My main interest is how plants are morphologically and physiologically adapted to their environment, in particular leaf-atmosphere interactions. When properly examined in modern leaf material collected in the field or grown under controlled environments, the challenge is to use these traits in fossil leaf material for paleoenvironmental reconstructions. My research combines modern plant physiology, ecology and anatomy with climatology in order to understand the contribution of terrestrial plant life in System Earth through geologic time. A major focus in my current research is the study of morphological and physiological leaf adaptations to elevation stress in order to develop paleobotanical proxies for past elevations, specifically in the Western US. A selection of projects I am involved in is listed below.
Many species of woody plants, including conifers, show an inverse relation between stomatal frequency (stomatal density or index) and atmospheric CO2
. For my PhD work (2)
at Utrecht University
, I used stomatal analysis on subfossil conifer needles to reconstruct Late Holocene atmospheric CO2
levels. Centennial scale variations were present concurrent with changes in Northern Hemisphere temperature (5)
. Validation of this stomatal proxy included morphological studies on modern developing needles (4)
, carbon isotope analysis to track potential influence of volcanic CO2
outgassing and local vegetation reconstruction through macrofossil analysis (2)
The reconstruction of paleoelevation is an exciting field, as the presence of a high mountain range affects atmospheric circulation and the position of monsoonal systems, and influences global temperatures through the drawdown of the greenhouse gas CO2
by increased weathering rates. Also, quantitative estimates of the timing and amount of surface uplift are needed to test tectonic models. One of my current research goals is the development of new paleobotanical methods to estimate paleoelevation. I have collected modern leaf material from five elevation transects in Washington State in 2007 to measure leaf traits that may show a promise as paleoelevation proxies under different climate regimes. The main focus is currently on paleoelevation of the Western US, but expansion to other regions is certainly a future option.
The predictable, constant decrease in CO2
partial pressure with increasing elevation theoretically allows stomatal frequency on leaves to be used to reconstruct past elevations. In collaboration with Jennifer McElwain
(University College Dublin) I have measured stomatal density on fossil California black oak leaves to show that the Sierra Nevada mountain range in California was already at present elevation in the early Miocene (8)
. These estimates are validated by other paleobotanical proxies measured on the same floras by Julie Broughton and Bruce Tiffney
(UC Santa Barbara).
A higher stomatal conductance and therefore maximum photosynthetic rates may be adaptive at higher elevations for various reasons (7)
. Changes in conductance can be the result of differences in stomatal density, but also by variation in stomatal patterns on the leaves. Currently, I am looking at the stomatal patterning on leaves from five elevation transects from Washington State (located on a gradient between the wet western and the dry eastern part to account for different climates) to test if such a relation between stomatal patterning and elevation (or other environmental parameters) is present.
Fluctuating asymmetry (FA) is the non-directional, random variation in bilateral symmetry in a morphological trait. It is thought to be the effect of disturbances due to either genetic or environmental stress during the development of leaves (or any other body part). Together with Surangi Punyasena
(UIUC) I have looked at changes in FA in five woody species from elevation transects from Washington State to test whether there is a relation with elevation, as indicated in literature, or other environmental factors.
The differences in leaf size and shape, vein density and epidermal features in leaves that were traditionally described as sun and shade leaves, have recently been interpreted to be due to hydraulic stress in different parts of the canopy. Leaf material from four species of oaks and sycamores were grown in controlled environments under high and low levels. I plan to test if these sun/shade features persist when hydraulic differences are eliminated.
Several morphological features in leaves of fossil trees have been traditionally interpreted as sun/shade characteristics or evidence for drought stress. Todd Dawson, Cindy Looy and I would like to test if these features are indeed related to environmental factors. Climatic and physiological data collected within the canopy of California's giant redwoods by the Dawson Lab show a wide range of microclimate variation. This provides a unique opportunity to study the variation in leaf shape and particularly epidermal features in Coastal Redwood and Giant Sequoia for the same genotype in very different levels of drought stress and light in their natural environment. This study is important to improve the reconstruction of paleoenvironments based on plant fossils. We are currently looking for two undergraduate students to assist in this project through the URAP program.
During the Permian the more derived voltzian conifers (voltzian Voltziales) gain dominance over the walchian conifers (walchian Voltziales), in concurrence with an equatorial drier climate. Voltzian and walchian conifers are very different, not just in reproductive structures but also in vegetative organs. Cindy Looy and I plan to study both conifer types on a whole-plant basis (but special emphasis on leaves and cuticle) and attempt to draw inferences on physiological and ecological capabilities of the groups from morphological features associated with physiological performance. This may elucidate why the voltzian conifers became so successful during this particular time period.
- Kouwenberg LLR, McElwain JC, Kürschner WM, Wagner F, Beerling DJ, Mayle FE & Visscher H. 2003. Stomatal frequency adjustment of four conifer species to historical changes in atmospheric CO2. American Journal of Botany 90, 610-619.
- Kouwenberg LLR. 2004. Application of conifer needles in the reconstruction of Holocene CO2 levels. PhD Thesis. LPP Contributions series 16. LPP Foundation, Utrecht. Download
- Wagner F, Kouwenberg LLR, van Hoof TB & Visscher H. 2004. Reproducibility of Holocene atmospheric CO2 records based on stomatal frequency. Quaternary Science Reviews 23, 1947-1954.
- Kouwenberg LLR, Kürschner WM & Visscher H. 2004. Changes in stomatal frequency and size during elongation of Tsuga heterophylla needles. Annals of Botany 94, 561-569.
- Kouwenberg LLR, Wagner F, Kürschner WM & Visscher H. 2005. Atmospheric CO2 fluctuations during the last Millennium reconstructed by stomatal frequency analysis of Tsuga heterophylla needles. Geology 33, 33-36.
- Kouwenberg LLR, Hines RR & McElwain JC. 2007. A new transfer technique to extract and process thin and fragmented fossil cuticle using polyester overlays. Review of Palaeobotany and Palynology 145, 243-248.
- Kouwenberg LLR, Kürschner WM & McElwain JC. 2007. Stomatal frequency change over altitudinal gradients: prospects for paleoaltimetry. Reviews in Mineralogy and Geochemistry 66, 215-241.
- Kouwenberg LLR, Broughton JD, Tiffney BH & McElwain JC. In revision. Ancient elevation of Northern Sierra Nevada Mountains detected from stomatal analyses of 16 - 23 million year old fossil leaves. Proceedings of the National Academy of Sciences.
me for a reprint !