Deep History and Biogeography of La Frontera - Design with the Desert: Conservation and Sustainable Development - page 121

Sonoran deserts as far north as La Frontera . Unfortunately, the vegetation terms “thorn-
scrub,” “thorn forest,” and “tropical deciduous forest” have often been confused in the
literature; for example, throughout the excellent historical biogeographical discussion in
Morafka et al., 18 “thornscrub” was mistakenly used instead of “tropical deciduous forest.”
A new method of estimating paleoelevations has challenged the aforementioned
scenario. Wolfe 19 used correlations of leaf morphology in fossil floras in modern floras
to modern climates to estimate the history of temperature in the Tertiary. Recently, his
analyses were expanded to include regressions between foliar morphology and various
aspects of climate, and were developed into a method of estimating the elevations of
fossil floras at the time of deposition. 20 In this approach, the physiological tolerances and
limits of the living populations or closest relatives of the fossil taxa are not considered
because they are extinct species that might have been different in the past. The leaf
morphology-climate relationship based on worldwide floras is thought to be a better
indicator of climate than the physiological tolerances of the living relatives of the
fossil taxa. The first studies using this methodology reached dramatically different
paleoelevation estimates than previous studies based on floristic affinities. For example,
MacGinitie 21 inferred a paleoelevation of 3001 ft (now at 8202 ft) for the latest Eocene
(35 mya, formerly called Oligocene) Florissant Beds in Colorado based on a paleoflora
closely allied with the highlands of northeastern Mexico. The flora was a mixture
of plants now found in tropical and montane areas and a sequoia ( Sequoia affinis). ). In
contrast, Gregory 22 using Wolfe's multivariate climate analysis techniques 20 estimated
that the Florissant Beds were at 7,545-10,826 ft elevation. The climatic implications of the
additional 4543-7824 ft elevation in the late Eocene of the Rocky Mountains are profound,
especially the inferences about cold temperatures. All of the ecological, evolutionary,
and biogeographic changes in the biota discussed for the Oligocene-middle Miocene in
the Axelrod model should have occurred earlier if Gregory were correct. It is difficult
to accept this in light of the persistence of tropical plants such as cedar ( Cedrela ), palms,
and piocha ( Tr i c h i li a ) into the early Oligocene, or the gradual modernization of the Rocky
Mountain flora from the Oligocene to the early Miocene. 15
A reexamination of Gregory's study 22 of the Florissant flora is enlightening. The living
relatives of at least 34% of the 29 taxa used in her paleoclimatic analysis are today restricted
to elevations and latitudes lower than Florissant. Cedar, hopbush ( Dodonaea ), mesquite
( Prosopis ), and soapberry ( Sapindus ) are genera with tropical affinities whose extant species
live in areas with higher mean annual temperatures than the upper elevations of the
Rockies. Tr i c h i li a in particular is a tropical grass genus in the Meliaceae that reaches its
northern limit in southern Sonora at 27°N Lat., where Tilia americana and Tilia hirta live
in tropical deciduous forest below about 3280 ft elevation. The inescapable conclusions
of a paleoelevation of 7,545-10,826 ft for the Florissant Beds are that a third of the flora
had greater cold tolerances than their living relatives, and they were more vulnerable to
extinction than their tropical descendants.
Wolfe 23 envisioned a “Big Chill,” a cold period of 1-2 million years in length in the earliest
Oligocene that had a profound impact on the flora. Other climatic reconstructions for the
Eocene\Oligocene boundary, primarily based on the climatic relationships of surviving
taxa were very different. For example, analyses of fossil reptiles and amphibians 24 and
of pollen and leaf floras13 13 indicated increasing aridity and seasonality with little cooling.
If Wolfe's multivariate climate analyses of leaf floras systematically underestimate mean
annual temperatures, then paleoelevations are overestimated, resulting in questionable
landscape and paleoclimatic reconstructions. I feel that important paleoecological signals
from plant and animal taxa in these fossil deposits must be considered. For the present,