Geog. 143 -- Global Environmental Change

Exercise 5:  Global biomes

The terrestrial biosphere is one of the major components of the climate system and of the carbon cycle, and is likely to respond significantly to global climate change, with the potential for both damping or amplifying feedback.  The aim of the exercise is to examine the moisture and energy controls of the distribution of some of the major global biomes.

This analysis involves the examination of maps of global "potential" vegetation (or the vegetation that would grow in a region under present-day climates, but without human modification of land cover.  The maps examined here come from the Atlas of the Biosphere provide by the Center for Sustainability and the Global Environment at the University of Wisconsin -- Madison (http://www.sage.wisc.edu/atlas/).

The maps necessary for answering the questions below have been downloaded and combined into a .pdf document, which can be downloaded from the following link:

http://geography.uoregon.edu/bartlein/courses/geog143/labs/UW_SAGE-atlas.pdf

It will be better to download the .pdf file and open it with Acrobat Reader than to open it directly in a browser.  You can do this by right-clicking on the link, and then saving the file locally.

(Poor man's GIS):  In the full versions of Acrobat Reader, it's possible to split the window to see two pages from the same document at the same time, which will make it easier to compare maps.  In the free version of Acrobat Reader, much the same thing can be accomplished by opening a second copy (which must have a different name) of the file.  Here's a link to a duplicate copy of the .pdf file:

http://geography.uoregon.edu/bartlein/courses/geog143/labs/UW-SAGE-atlas-copy.pdf

Then different maps from the .pdf file can be examined side by side using the Window > Cascade, or Window > Tile menu commands in Acrobat Reader.

1.  Moisture and energy controls of biomes

There are four moisture-related variables mapped in the atlas:  1) annual total precipitation (P), 2) annual evapotranspiration (ET), 3) soil moisture storage (SM), and 4) runoff (RO).  The variables are related to one another as follows:

P = ET + RO + SM

which is another way of saying that annual precipitation is partitioned into moisture that can be lost by evapotranspiration, lost through runoff into rivers or streams, or stored in the soil.  This equation can be arranged several ways to describe the controls of the different variables, for example:

SM = P - ET - RO (soil moisture storage is what remains after precipitation
                               evaporates or runs off), etc.

There are two energy-related variables included:  annual average temperature, and annual growing degree-days above a 5ºC base (the sum over the year of the number of degrees each day's average temperature is above 5ºC), a measure of the growing-season length and warmth.

Note:  Ignore Greenland and Antarctica when you examine the maps.

Q1:  This is extremely subjective, but which of the moisture variables has the greatest general resemblance to the overall pattern of potential vegetation?  Which of the two energy variables?

2.  Grasslands, shrublands, and deserts

Examine the map pattern of grasslands, deserts and shrublands (dry vegetation types), and compare it to the soil-moisture map.

Q2:  Is there a distinct soil-moisture level that determines where in "climate space" these dry biome-types occur?  Do all locations that are relative dry necessarily feature these biome types? If not, why not?

3.  Taiga (boreal forest) and tundra

Examine the map patterns of the boreal evergreen forest/woodland, boreal deciduous forest/woodland and evergreen/deciduous mixed forest biomes.

Q3:  What are the main climatic constraints on the occurrence of these biomes?  Why do you think that boreal deciduous forest (i.e. larch forests) are more prevalent in eastern Eurasia than in North America?

4.  Tropical evergreen forest/woodland (tropical rain forest)

Compare the map pattern of this biome with both the moisture and energy controls.

Q4:  Is there a compound or multiple-variable (i.e. first one constraint, then another) control of the distribution of this biome or is its distribution related to a single control?  Describe the control(s).