Geoscience Reference
In-Depth Information
Fossil leaves have also been used to indicate past rainfall. This is based upon
the observation that the morphology of a living leaf, and especially the size of
the leaf, is influenced by the available moisture. Plants found in more arid areas
tend to have smaller leaf size than plants found in more humid regions. This
occurs as plants transpire water into the atmosphere through the surface area
of the leaf, thus a larger leaf has greater surface area and consequently loses
more water (Wilf et al ., 1998).
Estimates of palaeoprecipitation based on leaf morphology are often based
on a method called Wolfe's Climate Leaf-Analysis Multivariate Program (CLAMP).
This method ordinates a multivariate data set of leaf-morphology characteris-
tics scored from modern vegetation samples that are associated with climate
stations. In this way, a quantitative framework can be developed for estimates
of climatic variables. At present, most data of this kind is based on vegeta-
tion from North American forests and there is a profound lack of data from
tropical regions. The technique has been found to overestimate precipitation,
however, and such tendencies have to be considered when palaeoclimates are
reconstructed. The overestimation is larger when data is extrapolated from the
original data in North America to other vegetation communities in warmer
climates. Due to these issues with the CLAMP method, Wilf et al .(1998)investi-
gatedaunivariate method of climate reconstruction based on leaf-area analysis.
This method produced more significant results than the multivariate models
based on the CLAMP data set. Although the univariate method has proved to be
more suitable than the multivariate method, it is advisable that additional data
should be used where possible.
Tree-ring analysis (dendrochronology)
Tree-ring analysis can be used to indicate precipitation levels for his-
toric or prehistoric times on temporal scales of decades to millennia (Hughes
et al. , 1994). Most commonly, the method is used to estimate precipitation levels
ranging over the scale of centuries. The use of tree-ring analysis is particularly
applicable to temperate and subpolar regions. Here, several long-lived species
have been found to leave identifiable traces of past precipitation events (Loagi-
cia et al ., 1993).
Dendrohydrology is the study of long-term hydrological variability using tree
rings. The technique is dependent upon a prior understanding of the relation-
ship between hydrological variables and the biological response of trees to these
variables. The hydrological variables include precipitation, run-off, near surface
temperature, evapotranspiration, ground water and soil moisture. The response
of a tree to changes in these variables is recorded mainly in the width of
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