Production
Answers to Activity 2 |
Note: Answers to the simulations are hot linked below.
Activity 2 Nutrient
Cycling Simulation Model
By manipulating the nutrient cycling model, students should gain insight
into a variety of human interactions with the environment. Three
distinct (and in some respects extreme) environments are chosen.
Below are links to the summary tables of each simulation and the respective
embedded graphs for:
-
an original selva
-
a conversion of selva to grass for grazing
-
selva occasionally and partially cut and burned for swidden agriculture
-
an original steppe
-
a conversion of steppe to continuous corn production with fertilizer added
-
a conversion of steppe to a four-year crop rotation
-
an original tundra and
-
a tundra altered by global warming that thaws the permafrost, but does
not allow for new species to invade yet.
The model values used to generate the graphs are contained in the summary
spreadsheets. The values for year 270 are summarized in the Year 270
Summary table below.
Where computer facilities are not available for the demonstration or
the hands-on use of the NUTCY4 model, the outputs for each
of the scenarios can be used instead. Instructors may make print-outs of
the table containing the initial conditions in each biome, run the simulation
for each to obtain the associated graphs for each biome, and then provide
students with these and the tables of the human-altered cases to use as
a basis to answer the questions. For the Selva3 case it may be preferable
to use the line graph rather than the histogram because the impact is more
obvious there.
Notes on the simulations
Students should have taken notes like the following while running the
simulation. They form the basis for their final assessment (summary paper).
Selva
Litter storage rates stabilize after a couple of years, soil storage
rates stabilize after 15-20 years, and biomass storage values stabilize
after about 120 years. The first rate reflects the time it takes under
tropical climate conditions for litter to be produced, decomposed, and
taken up via the soil. The time it takes for the nutrient storage in soils
to stabilize reflects the time it takes to establish a dynamic equilibrium
of edaphic (soil) processes, including physical processes, biochemical
processes, and the establishment of a viable soil fauna and flora in the
face of fast-growing biomass. Finally, nutrient stabilization in biomass
takes the longest because tropical tree species need this amount of time
to reach full maturity and for the ecosystem to establish itself as a mature,
self-organizing web of physical and biotic interactions in dynamic equilibrium.
Students should be able to predict that because most of the nutrients
are stored in the biomass, and not in the soil, that in the case of a large-scale
deforestation, most of the fertility of this biome would be lost. (You
may want to compare this to a temperate forest situation where nutrients
are largely stored in the soil.)
Selva2
This simulation shows what students predicted for a deforestation
of Selva. The invoked change results in a severe drop in biomass
and a drop in uptake rates because the grasses do not extract as much water
and nutrients from the soil as do large trees. Similarly, the standing
biomass is almost completely lost to fallout on an annual basis. This is
why the fallout rate is more than 99%. It reflects the character of a grassland
where grazing and leaf decomposition take away most of the standing biomass.
Selva3
Because the soil depletion is not total and a plot is commonly only
a few tens of meters across, which facilitates recolonization by neighboring
species, the natural recovery after a slash-and-burn and farming episode
is fairly rapid. In comparison to the Selva2 situation where the
biomass is removed completely and continuously, biomass nutrient storage
is able at least to approach pre-cutting values. But the simulation of
repeated slash-and-burn cycles shows that biomass storage doesn't quite
reach pre-cutting levels. A shortening of the fallow-period would result
in an ever smaller nutrient recovery, i.e., an eventual loss of fertility.
Steppe
Soil storage of nutrients is highest in this biome, with litter and
biomass falling vastly behind. The storage differences reflect both the
relatively dry climate, which doesn't allow much soil leaching and slows
down litter decomposition, and the biomass that in and of itself isn't
able to store large amounts of nutrients, but which produces biomass year
after year and then accumulates as litter and eventually as a nutrient
reservoir in these most fertile soils. Students should be able to conclude
that cultivation of a steppe soil would result in a quick nutrient loss
whose speed is determined by the uptake of nutrients by grain, the increase
of leaching and erosion, and the lack of supply of litter for decomposition,
hence replenishment of soil fertility.
Steppe2
The cultivation of corn affects the steppe in several ways: as predicted
at the end of the Steppe simulation, there is a quick loss of nutrient
storage from the soil. The initial boost in soil storage is simply the
result of applying water (irrigation) and fertilizer and plowing the soil,
all of which combine to accelerate the decomposition process. The initial
boost in litter storage reflects the plowing under of grasses. Both erosion
and leaching increase with a more open soil (corn is renowned for its accompanying
erosion because it covers the ground so incompletely), but are still much
less than in the humid tropical climates (much less rainfall). Fallout
rates close to 100% reflect the annual harvesting of corn (total removal
of biomass), and the increase in the uptake rate shows that corn needs
more nutrients than grasses.
Steppe3
The pre-1950s cultivation and crop rotation shares some of the characteristics
described above but generally seems to be more conserving of soil fertility.
Litter and biomass storage values hardly differ after 270 years of this
type of cultivation, but the soil fertility has declined, if more slowly
than in Steppe2. Because average soil coverage is higher, leaching
and runoff are less than in Steppe2. Similarly, biomass uptake is
an average over a four-year crop rotation that includes lower-yield (less
nutrient-intensive) crops. Finally, the return of some nutrients via manuring
also slows down the loss of nutrients from the soil.
Tundra
In this biome, litter stores most of the nutrients, with soil storing
much less and only little more than biomass. Plants, as mentioned on the
student worksheet, grow slowly and generally are small and low to the ground.
Soils are mostly frozen, and generally thin and poor in nutrients, allowing
soil processes to proceed only slowly. Litter decomposition is restricted
by temperatures and hard plant material, which explains the relatively
large accumulation of nutrients in litter. The most important effect of
climate change for the tundra may be the increase in temperatures which
will enhance plant growth, evapotranspiration, litter decomposition, and
nutrient leaching from the thawing ground. Thus, students could predict
an increase of nutrient storage in biomass along with a large loss of nutrients
from the soil. Because litter decomposes faster, the storage there will
decrease as well.
Tundra2
Interestingly, the changes predicted at the end of Tundra hold
only for the first few years. Over time, litter storage of nutrients and
biomass accumulation all increase, if slowly. This can be explained by
a general amelioration of the growing conditions for plants given higher
temperatures. Soil processes accelerate, leading to more fertile and better
drained soils. Plant growth is stimulated by increased soil fertility and
a longer growing season (light conditions stay the same, but frost leaves
sooner and comes later in the year). And the increase in biomass translates
to larger litter amounts. In short, assuming current species can adapt
rapidly enough (as this model does), global warming is expected to increase
the productivity of the tundra biome.
In the short essay that concludes this activity, students should summarize
and discuss these observations and explanations. Check their essays for:
-
a critical understanding of human interactions with the natural environment
-
the ability to think in systems terms, i.e., the ability to see how changes
in one compartment of the ecosystem affect other compartments and processes
-
a good understanding of the differences among the three biomes discussed
-
the ability to abstract from the numbers they simulated to the larger underlying
principles and processes that bring about the simulation results.
Table 2: Year 270 Summary (Results)
|
|
Litter |
Soil |
Biomass |
Runoff |
Leach |
Removal |
|
Selva |
18 |
16 |
270 |
5.80 |
6.20 |
0 |
|
Selva2 |
17 |
18 |
6 |
7.58 |
4.42 |
0 |
|
Selva3 |
17 |
15 |
213 |
5.74 |
6.15 |
0 |
|
Steppe |
17 |
207 |
10 |
0.85 |
4.15 |
0 |
|
Steppe2 |
32 |
130 |
26 |
3.18 |
6.49 |
0.3 |
|
Steppe3 |
11 |
136 |
7 |
0.77 |
4.07 |
0.2 |
|
Tundra |
60 |
6 |
2 |
3.00 |
0.00 |
0 |
|
Tundra2 |
92 |
72 |
38 |
1.85 |
2.15 |
0 |
The following pages are print-outs of the year-270 summary graphs that
students should produce in this activity.