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.
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Phytolith assemblages reflect variability in human land use and the modern environment
Phytoliths preserved in soils and sediments can be used to provide unique insights into past vegetation dynamics in response to human and climate change. Phytoliths can reconstruct local vegetation in terrestrial soils where pollen grains typically decay, providing a range of markers (or lack thereof) that document past human activities. The ca. 6 million km2of Amazonian forests have relatively few baseline datasets documenting changes in phytolith representation across gradients of human disturbances. Here we show that phytolith assemblages vary on local scales across a gradient of (modern) human disturbance in tropical rainforests of Suriname. Detrended correspondence analysis showed that the phytolith assemblages found in managed landscapes (shifting cultivation and a garden), unmanaged forests, and abandoned reforesting sites were clearly distinguishable from intact forests and from each other. Our results highlight the sensitivity and potential of phytoliths to be used in reconstructing successional trajectories after site usage and abandonment. Percentages of specific phytolith morphotypes were also positively correlated with local palm abundances derived from UAV data, and with biomass estimated from MODIS satellite imagery. This baseline dataset provides an index of likely changes that can be observed at other sites that indicate past human activities and long-term forest recovery in Amazonia.
more » « less- Award ID(s):
- 2148984
- PAR ID:
- 10501868
- Publisher / Repository:
- Vegetation History and Archaeobotany
- Date Published:
- Journal Name:
- Vegetation History and Archaeobotany
- Volume:
- 33
- Issue:
- 2
- ISSN:
- 0939-6314
- Page Range / eLocation ID:
- 221 to 236
- Subject(s) / Keyword(s):
- Amazonia UAV imagery Human–environment interactions Suriname Palaeoecology Phytoliths
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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