Review questions and answers for the 3rd exam for PLS144, Trees and Forests, at UCD
What are four major soil forming processes responsible for development of soil horizons/profiles?
1. Transformation: alterations of things, chemical weathering
2. Translocation: primarily downward due to leeching
3. Additions: precipitations, dust particles
4. Losses: denitrification, erosion of solid materials, leeching
What are the 5 forming factors that determine the kind of soil which forms at a given location?
How are A horizons formed?
Enriched in organic matter (dead plants and animals)
Formed relatively quickly (100s of years)
How are B horizons formed?
Requires translocation process
Takes a long time (10s of 1000s of years)
Why do O horizons preferentially form in forest soils?
Distinguished by a degree of decomp (Oi, less decomposed, more N and C; Oa: more decomposed)
1. litter falls on soil
2. lignins and tannins of organic forest material takes a while to decompose
3. higher C to N ratio; slows decomp because N are limited
4. fewer soil mixers
Will soils tend to form more quickly on consolidated or unconsolidated parent material and why?
Consolidated: hard bedrock
Unconsolidated better: glacial till, has porosity and surface area for soil formation, deep area for trees to anchor roots
Will forest productivity tend to be higher on consolidated or unconsolidated parent material and why?
Productivity will be higher on unconsolidated as well because there is more openness for development
In general, how do soil horizons (the number and type) tend to change as soils grow older?
As soils get old, they tend to get grey
Fresh material begins with volcanic ash (C horizon), will take a few hundred years to form A, then we get Bt after 10s of thousands, a few hundreds of thousands of years intense chemical weathering and chemical leeching forms the E
Will forest productivity tend to be higher on young, medium-age, or old soils? Why?
Medium-age because of the organic matter and structure
big Bt horizons prevent roots from going downward through the clay
chemical weathering will consume primary mineral over tim
What is soil texture? Can it be easily changed? Why is it so important?
No, would need to add a lot of nutrients and components
It determines the aeration and water holding ability of soil
Soil structure: the combination of all the different sand silt and clay particles that make up the soil
Macro and micropores
With regard to plant growth, what are the four main roles that soils provide?
1. support for roots
What is bulk density? How does it affect root growth? Water movement? Soil gas movement?
Bulk density: wt of soil per unit volume, compacting soil increases Bulk Density
compacting also decreases macropores (needed for transport of water and gas)
too dense and fine roots cannot penetrate
How does soil moisture content affect the potential for soil compaction during forest harvest?
Compaction greatest near the surface
The wetter the soil, the greater the potential for compaction
particles can slide by more efficiently and to greater depths
best to do when soil is dry or frozen to minimize compaction
In what size pores is plant available water held?
Too big: water drains out
Too small: water not available to plants because the surfaces are holding the water too tightly
Which soil textures hold the most plant-available water? Why?
Silt-Loam because it can hold the water without trapping it, med pores
Clay is compacted and traps the water in too tightly
Sand can't hold the water at all because the macropores are too big
How do concentrations of oxygen and carbon dioxide vary with depth in a soil profile? Why?
Inverse relationship of soil, always add to 21%
CO2 comes from plant and microbial respiration
Oxygen moves down in soil
Are macropores or micropores more important for water and gas transport?
Macropores are more important for transport
Sand higher macropore count
Water-filled pores transport gas slowly, so more wet soil will transport O slowly
Diffusion goes up with increased sand in soil
Decreased diffusion with compaction
What are the major sources of the 17 essential plant nutrients?
1. rock-derived (chemical weathering), young soil has a lot but chemical weathering can take these away
2. N-fixation from atmosphere
3. air and water provide O, C, H
4. dust inputs
What are the major soil nutrient storage pools? What is the relative availability of nutrients to plants from each of these soil pools?
Soil solution, organic matter, rocks and mineral, phosphate, cations
Soil solution in water most available
Cations next most available
Phosphate next, then organic and then rocks are the least available since they need chemical weathering
What controls the concentrations of organic matter in soils?
Balance between production of organic matter and the speed of decomp via the respiration process
Climate: moderately wet climates will have the most decomp
Grassland soils tend to have more because of
the availability of organic material
What is the relationship between the complexity of an organic molecule and its decomposition rate by soil microbes?
More enriched material is going to decompose more slowly
Pine needles will decompose more slowly because of the tannins and lignins
Whereas deciduous leaves will decompose relatively quickly as they lack these things
What does "mineralization" refer to?
the release of nutrients that are organically bound
such as the release of proteins into nitrogen
mineralized material are more available for use by plants than organic materials
Why do forest soils tend to be acidic (pH<7)?
Lots of rainfall which causes chemical weathering and leeching
Deserts tend to be more basic since they don't have the water for leeching
What soil factors contribute to high productivity of forests?
1. deep soil (more volume for water and air)
2. loamy textures
3. free-draining profile (no saturation wanted)
4. soil pH (5 or 6 due to the adaptations of the forest plants)
5. moderately weathered soil to release nutrients
6. low to moderate slopes
7. organic matter
Why are actual evapotranspiration rates such a good predictor of net primary productivity (NPP) at the global scale?
Evapotranspiration requires water and sun's energy for evaporation of water
Climates with both sun and water are going to be more productive
What is watershed? What is the riparian zone?
an area of land in which all water within drains out to a common river
the innerface between forest and river with groundwater influences which is why trees in riparian zone survive, more dense forest usually found in these zones in CA
What important roles do riparian corridors play in California forests? How do they affect fire dynamics?
What are the major hydrologic inputs into forest ecosystems? What are the major hydrologic losses from forest ecosystems?
What do canopy throughfall, stemflow, and interception refer to?
water that falls through canopy
water that gets trapped in stem
water that gets evaporated right back from canopy into the air
How does canopy throughfall and stemflow affect the amount and distribution of water entering the soil profile?
a lot less water is reaching the ground in a canopy then in an open environment
majority of water is evaporated back up from the canopy
large part of water that enters soils is taken up into the stem of the plant
Is California's Mediterranean climate conducive to optimizing forest growth? During what seasons of the year are plant growth and soil microorganism activity greatest?
it's usually either too cold or too dry
but the most activity is during the spring after snowpack melts and the fall when the soil is warmed still from summer
If the prediction for global warming come true, how will the Sierra Nevada snowpack be affected? How might this affect Sierra Nevada forest productivity?
A lot of conifer is going to be converted to mixed hardwood conifer while a lot of the oak woodlands will become grassland will no longer be getting sufficient water
There will probably be more fire
Forest productivity will decrease by ~20%
How do the rates of water infiltration into a soil and water runoff from a soil surface change with increasing length and intensity of a precipitation event? How will snowpack vs runoff affect runoff?
infiltration: water into the soil
runoff: water moving along surface and NOT into soil, creates erosion so we want to avoid, not a lot in intact forests due to the sponge-like O horizon
without runoff, there are not sediments coming off via erosion
so a higher snowpack
Is surface runoff a major process in a mature forest? How might runoff change following forest clearcutting?
What soil characteristics must you measure in order to determine that total plant-available water holding capacity of a soil?
want a deep soil
no coarse fragments such as rocks and gravels
What is water-use efficiency? Will water-use efficiency be greater for plants with lower stomatal conductance or those with higher stomatal conductance?
how much water it takes to generate a net unit of primary productivity
water use efficiency is better in lower stomatal conductance because of less water loss
What is hydraulic lift? How might hydraulic lift influence nutrient uptake in a forest?
during day the water is being taken up into the canopy and being removed via evapotranspiration
at night it is not taken from evapotranspiration
at night taken from deep in the soil profile to higher in the soil profile where it is dry
this brings nutrients up from below to the upper soils where it can be utilized
How would you expect the shape of a stream hydrograph during a storm event to change after clearcutting a mature forest?
a measure of how high the water is during a given storm event
in an intact forest, the initial impact of the rain is not changing much since the water is being taken in by the forest, and then it will increase as the ground becomes saturated (lower over a longer time)
in a clearcut area, the peak height is very high because there is less to absorb the water and over a shorter amount of time
How do trees in a forest contribute to the aquatic habitat of a stream running through a forest?
large, woody debris create pools and ripples for a variety of habitats, providing niches
shade the streams
What is the major source of energy driving the nutrient cycle?
How are carbon and nitrogen linked cycles?
Nitrogen gas in atmosphere is fixed and forms ammonium which then can be used in plants (3 extra electrons, 3 units of energy)
Carbon cycle provides energy to reduce nitrogen
Where does the energy for symbiotic N-fixation originate from? How many electrons are transferred for each mole of N fixed?
From ammonium to nitrate we release 8 electrons
this energy goes to chemoautotrophs
denitrification energy is coming from organic matter
Where does the energy for N-fixation by free-living bacteria originate from?
energy comes from organic matter
What was the most limiting nutrient to forest growth in the devastated area around Mt. St. Helens following the 1980 eruption?
How did nitrogen enter the ecosystem following the Mt. St. Helens eruption?
via lupins which could fix the N
How did soil erosion aid in the reestablishment of vegetation in some areas after the Mt. St. Helens eruption?
eventually eroded down to the buried soil O horizon which gave access to nitrogen and organic materials
In what forms can atmospheric deposition of nitrogen occur in a forest ecosystem?
can be dissolved in precipitation, fog
In northern California, how many years does it take to approach steady-state carbon and nitrogen in soils during primary succession?
about 200-300 years
comes from nitrogen fixation and atmospheric deposition
What controls whether N mineralization or N immobilization will occur in the litter layer of a forest soil?
when microbes eat organic matter they can either return the nutrients to the soil or keep it
what determines this is the C:N ratio
if we are N-rich, extra is given off to soil
C-rich means there isn't enough N to supply their body tissues so they must take N from soil which means that N is stuck until the microbes die
Compare ammonium vs nitrate: which is more susceptible to loss from a forest ecosystem?
Nitrate, an anion, which can move very quickly (susceptible to leeching)
Ammonium is held to cation exchange sites, cant' move very quickly
Compare ammonium vs nitrate: which form is more energetically favorable for protein synthesis by plants?
listen from 1 hour
What conditions favor denitrification in forest ecosystems?
heterotrophs under anaerobic conditions
1. carbon that microbes can eat quickly
3. warmer temperatures
4. higher levels of water
What is the ultimate source of phosphorus and calcium in forest soils?
rocks and minerals
What are the major soil nutrient pools for phosphorus and calcium in a typical forest soil?
organic matter cycling
How do total P and the plant availability of P change with increasing soil age (or degree of soil development/weathering)?
it decreases as is becomes trapped or weathered away
lots of P in young soils
old, weathered soils do not have available P and must rely on turnover of organic matter
What factors regulate how quickly nutrient mineralization occurs in forest soils?
1. climate: temp, moisture
2. organic matter quality: limitations of N, high lignin and low N is a slow decomp
Do deciduous or conifers tend to have a greater nutrient-use efficiency?
Conifer more nitrogen-rich efficient
How do mycorrhizae fungi increase the availability of nutrients to trees?
Produce exoenzymes which break down nutrients
increase surface area for absorption
enhanced water uptake because of SA
prevent uptake of toxic compounds
Which nutrients have the largest percentages of reabsorption?
90% of N/L will be reabsorbed
Ca not reabsorbed
Why is calcium not significantly reabsorbed?
Needed to keep support for...
At what point during forest development are nutrients required in the greatest amounts?
Between 20-40 years
right after clearcut will just leech out of soil
How does soil nutrient availability affect the partitioning of photosynthate between above-ground and below-ground components?
Fewer roots needed to supply tree with nutrients which means that the upper parts can grow
How do forest ecosystems contribute to the food web of aquatic (stream) ecosystems within a forest?
adds nutrient fueling primary productivity of algae
provides organic matter to the microbial food loop which feeds zooplankton which are fish food
How do "islands of soil fertility" develop in California oak woodlands?
interactions between all the spheres
atmosphere: canopy captures extra water from rainfall and dust
tree provides a lot of organic matter
plenty of weathering adding minerals
geosphere: enhanced chemical weathering
more water lost to atmosphere which means less leeching
lower bulk density beneath canopy, more macropores which leads to more infiltration and less runoff
Why do these "islands of soil fertility" disappear so quickly following tree removal?
What does desertification refer to and how does it occur?
once had a lush tree canopy, cut it down
led to higher rates of erosion
loss of A horizon
infiltration rate decreases at B horizon because of high bulk density, which means more runoff
more runoff means loss of groundwater and loss of soil support ability
How does forest clearcutting affect nutrient cycling processes?
litter and organic matter go up
soil temp increases
soil moisture goes up as there is no transpiration or canopy interception
increases microbial activity = lots of mineralization
nutrients build up in soil
if no plants to take minerals it will just be lost from system
in a fire cut system there would be less organic material, but mostly the same steps
How is nitrogen lost from a forest ecosystem following clearcutting?
nitrogen is leeched
denitrification as it's wetter
P would not have gaseous loss but still the leeching and erosion
Does nitrogen loss from northern hardwood forests (New England) respond similar to California coastal redwood forests following clearcutting?
less nitrate loss in redwood forest
What happens to streamflow following clearcutting of a northern hardwood forest (New England)?
increase in streamflow because of a decrease in evapotranspiration
when canopy is cut, the fog levels go down
How might the response of streamflow to clearcutting be different for Pacific Coastal forests?
How might soil physical properties be changed by forest harvest practices?
full cut: O horizon removed, E poke through
a lot more erosion happens
heavy traffic causes compaction and breaks down soil structure which breaks down macropores which decreases infiltration and increases runoff and erosion is rampant
Compared to an open-canopy forest, how is forest health affected by a closed-canopy structure?
huge competition between trees
water limitations weaken trees and make them more susceptible to trees
Which nutrients are most strongly affected by forest fires?
nitrogen is susceptible to volitalization
How is a nutrient loss affected by the severity (temperature) of a forest fire?
How are nutrients lost during and following a forest fire (by what processes)?