Latitudinal trends in productivity

the productivity of forests, grasslands and lakes follows a latitudinal pattern

In the forest biomes of the world a general latitudinal trend of increasing productivity can be seen from boreal, through temperate, to tropical conditions (Table 17.2). However, there is also considerable variation, much of it due to differences in water availability, local topography and associated variations in microclimate. The same latitudinal trend (and local variations) exists in the above-ground productivity of grassland communities (Figure 17.1). Note the considerable differences in the relative importance of above-ground and below-ground productivity in the different grassland biomes. It is technically difficult to estimate below-ground productivity and early reports of NPP often ignored or underestimated the true values. As far as aquatic communities are concerned, a latitudinal trend is clear in lakes (Brylinski & Mann, 1973) but not in the oceans, where productivity may more often be limited by a shortage of nutrients - very high productivity occurs in marine communities where there are upwellings of nutrient-rich waters, even at high latitudes and low temperatures.

Table 17.2 Gross primary productivity (GPP) of forests at various latitudes in Europe and North and South America, estimated as the sum of net ecosystem productivity and ecosystem respiration (calculated from CO2 fluxes measured in the forest canopies - only one estimate for tropical forest was included by the reviewers). (From data in Falge et al., 2002.)

Forest type

Range of GPP estimates (g C m-2 year1)

Mean of estimates (g C m-2 year

Table 17.2 Gross primary productivity (GPP) of forests at various latitudes in Europe and North and South America, estimated as the sum of net ecosystem productivity and ecosystem respiration (calculated from CO2 fluxes measured in the forest canopies - only one estimate for tropical forest was included by the reviewers). (From data in Falge et al., 2002.)

Forest type

Tropical rainforest

3249

3249

Temperate deciduous

1122-

-1507

1327

Temperate coniferous

992

-1924

1499

Cold temperate deciduous

903

-1165

1034

Boreal coniferous

723-

-1691

1019

Figure 17.1 (a) The location of 31 grassland study sites included in this analysis. (b) Above-ground net primary productivity (ANPP) and below-ground net primary productivity (BNPP) for five categories of grassland biomes (BNPP not available for temperate steppe). The values in each case are averages for 4-8 grassland studies. The technique involved summing increments in the biomass of live plants, standing dead matter and litter between successive samples in the study period (average 6 years). (From Scurlock et al., 2002.)

Figure 17.1 (a) The location of 31 grassland study sites included in this analysis. (b) Above-ground net primary productivity (ANPP) and below-ground net primary productivity (BNPP) for five categories of grassland biomes (BNPP not available for temperate steppe). The values in each case are averages for 4-8 grassland studies. The technique involved summing increments in the biomass of live plants, standing dead matter and litter between successive samples in the study period (average 6 years). (From Scurlock et al., 2002.)

The overall trends with latitude suggest that radiation (a resource) and temperature (a condition) may often limit the productivity of communities. But other factors frequently constrain productivity within even narrower limits.

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