Oct. 8 Movie 'Avatar'
Oct. 15 Class cancelled
Oct. 22 Mid-term exam
Oct. 29 Draft paper due
Nov. 19 Papers due
Dec. 3 Class presentations
Review productivity.
Are we measuring productivity
at a single point in time, or production over time?
Production – rate of formation of new material over
time. Reported as calories/m2/yr
(energy) or grams/m2/year (dry organic matter)
Standing crop – amount of biomass at a single point in time.
Cal/m2 or gm/m2
Need to know these measurements so that we can decide how to manage a
forest.
We calculated standing crop:
1.
Determine the size of the forest you are
measuring.
2.
Count the number of trees in the forest
3.
Convert the number to density of trees per meter
squared
Then to determine production, count the trees one year later, and
determine the difference between standing stock in years 1 and 2.
Forest A =
1000 m2. Year 1 it has 1000 trees.
Density is 1 tree/m2. This is
standing crop.
Year 2 it
has 1500 trees. Density is 1.5 trees/m2.
Year 2 minus year 1 = 0.5 trees/m2.
Thus production is 0.5 trees/m2/year
Forest B =
100 m2. Year 1 it has 200 trees. Density or standing crop is 2 trees/m2.
Should
we harvest from Forest B instead of Forest A?
Look at production before deciding.
Year 2 it
has 200 trees. Density or standing crop
is 2 trees/m2.
Year 2 minus year 1 = 0 trees/m2.
Thus production is 0 trees/m2/year
Perhaps
Forest B cannot sustain harvest! It has
no production.
Which should a forest manager look at – standing crop or production
over time? If he wants the greatest
yield of trees from a plot this year, then look at standing crop, and
harvest trees when it is highest. SHORT
TERM INVESTMENT.
If he wants to continue to harvest trees over time, he needs to
look at rate of biomass production. Also,
don’t harvest more than the forest can produce.
In fact, harvest less?
Do the tropics support good
agriculture?
Tropics have thin soils, below
that weathered rocks – devoid of soluble material – high temperature and
rainfall washed it all away. Only 20% of
humid tropics has soils that can support agriculture!
Tropics have high temperature and moist conditions year round, thus
decomposition takes place quickly, and trees quickly absorb the nutrients.
Thus the soils are poor in
nutrients and organic matter!
Temperate forests are in colder, drier climates, thus decomposition is
slower, and organic matter builds up in the soil.
Destroy a tropical forest is the same as removing the nutrients. The soil doesn’t have much nutrients. Yields of crops planted there will decrease
every year.
And to grow back a forest as complex and diverse as the original
primary tropical forest will take thousands of years.
Do the tropical soils support
good agriculture? NO!
But tropics do have warm, rainy weather that is good for growing crops
all year.
Forest food webs = a diagram of the feeding relationships in an
ecosystem. It shows the flow of
metabolic energy.
Decomposers – invertebrate animals (worms, insects, etc.) and
microbes (bacteria, etc.) that feed on dead tissue.
In general - Decomposers --> nutrients for are released --> plants --> herbivores --> carnivores
90% of energy is lost in each step up the tropic ladder.
Each level supports 10% as much of the biomass below it.
Biomass of primary producers is the greatest, then decreases as
you move up the food web.
It takes 1000 ha
(3.75 square miles) of temperate evergreen forest to support one pair of
spotted owls.
