Soils and Vegetation—Midterm Exam Prep.
Species: a group of individuals that 1) share a common ancestor, 2) share
significant amounts of genetic material, 3) can interbreed with each other 4) are
usually reproductively isolated from members of other species
Plant Population: group of individuals of the same species that live in one location
and actively interbreed, includes individuals at different life stages, sapling to
Flowering Plants Conifers, replaced by angiosperms in
Emerged in the Cretaceous period Were dominant in the Mesozoic era
shrubs, vines, trees, and herbs Trees and shrubs, and cones, ie pine trees,
Divided into monocots and dicots
Outnumber gymnosperms by 20:1 Marginalized in stressful or less productive
environments, like tundra
Abiotic/physical factors are those which are part of the physical environment. They typically
occur as gradients across regions of the earth’s surface. They limit species’ distribution when
they reach certain levels, and it depends on the species’ evolved physiological response to
them. Abiotic factors tend to affect a plant’s tolerance range, which is the growth rate or
performance along the gradient of a single physical factor, like pH, moisture, or temperature.
Tolerance ranges explain the gradient and range of tolerance. For example, an increase in
moisture increases plant growth. At low moisture levels, the plant does not grow, but at high
moisture levels, this impairs plant growth.
1) Insolation (Light)
-This is the most important factor in plant photosynthesis. Light in the PhAR portion of
the electromagnetic spectrum provides photosynthetic energy. PhAR= Photosynthetic active radiation, what plants use to photosynthesize. It is a small
portion of the light spectrum. It reflects the most green light, but red and a bit of blue
light is important for plants.
Note that photooxidation is only occurring in C3 plants, and produces photorespiration
where CO2 is released without the production of chemical energy for the plant. The rate
of photorespiration increases with increasing temperature and light intensity. C4 plants
do not have photorespiration, so have higher rates of photosynthesis at high light
intensities and temperatures.
The two categories that plants are classified into with espect to their light requirements:
a) heliophytes-need full sunlight to grow optimally. They are better equipped to handle
large amounts of sunlight.
b) sciophytes-grow best in shade, shade-tolerant spp. High light intensities cause
photoinhibition for them.
*As light intensity increases, photosynthesis increases until it reaches a maximum. Then,
it actually goes down after the compensation point. Pg 101
*Max net photosynthesis occurs when light intensities reach 1/3 to 2/3 the strength of
*Aquatic plants and algae only need low levels of light, like 5% to photosynthesise.
However, at depth there is not enough light, and plants cannot grow high.
Shade-tolerant plants are perennials that live over a year. Annuals germinate within a
-It is the second most important environmental component.
Plants are poikilotherms and assume the temp of their environment. But, temperature
within plants varies.
At temperatures below 0C, plants can get discoloration, depressed growth rates, and
damage to cells. When ice crystals form inside cells, this is lethal to the cell.
Often, during the cold season, plants go dormant, esp deciduous trees.
*For plants like conifers, spruce, and firs to escape freezing, they need to chemically
alter their liquids. This is known as supercooling, and the plants metabolically synthesize
*At high temperatures, some plants just cant survive either. They can cause soomatal
closure, and inhibit CO2, water, and oxygen exchange. Plants adapt to high
temperatures by either losing their leaves or having dimorphic leafing patterns. In hot
periods, they have small leaves and cool season have larger leaves. This occurs in SW US
and Mexico. Specific examples:
Saguaro cactus-very reactive at its northern limit, does well at high temperatures and
low moisture. Young saguaro cacti hide under larger shrubs. As the cactus grows, it rises
above the shrub. Radiation of heat from the stalk of the large mature cactus prevents
freezing. Note that this is a facilitation example,and saguaros do not survive in
temperatures of 0C for over 12-24 hrs.
Ex 2: Palm family
-universally associated with tropics and subtropics, and can never survive in areas where
temperatures drop below -10C to -15C
-palm trees are very widely distributed plants. Why? Because it can tolerate the
subtropics and it gets around well. This wide distribution is due to its ability to disperse.
-Most palm species have large seeds.
-Palm species are absent from temperate locales.
Ex 3: Arctic timberline
-The temperature effect on timberline? Black and white spruce are very good at
withstanding freezing. However, at any point above 10 to 12 deg C, the plants cannot fix
enough carbon. It is too cold for them to be able to fix enough carbon.
-They cant eat enough when it gets too cold.
What is CAM photosynthesis?
Crassulacean acid metabolis is CAM photosynthesis. The plants do not open their
stomata, they keep their stomata tightly closed. At night when it is cool and dark, the
plants photosynthesize, and take in CO2, store it, convert it to malic acid, and store it in
their stem. Then, they close their stomata, the sun comes out, and the sun’s energy
converts this malic acid into carbohydrates and energy. They take the CO2 at night and
avoid the transpiration, which is the loss of moisture through the pores, and they can
CAM idle (keep stomata closed for day and night for weeks during droughts and recycle
the CO2 without releasing any moisture during droughts.) Saguaro cacti undergo CAM
*Water is needed for photosynthesis, cell growth, and cooling. In vascular plants, it is
needed for the movement of nutrients. There are three classifications for defining plants
by their water demand: xerophytes, mesophytes, hydrophytes.
xerophytes-can exist in dry environments mesophytes-need moderately moist conditions
hydrophytes-grow in water or wet soils
xylem-vascular tissues that conduct water
phloem-tissues that transfer nutrients
transpiration-the release of water to the atmosphere by plants, depends on humidity,
wind conditions, and air temperature
Stomata are most commonly found on leaves, but can also occur on the top of leaves,
on stems, and flower parts.
Water stress occurs when rate of water loss is high relative to absorption through the
The three categories that plants use to deal with dry conditions are water stress
escapees, avoiders, and tolerators.
1) water stress escapees –Plants of seasonally dry environments survive the dry season
as dormant seeds. They just go dormant during the dry season, like desert plants.
Example: annual plants in the Sahara may germinate in periods as short as 8 days
3) avoiders-They adapt in different ways, like having a hard and waxy cuticle that
decreases moisture loss. Another way