Erik L. Gulbranson

Permian polar forests: deciduousness and environmental variation.

Forests are expected to expand into northern polar latitudes in the next century. However, the impact of forests at high latitudes on climate and terrestrial biogeochemical cycling is poorly understood because such forests cannot be studied in the modern. This study presents forestry and geochemical analyses of three in situ fossil forests from Late Permian strata of Antarctica, which grew at polar latitudes. Stem size measurements and stump spacing measurements indicate significant differences in forest density and canopy structure that are related to the local depositional setting. For forests closest to fluvial systems, tree density appears to decrease as the forests mature, which is the opposite trend of self-thinning observed in modern forests. We speculate that a combination of tree mortality and high disturbance created low-density mature forests without understory vegetation near Late Permian river systems. Stable carbon isotopes measured from permineralized wood in these forests demonstrate two important points: (i) recently developed techniques of high-resolution carbon isotope studies of wood and mummified wood can be applied to permineralized wood, for which much of the original organic matter has been lost and (ii) that the fossil trees maintained a deciduous habit at polar latitudes during the Late Permian. The combination of paleobotanical, sedimentologic, and paleoforestry techniques provides an unrivaled examination of the function of polar forests in deep time; and the carbon isotope geochemistry supplements this work with subannual records of carbon fixation that allows for the quantitative analysis of deciduous versus evergreen habits and environmental parameters, for example, relative humidity.

A Proxy for Humidity and Floral Province from Paleosols

We develop a proxy to infer humidity from the geochemistry of paleosols in order to enhance reconstructions of ancient paleoclimate beyond trends in mean annual precipitation. Geochemical transfer functions, developed herein, are used to estimate net primary production and evapotranspiration along three latitudinal transects of modern soils in the coterminous United States. Mean annual precipitation and the degree of chemical weathering are estimated from the major element concentrations in soils. The ratio of evapotranspiration, estimated from our proxy, and mean annual precipitation provides a method of determining the humidity province of ancient climates that is more robust and meaningful than previous methods, and our approach differs from existing methods since both the influx and efflux of moisture are explicitly determined. The required input parameters for application of this proxy are (1) the soil morphology, (2) accurate and complete major element concentration data for the active soil and parent material, and (3) the latitude or mean annual temperature (±5°C) of the soil or paleosol. The correlation coefficient between the measured climate and the modeled climate regime using only the latitude, morphologic, and major element data of soils is . We conclude that this proxy provides a refined determination of humidity and floral regimes that can be applied to paleosols.

PALEOBOTANICAL AND GEOCHEMICAL APPROACHES TO STUDYING FOSSIL TREE RINGS: QUANTITATIVE INTERPRETATIONS OF PALEOENVIRONMENT AND ECOPHYSIOLOGY

Fossilized remains of wood are an invaluable source of information about paleoecology, paleoclimate, and the evolution of plants. Traditional studies on fossil wood focus on the anatomy of tracheid cells and the use of modern dendrochronological techniques to determine the environmental factors that have influenced the cell structure of wood (Fritts, 1976; Jefferson, 1982; Creber and Chaloner, 1984). The proportion of cell lumen to cell wall reflects the environment in which the trees grew. Information that can be garnered from the cell measurements includes the earlywood-latewood boundary (Fritts, 1976; Creber and Chaloner, 1984; Denne, 1989), the wood density (Jefferson, 1982), leaf retention time in evergreen trees (Falcon-Lang, 2000), and incidences of dormancy in a plant (Fritts, 1976). The earlywood-latewood boundary is considered to be the point where the individual tree has converted energy resources from expansive growth (earlywood: large cell lumen, thin cell wall) to preparation for dormancy (latewood: small cell lumen, thick cell wall). The proportion of these two factors can indicate if: (1) a given tree never entered dormancy (no latewood); (2) the expansive growth took up the majority of the growing season (little latewood); (3) or if the time of expansive growth was short while much of the growing season was dedicated to preparing for dormancy (a lot of latewood). The proportions found in fossil wood can then be compared to ring structures of modern wood to extrapolate the environmental causes determining ring formation. Density is an important factor in wood structure, as it determines how resilient the wood is to external pressures; the greater proportion of latewood, the denser the wood (USDA Forest Service, 2005). Several methods have been proposed to calculate density in fossil woods (Creber and Francis, 1989; Brea et al., 2008; Williams et al., 2009), but as …

Leaf habit of Late Permian Glossopteris trees from high-palaeolatitude forests

The leaf longevity of trees, deciduous or evergreen, plays an important role in climate feedbacks and plant ecology. In modern forests of the high latitudes, evergreen trees dominate; however, the fossil record indicates that deciduous vegetation dominated during some previous warm intervals. We show, through an integration of palaeobotanical techniques and isotope geochemistry of trees in one of the earliest polar forests (Late Permian, c. 260 Ma, Antarctica), that the arborescent glossopterid taxa were both deciduous and evergreen, with a greater abundance of evergreen trees occurring in the studied forests. These new findings suggest the possibility that deciduousness was a plastic trait in ancient polar plants, and that deciduous plants, migrating poleward from lower latitudes, were probably better adapted to high-disturbance areas in environments that were light-limited. Supplementary material: Wood anatomy descriptions (supplemental file 1), stable carbon isotope data for tree rings (supplemental files 2–4), and method and parameters for modelled Δ13C (supplemental file 5) are available at www.geolsoc.org.uk/SUP18744.

Nitrogen-fixing symbiosis inferred from stable isotope analysis of fossil tree rings from the Oligocene of Ethiopia

The acquisition of reduced nitrogen (N) is essential for plant life, and plants have developed numerous strategies and symbioses with soil microorganisms to acquire this form of N. The evolutionary history of specific symbiotic relationships of plants with soil bacteria, however, lacks evidence from the fossil record confirming these mutualistic relationships. Here we use modern plants in the N-fixing clade of rosids to develop a geochemical method to assess the presence of symbiotic relationships with N-fixing soil bacteria via δ 15 N values of tree rings. Application of this method to Oligocene tree rings confirms the symbiosis of certain arborescent legumes with N-fixing soil bacteria. The results suggest actinorhizal symbiosis for some Oligocene non-leguminous trees. The specific age, genera, and presence or absence of bacterial symbiosis of these fossil trees provide new information on genera that have maintained or lost the ability to form symbioses in the N-fixing clade. We envision that this approach, as applied to paleoecology, can lead to greater understanding of the response of plant symbioses under variations in atmospheric chemistry for N-limited ecosystems.