[Laboratory XI - Interpreting past Climate from Fossil Leaf Assemblages]

In 1918, Wladimir Köppen published the first comprehensive classification of global climate zones. Köppen's system recognized five major climate groups, each with a host of subdivisions that reflected different aspects of seasonality in temperature and precipitation. The system was of particular use to geographers because it defined climate zones based on quantitative temperature and precipitation parameters. Consequently, Köppen's zones could predict agricultural use and many other aspect of human activity. This system, slightly modified, is still in use today.

Surprisingly, Köppen was neither a geographer nor a meteorologist; he was a botanist. At the time that Köppen devised his system, temperature and precipitation data were available for only tiny swatches of the globe. However, descriptions of vegetation existed for most of the explored world. Köppen recognized that plants were useful barometers of climate; thus, mapping the distribution of vegetation types would ultimately map climate as well. Today, we have detailed meteorological data from most of the planet. Even with this wealth of additional information, Köppen's original climate zones have remained intact and useful.

If vegetation can be used to map modern climate, fossil floras might also be useful for reconstructing ancient climate. There are three basic approaches used by paleobotanists to reconstruct ancient climate from fossil assemblages: (1) nearest living relative or the coexistence model; (2) Climate-Leaf Analysis Multivariate Program (CLAMP); and (3) leaf margin analysis.

Nearest Living Relative or the Coexistence Model

This approach to interpreting past climate uses the climatic preferences of modern plants to interpret the past. Climate reconstruction using ancient vegetation then requires three bits of information: 1) a living relative (preferably a close one) for each fossil species; 2) the autecology of the living relative of each fossil species; and 3) a modern association of species similar to the fossil flora. Ideally, the modern analog community should be similar to the fossil assemblage in both species composition and relative abundance of taxa. For example, in the hardwood forests of eastern North America, sugar maple (Acer saccharum) can tolerate cooler winters than can red maple (Acer ruburm). This autecological fact coupled with the relative distributions of the two species allow us to map winter temperatures. If the two species were then recovered from the fossil record, in the same relative abundances, a similar conclusion might be drawn about winter temperature. Similarly, coastal micro-climates in California can be mapped by the presence of the Coastal Redwood (Sequoia sempervirens). If Coastal Redwood is found in the fossil record, one might conclude that the area had a mild and foggy climate at some time in the past.

This method relies heavily on the paleobotanist being able to correctly identify each fossil taxon and to match it to an appropriate living relative. The nearest living relative approach runs into difficulty when fossil groups have no living relatives or when one is uncertain about which living plant might be most closely related to a fossil form. The former problem is well-illustrated by any of the hundreds of extinct taxa we have studied in lab. The latter problem is exemplified by the oaks of California. The modern analogs of many fossil Quercus are strongly debated and the climate preferences of any set of candidates may differ significantly. More importantly, when extrapolating to ancient floras, you cannot guarantee that the autecological preferences of ancestral plants resemble those of their extant descendants. In fact, we assume that ecology does change with evolution-probably faster than does morphology. Furthermore, fossil plant assemblages seldom exactly parallel those of living communities. For example, several cypress genera occur together in the early Tertiary of Spitsbergen (eastern Greenland), but have temperature and precipitation requirements that are, in their modern analogs, mutually exclusive. Consequently, the paleobotanist is at a loss to interpret the climate requirements of extinct taxa when only taxonomic affinities and modern distributions are considered.

Clearly, a taxonomic approach to reconstructing ancient climate has problems. Many of these problems can be avoided by looking at specific features of the plant that might indicate climatic parameters. Webb (1959) classified Australian rain forests based on the physiognomic characteristics of the plants. Physiognomy uses morphological features to reflect some functional or physiological feature of the plant. For example, thick, waxy, succulent leaves indicate arid environments in which the plant must conserve water. These features are convergent (homoplastic) across many lineages. Although homoplasy has been our enemy in the past, here it is very helpful. Leaf physiognomy is particularly useful for deducing temperature and precipitation patterns because the leaf is instrumental in maintaining plant water and temperature balances. Two methods have made use of leaf physiognomy to reconstruct ancient climate.

Climate-Leaf Analysis Multivariate Program: CLAMP

In a series of elegant papers, paleobotanist Jack Wolfe (see reference list, especially Wolfe, 1995, for a summary and overview) has studied the physiognomic features of modern angiosperm leaves and correlated them with climate in hundreds of communities throughout the world. Wolfe's work takes a multivariate approach, meaning that he compares many combinations of characters in concert using computer programs. He chose this approach on the assumption that different combinations of environmental factors would interact to produce the pattern of leaf form observed in nature. For example, a hot, moist climate might produce leaf form similar to a cooler, dry climate because heat exacerbates water stress.

Wolfe's work is based on pioneering studies conducted by Bailey and Sinnott (1916), who recognized that certain features of angiosperm leaves had discrete climatic distributions. Wolfe then took these physiognomic parameters and applied them to study fossil leaf floras from the Tertiary of North America. Wolfe's method scores each fossil taxon for 29 leaf characters. The fossil assemblage is then ordinated within the multivariate space of the modern database and climate interpretations are developed from climate vector scores. Because Wolfe's approach is ataxonomic, it avoids the pitfall of changing autecological preference through time. Instead, it relies on the pervasive homoplasy between lineages. CLAMP also allows each fossil flora to speak for itself as the relative abundance of particular features recalls the ancient climate. This approach does, however, require the adaptationist assumption that leaves will be modified to fit prevailing climatic parameters or, alternatively, that plants bearing certain types of leaves will track their preferred climatic conditions. Furthermore, the current CLAMP database of living plants and climate has difficulties with some climate-vegetation types, like rain forests and alpine vegetation. Another difficulty with this method is a practical consideration: scoring the fossils for all the characters (29 in the latest incarnation of CLAMP) requires skill and different workers are sometimes unable to replicate one another's work.

Leaf Margin Analysis

A third approach to climate reconstruction is directly based on the work of Bailey and Sinnott (1916), who had found a robust relationship between the leaf margin (entire vs. toothed) and climate. Peter Wilf (1997, 1998) has refined this method, which is similar to Wolfe's method, but has the advantage of scoring only one character and therefore minimizing mistakes. Another advantage of looking at only one leaf feature at a time is that the statistics used allow Wilf to calculate confidence intervals for his prediction of climate. Wilf has tested his assumptions with the CLAMP modern-plant database as well as with independently collected modern floras (mostly herbarium specimens) and found good correlations between leaf margin type and mean annual temperature, and leaf area with mean annual precipitation. One disadvantage to this method is that we can evaluate only two parameters of climate: Mean annual temperature (MAT) and mean annual precipitation (MAP). From the plants' perspective, there are many more parameters that may be equally important in survival and reproduction.

In this lab you will use Wilf's method to develop a climate interpretation for a Miocene leaf flora from Buffalo Canyon, Nevada. Wilf's technique will allow you to reconstruct MAT and MAP.

As you work your way through the lab, think about the advantages and disadvantages that this method may have. What potential sources of error are embedded in this method? Do you think that the two climatic parameters of MAT and MAP are enough information to reconstruct ancient climate? Why or why not?

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