Understanding how plant communities of the past have responded to disturbance events can
provide valuable insights when managing our natural resources and assessing human impacts
on ecosystems. The geologic record has the potential to reflect these responses through the
analysis of functional traits, which relate directly to plant function and ecosystem strategy.
There is currently little evidence of how functional traits measurable in fossil leaves vary
across succession in different forest types. Because of this, there is a limited ability to identify
disturbance as the primary driver of vegetation change within the fossil record. To improve
this ability, this study analyzes the carbon stable isotopic composition (δ 13C) of bulk organic
matter sampled at the community-scale across successional gradients in a temperate deciduous
forest (North Carolina, USA) and compares them against values from a previous study across
succession in a tropical evergreen forest (Malaysian Borneo). Leaf δ13C is representative of a plant's water use efficiency (WUE), an important axis of ecological strategy representing the carbon assimilated per water lost in a plant during photosynthesis. Leaf δ13C as a functional trait has the advantage that it is often preserved during leaf fossilization and, integrated across a plant community, can be informative about prevalent ecological strategies, functional diversity , and community assembly dynamics. In Borneo, the community-weighted mean of leaf δ13C to be highest in early-succession plots, indicative of a higher WUE in plant communities closely following a disturbance event. Old growth plots were found to have a lower δ13C, and thus a more conservative WUE. This study will further investigate if this
trend is followed within temperate forests, which is important as many mid-late Cenozoic
plant assemblages come from what would have been temperate regions. Developing a method of identifying disturbances within the geologic record, will improve the ability to discern drivers of plant community change in the past. This improved knowledge will help guide management decisions across a range of ecosystems.
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Canopy Structure in Late Cretaceous and Paleocene Forests as Reconstructed from Carbon Isotope Analyses of Fossil Leaves
While modern forests have their origin in the diversification and expansion of angiosperms in the late Cretaceous and early Cenozoic, it is unclear if the rise of closed-canopy tropical rainforests preceded or followed the end-Cretaceous extinction. The “canopy effect” is a strong vertical gradients in the carbon isotope (δ13C) composition of leaves in modern closed-canopy forests that could serve as a proxy signature for canopy structure in ancient forests. To test this, we report measurements of the carbon isotope composition of nearly 200 fossil angiosperm leaves from two localities in the Paleocene Cerrejón Formation and one locality in the Maastrichtian Guaduas Formation. Leaves from one Cerrejón fossil assemblage deposited in a small fluvial channel exhibited a 6.3‰ range in δ13C, consistent with a closed-canopy forest. Carbon isotope values from lacustrine sediments in the Cerrejón Fm. had a range of 3.3‰, consistent with vegetation along a lake edge. An even narrower range of δ13C values (2.7‰) was observed for a leaf assemblage recovered from the Cretaceous Guaduas Fm., and suggests vegetation with an open canopy structure. Carbon isotope fractionation by late Cretaceous and early Paleogene leaves was in all cases similar to modern relatives, consistent with estimates of low atmospheric CO2 during this time period. This study confirms other lines of evidence suggesting closed-canopy forests in tropical South America existed by the late Paleocene, and fails to find isotopic evidence for a closed-canopy forest in the Cretaceous.
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- Award ID(s):
- 1829299
- NSF-PAR ID:
- 10111383
- Date Published:
- Journal Name:
- Geology
- ISSN:
- 1859-0659
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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INTRODUCTION Inherent in traditional views of ape origins is the idea that, like living apes, early large-bodied apes lived in tropical forests. In response to constraints related to locomoting in forest canopies, it has been proposed that early apes evolved their quintessential upright torsos and acrobatic climbing and suspensory abilities, enhancing their locomotor versatility, to distribute their weight among small supports and thus reach ripe fruit in the terminal branches. This feeding and locomotor transition from a quadruped with a horizontal torso is thought to have occurred in the Middle Miocene due to an increasingly seasonal climate and feeding competition from evolving monkeys. Although ecological and behavioral comparisons among living apes and monkeys provide evidence for versions of terminal branch forest frugivory hypotheses, corroboration from the early ape fossil record has been lacking, as have detailed reconstructions of the habitats where the first apes evolved. RATIONALE The Early Miocene fossil site of Moroto II in Uganda provides a unique opportunity to test the predictions of terminal branch forest frugivory hypotheses. Moroto II documents the oldest [21 million years ago (Ma)] well-established paleontological record of ape teeth and postcranial bones from a single locality and preserves paleoecological proxies to reconstruct the environment. The following lines of evidence from Moroto II were analyzed: (i) the functional anatomy of femora and a vertebra attributed to the ape Morotopithecus ; (ii) dental traits, including molar shape and isotopic profiles of Morotopithecus enamel; (iii) isotopic dietary paleoecology of associated fossil mammals; (iv) biogeochemical signals from paleosols (ancient soils) that reflect local relative proportions of C 3 (trees and shrubs) and C 4 (tropical grasses and sedges that can endure water stress) vegetation as well as rainfall; and (v) assemblages of phytoliths, microscopic plant-derived silica bodies that reflect past plant communities. RESULTS A short, strong femur biomechanically favorable to vertical climbing and a vertebra indicating a dorsostable lower back confirm that ape fossils from Moroto II shared locomotor traits with living apes. Both Morotopithecus and a smaller ape from the site have elongated molars with well-developed crests for shearing leaves. Carbon isotopic signatures of the enamel of these apes and of other fossil mammals indicate that some mammals consistently fed on water-stressed C 3 plants, and possibly also C 4 vegetation, in a woodland setting. Carbon isotope values of pedogenic carbonates, paleosol organic matter, and plant waxes all point to substantial C 4 grass biomass on the landscape. Analysis of paleosols also indicates subhumid, strongly seasonal rainfall, and phytolith assemblages include forms from both arid-adapted C 4 grasses and forest-indicator plants. CONCLUSION The ancient co-occurrence of dental specializations for leaf eating, rather than ripe fruit consumption, along with ape-like locomotor abilities counters the predictions of the terminal branch forest frugivory hypotheses. The combined paleoecological evidence situates Morotopithecus in a woodland with a broken canopy and substantial grass understory including C 4 species. These findings call for a new paradigm for the evolutionary origins of early apes. We propose that seasonal, wooded environments may have exerted previously unrecognized selective pressures in the evolution of arboreal apes. For example, some apes may have needed to access leaves in the higher canopy in times of low fruit availability and to be adept at ascending and descending from trees that lacked a continuous canopy. Hominoid habitat comparisons. Shown are reconstructions of a traditionally conceived hominoid habitat ( A ) and the 21 Ma Moroto II, Uganda, habitat ( B ).more » « less
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The end-Cretaceous event was catastrophic for terrestrial communities worldwide, yet its long-lasting effect on tropical forests remains largely unknown. We quantified plant extinction and ecological change in tropical forests resulting from the end-Cretaceous event using fossil pollen (>50,000 occurrences) and leaves (>6000 specimens) from localities in Colombia. Late Cretaceous (Maastrichtian) rainforests were characterized by an open canopy and diverse plant–insect interactions. Plant diversity declined by 45% at the Cretaceous–Paleogene boundary and did not recover for ~6 million years. Paleocene forests resembled modern Neotropical rainforests, with a closed canopy and multistratal structure dominated by angiosperms. The end-Cretaceous event triggered a long interval of low plant diversity in the Neotropics and the evolutionary assembly of today’s most diverse terrestrial ecosystem.
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Degree of canopy cover is linked to transpiration, carbon cycling and primary productivity of an ecosystem. In modern ecology, canopy structure is often quantified as Leaf Area Index (LAI), which is the amount of overstory leaf coverage relative to ground area. Although a key aspect of vegetation, the degree of canopy cover has proven difficult to reconstruct in deep time. One method, Reconstructed Leaf Area Index (rLAI), was developed to infer canopy structure using the relationship between non-grass leaf epidermal phytolith (plant biosilica) morphology, and leaf coverage in modern forests. This method leverages the observed correlation between epidermal phytolith size, shape (margin undulation), and light availability. When more light is available in a canopy, epidermal phytoliths tend to be smaller and less undulate, whereas less light availability is linked to larger and more undulate epidermal phytoliths. However, the calibration set used to develop this method was compiled from field sites and samples from localities in Costa Rica and it remains unclear how applicable it is to temperate North American fossil sites due to lack of data from relevant vegetation types and taxonomic differences between plant communities in the Neotropics vs. mid-latitude North America. For example, preliminary results measuring rLAI in phytolith assemblages from the Miocene of the North American Great Plains have yielded surprisingly high degrees of canopy density despite containing high relative abundances of open-habitat grasses. To test whether vegetational and taxonomic differences impact the calibration set, we constructed a new North American calibration using 24 quadrats from six sites, representing reasonable modern analogs for Miocene vegetation in eastern North America. Specifically, we sampled in Bennett Springs State Park in Lebanon, MO; Mark Twain National Forest in Rolla, MO; Tellico in Franklin, NC and Congaree National Park in Hopkins, SC. All sites include a range of canopy covers and vegetation types, from oak savannas and oak woodlands to mixed hardwood forests, pine savannas, and old growth bottomland forests. From each quadrat, we collected a soil sample and took hemispherical photos of the local canopy. From modern soil samples, biosilica was extracted in the lab, yielding phytolith assemblages which were scanned for epidermal phytoliths using a compound microscope. Recovered epidermal phytoliths size and margin undulation were measured and assemblage averages were used to predict measured LAI at each quadrat. Hemispherical photographs were processed using the software Gap Light Analyzer to obtain LAI values. We hypothesize there will be a linear relationship between actual LAI and LAI calculated from epidermal phytolith morphology, but its relationship will differ from that found in South America. Results will be used to reevaluate canopy coverage in sites within the Great Plains Miocene as well as applied to Pacific Northwest Miocene sites, both to understand changes to vegetation during global climatic events in their respective regions.more » « less
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