Molecular and isotopic indicators of canopy closure in ancient forests and the effects of environmental gradients on leaf alkane expression

Open Access
Author:
Graham, Heather Valeah
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 26, 2013
Committee Members:
  • Katherine Haines Freeman, Dissertation Advisor
  • Lee Kump, Committee Member
  • MARK E PATZKOWSKY, Committee Member
  • Peter Daniel Wilf, Committee Member
  • Erica A H Smithwick, Committee Member
  • Scott W Wing, Special Member
Keywords:
  • n-alkanes
  • canopy
  • isotope fractionation
  • Cerrejon
  • Guaduas
Abstract:
Dense forests with closed canopies are of enormous climatic and ecological significance. Three-dimensional forest structure affects surface albedo, atmospheric circulation, hydrologic cycling, soil stability, and the carbon cycle, both on the local and global scale. Increasingly, dense canopy forests are recognized for their role in plant and animal evolution and provide a great variety of microhabitats into which much of the diversity of animal and plant life has specialized. The geologic history of three dimensional forest structure is poorly known, however, because fossil assemblages seldom reflect tree spacing, size, or branching and adaptations specific to closed-canopy forests – such as large, fleshy fruits or branchless boles – rarely preserve. The best preserved and most abundant plant fossils are leaves and leaf fragments, however leaf morphology alone does not indicate canopy conditions. This study focuses on leaf biochemical signatures that have potential to record light environment, preserve in geologic archives, and ultimately serve as proxies of canopy density. Dense, closed-canopy forests are defined by strong vertical light and humidity gradients, and a pronounced decrease in photosynthetic rate in the understory. This rate difference results in an enrichment in the carbon isotope composition of leaves (δ13Cleaf) at the top of the canopy relative to the lower levels. Our study found that leaves from a deeply-shaded closed-canopy forest in Panama had a ~10‰ vertical range in δ13Cleaf values, vertically, while leaves from a seasonal canopied forest in Maryland expressed only a ~6‰ range. A Monte Carlo model that resampled δ13Cleaf values indicated that canopy closure could be identified by isotopic range from a relatively small (~50) number of leaves. Based on this result, we used δ13Cleaf range as a diagnostic feature of canopy coverage and measured δ13Cleaf values of fossil leaves to identify canopy density characteristics of three ancient forests. Our results confirm the earliest closed-canopy Neotropical forest in a Paleocene fossil assemblage and identified open canopy features in a Cretaceous fossil assemblage and a forest edge in another Paleocene assemblage. Identifying canopy closure in fossil leaves by analogy by isotopic leaf features observed in modern plants assumes that patterns of photosynthetic carbon isotope fractionation (∆leaf) are the same in ancient plants and their extant relatives. ∆leaf values calculated for modern leaves had a wide range of values from canopy to understory. ∆leaf values calculated from fossil leaf data indicated that the majority of leaves in the assemblages were from the upper canopy. A similar analysis of n-alkanes from modern found that modern leaves express a wide range of δ13Clipid values, in correlation with humidity, canopy height, and irradiance. δ13Clipid values from fossil leaves had smaller ranges than modern relatives but a mixing model that used the relative abundance of leaves in the assemblage and the fossil δ13Clipid data closely reproduced the chain-length distribution and δ13Clipid of alkanes from bulks sediments from the fossil assemblage. Calculated fractionation during lipid synthesis (εlipid) in fossil leaves was less negative than modern leaf values and may reflect drier climates or the bias in fossil leaf assemblages toward upper canopy leaves. The amount of n-alkanes made by a leaf is greater in the understory than it is in the canopy. Understory leaves also have more depleted δ13Clipid values. Alkanes extracted from fossil leaves in the closed-canopy assemblage also exhibited depleted δ13Clipid values in leaves with higher alkane content. An analysis of the published literature found leaf higher angiosperms produce greater concentrations of leaf alkanes. The systematic increase in alkane production as well as the higher alkane amounts in understory leaves may be related to the enhanced fungal resistance proffered by alkanes that would be necessary in humid understories. Increased alkane production may then also be related to shade tolerance and early adaptation by higher angiosperms to closed-canopy conditions.