Robyn J. Burnham

Relationships between standing vegetation and leaf litter in a paratropical forest: implications for paleobotany

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 homogenous 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.

Habitat-related error in estimating temperatures from leaf margins in a humid tropical forest

Leaf margin characters are strong predictors of mean annual temperature (MAT) in modern plant communities and widely used tools for reconstructing paleoclimates from fossil floras. However, the frequency of nonentire-margined species may vary dramatically between different habitats of the same forest. In this paper we explore the potential for this habitat variation to introduce error into temperature reconstructions, based on field data from a modern lowland forest in Amazonian Ecuador.The data show that the provenance of leaves can influence temperature estimates to an important degree and in a consistent direction. Woody plants growing along lakes and rivers underestimated MAT by 2.5°-5°C, while those in closed-canopy forest provided very accurate predictions. The high proportion of liana species with toothed leaves in lakeside and riverside samples appears to be responsible for a large part of the bias. Samples from closed-canopy forest that included both lianas and trees, however, were more accurate than tree-only or liana-only samples.We conclude that paleotemperature reconstructions based on leaf margin characters will be misleading to the extent that fossilization provides a better record of certain habitats than others. The preponderance of lake and river deposits in the angiosperm fossil record suggests that underestimation of mean annual paleotemperature may be common.

Dominance, diversity and distribution of lianas in Yasuní, Ecuador: who is on top?

Lianas of YasuniNational Park and the Huaorani Ethnic Reserve, Ecuador were sampled using 0.2-ha subsamples from 12 1-ha plots. Using surveys and herbarium collections, the total species richness of lianas in the park and reserve is estimated to be close to 500 species. Terra firme habitats are significantly more species rich than floodplain habitats, but density of stems is not significantly different between habitats. The most abundant liana species is Machaerium cuspida- tum Kuhlm. & Hoehne (Fabaceae) in both floodplain and terra firme habitats, repres- enting more than 10% of all stems in census plots. Analysis of rarity indicates that none of the liana species encountered on the 12 plots is rare in terms of geographic range, and many species are not rare in terms of population size or habitat specifi- city. The rank order of abundance, combined with the coefficient of variability in species distribution among plots, is used to identify the 38 species (an oligarchy) that dominate forests in Yasuni ´.

Reconstructing richness in the plant fossil record

A comparison of species richness of leaf litter samples derived from a variety of modern forest types provides a means for estimating diversity of source forests in the fossil record. Single samples of temperate forest litter relatively consistently record about three-quarters of the source tree species larger than 10 cm diameter at breast height within the surrounding hectare. Tropical and subtropical samples, in contrast, contain a very small proportion of leaves of the tree species in the surrounding hectare of source forest, and demonstrate that the forest sampled by a single collection is about 0.1 to 0.125 hectare

Patterns in tropical leaf litter and implications for angiosperm paleobotany

Abstract One hectare of undisturbed Amazonian forest, containing about 175 species of trees larger than 10 cm diameter at breast height, was studied to determine the relationship between high-richness forest and the autochthonous litter produced by the forest. Litter samples contained up to 52 species, of which one-third represented epiphytes, vines, and lianas. These modern leaf litter studies from southeast Amazonian Peru indicate that reconstructions of ancient high-diversity forests are possible using autochthonous leaf litter deposits. In comparison to temperate litter samples, however, more sampling must be done to recreate fairly simple descriptors of ancient communities such as species richness and heterogeneity. Samples must be large, relatively closely spaced, and maintained as distinct collecting localities to retrieve the maximum amount of data from rich, angiosperm-dominated localities. There are many advantages justifying more intensive collections. For example, biomass contribution of major life-form categories in the source forest is reflected in leaf litter accumulating under tropical forest canopies. Tropical forests, because of their extreme heterogeneity, also can provide the opportunity to reconstruct individual species characteristics from litter signatures. The relative rarity of most species creates distinct leaf shadows from which the canopy breadth and volume of many individuals can be estimated. The principles derived from modern tropical litter studies can be applied to existing fossil collections; however, their power lies with those collections originating from autochthonous assemblages, for which spatial control during collecting has been maintained, and time averaging has been kept to a minimum. These reflections of community structure available from the leaf litter provide a means for paleobiologists to contribute significantly to the study of community evolution and stability.

Miocene winged fruits of Loxopterygium (Anacardiaceae) from the Ecuadorian Andes

A new species of asymmetrically winged fruit is described from Miocene sediments of Andean Ecuador. The new fruit is readily placed in the genus Loxopterygium of the Anacardiaceae based on the size, position of the stigma, wing venation, and serration of the wing tip. The new fossil species is very similar to extant species of Loxopterygium now distributed in dry habitats of coastal Ecuador and Peru, as well as dry interior forests of Bolivia and northern Argentina. We use the fossil to calibrate a molecular-based phylogeny of some members of the Anacardiaceae, showing that dry forest habitats may have been present in South America for more than 10 million years.

CRITICAL ISSUES OF SCALE IN PALEOECOLOGY

In mid-September 2007, 32 paleontologists gathered at the Smithsonian Institution to spend four days discussing research frontiers in paleoecology, particularly at the interface with neoecology. They represented expertise throughout the Phanerozoic and in all major groups of fossilizable organisms. This meeting was timely, given the increasing evidence of the impact of climate change on ecosystems in our modern world. The vast repository of paleoecological data on past environmental change and concomitant ecological responses, observed at many different spatial, temporal, and taxonomic scales, is of potentially great value for understanding and predicting how modern ecosystems will respond to climate change. Of particular interest to the participants of this meeting were questions of how ecological data collected at different scales could be reconciled so that our knowledge of ecological change in the past can better inform our understanding of the present and our predictions of how ecosystems will change in the future. Certainly, this is one of the most exciting research frontiers in paleoecology. Data collected for different ecological studies (both paleoecological and neoecological) encompass a wide range of spatial, temporal, and taxonomic scales. Understanding the scales inherent in an ecological research question is critical to designing a sampling protocol that will yield data capable of resolving that question, yet these scales are often not adequately evaluated or presented in published paleoecological reports. Furthermore, for any body of paleoecological research to be rescued from isolation and integrated with other studies, the various scales encompassed by the research questions and data must be understood and reported. The greatest barrier to communicating and collaborating with neoecologists is not that data collected from extant ecosystems are necessarily different or more complete than paleoecological data but, rather, that these two data sets commonly represent or are collected at different scales. If such differences of scale can …

Ecology of lianas

A liana is a long-stemmed, woody vine that is rooted in the soil at ground level and uses trees to climb up to the canopy to get access to well-lit areas of the forest. The main goal of this book is to present the current status of liana ecology in tropical and temperate forests. In essence, it is a forum to summarize and synthesize the most recent research in liana ecology and to address how this research fits into the broader field of ecology.

Modern tropical forest taphonomy: Does high biodiversity affect paleoclimatic interpretations?

Abstract Modern biodiversity hotspots are characterized by high species diversity and by biotas facing a substantial threat of extinction, largely due to a high proportion of endemic taxa living in these regions. Theoretically, hotspots of biodiversity are areas of particular interest in the fossil record because of the relatively high quality and quantity of data that they may contribute to a global understanding of vegetational response to changes in climate, tectonic uplift, and ecological disturbance. Current models for climatic reconstruction that depend on leaf physiognomy are based on data sets in which species-rich tropical floras are less well represented, relative to temperate floras. Eight modern Neotropical floras from a range of precipitation regimes were evaluated to determine the influence that high source floral diversity has on reconstruction of mean annual temperature (MAT) and mean annual precipitation (MAP). Floras are drawn from sites in Costa Rica to southern Peru, having species richness from 55 to >400 species per plot. MAT of the sites spans a range of 24 to 28°C, and MAP ranges from ∼1600 mm to 3000 mm. By subsampling the modern floras in rank order of dominance (basal area), the importance of collecting intensity and completeness on subsequent assessments of MAT and MAP is evaluated. Biodiverse floras are good at reconstructing MAT if at least 50% of the species are included. When only 25 species are used for temperature calculations, the accuracy of the parameter is compromised, but a ±3°C error encompasses the majority of the deviation. Application to the early Paleocene Castle Rock fossil flora of Colorado confirms the validity of subsampling in high-diversity fossil applications. However, reconstruction of MAP is fraught with problems that do not appear to be related to biodiversity of the floras. Errors on estimates of MAP currently are so large as to make the values too vague to be useful in most applications. This study has accepted a 20% error as necessary, but the applicability of data with errors > 20% is questionable in situations where rainfall is >1500 mm per year. MAP estimates using leaf area are almost universally underestimates of actual MAP, and frequently are >400 mm in error. Exploration of these data indicates that effort would be well placed in investigating the relative importance of precipitation parameters in altering leaf morphology before choosing one to reconstruct climates of the past.

HIDE AND GO SEEK : WHAT DOES PRESENCE MEAN IN THE FOSSIL RECORD?

Abstract Our efforts to reconstruct accurate, complete records of events in vegetation history and in plant evolutionary history depend on accuracy in dating sediments, interpretation of structures preserved, reconstruction of whole organisms or communities from the preserved material, and interpretation of the interaction between past abundance and fossil presence. This contribution examines the interaction between past abundance of a target plant and the probability of retrieval of that species in the fossil record. By examining records of recolonization in volcanic areas, records of invasive species spread, succession in disturbed habitats, and historical migration patterns, we can provide estimates of the likelihood of appearance in the potential fossil record of newly evolved and reasonably successful species. The lag in discovery, recognition, and publication of a fossil as an important representative of a critical clade is also evaluated and is highlighted as a more important constraint on the use of fossils in testing evolutionary and ecological hypotheses than the recolonization rate. The lag between discovery and publication is particularly relevant in areas of the modern world where fossil plant–bearing deposits are either rare or inaccessible. Greater awareness of the density and reliability of the plant record should allow evolutionary biologists and paleoecologists to bracket not only time intervals but also geographic regions where the fossil record can be interpreted largely at face value. At the same time, more effort should be focused on intense collecting efforts and training in areas where fossil deposits are potentially present, but poorly collected and evaluated.