Week 6 - 01 Oct. 2013 Productivity continued, forest food webs

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.