NPP & Decomposition

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Part 1: Estimating NPP and Decomposition from AE

Annual AE for Green Bay, Wisconsin is 504 mm for fine sand soil.

1. Using the Figure 11.1 graph, what is Green Bay’s NPP? ~800 g/m²/yr

2. Using equation 1, what is Green Bay’s NPP? 787 g/m²/yr

3. a. Complete the NPP column for Table 11.2 below. Use equation 1 and round your answer to the nearest whole number.

b. Use equation 2 to calculate the decomposition time to the nearest 0.01 year for each site.

TABLE 11.2 Selected North American Sites

# and Site	Location	Annual AE (mm)	Annual NPP (g/m²/yr)	Decomposition Rate (%/yr)	Decay Time (years)
70. Green Bay, WI	45N/88W	504	787	49	2.0
71. Dryden, Ont.	50N/93W	424	604	44	2.3
72. New Orleans, LA	30N/90W	1008	1942	77	1.3
73. Lamar, CO	39N/103W	374	490	41	2.4
74. Painted Desert	35N/111W	240	183	32	3.1

4. Some tropical rainforests have an annual AE of 1900 mm.

What would be their estimated NPP? 3984 g/m²/yr

5. Some deserts have an annual AE of only 80 mm.

What would be their estimated NPP? - 184 g/m²/yr

6. Some polar areas have an annual AE of 150 mm.

What would be their estimated NPP? - 23 g/m²/yr

7. The AE:NPP relationship in Figure 11.1 and equation 1 derives from direct measurements. Negative NPP values mean that a net loss of vegetation should normally occur every year. If so, no vegetation should exist at all after only a few years, yet there is vegetation in most deserts and polar areas. How do you explain the apparent mis-prediction? (Hint - where was the source of Figure 11.1’s data?) The equation derives from a subtropical swamp, not desert or tundra. Desert and tundra plants have their own adaptations to survive and have some productivity, except in truly barren areas such as active dunes or icecaps. Local variability, such as in highlands, or human modifications, such as irrigation agriculture, also would render the equation less accurate.

8. a. At which Table 11.2 site should a kilogram of leaf litter decompose most rapidly? New Orleans

b. Why? Higher heat and humidy promote chemical reactions, including microscopic growth and the resultant decomposition

9. Although decomposition values in Table 11.2 might suggest otherwise, Dryden Ontario experiences the most rapid accumulation of fuel load. Why? (Hint: consult a vegetation map.) Dryden is in subarctic needleleaf forest, which produces much leaf litter. Being cold most of the year, decomposition is relatively slow, so the litter accumulates.

10. Of the five sites in Table 11.2, where should soil nutrients most accumulate due to lack of plant consumption of them? Petrified Forest; the lower productivity of the desert indicates fewer, and more widely spaced, plants. The aridity also slows growth rates, and prevents leaching of soluable nutrients. Fewer competitors, slower uptake, and lack of hydrologic removal would result in greater accumulation.

11. Considering the predictor variable AE, name two climatic conditions that seem to inhibit decomposition, and indicate an example location (e.g. Amazon rainforest) of where each inhibition factor is likely to prevail.

Condition Aridity Location southwest USA deserts; Great Plains grasslands

Condition Cold Location Alaska/Canada tundra; Rocky Mountain summits

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part 2: mapping & map interpretation of npp & decomposition

1. NPP values from Table 11.1 are plotted on the North America map in Figure 11.3.

a. Add the five NPP values you calculated in Table 11.2 to the map blanks.

b. Draw NPP isolines for 0, 250, 500, 750, 1000, 1500, and 2000 g/m²/yr on the North America map. (Refer to Appendix B for information on drawing isolines.)

2. a. What NPP value corresponds with a 2.5-year decay time (40% decay rate)? 450

b. In a distinct color, draw approximate boundaries separating the more-than from less-than 2.5-year decay time area on the map.

3. a. In about what fraction (e.g., ½, ¹/₁₀, etc.) of North America does one full year's dead organic materials decompose in 2.5 years or longer (i.e., the decomposition rate is slow)? about 60%, including Alaska

b. Where do these areas occur? Alaska/Canada tundra; southwest USA deserts

c. What climate conditions (temperature and precipitation) from Figure 11.4 occur there? Cold and/or arid

d. Compare these areas with a vegetation map in an atlas. What kinds of vegetation correspond with slow decomposition? Tundra, desert, some subarctic needleleaf forest, schlerophyl woodland

e. Describe how these places seem to correspond to areas on the potential wildfire map (Figure 11.4). The desert areas match well, but the tundra areas do not.

4. Your isoline map in Figure 11.3 should depict a large vertical band in western North America that has less than 500 g/m²/yr. Compare this area with a vegetation map in an atlas. What biome(s) or formation group(s) coincide with this low-production zone:

a. north of about 55°N? subarctic needleleaf forest and tundra

b. between about 40°N and 55°N? desert and interior mountains

c. between about 25°N and 40°N? deserts

5. a. Compare your NPP map in Figure 11.3 with a vegetation map in an atlas. What would be good approximate cutoff values for:

Biome/Formation Group	Annual NPP (g/m²/yr)	Decompostion Rate (%/yr)
Tundra	200 g/m²/yr	32%
Deciduous forest	800 g/m²/yr	50%
Desert	300 g/m²/yr	35%

b. Tundra and desert latitudes are quite different, but their NPP and decomposition rates are similar. What climatic conditions account for this similarity?

Aridity: lack of moisture inhibits chemical reactions, including growth and decomposition

Cold: lack of effective moisture slows growth and decomposition, and shortens growing season

6. a. The NPP isolines in some areas on your Figure 11.3 map should pack quite closely together, indicating a sharp change of growing conditions within a short distance. For example, notice the drastic change of the NPP index between La Paz and Palos Blancos, Mexico, over a distance comparable with Washington DC to New York City. Two other areas with high production gradients are listed below. Determine their separating distances and NPP differences to indicate how sharply growing conditions change. Use a physiographic map (Figure 11.5) to identify landscape features that seem to coincide with these steep NPP gradients.

From	To	Kilometers	NPP Difference	Physical Feature
La Paz	Palos Blancos	500	1030	salt-water gulf
Eureka	Wendover	800	858	Sierra rainshadow
Corpus Christi	El Paso	1000	1126	Coast to desert interior

Figure 11.3 Estimated North American NPP (g/m²/yr) [at left]


Estimated North American NPP (g/m²/yr)	Decomposition Rate (40% = 2.5 year decay time)

b. Why should these types of features exhibit steep production gradients? The coast-to-interior transect (Texas) has a steep moisture gradient. The salt-water gulf (Mexico) has steep altitude changes. The rainshadow (California) has steep gradients of both moisture and altitude.

7. a. Consult your maps, Table 11.1, and use your knowledge of PE and AE to interpret the data below. Pick a likely North American place (town, state, region, etc.) where you might expect each of the following sets of annual conditions to occur.

Approxi-mate PE	Approxi-mate AE	Approxi-mate NPP	Decay Time	Vegetation	Possible North American Place
550 mm	400 mm	500 gm	2.4 yr	grassland	eastern Colorado, western Nebraska
775 mm	775 mm	1400gm	1.5 yr	mixed forest	Inland Georgia, Alabama, Carolinas
1275 mm	180 mm	125 gm	3.8 yr	desert	Sonoran desert
250 mm	250 mm	200 gm	3.1 yr	tundra	northern Alaska, Canada
550 mm	500 mm	775 gm	2.0 yr	evergreen forest	Pacific Northwest
650 mm	650 mm	1125 gm	1.7 yr	[NcH1] deciduous forest	Ohio River Valley-DelMarVa

b. What sequence of vegetation should you expect to cross if you travel straight from Corpus Christi to El Paso? What probably accounts for this vegetation transition? Forest, Grassland, Desert

c. What sequence of vegetation should you expect to cross if you travel straight from Eureka to Wendover? What probably accounts for this vegetation transition? Forest, Tundra, Desert

Sonoran desert

Pacific Northwest