing of aerial debris from the swamp. Similar processes may have been responsible for the shoot-poor nature of some ancient peats. For example, Cordaites-dominated coal balls from the Carboniferous of Iowa contain little aerial debris. Cridland's reconstruction (1964) of Cordaitalean plants resembles the modern mangrove genus, Rhizophora. Both that study and Raymond and Phillips (1983) accorded a mangrove habitat to Cordaitalean plants, DiMichele et al. (1985) suggested that shootpoor peats can also form in seasonally-dry swamps. This might happen if lowered water tables and increased soil aerobic zones during dry periods result in an increased decomposition rate. That study also suggested that the increasing abundance of shoot-poor peats during the Pennsylvanian is associated with intervals of declining precipitation. Similarly, Phillips and Peppers (1984) argued that such ancient peats may result from accumula-
tion under seasonally-variable precipitation regimes. Raymond (1987) evaluated reported environmental causes of shoot-poor peats by comparing modern freshwater peats (relatively shoot-rich) from the Okefenokee and inland Everglades to saltwater peats (relatively shootpoor) from coastal Everglades mangrove swamps. That study rejected a seasonal precipitation regime as a cause of shoot/root ratio variability by pointing out that both relatively high-shoot and low-shoot peats accumulated in climates with a pronounced dry season. Likewise, a lowered water table (and presumably increased aerobic decomposition rates) was discounted as both freshwater and saltwater peats with relatively high shoot/root ratios could be found in locations with comparatively low water tables. That study concluded that salinity and the factors correlated with it (tidal flushing of aerial debris and increased rates of decomposition due to the relatively higher frequency of detritivores in saline environments) were the most reasonable cause for shoot-poor peats. Consequently, shoot/root ratios of ancient peats can be used as an indicator of paleosalinities. The suggestion that shoot/root ratios are determined by salinity requires further evaluation. The differences between freshwater peats with high shoot/root ratios and saltwater mangrove peats with low shoot/root ratios may not be solely attributable to environmental differences as different plant species with disparate morphologies are being compared. Shoot/root ratio variability may be as much the result of taxon-specific morphological characteristics as of any particular physical environmental parameter. Further, plant/environment interactions operate bi-directionally. Not only does the physical environment affect community taxonomic composition, structure, and metabolism, but a plant community has profound effects on its physical environment. It is difficult therefore to evaluate the validity of inferences about cause made from comparisons among plant communities that differ both in taxonomic composition and physical en-
87 vironment. There are no a priori reasons to accept the supposition that processes of peat formation are the same (or even similar) in taxonomically different communities. Inferences about cause derived from observed variation of peat characteristics are strongest when the peats are produced by taxonomically similar communities whose taxa possess sufficient ecological amplitudes to assess the role of specific environmental parameters by amongsite comparisons. Observed cause-effect patterns of peat variation in such communities can then be applied to and evaluated in taxonomically dissimilar communities. If the patterns pertain in both taxonomically and ecologically diverse modern communities, then these patterns may be appropriate in the interpretation of ancient peats. Thus, the relationship between shoot/root ratios of ancient peats and paleosalinities requires further study. Our present research efforts involve the study of peat formation in a number of mangrove swamps, which are ecologically variable yet taxonomically simple communities. Here we report the preliminary data gathered from reconnaissance trips to our study sites in Belize, Central America.
Caribbean Sea increase the likelihood that a sufficient number of ecologically varying mangrove swamps could be found to evaluate ecological effects on peat formation. The northern half of the country is located on a carbonate platform, while the southern half is dominated by the siliceous Maya Mountains. Thus, the nature of the sediments entering the coastal system varies and this has been suggested to influence peat deposition (Wanless, 1974). Finally, Belize is characterized by a rather steep precipitation gradient with the northern portionsreceiving an average of 1500 mm/y, while the extreme southern portion receives more than 4000 mm/y (Walker, 1973). The difference in rainfall could be expected to have a large effect on the hydrology of mangrove swamps. Mangrove swamp sites were selected to test the possibility that salinity and the factors correlated with it determine the shoot/root ratio of peat. Since the sites vary in other ecological factors as well, it was possible to evaluate a number of other physical ecological parameters.
Methods
An acrylic piston corer (5 cm diam.) was used to extract cores, 0.5 m in length, from each of the sites. The cores were extruded in the field into mesh bags and wrapped in plastic for transport to the laboratory. Peats from the Twin Cays site were available from a different project conducted by the Smithsonian Institution. Vibracores of the deep peats in this site were taken in 1984 and 1985. In the laboratory, the cores were longitudinally halved: one half was archived by wrapping in plastic while the other was used for analysis. Portions of the cores, 2 cm in length, were removed at 10-cm intervals. These portions were contained within fiberglass mesh bags, dehydrated, and infiltrated with paraffÉn following standard procedures (Sass, 1958). The embedded peat samples were thin sectioned on a sliding microtome, mounted on microscope slides, and examined for organ
Site selection The mangrove swamps of Belize were selected as test sites for several reasons. There are only three species which may be present in our sites: Rhizophora mangle L., Avicennia germinans L., and Conocarpus erecta L. which will hereinafter be referred to by their generic names. Belize is subtropical and it is thought that most Paleozic Euramerican coals accumulated in tropical or subtropical climates. The offshore barrier reef(see F i g . l ) m a k e s possible the inclusion of isolated salt water swamps, located on leeward islands, which are free of major terrestrial effects. The variety of coastal landforms found in Belize (paralic swamps, lagoons, deltas, beach ridges) and the presence of several small rivers draining into the
Core evaluation
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Fig.1. Map of Belize showing the localities of the mangrove swamps studied.
content. The linear transect method of Schopf (1938) was used to evaluate the shoot/root ratio of the peats. Additional samples (approximately 2 g dry weight) of the test cores were removed, dried at 60°C to constant weight, weighed, and ashed in a muffle furnace at 575°C for three hours. The remaining ash was weighed, and this value subtracted from pre-ashing weight to determine non-carbonate organic matter. The ash was then digested with HC1 to determine
carbonate content. The remainder was assumed to be the silicate fraction. Results
Site descriptions Brief descriptions of the geomorphological setting and composition and structure of the plant community are provided for each of the sites. These characterizations are summarized
89
in Table I. The first three sites have freshwater influence of variable magnitude,
Hattieville (A, Fig. 1): Located approximately 17 km from the sea, this site is in a localized catchment basin and is only a few acres in extent. No apparent connections exist with either the sea or other mangrove swamps. Surrounding vegetation is typical of the seasonally wet, poorly drained carbonate terrain of n o r t h e r n Belize. Microtopography is variable. Small hummocks are common and these are vegetated by upland species. Rhizophora (1-1.5 m) is the dominant species with infrequent Conocarpus. The site has standing water for ten months of the year, but even at the height of the dry season (April and May), the soils remain waterlogged. Peat distribution is patchy. Temash River I (B, Fig. 1): This site is located at the confluence of the Temash River and two small t r i b u t a r y creeks approximately 10 km from the sea. No well-defined banks are present in this area. During the dry season, salt water intrudes up the river channel. No interstitial salinity data are available. Rhizophora (1-2.5 m) is the only tree species present although
herbs typical of fresh water environments are common. Rainfall is high (4500 mm/y) although there is a definite dry season (April and May).
Temash River H (C, Fig. 1): Located approximately 5 km downstream from Temash River I, this site is situated on narrow, well-defined river banks. Large Rhizophora (up to 13 m) are the only mangrove species present, although non-mangrove species occur infrequently as small understory trees. Saline intrusion occurs during the dry season, and is probably more pronounced t han at Temash River I. The site is probably inundated only during floods as it is higher t han the previous site relative to river level.
Anderson's Lagoon I (D, Fig. 1): Anderson's Lagoon is located just to the south of the Sittee River delta. The barrier forming the lagoon is a depositional feature with constituent sediments probably derived from the river and from n o r t h e r n sources which have been transported by the southwards-flowing long shore c u r r e n t (High, 1975). The Sittee River drains the eastern flank of the siliceous Maya Mountains. The site is located at the mouth of the channel which connects the lagoon to the open sea. Sea water
TABLE I Site ecological and geomorphological characterizations Site
Map
Landform
Salinityi
Vegetationb
Hattieville
A
FW
Temash river I Temash river II Anderson's lagoon I Anderson's lagoon II Placentia lagoon I Placentia lagoon II
B
isolated, inland freshwater pothole riverine-swamp riverine-bank paralic: open sea lagoonal paralic: open sea lagoonal
Twin cays
H
Rhizophora (1-1.5 m), ((Conocarpus)) (freshwater species) Rhizophora(1-1.5 m), rushes, ((palmetto)) Rhizophora(10-13 m), (freshwater species) Avicennia (5-6 m), (Rhizophora fringe) Rhizophora (3-5 m), (Acrostichum) Avicennia (54 m), (Rhizophora fringe) Rhizophora (5-8 m), (Avicennia, Laguncularia) Rhizophora (dwarf-6 m), (Avicennia)
C D
E F G
isolated,saltwater islets
FW-B FW-B SW+ SW SW+ SW SW
aFW= freshwater, B = brackish, SW= oceanic salinity, SW+ = hypersaline. b( )= infrequent, (()) rare.
9O flOW into the lagoon is probably restricted as the channel is long (ca. 2 kin) and narrow (less than 10 m). The landform of this site is characterized as a paralic swamp. It has a fringe of medium-sized Rhizophora (4-6 m) grading landwards to a stand of pure Avicennia (5-6 m). This zonation pattern commonly occurs in swamps with topographical relief sufficient to prevent regular tidal inundation, Landward areas in such swamps are characteristically hypersaline due to evaporation. Species distribution is affected. Species able to tolerate higher interstitial salinities, such as Avicennia, grow in these areas, while species intolerant of hypersalinities, such as Rhizophora, are restricted to swamp shore margins where constant contact with the sea maintains interstitial salinities at or near mean oceanic levels (Teas, 1979).
Anderson's Lagoon H (E, Fig. 1): This site is on the mainland shore of the lagoon. The lagoon, rather than the sea, affects the site directly. There is no significant freshwater influx into the area with the exception of a small, man-made channel (2 m in width) which connects the lagoon to the Sittee River. Rhizophora (to 5 m) is the only tree species present although a fern, Acrostichum sp., is common.
Placentia Lagoon I (F, Fig. 1): This site is located on the mainland just outside of the lagoon so, like Anderson's Lagoon I, it is best characterized as paralic. The zonation of species is also similar, A shoreward fringe of Rhizophora (4-6 m) grades sharply into a pure Avicennia stand (4-6 m) landwards. The lagoon itself is large and is connected to the sea by a large mouth so lagoonal salinities are probably similar to those of the open sea.
Placentia Lagoon H (G, Fig. 1): The site is on the mainland approximately 10 km north of the lagoon mouth. There is no major focus of fresh water discharge. It is dominated by medium-sized Rhizophora (5-8 m).
Twin Cays (H, Fig. 1): This site is a carbonate-mangrove peat buildup up in the lee of the barrier reef about 20 km east of the Sittee River Delta. Rainfall is the only source of freshwater on the island. The island is isolated from terrigenous sediment input. Low-lying interior ponds are vegetated by dwarf forms of Rhizophora. These areas are probably hyper-saline due to restricted access to open water. Larger mangrove trees occur on the island fringes and in inland areas flushed at least occasionally by the tides. Rhizophora (dwarf-6 m) is the dominant species with occasional Avicennia.
Core analyses The composition analyses (Table II) indicate that siliciclastics constitute the majority of the inorganic fraction of mainland swamp peats. The carbonate fraction at these sites may be derived from storm deposits or deposited in situ by calcareous organisms. The isolated offshore site, Twin Cays, has only carbonate in its ash fraction.
Shoot-rootvalues Shoot/root ratios including the average and range of values for each core are indicated in Table II. All of the peats measured were shootpoor except for those from Anderson's Lagoon I. The surface peat from this site contained two large pieces of stem wood and yielded a shootroot value of 2.7. The Placentia I core contained a buried leaf mat between 20 and 22 cm deep, which yielded a shoot-root value of 0.32 at this depth. The lowest shoot-root value measured for an individual core increment was zero. The highest value was 2.7 at the surface of the Anderson's Lagoon I core.
Discussion Mangrove swamps can grow in a diversity of ecological settings yet mangrove peats are apparently shoot-poor regardless of environment (Tables II and III). Since our sites
91 TABLE II Core analyses summaries Site
Number of samples
Shoot/root mean (range)
Organic (%)
Carbonate (%)
Silicate (%)
Total ash
Hattieville Temash river I Temash river II Anderson's lagoon I Anderson's lagoon II Placentia lagoon I Placentia lagoon II Twin cays
4 5 3 2 3 4 2 6
0.05 (0-0.14) 0.08 (0-0.26) 0.07 (0-0.18) 1.5 (0.06-2.7)l 0 (0) 0.09 (0-0.32) 0 (0) 0.05 (0-0.11)
41.6 57 90 27 71 58.3 57.1 61.9
2.7 6.3 4.0 12.7 4.6 33.2 17.5 38.1
55.7 36.7 6.0 60.3 24.4 8.5 25.4 0.0
58.4 43.0 10.0 73.0 29.0 41.7 42.9 38.1
aThis extremely high shoot-root ratio resulted from the occurrence of two pieces of wood at the surface of the core. e n c o m p a s s a b r o a d e n v i r o n m e n t a l range, the f a c t o r s w h i c h c o n t r o l s h o o t / r o o t r a t i o s in m a n g r o v e p e a t s are i n d e p e n d e n t of locally v a r y i n g ecology. We c a n identify w i t h confiTABLE III Peat type
Environment
Okefenokee (freshwater) Taxodium b Okefenokee (freshwater) Tree IslandI Okefenokee (freshwater) Nyssa ~ Okefenokee (freshwater) Cyrilla b Okefenokee (freshwater) Myrica-Persea Salix ~ Everglades (freshwater) ConocarpustransitionaP Everglades (saltwater) C o n o c a r p u s transitional ~ Everglades (saltwater) Rhizophora a Everglades (saltwater) Rhizophora c Everglades (saltwater) Rhizophora-Acrostichum Everglades Mariscus c (saltwater) Avicennia a Everglades (saltwater) Avicennia-Rhizophora a Everglades (saltwater) Taxodium a
Shoot/ root ratio
Swamp 4.3 Swamp 1.5 Swamp 3.0 Swamp 1.2 Swamp 1.9 1.9 0.3 0.04 0.2 0.4 0.02 0.4 0.07
Source: a: Raymond (1983, 1987). b: Spackman et al. (1976). c: Cohen and Spackman (1977).
d e n c e w h i c h e c o l o g i c a l p a r a m e t e r s do n o t affect s h o o t / r o o t ratios; we a r e m o r e c a u t i o u s in i d e n t i f y i n g t h e factor(s), u n i v e r s a l in m a n g r o v e swamps, w h i c h do affect s h o o t / r o o t ratios. O u r sites w e r e l o c a t e d on a n u m b e r of different c o a s t a l l a n d f o r m t y p e s (Table I). S h o o t / r o o t r a t i o s a r e n o t affected by e c o l o g i c a l p a r a m e t e r s d e t e r m i n e d , at l e a s t in part, by g e o m o r p h o l o g i c a l setting. This h a s b e e n s h o w n to affect s w a m p t a x o n o m i c c o m p o s i t i o n (Thom, 1967), c o m m u n i t y p h y s i o g n o m y a n d m e t a b o l i s m (Lugo a n d S n e d a k e r , 1974), a n d t h e a e r i a l e x t e n t a n d d e p t h of p e a t deposits (Wanless, 1974). C o h e n (1968) s u g g e s t e d t h a t tidal flushing of a e r i a l debris w a s p a r t l y r e s p o n s i b l e for the low s h o o t / r o o t r a t i o of m a n g r o v e peats. Tidal s c o u r i n g of a e r i a l debris is a p p a r e n t l y n o t a n e c e s s a r y c o n d i t i o n for s h o o t - p o o r p e a t s as H a t t i e v i l l e is c o m p l e t e l y u n a f f e c t e d by tides, t h e T e m a s h R i v e r sites a r e o n l y infreq u e n t l y s c o u r e d by r i v e r flooding, w h i l e t h e c o a s t a l sites a r e r e g u l a r l y i n u n d a t e d . Phillips a n d P e p p e r s (1984) a n d D i M i c h e l e et al. (1985) s u g g e s t e d t h a t r o o t - r i c h p e a t s of P e n n s y l v a n i a n a g e a c c u m u l a t e d in s w a m p s t h a t w e r e s e a s o n a l l y dry. R e s u l t s of o u r p r e l i m i n a r y i n v e s t i g a t i o n do n o t i n d i c a t e t h a t this is t r u e for m a n g r o v e peats. A l t h o u g h r a i n f a l l is h i g h l y s e a s o n a l in the v i c i n i t y of H a t t i e v i l l e , this s w a m p g r o w s in a local c a t c h m e n t basin, a n d t h e s u r f a c e of t h e p e a t is continually waterlogged, even during the dry
92 season. High shoot root ratios are the expected pattern in such continually-flooded swamps; the Hattieville site had a low shoot/root ratio, The shoot/root ratio of peat from the Temash River localities supports the idea that seasonal drying of the swamp surface does not cause low shoot-root ratios in peat. These sites are similar to the Hattieville site in t h a t they are influenced by fresh water processes for most of the year. However, in contrast to the Hattieville site, the surfaces of Temash River peats are drained during the dry season, even though annual rainfall is higher at Temash River. If seasonal drying of the swamp surface affected shoot-root ratios, the shoot/root ratios of Temash River peats should be equal to, or lower than those observed at Hattieville. The opposite is true. The shoot/root ratio of Temash River I peats is actually higher, primarily due to the presence of a surface leaf mat. Raymond (1983) observed the same phenomenon in the Okefenokee swamp and the mangrove swamps of the Florida Everglades (see Table III). For both mangrove and nonmangrove freshwater swamps, the sites with the highest shoot/root ratios (Taxodiurn swamp in the Okefenokee and the Conocarpus transitional swamp along the Joe River in Florida) were also the best drained sites in that study. Seasonally-drained peats may have a better chance of developing leaf mats than continuously water-logged peats because decomposition rates are lower in dry areas. In addition, the process of drying the swamp surface may render the leaf mats of seasonallydrained sites more impervious to decay (Cohen and Spackman, 1977). Although such surficial leaf mats may be rare in peat, episodic sedimentation could preserve these mats. For example, the relatively high shoot-root value (0.32) of the Placentia I core at 20 cm depth probably resulted from episodic preservation of a surficial litter layer. In addition, cuticular shales or coals, such as t h a t described from the Pennsylvanian-aged Block Coal of Indiana by DiMichele et al. (1984) and Eggert and Phillips (1982), may have resulted when sediments covered leaf mats t h a t accumulated
in drained areas of the swamp. Abundant cuticular shales or coals may indicate seasonally-dry swamp conditions. Since Rhizophora peats are shoot-poor, it may be possible t h a t the growth habit of the plant itself determines shoot/root ratios. The reported shoot/root ratios for living mangrove trees range from 0.57 to 1.47 (Clough and Attiwill, 1982; Saenger, 1982). The shoot-root ratios reported for in situ Rhizophora peats are lower, ranging from 0.22 to 0.02 (Table III) so the production of Rhizophora peat involves the destruction of considerable shoot material relative to root material. However, Rhizophora communities form shallow and dense root mats (Teas, 1979). Do these root mats confine aerial debris to the surface of the peat where decomposition is inevitable? The influence of root mat formation on peat shoot/root ratios is more difficult to assess than physical ecological parameters because detailed information on the root system morphologies of mangrove species other than Rhizophora is lacking. Further, the density of Rhizophora in a mixed mangrove stand required for formation of a root mat is also unknown. Mixed mangrove communities with appreciable densities of Avicennia,Laguncularia racemosa (L.) Gaertner, and Conocarpus from both fresh and saline habitats have universally shoot-poor peats. It is possible that such peats may be the result of the root system strategies of these species, or that a relatively low frequency of Rhizophora in a mixed mangrove community can form a root mat sufficient to exclude aerial debris from sub-surface anaerobic zones. Successional changes in species composition make the situation even more problematic as a persistent root mat from a dwindling Rhizophora community may still prevent the incorporation of aerial debris into the peat. In addition, root mat formation may be a near-universal property of mangrove species as they occupy habitats characterized by unstable substrates, have a high percentage of their total biomass as roots, and generally have shallow root systems (Saenger, 1985). If so, ancient plants
93 with the ability to accumulate peat in saltwater might produce shoot-poor peats even in freshwater swamps, The data indicate that shoot/root ratios of mangrove peats do not correlate to salinity: all investigated mangrove peats are shoot poor even if they accumulate in freshwater regimes, Since other freshwater, non-mangrove, peats do exhibit high shoot/root ratios (Raymond, 1987), processes of peat formation in taxonomically different swamps may not be strictly comparable. Comparisons between mangrove and non-mangrove swamps may indicate how these processes differ and how this information can be used in reconstructions from ancient peats, Decomposition rates of aerial debris correlate with the presence of water in mangrove swamps (Heald, 1971), freshwater alluvial swamps (Brinson, 1977), and lowland tropical rainforests (Stout, 1980). In a review of litter decomposition rates in upland environments, Singh and Gupta (1977) conclude that the same correlation pertains. These high rates of decomposition suggest that all peats should be shoot-poor. Since shoot-rich peats do exist, there must also exist factors which retard the expected high rates of litter decomposition, These factors are apparently inoperative in mangrove swamps either as a result of their absence, or, because attributes of the mangrove e n v i r o n m e n t i t s e l f o f f s e t t h e i r o p e r a t i o n , An example of t h e latter situation would be root mat exclusion of aerial debris discussed above. An example of the former situation is acid water which is not likely to occur in saline mangrove swamps due to the buffering influence of the sea. Our freshwater mangrove sites also had nearly neutral pH, and this neutral pH could also have contributed to the rapid decomposition of aerial debris in these swamps. Janzen (1977) discusses tropical black water swamps characterized by acid water conditions and low rates of decomposition. If low pH conditions are sufficient to inhibit the activity of decomposers, and if high rates of decomposition are responsible for the paucity of aerial debris in some peats, then we should
expect to find shoot-rich peats accumulating in acid water swamps. An example of such a swamp is the lowland dipterocarp forest of Malesia (Anderson, 1983)but quantitative data on the organ composition of these peats are currently not available. The suggestion that low pH is a necessary and sufficient condition for the formation of shoot-rich peats remains unverified, but also is not falsified by any of our results. Conclusions The shoot/root ratios of peats have been suggested to vary directly with the salinity of the environment of deposition. The basis for this suggestion was a comparison between modern swamps which vary not only in their salinity regimes, but also in their taxonomic composition. We made the assumption that taxonomically dissimilar communities may have dissimilar processes of peat formation, so that inferences derived from comparisons between them may not be valid. It is necessary to test the suggestion that shoot/root ratios reflect salinity in taxonomically similar swamps which occur in environments with different salinity regimes. Mangrove swamps were chosen as they occur in saline, brackish, and freshwater habitats. Mangrove peats exhibit universally low shoot/root ratios so the suggestion that shoot/root ratios of peat reflect the salinity of depositional environment is not true for them. Because modern freshwater, non-mangrove swamps do accumulate shoot-rich peats, we conclude that different processes of peat formation operate in taxonomically different swamps. We discussed plausible factors which may account for the shoot-poor nature of mangrove peats (root mat exclusion of aerial debris) and the shoot-rich nature of nonmangrove freshwater peats (acid water inhibition of decomposition). Our present data cannot verify if these two suggestions are necessary and sufficient to account for observed variation in peat shoot/root ratios in taxonomically different swamps.
94
Acknowledgements We would like to acknowledge the field assistance of the following people: Robin Lightey, Ian Macintyre, Gregor Bond, Keith Bowers, and Scott Cross. This research was funded by the American Chemical Society, Petroleum Research Fund and the Whitehall Foundation. The Smithsonian Institute Carrie Bow Cay Research Station provided field support and sampling equipment. Marilyn Green assisted in manuscript preparation. References Anderson, J.A.R., 1983. The tropical peat swamps of western Malesia. In: A. P. Gore (Editor), Mires: Swamp, bog, fen and moor (Ecosystems of the World, 4 B). Elsevier, Amsterdam, pp.181-199. Brinson, M., 1977. Decomposition and nutrient exchange of litter in an alluvial swamp forest. Ecology, 58: 601-609. Clough, B.F. and Attiwill, P.M., 1982. Primary productivity of mangroves. In: B.F. Clough (Editor), Mangrove Ecosystems in Australia: Structure, Function and Management. Aust. Nat. Univ. Press, Canberra, pp.213-222, Cohen, A.D., 1968. The petrology of some peats of southern Florida (with special reference to the origin of coal). Thesis Pennsylvania State Univ., 352 pp. 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. Palaeontographica, 162B: 71-114. Cridland, A.D., 1964. Amyelon in American coal-balls. Paleontology, 7: 186-209. DiMichele, W.A., Rischbieter, M.O., Eggert, D.L. and Gastaldo, R.A., 1984. Stem and leaf cuticle of Karinopteris: source of cuticles from the Indiana "paper coal". Am. J. Bot., 7: 626-637. 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), Geological Factors in the Evolution of Plants. Yale Univ. Press, New Haven, Conn., pp.223 256. Eggert, D.L. and Phillips, T.L., 1982. Environments of deposition coal balls, cuticular shale and gray-shale floras in Fountain and Parke Counties Indiana. Ind. Dept. Nat. Resour. Geol. Surv. Spec. Rep., 30, 43 pp. Heald, E.J., 1971. The Production of Organic Detritus in a South Florida Estuary. Univ. Miama Sea Grant, Techn. Bull., 6. High, L.R., Jr., 1975. Geomorphology and sedimentology of Holocene coastal deposits. In: K.F. Wantland and W.C. Pusey III (Editors), Belize: Shelf-Carbonate Sediments, Clastic Sediments and Ecology. Am. Assoc. Petrol. Geol. Tulsa, Okla., pp.53-96,
Janzen, D,H., 1977. Tropical blackwater rivers, animals, and mast fruiting by the Dipterocarpaceae. Biotropica,
6: 69-103.
Lugo, A.E. and Snedaker, S.C., 1974. The ecology of mangroves. Ann. Rev. Ecol. Syst., 5: 39-64. 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. Praeger, New York, N.Y. I, 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. Givens and A.D. Cohen (Editors), Interdisciplinary Studies of Peat and Coal Origins. Geol. Soc. Microform Publ., 7: 18-49. Raymond, A., 1983. Peat taphonomy of Recent mangrove peats and Upper Carboniferous coal-ball peats. Thesis, Univ. Chicago, Ill., 293 pp. Raymond, A., 1987. Interpreting ancient swamp communities: Can we see the forest in the peat? Rev. Palaebot. Palynol., 52: 217-231. Raymond, A. and Phillips, T.L., 1983. Evidence of an Upper Carboniferous mangrove community. In: H. Teas (Editor), 2nd Int. Symp. Biology Manage. Mangroves. Junk, The Hague, pp.19 30. Saenger, P., 1982. Morphological, anatomical and reproductive adaptations of Australian mangroves. In: B.F. Clough (Editor), Mangrove Ecosystems in Australia: Structure, Function, and Management. Aust. Nat. Univ. Press, Canberra, pp.153-192. Sass, J.E., 1958, Botanical Microtechnique, Iowa State Univ. Press, Ames, Iowa, 3rd ed. 228 pp. Schopf, J.M., 1938. Coal balls as an index to the constitution of coal. Trans. Ill. Acad. Sci., 31: 187-189. Singh, J.S. and Gupta, S.R., 1977. Plant decomposition and soil respiration in terrestrial ecosystems. Bot. Rev., 43 (4): 449-528. 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 Evergladesmangrove swamp-marsh complex of southern Florida. Coal. Res. Stn., Pennsylvania State Univ., College Park, Pa., 403 pp. Stout, J., 1980. Leaf decomposition rates in Costa Rican lowland tropical rainforest streams. Biotropica, 12 (4): 264-272. Teas, H. J., 1979. Silviculture with saline water. In: A. Hollaender (Editor), The Biosaline Concept. Plenum, New York, N.Y., pp.l17-161. Thorn, B.G., 1967. Mangrove ecology and deltaic geomorphology, Tabasco, Mexico. J. Ecol., 55:301 343. Walker, S.H., 1973. Summary of climatic records for Belize. Land Res. Div., Surbiton, Surrey, Suppl.no. 3. Wanless, H.R., 1974. Mangrove sedimentation in geologic perspective. In: Environments of South Florida: Present and Past. Miami Geol. Soc. Mem., 2, pp.190-200.
85
TAXONOMIC UNIFORMITARIANISM: THE PROBLEM WITH SHOOT/ROOT RATIOS OF PEATS DANIEL COVINGTON 1 and ANNE RAYMOND 2 1Department of Biology, Texas A & M University, College Station, TX 77843 (U.S.A.) 2Department of Geology, Texas A & M University, College Station, TX 77843 (U.S.A.) (Received December 10, 1987; revised and accepted February 26, 1988)
Abstract Covington, D. and Raymond, A., 1989. Taxonomic uniformitarianism: the problem with shoot/root ratios of peats. Rev. Palaeobot. Palynol., 58: 85-94. The ratio of shoot to root debris is readily measured in both ancient and modern peats. Several ecological causes of shoot-poor peats have been suggested, but these are based on comparisons between taxonomically dissimilar peatforming swamps. Here we evaluate the ecological significance of shoot/root ratios in taxonomically similar, yet ecologically variable communities: mangrove swamps. Our preliminary data indicate that all mangrove peats are shoot-poor regardless of environment of deposition. The shoot-poor nature of mangrove peats may be the result of some factor which is universally present in mangrove swamps and does not vary with local environment. We discuss the possibility that the growth habit of mangroves species themselves may account for the scarcity of aerial debris in mangrove peats. For example, mangrove species tend to have a larger portion of their living biomass as roots when compared to non-mangrove tree species. Further, mangrove species tend to form root mats which would serve to exclude aerial debris from sub-surface layers until aerobic degradation is complete. Neutral pH, which occurred in both the freshwater and saline mangrove localities studied, may also contribute to the production of shoot-poor peats. Since freshwater, shoot-rich peats do occur, we conclude that processes of peat formation vary. Universal statements correlating environments of deposition to peat characteristics should be applied with caution.
Introduction Permineralized peats from the Upper Carboniferous have been used to reconstruct p a l e o c o m m u n i t i e s ( P h i l l 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; R a y m o n d a n d P h i l l i p s , 1983). T h e s e s t u d i e s h a v e a s s u m e d t h a t plant frequencies in the community are faithfully reflected in the peat while studies of m o d e r n p e a t - f o r m i n g s y s t e m s ( C o h e n , 1968; C o h e n a n d S p a c k m a n , 1977; R a y m o n d , 1987) indicate that a taphonomic filter exists between a plant community and the peat it produces. If ancient peats are to be considered reliable repositories of paleoecological infor0034-6667/89/$03.50
mation, then taphonomic processes resulting in preservational bias have to be considered. Reconstructions of communities and environments of deposition depend on an imperfect record: only if the imperfections have systematic variation and understood causes will reconstructions have validity. We assess the reliability of the ratio of aerial to subterranean organs (shoot/root ratio)preserved in mangrove peats as an indicator of d e p o s i t i o n a l e n v i r o n m e n t . I f c o r r e l a t i o n s between ecological factors and differential preservation can be discerned in modern mangrove swamps, the consideration of such correlations may prove useful for reconstruction from
© 1989 Elsevier Science Publishers B.V.
86 ancient peats. Shoot/root ratios are readily determined and vary considerably in both ancient and modern peats. Ancient peats composed of arborescent lycopods, ferns, and seed ferns generally have high shoot/root ratios (1.1-1.9)(Phillips and DiMichele, 1981), which may be indicative of fresh water swamps. In contrast, ancient peats with high frequencies of the Cordaitalean seed plants have much lower shoot/root ratios (0-0.1) which may be characteristic of brackish-tomarine swamps (Raymond, 1987). Shoot/root ratios of modern peats have been investigated by Cohen (1968), Cohen and Spackman (1977), and by Raymond (1987). These studies cornpared peats formed in a variety of environments in the Okefenokee Swamp and the Everglades. In all cases, peats formed by freshwater communities (Okefenokee and inland Everglades) have higher shoot/root ratios than those formed by saltwater communities (mangrove of coastal Everglades). Causes of variable shoot/root ratios have been suggested for both ancient and modern peats. Cohen and Spackman (1977) suggested that root-rich modern saline peats result from rapid decomposition and tidal flushing of aerial debris from the swamp. Similar processes may have been responsible for the shoot-poor nature of some ancient peats. For example, Cordaites-dominated coal balls from the Carboniferous of Iowa contain little aerial debris. Cridland's reconstruction (1964) of Cordaitalean plants resembles the modern mangrove genus, Rhizophora. Both that study and Raymond and Phillips (1983) accorded a mangrove habitat to Cordaitalean plants, DiMichele et al. (1985) suggested that shootpoor peats can also form in seasonally-dry swamps. This might happen if lowered water tables and increased soil aerobic zones during dry periods result in an increased decomposition rate. That study also suggested that the increasing abundance of shoot-poor peats during the Pennsylvanian is associated with intervals of declining precipitation. Similarly, Phillips and Peppers (1984) argued that such ancient peats may result from accumula-
tion under seasonally-variable precipitation regimes. Raymond (1987) evaluated reported environmental causes of shoot-poor peats by comparing modern freshwater peats (relatively shoot-rich) from the Okefenokee and inland Everglades to saltwater peats (relatively shootpoor) from coastal Everglades mangrove swamps. That study rejected a seasonal precipitation regime as a cause of shoot/root ratio variability by pointing out that both relatively high-shoot and low-shoot peats accumulated in climates with a pronounced dry season. Likewise, a lowered water table (and presumably increased aerobic decomposition rates) was discounted as both freshwater and saltwater peats with relatively high shoot/root ratios could be found in locations with comparatively low water tables. That study concluded that salinity and the factors correlated with it (tidal flushing of aerial debris and increased rates of decomposition due to the relatively higher frequency of detritivores in saline environments) were the most reasonable cause for shoot-poor peats. Consequently, shoot/root ratios of ancient peats can be used as an indicator of paleosalinities. The suggestion that shoot/root ratios are determined by salinity requires further evaluation. The differences between freshwater peats with high shoot/root ratios and saltwater mangrove peats with low shoot/root ratios may not be solely attributable to environmental differences as different plant species with disparate morphologies are being compared. Shoot/root ratio variability may be as much the result of taxon-specific morphological characteristics as of any particular physical environmental parameter. Further, plant/environment interactions operate bi-directionally. Not only does the physical environment affect community taxonomic composition, structure, and metabolism, but a plant community has profound effects on its physical environment. It is difficult therefore to evaluate the validity of inferences about cause made from comparisons among plant communities that differ both in taxonomic composition and physical en-
87 vironment. There are no a priori reasons to accept the supposition that processes of peat formation are the same (or even similar) in taxonomically different communities. Inferences about cause derived from observed variation of peat characteristics are strongest when the peats are produced by taxonomically similar communities whose taxa possess sufficient ecological amplitudes to assess the role of specific environmental parameters by amongsite comparisons. Observed cause-effect patterns of peat variation in such communities can then be applied to and evaluated in taxonomically dissimilar communities. If the patterns pertain in both taxonomically and ecologically diverse modern communities, then these patterns may be appropriate in the interpretation of ancient peats. Thus, the relationship between shoot/root ratios of ancient peats and paleosalinities requires further study. Our present research efforts involve the study of peat formation in a number of mangrove swamps, which are ecologically variable yet taxonomically simple communities. Here we report the preliminary data gathered from reconnaissance trips to our study sites in Belize, Central America.
Caribbean Sea increase the likelihood that a sufficient number of ecologically varying mangrove swamps could be found to evaluate ecological effects on peat formation. The northern half of the country is located on a carbonate platform, while the southern half is dominated by the siliceous Maya Mountains. Thus, the nature of the sediments entering the coastal system varies and this has been suggested to influence peat deposition (Wanless, 1974). Finally, Belize is characterized by a rather steep precipitation gradient with the northern portionsreceiving an average of 1500 mm/y, while the extreme southern portion receives more than 4000 mm/y (Walker, 1973). The difference in rainfall could be expected to have a large effect on the hydrology of mangrove swamps. Mangrove swamp sites were selected to test the possibility that salinity and the factors correlated with it determine the shoot/root ratio of peat. Since the sites vary in other ecological factors as well, it was possible to evaluate a number of other physical ecological parameters.
Methods
An acrylic piston corer (5 cm diam.) was used to extract cores, 0.5 m in length, from each of the sites. The cores were extruded in the field into mesh bags and wrapped in plastic for transport to the laboratory. Peats from the Twin Cays site were available from a different project conducted by the Smithsonian Institution. Vibracores of the deep peats in this site were taken in 1984 and 1985. In the laboratory, the cores were longitudinally halved: one half was archived by wrapping in plastic while the other was used for analysis. Portions of the cores, 2 cm in length, were removed at 10-cm intervals. These portions were contained within fiberglass mesh bags, dehydrated, and infiltrated with paraffÉn following standard procedures (Sass, 1958). The embedded peat samples were thin sectioned on a sliding microtome, mounted on microscope slides, and examined for organ
Site selection The mangrove swamps of Belize were selected as test sites for several reasons. There are only three species which may be present in our sites: Rhizophora mangle L., Avicennia germinans L., and Conocarpus erecta L. which will hereinafter be referred to by their generic names. Belize is subtropical and it is thought that most Paleozic Euramerican coals accumulated in tropical or subtropical climates. The offshore barrier reef(see F i g . l ) m a k e s possible the inclusion of isolated salt water swamps, located on leeward islands, which are free of major terrestrial effects. The variety of coastal landforms found in Belize (paralic swamps, lagoons, deltas, beach ridges) and the presence of several small rivers draining into the
Core evaluation
88
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Fig.1. Map of Belize showing the localities of the mangrove swamps studied.
content. The linear transect method of Schopf (1938) was used to evaluate the shoot/root ratio of the peats. Additional samples (approximately 2 g dry weight) of the test cores were removed, dried at 60°C to constant weight, weighed, and ashed in a muffle furnace at 575°C for three hours. The remaining ash was weighed, and this value subtracted from pre-ashing weight to determine non-carbonate organic matter. The ash was then digested with HC1 to determine
carbonate content. The remainder was assumed to be the silicate fraction. Results
Site descriptions Brief descriptions of the geomorphological setting and composition and structure of the plant community are provided for each of the sites. These characterizations are summarized
89
in Table I. The first three sites have freshwater influence of variable magnitude,
Hattieville (A, Fig. 1): Located approximately 17 km from the sea, this site is in a localized catchment basin and is only a few acres in extent. No apparent connections exist with either the sea or other mangrove swamps. Surrounding vegetation is typical of the seasonally wet, poorly drained carbonate terrain of n o r t h e r n Belize. Microtopography is variable. Small hummocks are common and these are vegetated by upland species. Rhizophora (1-1.5 m) is the dominant species with infrequent Conocarpus. The site has standing water for ten months of the year, but even at the height of the dry season (April and May), the soils remain waterlogged. Peat distribution is patchy. Temash River I (B, Fig. 1): This site is located at the confluence of the Temash River and two small t r i b u t a r y creeks approximately 10 km from the sea. No well-defined banks are present in this area. During the dry season, salt water intrudes up the river channel. No interstitial salinity data are available. Rhizophora (1-2.5 m) is the only tree species present although
herbs typical of fresh water environments are common. Rainfall is high (4500 mm/y) although there is a definite dry season (April and May).
Temash River H (C, Fig. 1): Located approximately 5 km downstream from Temash River I, this site is situated on narrow, well-defined river banks. Large Rhizophora (up to 13 m) are the only mangrove species present, although non-mangrove species occur infrequently as small understory trees. Saline intrusion occurs during the dry season, and is probably more pronounced t han at Temash River I. The site is probably inundated only during floods as it is higher t han the previous site relative to river level.
Anderson's Lagoon I (D, Fig. 1): Anderson's Lagoon is located just to the south of the Sittee River delta. The barrier forming the lagoon is a depositional feature with constituent sediments probably derived from the river and from n o r t h e r n sources which have been transported by the southwards-flowing long shore c u r r e n t (High, 1975). The Sittee River drains the eastern flank of the siliceous Maya Mountains. The site is located at the mouth of the channel which connects the lagoon to the open sea. Sea water
TABLE I Site ecological and geomorphological characterizations Site
Map
Landform
Salinityi
Vegetationb
Hattieville
A
FW
Temash river I Temash river II Anderson's lagoon I Anderson's lagoon II Placentia lagoon I Placentia lagoon II
B
isolated, inland freshwater pothole riverine-swamp riverine-bank paralic: open sea lagoonal paralic: open sea lagoonal
Twin cays
H
Rhizophora (1-1.5 m), ((Conocarpus)) (freshwater species) Rhizophora(1-1.5 m), rushes, ((palmetto)) Rhizophora(10-13 m), (freshwater species) Avicennia (5-6 m), (Rhizophora fringe) Rhizophora (3-5 m), (Acrostichum) Avicennia (54 m), (Rhizophora fringe) Rhizophora (5-8 m), (Avicennia, Laguncularia) Rhizophora (dwarf-6 m), (Avicennia)
C D
E F G
isolated,saltwater islets
FW-B FW-B SW+ SW SW+ SW SW
aFW= freshwater, B = brackish, SW= oceanic salinity, SW+ = hypersaline. b( )= infrequent, (()) rare.
9O flOW into the lagoon is probably restricted as the channel is long (ca. 2 kin) and narrow (less than 10 m). The landform of this site is characterized as a paralic swamp. It has a fringe of medium-sized Rhizophora (4-6 m) grading landwards to a stand of pure Avicennia (5-6 m). This zonation pattern commonly occurs in swamps with topographical relief sufficient to prevent regular tidal inundation, Landward areas in such swamps are characteristically hypersaline due to evaporation. Species distribution is affected. Species able to tolerate higher interstitial salinities, such as Avicennia, grow in these areas, while species intolerant of hypersalinities, such as Rhizophora, are restricted to swamp shore margins where constant contact with the sea maintains interstitial salinities at or near mean oceanic levels (Teas, 1979).
Anderson's Lagoon H (E, Fig. 1): This site is on the mainland shore of the lagoon. The lagoon, rather than the sea, affects the site directly. There is no significant freshwater influx into the area with the exception of a small, man-made channel (2 m in width) which connects the lagoon to the Sittee River. Rhizophora (to 5 m) is the only tree species present although a fern, Acrostichum sp., is common.
Placentia Lagoon I (F, Fig. 1): This site is located on the mainland just outside of the lagoon so, like Anderson's Lagoon I, it is best characterized as paralic. The zonation of species is also similar, A shoreward fringe of Rhizophora (4-6 m) grades sharply into a pure Avicennia stand (4-6 m) landwards. The lagoon itself is large and is connected to the sea by a large mouth so lagoonal salinities are probably similar to those of the open sea.
Placentia Lagoon H (G, Fig. 1): The site is on the mainland approximately 10 km north of the lagoon mouth. There is no major focus of fresh water discharge. It is dominated by medium-sized Rhizophora (5-8 m).
Twin Cays (H, Fig. 1): This site is a carbonate-mangrove peat buildup up in the lee of the barrier reef about 20 km east of the Sittee River Delta. Rainfall is the only source of freshwater on the island. The island is isolated from terrigenous sediment input. Low-lying interior ponds are vegetated by dwarf forms of Rhizophora. These areas are probably hyper-saline due to restricted access to open water. Larger mangrove trees occur on the island fringes and in inland areas flushed at least occasionally by the tides. Rhizophora (dwarf-6 m) is the dominant species with occasional Avicennia.
Core analyses The composition analyses (Table II) indicate that siliciclastics constitute the majority of the inorganic fraction of mainland swamp peats. The carbonate fraction at these sites may be derived from storm deposits or deposited in situ by calcareous organisms. The isolated offshore site, Twin Cays, has only carbonate in its ash fraction.
Shoot-rootvalues Shoot/root ratios including the average and range of values for each core are indicated in Table II. All of the peats measured were shootpoor except for those from Anderson's Lagoon I. The surface peat from this site contained two large pieces of stem wood and yielded a shootroot value of 2.7. The Placentia I core contained a buried leaf mat between 20 and 22 cm deep, which yielded a shoot-root value of 0.32 at this depth. The lowest shoot-root value measured for an individual core increment was zero. The highest value was 2.7 at the surface of the Anderson's Lagoon I core.
Discussion Mangrove swamps can grow in a diversity of ecological settings yet mangrove peats are apparently shoot-poor regardless of environment (Tables II and III). Since our sites
91 TABLE II Core analyses summaries Site
Number of samples
Shoot/root mean (range)
Organic (%)
Carbonate (%)
Silicate (%)
Total ash
Hattieville Temash river I Temash river II Anderson's lagoon I Anderson's lagoon II Placentia lagoon I Placentia lagoon II Twin cays
4 5 3 2 3 4 2 6
0.05 (0-0.14) 0.08 (0-0.26) 0.07 (0-0.18) 1.5 (0.06-2.7)l 0 (0) 0.09 (0-0.32) 0 (0) 0.05 (0-0.11)
41.6 57 90 27 71 58.3 57.1 61.9
2.7 6.3 4.0 12.7 4.6 33.2 17.5 38.1
55.7 36.7 6.0 60.3 24.4 8.5 25.4 0.0
58.4 43.0 10.0 73.0 29.0 41.7 42.9 38.1
aThis extremely high shoot-root ratio resulted from the occurrence of two pieces of wood at the surface of the core. e n c o m p a s s a b r o a d e n v i r o n m e n t a l range, the f a c t o r s w h i c h c o n t r o l s h o o t / r o o t r a t i o s in m a n g r o v e p e a t s are i n d e p e n d e n t of locally v a r y i n g ecology. We c a n identify w i t h confiTABLE III Peat type
Environment
Okefenokee (freshwater) Taxodium b Okefenokee (freshwater) Tree IslandI Okefenokee (freshwater) Nyssa ~ Okefenokee (freshwater) Cyrilla b Okefenokee (freshwater) Myrica-Persea Salix ~ Everglades (freshwater) ConocarpustransitionaP Everglades (saltwater) C o n o c a r p u s transitional ~ Everglades (saltwater) Rhizophora a Everglades (saltwater) Rhizophora c Everglades (saltwater) Rhizophora-Acrostichum Everglades Mariscus c (saltwater) Avicennia a Everglades (saltwater) Avicennia-Rhizophora a Everglades (saltwater) Taxodium a
Shoot/ root ratio
Swamp 4.3 Swamp 1.5 Swamp 3.0 Swamp 1.2 Swamp 1.9 1.9 0.3 0.04 0.2 0.4 0.02 0.4 0.07
Source: a: Raymond (1983, 1987). b: Spackman et al. (1976). c: Cohen and Spackman (1977).
d e n c e w h i c h e c o l o g i c a l p a r a m e t e r s do n o t affect s h o o t / r o o t ratios; we a r e m o r e c a u t i o u s in i d e n t i f y i n g t h e factor(s), u n i v e r s a l in m a n g r o v e swamps, w h i c h do affect s h o o t / r o o t ratios. O u r sites w e r e l o c a t e d on a n u m b e r of different c o a s t a l l a n d f o r m t y p e s (Table I). S h o o t / r o o t r a t i o s a r e n o t affected by e c o l o g i c a l p a r a m e t e r s d e t e r m i n e d , at l e a s t in part, by g e o m o r p h o l o g i c a l setting. This h a s b e e n s h o w n to affect s w a m p t a x o n o m i c c o m p o s i t i o n (Thom, 1967), c o m m u n i t y p h y s i o g n o m y a n d m e t a b o l i s m (Lugo a n d S n e d a k e r , 1974), a n d t h e a e r i a l e x t e n t a n d d e p t h of p e a t deposits (Wanless, 1974). C o h e n (1968) s u g g e s t e d t h a t tidal flushing of a e r i a l debris w a s p a r t l y r e s p o n s i b l e for the low s h o o t / r o o t r a t i o of m a n g r o v e peats. Tidal s c o u r i n g of a e r i a l debris is a p p a r e n t l y n o t a n e c e s s a r y c o n d i t i o n for s h o o t - p o o r p e a t s as H a t t i e v i l l e is c o m p l e t e l y u n a f f e c t e d by tides, t h e T e m a s h R i v e r sites a r e o n l y infreq u e n t l y s c o u r e d by r i v e r flooding, w h i l e t h e c o a s t a l sites a r e r e g u l a r l y i n u n d a t e d . Phillips a n d P e p p e r s (1984) a n d D i M i c h e l e et al. (1985) s u g g e s t e d t h a t r o o t - r i c h p e a t s of P e n n s y l v a n i a n a g e a c c u m u l a t e d in s w a m p s t h a t w e r e s e a s o n a l l y dry. R e s u l t s of o u r p r e l i m i n a r y i n v e s t i g a t i o n do n o t i n d i c a t e t h a t this is t r u e for m a n g r o v e peats. A l t h o u g h r a i n f a l l is h i g h l y s e a s o n a l in the v i c i n i t y of H a t t i e v i l l e , this s w a m p g r o w s in a local c a t c h m e n t basin, a n d t h e s u r f a c e of t h e p e a t is continually waterlogged, even during the dry
92 season. High shoot root ratios are the expected pattern in such continually-flooded swamps; the Hattieville site had a low shoot/root ratio, The shoot/root ratio of peat from the Temash River localities supports the idea that seasonal drying of the swamp surface does not cause low shoot-root ratios in peat. These sites are similar to the Hattieville site in t h a t they are influenced by fresh water processes for most of the year. However, in contrast to the Hattieville site, the surfaces of Temash River peats are drained during the dry season, even though annual rainfall is higher at Temash River. If seasonal drying of the swamp surface affected shoot-root ratios, the shoot/root ratios of Temash River peats should be equal to, or lower than those observed at Hattieville. The opposite is true. The shoot/root ratio of Temash River I peats is actually higher, primarily due to the presence of a surface leaf mat. Raymond (1983) observed the same phenomenon in the Okefenokee swamp and the mangrove swamps of the Florida Everglades (see Table III). For both mangrove and nonmangrove freshwater swamps, the sites with the highest shoot/root ratios (Taxodiurn swamp in the Okefenokee and the Conocarpus transitional swamp along the Joe River in Florida) were also the best drained sites in that study. Seasonally-drained peats may have a better chance of developing leaf mats than continuously water-logged peats because decomposition rates are lower in dry areas. In addition, the process of drying the swamp surface may render the leaf mats of seasonallydrained sites more impervious to decay (Cohen and Spackman, 1977). Although such surficial leaf mats may be rare in peat, episodic sedimentation could preserve these mats. For example, the relatively high shoot-root value (0.32) of the Placentia I core at 20 cm depth probably resulted from episodic preservation of a surficial litter layer. In addition, cuticular shales or coals, such as t h a t described from the Pennsylvanian-aged Block Coal of Indiana by DiMichele et al. (1984) and Eggert and Phillips (1982), may have resulted when sediments covered leaf mats t h a t accumulated
in drained areas of the swamp. Abundant cuticular shales or coals may indicate seasonally-dry swamp conditions. Since Rhizophora peats are shoot-poor, it may be possible t h a t the growth habit of the plant itself determines shoot/root ratios. The reported shoot/root ratios for living mangrove trees range from 0.57 to 1.47 (Clough and Attiwill, 1982; Saenger, 1982). The shoot-root ratios reported for in situ Rhizophora peats are lower, ranging from 0.22 to 0.02 (Table III) so the production of Rhizophora peat involves the destruction of considerable shoot material relative to root material. However, Rhizophora communities form shallow and dense root mats (Teas, 1979). Do these root mats confine aerial debris to the surface of the peat where decomposition is inevitable? The influence of root mat formation on peat shoot/root ratios is more difficult to assess than physical ecological parameters because detailed information on the root system morphologies of mangrove species other than Rhizophora is lacking. Further, the density of Rhizophora in a mixed mangrove stand required for formation of a root mat is also unknown. Mixed mangrove communities with appreciable densities of Avicennia,Laguncularia racemosa (L.) Gaertner, and Conocarpus from both fresh and saline habitats have universally shoot-poor peats. It is possible that such peats may be the result of the root system strategies of these species, or that a relatively low frequency of Rhizophora in a mixed mangrove community can form a root mat sufficient to exclude aerial debris from sub-surface anaerobic zones. Successional changes in species composition make the situation even more problematic as a persistent root mat from a dwindling Rhizophora community may still prevent the incorporation of aerial debris into the peat. In addition, root mat formation may be a near-universal property of mangrove species as they occupy habitats characterized by unstable substrates, have a high percentage of their total biomass as roots, and generally have shallow root systems (Saenger, 1985). If so, ancient plants
93 with the ability to accumulate peat in saltwater might produce shoot-poor peats even in freshwater swamps, The data indicate that shoot/root ratios of mangrove peats do not correlate to salinity: all investigated mangrove peats are shoot poor even if they accumulate in freshwater regimes, Since other freshwater, non-mangrove, peats do exhibit high shoot/root ratios (Raymond, 1987), processes of peat formation in taxonomically different swamps may not be strictly comparable. Comparisons between mangrove and non-mangrove swamps may indicate how these processes differ and how this information can be used in reconstructions from ancient peats, Decomposition rates of aerial debris correlate with the presence of water in mangrove swamps (Heald, 1971), freshwater alluvial swamps (Brinson, 1977), and lowland tropical rainforests (Stout, 1980). In a review of litter decomposition rates in upland environments, Singh and Gupta (1977) conclude that the same correlation pertains. These high rates of decomposition suggest that all peats should be shoot-poor. Since shoot-rich peats do exist, there must also exist factors which retard the expected high rates of litter decomposition, These factors are apparently inoperative in mangrove swamps either as a result of their absence, or, because attributes of the mangrove e n v i r o n m e n t i t s e l f o f f s e t t h e i r o p e r a t i o n , An example of t h e latter situation would be root mat exclusion of aerial debris discussed above. An example of the former situation is acid water which is not likely to occur in saline mangrove swamps due to the buffering influence of the sea. Our freshwater mangrove sites also had nearly neutral pH, and this neutral pH could also have contributed to the rapid decomposition of aerial debris in these swamps. Janzen (1977) discusses tropical black water swamps characterized by acid water conditions and low rates of decomposition. If low pH conditions are sufficient to inhibit the activity of decomposers, and if high rates of decomposition are responsible for the paucity of aerial debris in some peats, then we should
expect to find shoot-rich peats accumulating in acid water swamps. An example of such a swamp is the lowland dipterocarp forest of Malesia (Anderson, 1983)but quantitative data on the organ composition of these peats are currently not available. The suggestion that low pH is a necessary and sufficient condition for the formation of shoot-rich peats remains unverified, but also is not falsified by any of our results. Conclusions The shoot/root ratios of peats have been suggested to vary directly with the salinity of the environment of deposition. The basis for this suggestion was a comparison between modern swamps which vary not only in their salinity regimes, but also in their taxonomic composition. We made the assumption that taxonomically dissimilar communities may have dissimilar processes of peat formation, so that inferences derived from comparisons between them may not be valid. It is necessary to test the suggestion that shoot/root ratios reflect salinity in taxonomically similar swamps which occur in environments with different salinity regimes. Mangrove swamps were chosen as they occur in saline, brackish, and freshwater habitats. Mangrove peats exhibit universally low shoot/root ratios so the suggestion that shoot/root ratios of peat reflect the salinity of depositional environment is not true for them. Because modern freshwater, non-mangrove swamps do accumulate shoot-rich peats, we conclude that different processes of peat formation operate in taxonomically different swamps. We discussed plausible factors which may account for the shoot-poor nature of mangrove peats (root mat exclusion of aerial debris) and the shoot-rich nature of nonmangrove freshwater peats (acid water inhibition of decomposition). Our present data cannot verify if these two suggestions are necessary and sufficient to account for observed variation in peat shoot/root ratios in taxonomically different swamps.
94
Acknowledgements We would like to acknowledge the field assistance of the following people: Robin Lightey, Ian Macintyre, Gregor Bond, Keith Bowers, and Scott Cross. This research was funded by the American Chemical Society, Petroleum Research Fund and the Whitehall Foundation. The Smithsonian Institute Carrie Bow Cay Research Station provided field support and sampling equipment. Marilyn Green assisted in manuscript preparation. References Anderson, J.A.R., 1983. The tropical peat swamps of western Malesia. In: A. P. Gore (Editor), Mires: Swamp, bog, fen and moor (Ecosystems of the World, 4 B). Elsevier, Amsterdam, pp.181-199. Brinson, M., 1977. Decomposition and nutrient exchange of litter in an alluvial swamp forest. Ecology, 58: 601-609. Clough, B.F. and Attiwill, P.M., 1982. Primary productivity of mangroves. In: B.F. Clough (Editor), Mangrove Ecosystems in Australia: Structure, Function and Management. Aust. Nat. Univ. Press, Canberra, pp.213-222, Cohen, A.D., 1968. The petrology of some peats of southern Florida (with special reference to the origin of coal). Thesis Pennsylvania State Univ., 352 pp. 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. Palaeontographica, 162B: 71-114. Cridland, A.D., 1964. Amyelon in American coal-balls. Paleontology, 7: 186-209. DiMichele, W.A., Rischbieter, M.O., Eggert, D.L. and Gastaldo, R.A., 1984. Stem and leaf cuticle of Karinopteris: source of cuticles from the Indiana "paper coal". Am. J. Bot., 7: 626-637. 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), Geological Factors in the Evolution of Plants. Yale Univ. Press, New Haven, Conn., pp.223 256. Eggert, D.L. and Phillips, T.L., 1982. Environments of deposition coal balls, cuticular shale and gray-shale floras in Fountain and Parke Counties Indiana. Ind. Dept. Nat. Resour. Geol. Surv. Spec. Rep., 30, 43 pp. Heald, E.J., 1971. The Production of Organic Detritus in a South Florida Estuary. Univ. Miama Sea Grant, Techn. Bull., 6. High, L.R., Jr., 1975. Geomorphology and sedimentology of Holocene coastal deposits. In: K.F. Wantland and W.C. Pusey III (Editors), Belize: Shelf-Carbonate Sediments, Clastic Sediments and Ecology. Am. Assoc. Petrol. Geol. Tulsa, Okla., pp.53-96,
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