Elizabeth A. Kowalski

Correlations of climate and plant ecology to leaf size and shape: potential proxies for the fossil record

The sizes and shapes (physiognomy) of fossil leaves are widely applied as proxies for paleoclimatic and paleoecological variables. However, significant improvements to leaf-margin analysis, used for nearly a century to reconstruct mean annual temperature (MAT), have been elusive; also, relationships between physiognomy and many leaf ecological variables have not been quantified. Using the recently developed technique of digital leaf physiognomy, correlations of leaf physiognomy to MAT, leaf mass per area, and nitrogen content are quantified for a set of test sites from North and Central America. Many physiognomic variables correlate significantly with MAT, indicating a coordinated, convergent evolutionary response of fewer teeth, smaller tooth area, and lower degree of blade dissection in warmer environments. In addition, tooth area correlates negatively with leaf mass per area and positively with nitrogen content. Multiple linear regressions based on a subset of variables produce more accurate MAT estimates than leaf-margin analysis (standard errors of ±2 vs. ±3°C); improvements are greatest at sites with shallow water tables that are analogous to many fossil sites. The multivariate regressions remain robust even when based on one leaf per species, and the model most applicable to fossils shows no more signal degradation from leaf fragmentation than leaf-margin analysis.

Warmer paleotemperatures for terrestrial ecosystems

Floras of predominantly wet-soil environments show a greater than expected proportion of toothed leaves, affecting the outcome of leaf physiognomically based temperature estimates. New analyses of foliar physiognomy of plants growing in predominantly wet soils in modern forests suggest that current methods of inferring paleotemperatures from fossil floras yield underestimates of 2.5–10°C. The changes we propose bring terrestrial paleotemperature estimates into agreement with temperatures inferred from other biological and geological proxies and strengthen the use of leaf physiognomy as a method for climate reconstruction.

Variation in Ginkgo biloba L. leaf characters across a climatic gradient in China

Fossil leaves assigned to the genus Ginkgo are increasingly being used to reconstruct Mesozoic and Tertiary environments based on their stomatal and carbon isotopic characteristics. We sought to provide a more secure basis for understanding variations seen in the plant fossil record by determining the natural variability of these properties of sun and shade leaf morphotypes of Ginkgo biloba trees under the present atmospheric CO2 concentration and a range of contemporary climates in three Chinese locations (Lanzhou, Beijing, and Nanjing). Climate had no major effects on leaf stomatal index (proportion of leaf surface cells that are stomata) but did result in more variable stomatal densities. The effects of climate and leaf morphotype on stomatal index were rather conserved (<1%) and much less than the response of trees to recent CO2 increases. Leaf carbon isotope discrimination (Δ) was highest for trees in Nanjing, which experience a warm, moist climate, whereas trees in the most arid site (Lanzhou) had the lowest Δ values. Interestingly, the variation in Δ shown by leaf populations of trees from China and the United Kingdom was very similar to that of fossil Ginkgo cuticles dating to the Mesozoic and Tertiary, which suggests to us that the physiology of leaf carbon uptake and regulation of water loss in Ginkgo has remained highly conserved despite the potential for evolutionary change over millions of years.

Mean annual temperature estimation based on leaf morphology: a test from tropical South America

Abstract Are models that predict mean annual temperature (MAT) from leaf morphology applicable globally? Fifteen models that predict MAT from leaf morphology were tested on thirty floral samples from tropical South America to determine the degree to which models based on published data that are primarily from other regions are applicable to floras from tropical South America. The models included are based on regional data from North America, South America, and Asia. Of the fifteen models tested, five are simple linear regressions, six are multiple linear regressions, two are canonical correspondence analyses, and two are correspondence analyses followed by nearest neighbor analyses. For the seven modern floras with MAT ≤21°C, every model overestimates MAT. For the 23 modern floras with MAT >21°C, all models produce variable results without a systematic error. The range of average model errors is 2.7–7.3°C, while the absolute extremes of error are 0 and 15.1°C. Average 95% predictive confidence intervals range from 1.6 to 6.9°C. Predicted MAT falls within the published standard error of the model for 0–67% of the South American test floras. Evaluating the seven sites with MAT ≤21°C separately from the 23 sites with MAT >21°C shows that no equation accurately estimates MAT of the majority of low-temperature sites, but that four equations accurately estimate >50% of high-temperature sites. The results suggest that at least for sites of unknown or high elevation, mean annual temperature of fossil floras from tropical South America may be better predicted from models based on the leaf morphology of tropical South American floras.

Glacial–interglacial variations in Quaternary production of marine organic matter at DSDP Site 594, Chatham Rise, southeastern New Zealand margin

Abstract CaCO3 and total organic carbon concentrations, organic matter C/N and carbon isotope ratios, and sediment accumulation rates in late Quaternary sediments from DSDP Site 594 provide information about glacial–interglacial variations in the delivery of organic matter to the Chatham Rise offshore of southeastern New Zealand. Low C/N ratios and nearly constant organic δ13C values of −23‰ indicate that marine production dominates organic matter supply in both glacial and interglacial times during oxygen isotope stages 1 through 6 (0–140 ka) and 17 through 19 (660–790 ka). Increased organic carbon mass accumulation rates in isotope stages 2, 4, 6, and 18 record enhanced marine productivity during glacial maxima. Excursions of organic δ13C values to ca. −29‰ in portions of isotope stage 2 suggest that the local concentration of dissolved CO2 was occasionally elevated during the last glacial maximum, probably as a result of short periods of lowered sea-surface temperature. Dilution of carbonates by clastic continental sediment generally increases at this location during glacial maxima, but enhanced delivery of land-derived organic matter does not accompany the increased accumulation of clastic sediments.

A climatic and taxonomic comparison between leaf litter and standing vegetation from a Florida swamp woodland.

One method to determine past climate has been the use of leaf morphological characteristics of fossil leaves quantified using modern climate and canopy leaf characteristics. Fossil assemblages are composed of abscised leaves, and climate may be more accurately determined by using leaves from leaf litter instead of the canopy. To better understand whether taphonomic processes make a difference in this relationship, a north-central Florida woodland was sampled to determine the morphologically based climate estimates from these leaves. Leaves from woody, dicotyledonous plants were collected and identified, then compared using presence/absence data and analyzed using several linear regression equations and the CLAMP data set. Although the majority of standing vegetation was reflected in leaf litter, some inconsistencies were observed, which may reflect plant community structure or sampling technique. Mean annual temperature (MAT) and growing season precipitation (GSP) were estimated from leaf litter morphological characters and living leaves. Overall, values for MAT estimated from litter and living leaves were cooler than actual MATs, although several accurate and high estimates were obtained depending on the predictive method used. Estimated GSP values were higher than actual GSPs. Statistically, no difference was observed between MAT and GSP estimates derived from leaf litter vs. estimates derived from living leaves, with one exception.