finally in the Northern Range in Trinidad (an island
today but once part of Venezuela; it became isolated
only when the sea level rose after the melting of the
glaciers several thousand years ago).
The complex topography of the Andes exerts strong
effects on the distribution and evolution of plants and
animals. The immensity of the overall range coupled
with the divisions of ridges and intervening altiplano,
plus the elevational differences along the mountain
slopes, have provided ideal conditions for evolutionary
divergence among many taxa. The fact that countries
such as Colombia, Ecuador, and Peru have so many
species is due in no small measure to the vicariance
potential (chapter 8) constantly present because of the
Andes.Elfin Forests
Trees and shrubs at high elevations (exact elevation
varies with latitude) are noticeably shorter in stature,
often very gnarled, and more heavily laden with
epiphytes, especially mosses, lichens, and bromeliads,
than those at lower elevations. Tree line, the elevation
above which trees no longer grow (also called
timberline), varies depending upon the location of
the mountain relative to other mountains. Tree line is
typically higher on mountains in close proximity with
other mountains, whereas tree line is lower (meaning
the climate is more severe) on isolated mountains.
The explanation for such distribution is the climatic
influence of the mountains themselves. Mountains
in close proximity moderate the influence of wind
and retain heat better than isolated, more exposed
mountains. This pattern is called the Massenerhebung
effect.
At higher elevations, you will find elfin forests of
twisted and stunted trees barely 3 m (9.8 ft) tall. An
abundance of lichens, green algae, and bryophytes
(mosses and their relatives) cover the trees’ branches,
along with larger epiphytes such as ferns and orchids.
These often dense, diminutive forests thrive in a
climate of near perpetual mist. Tree growth is slowed
by a shortage of sunlight as well as low temperatures.
Elfin forests have a lower species richness of trees than
lower elevation forests, probably due to greater climatic
stress. Prominent plant genera include Podocarpus,
Clusia, and Gynoxys.Plate 13- 23. Modern descendants of the Inca celebrating
traditions linked to the once great Incan Empire. Photo by
David Clapp.Plate 13- 24. Polylepis forest enshrouded, as it usually is, in mist.
Photo by John Kricher.Plate 13- 25. Giant Conebill. Photo by Edison Buenaño.chapter 13 scaling the andes 245