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Trees and Forests: Midterm 3
Instructions: Please write your name on your exam and on Scantron Form (also student ID#, version #). Read all questions carefully. This 100 point exam has two parts:
Part 1: 25 Questions, 2 points each. Choose the single best answer, mark on the Scantron Part 2: 10 Short answers. Answer questions in spaces provided on your exam.
Part 1. Multiple choice, true/false [A = TRUE, B = FALSE.]
_____ 1 True [a] or False [b] The plant-availability of nutrients immediately following a forest fire is greatly decreased.
_____ 2 Soil compaction during timber harvest activities will be most severe when the soil is [a] dry [b] wet [c] frozen
_____ 3 True [a] or False [b] Soil compaction by forest harvest activities will result in increased soil bulk density and an increased potential for soil erosion.
_____ 4 Which soil textural class would you expect to have the highest “plant-available” water holding capacity? [a] sand [b] sandy loam [c] silt loam [d] clay
_____ 5 Relative to the amount of incoming precipitation (rain or snow; do not consider fog inputs), the amount of canopy throughfall is typically [a] higher [b] about the same [c] lower
_____ 6 True [a] or False [b] Actual evapotranspiration is a better predictor of net primary productivity (NPP) than potential evapotranspiration.
_____ 7 Along an elevational transect from Sacramento to Lake Tahoe, where would you expect to find the most highly weathered soil? [a] high elevations (Lake Tahoe) [b] mid-elevations [c] low elevation (Sacramento)
_____ 8 Which soil textural class would tend to have the lowest soil oxygen concentrations? [a] clay [b] clay loam [c] loam [d] sandy loam [e] sand
_____ 9 True [a] or False [b] Foliage from a species with a higher nitrogen-use efficiency will tend to have a lower carbon:nitrogen ratio.
_____ 10 Which soil horizon would provide the most nutrients to a forest? [a] C horizon [b] B horizon [c] E horizon [c] A horizon
_____ 11 Compared to a mature forest ecosystem, which of the following is the most likely response of a stream hydrograph to a large rain event following a forest clearcut: [a] higher & earlier peak [b] higher & later peak [c] lower & earlier peak [d] lower & later peak [e] no significant change
_____ 12 With respect to the oxidation state of nitrogen, which form of nitrogen contains more energy? [a] nitrate (NO 3 -^ ) [b] di-nitrogen gas (N 2 (g)) [c] ammonium (NH 4 +)
_____ 13 In the nitrogen cycle, which of the following reactions release net energy from nitrogen compounds: [a] symbiotic N-fixation [b] free-living (asymbiotic) N-fixation [c] denitrification [d] nitrification [e] nitrate leaching
_____ 14 The primary source of “plant-available” phosphorus in very old, highly weathered soils is? [a] the mineral apatite [b] sorbed (or non-occluded) PO 4 [c] occluded PO 4 [d] soil organic matter
_____ 15 Prior to foliage senescence (e.g., leaf fall), which nutrient tends to have the greatest resorption (or retranslocation)? [a] calcium (Ca2+) [b] magnesium (Mg 2+) [c] nitrogen (N) [d] carbon (C)
_____ 16 As site quality increases (e.g., higher nutrient availability), a tree will allocate a greater percentage of its net primary productivity (NPP) to: [a] to foliage [b] to roots [c] to wood [d] to wood & roots [e] to foliage & wood
_____ 17 True [a] False [b] In areas with little fog input, clearcut harvesting of forests tends to increase streamflow for a short period of time following the harvest.
_____ 18 What is the dominant mechanism by which phosphorus is lost as a result of a forest fire: [a] volatilization [b] soil erosion [c] herbivory [d] leaching [e] wind blown particulate matter during fire
_____ 19 Hydrophobic soil layers form in soils due to: [a] soil erosion [b] clearcut harvesting [c] soil compaction [d] cold temperatures [e] fire
_____ 20 Which soil characteristic is not necessary to consider when calculating the total plant-available water holding capacity of a soil? [a] soil texture [b] soil depth [c] slope aspect [d] coarse fragment content
_____ 21 At the grassland-forest ecotone (i.e., the break between forest and grassland ecosystems) in California, trees tend to preferentially occur on: [a] north-facing slopes [b] south-facing slopes [c] west-facing slopes [d] east-facing slopes
_____ 22 True [a] False [b] Soil carbon dioxide concentrations will be higher in a wetter soil, everything else being equal.
_____ 23 Which type of tree is better adapted to low nutrient sites? [a] evergreen [b] deciduous [c] no difference
_____ 24 Nutrient demands during forest ecosystem development following a major disturbance are greatest at which stage? [a] immediately following disturbance (0-5 years) [b] prior to canopy closure (20-40 years) [c] following canopy closure (40-100 years) [d] an old growth forest (>200 years)
_____ 25 True [a] False [b] Changes in fire severity from pre-European settlement time compared to today vary from little or no change in fuel-limited forests to much greater severity today in climate-limited forests.
(12) 29. a. In the space below part “e”, provide a sketch of the nitrogen cycle that includes the important soil and ecosystem “pools” specific for nitrogen (pools are the boxes in my nutrient cycling diagram). Provide the names for each of these pools and show the proper arrows (you do NOT need to name the arrows) between the pools.
b. Indicate the two major external inputs into the nitrogen cycle (where does the N come from; inputs to the ecosystem?) Sources: Atmospheric deposition, N fixation, lightening
c. Indicate three potential losses from the nitrogen cycle (losses from the ecosystem).
Losses: leaching, denitrification, erosion
d. What soil pool contains the largest amount of nitrogen in forest ecosystems? Soil organic matter is the largest soil pool of nitrogen
e. Which flux (name of the arrow between pools) is most important in regulating the availability of nitrogen for plants in most forest ecosystems? Mineralization is the primary control on N availability in most forest as it regulates how much N is released from the soil organic matter pool.
(4) 30. In our future, what are the two primary reasons why “brush” (weeds and shrubs) is expected to replace trees in many Sierra Nevada forests following high severity fires?
#1 Current and future fires are expected to be very large in size. It is very difficult for the seeds of tree species to be spread at large distances away from the remaining living trees. Thus, it could take many years for sufficient seed to arrive to the middle of large fire areas. In the meantime, seeds from brush and weeds will quickly take advantage of the disturbed habitat.
#2 Global warming is expected to (and already has) result in summers that are hotter, drier, and longer in duration that in the past. If this is true, the longer dry period is expected to favor brush species, such as chapparel.
(4) 31. In his discussion of fire in the California Sierra Nevada Mountains, Hugh Safford indicated that fire frequency and intensity in some forests were “fuel limited” or “climate limited”.
a. Define what he meant by each of these terms
Fuel-limited refers to ecosystems in which the summer climate conditions (hot-dry) are almost always favorable to support a forest fire. Thus, what determines the actual fire frequency is how much fuel as accumulated to support a fire.
Climate-limited refers to ecosystems in which climate conditions (long snowpack; higher humidity) generally do not favor summer fires. The summer conditions are moist enough due to the long duration of snowpack to keep the fuels relatively non-combustible in most years. However, there will be plenty of fuels built up in these ecosystems due to the long duration of time between fires.
b. At what elevation zone do each of these fire type sytstems occur (lower, middle, upper)? Fuel-limited = lower elevation Climate-limited = upper elevation
c. Provide an example of a specific forest type (e.g., dominant tree species) for each of the categories. Fuel-limited = Ponderosa pine Climate-limited = Red fir/subalpine (Lodgepool, white pine, Jeffrey pine)
(5) 32. Nitrate concentrations in stream water tend to increase following clearcut harvesting. Provide a process-level (the mechanism) discussion for why nitrate leaching increases following clearcut harvesting.
Following a clearcut, there is often an addition of litter (foliage/twigs/dead roots) to the soil organic matter pool. Soil temperature increases due to the removal of the canopy. Soil moisture increases due to the lack of transpiration (vegetation removal). Thus, the combination of more food, higher temperature, and high moisture means much greater microbial activity and mineralization of nitrogen from the soil organic matter pool. Nitrate will build up in the soil solution pool because there is little demand for nitrogen from the vegetation, which is just beginning to regenerate. As nitrate builds up in the soil solution pool, it can be easily leached from the soil because it is an anion and not held be the soil CEC.
(5) 33. Why did the “islands of soil fertility” disappear so quickly in the 0-5 cm soil depth following removal of blue oak trees. Provide three mechanisms.
#1 Lack of canopy to capture atmospheric deposition #2 Lack of canopy litterfall to replace nutrients and organic matter leached from the surface soil layers. This represents nutrients taken up by the deep and wide expanding rooting system. #3 More water leaching through the soils because of the lack of canopy interception and transpiration. Thus, more leaching of nutrient from surface soil layers. #4 Possibly more soil erosion removing soil with its organic matter and nutrients. #5 Perhaps warmer and wetter conditions resulting in greater decomposition and mineralization of nutrients from the organic matter in the upper soil layers.
