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Review of Palaeobotany and Palynology, 58 (1989): 5-32 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

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RELATIONSHIPS BETWEEN STANDING VEGETATION AND LEAF LITTER IN A PARATROPICAL FOREST: IMPLICATIONS FOR PALEOBOTANY R O B Y N J. B U R N H A M '

Department of Botany KB-15, University of Washington, Seattle, WA 98195 (U.S.A.) (Received April 1, 1987; revised and accepted October 12, 1987)

Abstract Burnham, R.J., 1989. Relationships between standing vegetation and leaf litter in a paratropical forest: implications for paleobotany. Rev. Palaeobot. Palynol., 58: 5-32. A paratropical floodplain forest in southern Mexico was investigated to determine the effect of heterogeneity within the standing vegetation on the taxonomic composition of the accumulating plant litter. Twelve study sites over 20 km of the Rio San Pedro were sampled. Four subenvironments of the floodplain were defined on topography, geomorphology, and distance from the channel. Litter samples collected from the channel or in low-lying areas close to the channel (forebanks) generally show low species richness and are homogeneous throughout the study area. Litter samples taken from levees and overbank facies (back-levees) are more heterogeneous, species-rich, and tend to reflect the local flora with greater accuracy than do channel samples. This analysis of modern samples analogous to paleobotanical collections from rocks of fluvial origin indicates that paleofloras sampled for reconstructing regional paleoecology and making inferences regarding paleoclimate will be improved by multiple sampling from a range of different subenvironments.

Introduction R e c o n s t r u c t i n g the p a t t e r n of e v o l u t i o n a r y c h a n g e and the past e n v i r o n m e n t s in w h i c h t h a t c h a n g e t o o k place has been the t r a d i t i o n a l goal of p a l e o b o t a n i c a l research. With the refinement of p a l e o e c o l o g i c a l r e c o n s t r u c t i o n s and the r e c o g n i t i o n t h a t facies-specific distrib u t i o n s of p l a n t s c a n c o n f o u n d a p p a r e n t stratig r a p h i c sequences, t h e r e has been i n c r e a s i n g c o n c e r n over the biases p r e s e n t in the fossil record. Some biases h a v e been r e c o g n i z e d in g e n e r a l for m a n y years, p a r t i c u l a r l y differentim p r o d u c t i o n , s o r t i n g and p r e s e r v a t i o n of 'Present address: Department of Paleobiology, MRC 164, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, U.S,A. 0034-6667/89/$03.50

p l a n t organs. In the last ten y e a r s t h e r e has been an i n c r e a s e in studies a t t e m p t i n g to q u a n t i f y these biases. These studies demons t r a t e t h a t a c c u r a t e r e c o n s t r u c t i o n of the l o w l a n d v e g e t a t i o n of the past will rest on the ability of p a l e o e c o l o g i s t s to a c c o u n t for taphon o m i c a n d c o l l e c t i n g biases. This s t u d y focuses on the r e l a t i o n s h i p bet w e e n a c l i m a t i c a l l y p a r a t r o p i c a l forest (mean a n n u a l t e m p e r a t u r e of 20-25°C) a n d the accum u l a t i n g p l a n t litter in a m o d e r n fluvial e n v i r o n m e n t b e c a u s e s u c h a forest provides a good a n a l o g for m a n y T e r t i a r y - a g e fossil p l a n t deposits. P a r t i c u l a r l y c o m m o n in e a r l y Tertia r y floodplain sediments are p l a n t s i n d i c a t i n g w a r m t e m p e r a t e to t r o p i c a l climatic c o n d i t i o n s (MacGinitie, 1941, 1974; Hickey, 1977; Wolfe, 1978; Wing, 1981). A l t h o u g h the n a t u r a l hetero-

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geneity of tropical forests has been acknowledged for many years (Foster, 1980; Augspurger, 1983; Hubbell and Foster, 1983; Lieberman et al., 1985), the impact of heterogeneous source vegetation on fossil deposits has not been addressed because the magnitude of potential bias remains largely undocumented (but see Wing, 1981, p.93). A baseline is needed for estimating the amount of vegetational heterogeneity that can be preserved in coeval deposits of a single floodplain. The results of this study document the variability in species richness, floristic composition, structure of the vegetation and foliar physiognomy that can be expected among plant deposits derived from a fluvial environment in a seasonal climate, Paleobotanists have relied on several taphonomic working hypotheses, formulated largely by intuition, when interpreting fossil angiosperm floras (MacGinitie, 1941; Chaney, 1959; Hickey, 1977). Those hypotheses most frequently invoked are as follows, (1) Streamside species are over-represented in fossil floras, (2) Processes operating during long-distance transport act to fragment many angiosperm leaves beyond recognition, (3) The most abundant taxa (number of individual leaves) in fossil floras were derived from plants which grew close to the site of deposition, (4) The most abundant taxa in fossil floras were dominant at the site of deposition. (5) Floristic and vegetational differences between plant fossil collections are due to climatic or age differences of the two floras (an implied assumption that the fossil flora represents the standing flora), Vegetational heterogeneity and its effect on the potential plant deposits of a fluvial environment is the focus of the present study, Fossil deposits derived from a natural forest potentially are composed of many species, However, the leaf litter available for incorporation into sediments varies temporally in species composition and density due to the individual biological attributes of the source species (Rai and Proctor, 1986). This aspect of

vegetational heterogeneity may be expected to be averaged over several seasons. In areas of slow, continuous aggradation, temporal heterogeneity may have little effect on the potential fossil record. At the same time, the natural forest is a spatially heterogeneous environment. Individuals of a species may be found clumped, resulting from edaphic, light, or dispersal phenomena. Litter samples from a tropical forest may not incorporate the most common species of the region, but rather those that are common in the immediate area. In this study, subenvironments within the fluvial environment are characterized generally to make the results comparable to a wide variety of Cenozoic paratropical plant deposits. Results of the research indicate that certain subenvironments of a fluvial setting can reflect the surrounding local forest with a high degree of accuracy, although herbaceous litter frequently is not recovered. Topographic and hydrologic factors affect the distribution of plant species in the floodplain, thus affecting the accumulation of plant litter into potential phytocoenoses. Although patchy distributions of species were observed, the patchiness had a weaker effect on litter accumulations than did the hydrologic and topographic regime within the floodplain. Taxa retrieved from sample sizes of approximately 450 leaves reflect the local flora but never reflect as much as 50% of the regional flora. Background The field of plant taphonomy has shown logarithmic growth in the years following the definition oftaphonomy by Efremov (1940). The most common contributions have been from field and laboratory studies of modern depositional settings that have emphasized transport, event deposition, community mixing, and geomorphic position of depositional environments likely to preserve plant parts (Chaney, 1924; Mueller, 1959; McQueen, 1969; Spicer, 1981; Rex and Chaloner, 1983; Scheihing and Pfefferkorn, 1984; Ferguson, 1985; Gastaldo, 1986; Spicer and Wolfe, 1987). Modern macrofossil taphonomic investigations began on ponds and

lacustrine depositional settings (Birks, 1973; Rau, 1°76; Drake and Burrows, 1980; Spicer, 1981; Hill and Gibson, 1986; Spicer and Wolfe, 1987), and then expanded to deltas and volcanic settings (Fritz, 1980; Fritz and Harrison, 1985; Scheihing and Pfefferkorn, 1984; Burnham and Spicer, 1986). Although many fossil deposits appear to be the result of fluvial processes, only recently have modern plant taphonomic analyses been undertaken in river environments (Scheihing and Pfefferkorn, 1984; Catto, 1 9 8 5 ; Gastaldo, 1986). Most of the studies of modern taphonomy have investigated the types of environments that are most likely to preserve plants and the effect of transport or biodegradation bias on the potential deposits. A notable exception is the recent study by Spicer and Wolfe (1987) which indicates that litter samples can reflect accurately the species composition of surrounding coniferous forest in high-energy, lacustrine deltaic deposits, Plant taphonomy also has been investigated with reference to ancient plant deposits. Most of this research has been designed to investigate problems specific to the study area (Chaney, 1924, 1925; Hickey and Doyle, 1977; Scott, 1977; Wilson, 1980; Rex and Chaloner, 1983; Wing, 1984; Scheihing and Pfefferkorn, 1984; Boyd, 1985). The results have been encouraging in all cases, particularly in the ecological interpretations that can be drawn by associations of environment-specific species. As more taphonomic insights are gained on both modern and ancient floras, the models derived from studies of modern sediments can be applied to fossil deposits.

ally low level of aggradation. The bed of the channel is composed of a thin veneer of silt and clay (as thin as 0.5m) overlying limestone bedrock. In the study area, the gradient is low: less than 0.2 m]km. Sediment in the channel is almost exclusively fine-grained, with clay-sized particles as the dominant size class. Some silt or sand-sized grains are present. The river floodplain has a very low rate of aggradation, presumably due to a low sediment source. Scattered bedrock outcrops of Cenozoic limestone occur throughout the area, probably providing the only major source of sediment on the floodplain in the river system. Upland sediment sources are the Tertiary sandstones of the Sierra de Chama of northern Guatemala and the low-lying hills that run obliquely across the M e x i c a n - G u a t e m a l a n border. In the study area, the river channel is from 2-10 m deep and from 43-100 m wide. Levees are from 0.5-2.0 m above mean low water level (April). The levees are generally less well developed in the downstream sites. During normal high-water level (August to November) the water level rises to within 0.5m of the top of the levees but generally does not stand above them. Flood maxima were estimated to be 65 cm above the top of the levee, using observations of high water marks on trees. Local reports indicate that this level is reached every 3-4 years for a period of up to one month. Estimated discharge during April low-water is 18 25 m 3 s-1. At present there is no gauging data for the Rio San Pedro. The area shows no tidal influence. Climate

Description of the study area The Rio San Pedro drains the Peten Highlands of northern Guatemala and runs to the confluence with the larger Rio Usumacinta in the Gulf Coastal Plain of Mexico (Fig.l). The study area lies entirely within the state of Tabasco on the Gulf Coastal Plain and covers an area of approximately 20 by 0.5 km alongside the river. The Rio San Pedro is a high-sinuosity, mud-rich river with stable banks and a gener-

The climate of the southern Tabascan plain in the area of the Rio San Pedro is seasonal humid tropical (Aw), according to the classification of Koeppen (Rzedowski, 1981). Mean annual temperature is reported by Mosifio and Garcia (1974) as 22°C, although 25 years of climatic data obtained from Ingenio Hermen Hildo Galeana, S.A. (a cane refinery) of Tenosique, Tabasco, indicate that the mean yearly temperature may be higher, 24 25°C. Temper-

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