Download Study Questions, First set for Exam 2

Edited 2/25/14. Note: we may not get through all of chpt 7.
Study Questions for Exam 2 A number of questions are similar, but asked a bit
differently. Forgive the redundancies.
Productivity: chpts 5 and 6.
1. How do plants with different photosynthetic pathways differ in their photosynthetic responses
to water and nitrogen? Why do these differences exist (what is the biochemical basis for
these differences in response to environment?)
2. How does each major environmental variable (CO2, light, nitrogen, water, temperature,
pollutants) affect photosynthetic rate in the short term? How do the photosynthetic
properties of individual leaves change in response to changes in these factors to optimize
photosynthetic performance?
3. How does the response of photosynthesis to one environmental variable (e.g., water or
nitrogen) affect the response to other environmental variables (e.g., light, CO2, or
pollutants)? Considering these interactions among environmental variables, how might
anthropogenic increases in nitrogen inputs affect the response of Earth's ecosystems to
rising atmospheric CO2?
4. How do environmental stresses affect light use efficiency in the short term? How does
vegetation adjust to maximize LUE in stressful environments over the long term?
5. What factors are most important in explaining differences among ecosystems in GPP? Explain
your answers.
6. What factors most strongly affect leaf area and photosynthetic capacity of vegetation? (e.g.,
what explains LAI? What explains the nitrogen content of a leaf?)
7. How do the factors regulating photosynthesis in a forest canopy differ from those in individual
leaves? How does availability of soil resources (water and nutrients) and the structure of
the canopy influence the importance of these canopy effects?
More chpt 5:
Why is the C3 photosynthetic pathway the most common in forests? Where does the C4
pathway dominate?
What is the dominant photosynthetic pathway for a ‘winter-wet’ desert?
A ‘summer-wet’ desert? (obviously, it cannot be too wet or it wouldn’t be a desert…)
How can a leaf with half the enzymes per unit area of leaf be able to photosynthesize at
A rate equal to a leaf with twice the enzymes (on a unit area basis)?
We looked at global pattern of NPP. Compare and contrast the characteristics of the
highest and lowest NPP in terrestrial and aquatic systems. (recall for aquatics: Coral reef
and spartina marsh match those of tropical rain forest; open ocean matches up with the
tundra or desert.
Net Ecosystem Production, Net Ecosystem Exchange.
What determines an ecosystem value of Net Ecosystem Production (NEP)? How can
NEP increase if NPP does not change?
Why does the NEP of a successional system usually exceed that of a mature ecosystem?
What is the difference between Net Ecosystem Exchange (NEE) and NEP? Why do we
care about NEE? What would happen if terrestrial ecosystems reverse their current
patterns and become sources of carbon?
More from class notes related to chpt 5:
1. What resources are required for plants to capture solar energy?
1a. Why the high demand for nitrogen in plants?
2. What form of photosynthesis is the most water efficient? What are the
consequences of being water efficient in terms of energy capture?
3. Why is the primitive (C3) pathway for photosynthesis considered less than
perfect? How did the C4 pathway “fix” this problem?
4. Why, if the C4 pathway evolved to do so well under full light, hot conditions, are
tropical forests still filled largely with C3 species?
5. When can a C3 plant outcompete a C4 plant?, i.e., in what conditions (or biomes)
do we see largely C3 species?
5a. What current human-caused environmental changes would favor one
photosynthetic pathway over another?
6. How can leaf chemistry and leaf angle (direction towards sun at noon) affect the
amount of light that ‘saturates” a leaf (saturate = point of maximum carbon gain).
7. Why does the intercept of the line for the amount of sunlight needed to yield a
positive energy gain vary among plant species? (i.e., what is the “compensation
point” and why does it vary?) Can this intercept vary for different leaves of the
same species?
7a. Why should plants worry about light inputs that exceed their photosynthetic
capacity?
8. What’s a good definition for net primary productivity?
8a. List three very different ecosystems that have extremely high NPP. What is
common factor(s)?
8b. List three very different ecosystems that have extremely low NPP. What is
common factor(s)?
9. What are the components of plant respiration?
10. What determines the amount of energy (fixed carbon) that plants will allocate to
structures such as leaves and roots? Is there any difference if a plant chooses to
allocate energy to defense (say, the production of lignin) rather than leaves and
roots?
11. What variable(s) are good predictors of global patterns of NPP?
12. Why do plants deliberately ‘throw away’ (senesce) structures like leaves?
13. What’s the difference between net primary productivity and net ecosystem
productivity? In a plant grown in sterile culture, would these values differ?
14. How can the net ecosystem production of an old growth Douglas fir forest be
zero?
15. Most temperate forests are thought to currently exhibit positive net ecosystem
exchange. What does that mean?
16. Why does the northern hemisphere appear to show more seasonal dynamics in
atmospheric carbon dioxide concentrations than the southern hemisphere?
17. NPP can be very high, but net ecosystem production (NEP) can be negative at the
same time. How is that possible?
Questions related to Chpt 7 (Decomposition)
Questions on decomposition (text, chpt 7 and class notes).
1. Essay question: Explain the three main (and somewhat independent) factors that
control the rate of decomposition
of plant material. (say, leaf litter).
2. What's the difference between decomposition and mineralization?
3. How is it possible for mass loss (decomposition) to occur, where substantial amounts
of carbon dioxide are released back to the atmosphere, but none of the nutrients in the
plant material are released as minerals to the environment? What is that called (special
term)?
4. Roughly how much of NPP is transferred directly from plants to the detritus pool in
a) a temperate zone forest.
b) a well-managed grassland.
5.Litterbag studies show that fauna (invertebrates that eat microbes and detritus) can
speed the rate of decay of litter by up to 50%. Yet these animals only respire away about
5% of the litter carbon themselves. Explain the discrepancy.
6. How does RESORPTION or RETRANSLOCATION of nutrients by plants affect the
quality of plant litter? (Resorption = removing and retaining nutrients before tissues are
senesced.)
7. What's the term for the ability of microbes to retain nutrients from decomposing
litter?
8. If microbes are making a living of eating pure CHO (e.g., sugar) within the soil,
where are they getting their nutrients?
9. Why are many small molecules of organic carbon of "high quality" to consumers?
Why
is the net energy gain less when you 'eat' (think like a microbe) large molecules?
10. What's an exoenzyme, and why is the production of exoenzymes considered to be
"risky business"?
11. AET is a great predictor of NPP, but AET is not as good a predictor of plant decomposition
rates. Why the difference?
12. What is lignin? Why is lignin so recalcitrant (so hard to break down and decompose)?
13. Lignin contains only CHO compounds. Yet, when this material is finally converted
into humus, it contains almost 1 nitrogen atom for every 10-11 carbon atoms.
How does this happen? Where does the nitrogen come from?
14. What would be a reasonable percent decay estimate (percent per year loss) for
the most recalcitrant stuff found in the prairie or forest? Why is this decay so slow?
15. Decay in the tropics is relatively faster with respect to plant production than
is decay in the boreal forest or tundra. (this means the ratio of live plant/dead
stuff changes). Suggest some hypotheses (based on known facts) why this might happen.
(Note added in preparation. One additional hypothesis is that recalcitrant materials
are simply more decomposable under higher temperatures because temperature alters the
three dimensional structure of these molecules in ways that favor their breakdown
under warm conditions. Why would a change in three dimensional structure be important?)
17a. In a temperate forest, litterfall is estimated at 1000 grams per sq. meter per year,
and carbon storage in the soil is estimated at 10,000 grams per sq meter. What's the
average decomposition rate (as a % of the soil organic matter pool?).
17b. If the production of this forest was increased to 2000 grams per sq. meter per year,
How much larger would the soil carbon pool grow IF decomposition remained unchanged?
18. What is anaerobic decomposition? Who is involved? List the elements or ions or materials
that provide the electron acceptors in anaerobic decomposition (see Table 3.4 of text for a list of
these). What electron acceptors are 'preferred' in decomposition and what determines this
preference? (i.e., if you lived in an anaerobic environment, what material would you 'breathe' and
why would you choose this?)
19…Many microbes, including mycorrhizae, appear to release exoenzymes into the soil. What’s
the benefit and what’s the risk of this activity?
20…How do we know that “resynthesis” of organic compounds occurs during the decomposition
process (hint: compare humus to lignin).