STUDY QUESTIONS
1) Why are temperate rain forest coastal? Two reasons.
Coastal areas with onshore circulation patterns are wetter and more moderate in
temperature than inland areas. So, not only is there more water in precipitation and fog, but
since mean temps remain above freezing, plants have access to the water year-round.
2) Why do deciduous trees dominate in the deciduous forest, but evergreens dominate in the
boreal forest?
The growing season is long enough in deciduous forests (at lower latitudes) for trees to grow
an entire new set of leaves each year and still have enough time left to make the energy they
need to last the winter. With a short growing season at higher latitudes, trees in the boreal
forest cannot spend a month growing new leaves; that would leave only three months to
photosynthesize and that is not long enough to make the energy they need to survive
through the longer winter. So, evergreens dominate in the boreal forest; they can use every
day of the short growing season to make the glucose they need over the long winter. Of
course, they make different kinds of leaves. Since they have their leaves all year, those
leaves represent a metabolic COST in winter (to keep them alive), and they also lose water.
Selection has favored plants that minimize these losses by having small, waxy, cylindrical
leaves that use less energy and lose less water.
3) How do arctic and alpine tundra differ in water availability, and why?
They both have low water availability on a yearly basis because the water is frozen most the
year. In addition, however, alpine tundra has the additional problem of being on a mounitan
side – so the water runs downhill and further reduces water availability.
4) Describe how the productivity of a waterway changes along a river continuum.
At the headwaters of a river, streams are narrow and covered by a forest canopy. Less light
means less algal growth (less productivity). In addition, there are few nutrients in the water
because most nutrients come from decomposition and weathering of rock. There is little to
decompose because productivity is low (so not a lot of life to die and decompose), and the
stream has just started, so there is not much "upstream" that has eroded/weathered the
rock. Farther downstream, the river is wider and the sun reaches the water – increasing algal
growth. In addition, this river has all the nutrients eroded from upsteam rocks.
5) Explain why trees drop their leaves in tropical dry forest.
Trees drop their leaves to reduce water loss during times of low water availability. In the
temperate zone, this occurs in winter when water is frozen and unavailable. In the tropics,
this happens in the dry season when there is little precipitation. Leaves are thin and broad,
with a high SA/V ratio. So, they lose water fast. Dropping leaves eliminates this cost.
6) Describe how evapotranspirational stress changes with temperature and water availability.
Evaporational stress is the difference between water availability and water loss. If water
availability (from precipitation) is always greater than water lost by evaporation, then there
is no evaporational stress (rain forest). If the tendancy of plants to lose water is greater than
the amount of precipitation, then stress is high. Since evaporation increases with
temperature, stress will tend to increase unless more water is provided. This is why, in the
graph with the biomes and rainfall and temperature, the transition from grasslands to forest
is "at an angle"… 50cm of precipitation can support a forest at cool temps, but can't support a
forest at warm temps where the evaporative losses are greater.
7) Why are tropical rainforests equatorial, while dry forests and grasslands are at higher
latitudes?
The equator gets two rainy seasons a year, and never really has a really dry season. In
contrast, at higher latitudes IN THE TROPICS, there is a transition to one pronounced rainy
season and one pronounced dry season… this selects for deciduous trees and grasses that can
tolerate the dry season.
8) How do continental and maritime climates differ, and why?
Continental climates are wetter than maritime climates, and have more dramatic variation in
temperature. Maritime climates are buffered by neighboring oceans, which contribute
warmth in the winter, draw heat off the land in the summer, and contribute moisture.
9) Why is the Atacama desert the driest place on the planet? Consider three variables.
It is at 30 S, so air from the upper atmosphere is dropping. It is already dry, and as it drops it
heats up, further increasing its ability to suck moisture off the surface of the earth. In
addition, the Atacama is on the leeward side of the Andes… so air moving from the east is
stripped of its moisture as it crosses the mountains. Finally, although it is a coastal region,
the coastal winds are offshore – so moisture is not moving from the ocean onto land. So,
none of the prevailing atmospheric currents supply moist air to the region.
10) Why are there two rainy seasons at the equator but one at the tropic of
cancer/capricorn?
Rainy seasons occur where the sun's rays are perpendicular to the surface of the earth. They
are perpendicular over the equator TWICE a year (Mar 21, Sept 21: the vernal equinox and
autumnal equinox), but over the tropic of cancer (Jun 21) and capricorn (Dec 21) only once
per year.
11) Why are there algal blooms in deep temperate lakes in the spring and fall?
This is when the temperature of the water is 4C throughout, from the surface to depth. So,
surface breezes can mix the water, bringing nutrients up from the bottom that algae need to
grow. During the rest of the year, the temperature of surface waters is different from deep
water, and floats on top. This layering prevents mixing of the water column, and nutrients
don't reach the surface where the photosynthetic algae have to live (where light is).