Differences among biomes in NPP reflect climate and biotic variation
It is not surprising that NPP varies among biomes, since biomes are associated with latitudinal climate variation. For example, the high NPP in the tropics is associated with tropical forests, grasslands, and savannas.
The low NPP at high latitudes is associated with boreal forests and tundra. Tropical forests and savannas contribute approximately 60% of terrestrial NPP and around 37% of global NPP (TABLE 20.1). In the oceans, zones of upwelling have high rates of NPP, but they cover less than 5% of the ocean surface. Although they cover less area than the open ocean, shallow oceans account for almost half of oceanic NPP. Despite its low rate of NPP, the vast area of the open ocean accounts for the majority of oceanic NPP and approximately 40% of total global NPP.TABLE 20.1 Variation in NPP among Terrestrial Biomes and Oceanic Provinces
| NPP9(g C∕m2∕yr) | Total NPP (Pg C∕yr) | Percentage of global NPP | |
| Biome | |||
| 2,500 | 21.9 | 22.7 | |
| Tropical savanna | 1,080 | 14.9 | 15.4 |
| Temperate forest | 1,550 | 8.1 | 8.4 |
| Temperate grassland | 750 | 5.6 | 5.8 |
| Boreal forest | 390 | 2.6 | 2.7 |
| Temperate shrubland | 500 | 1.4 | 1.4 |
| Tundra | 180 | 0.5 | 0.5 |
| Desert | 250 | 3.5 | 3.6 |
| Crops | 610 | 4.1 | 4.2 |
| Total terrestrial | 62.6 | 64.8 | |
| Oceanic Province | |||
| Oligotrophic (e.g., open ocean) | 91 | 14.5 | 15.0 |
| Mesotrophic (e.g., shallow ocean) | 132 | 15.7 | 16.3 |
| Eutrophic (e.g., upwelling zone, coral reef) | 422 | 2.8 | 2.9 |
| Macrophytes (e.g., kelp beds, sea grass) | 1,500 | 1.0 | 1.0 |
| Total oceanic | 34.0 | 35.2 |
Source: B. Saugier et al.
2001. In Terrestrial Global Productivity, J. Roy et al. (Eds.), pp. 543-557. Academic Press: San Diego, CA.
As noted in Concept 20.1, much of the variation in NPP among terrestrial biomes is associated with differences in leaf area index. Similarly, the complex structure of ocean macrophyte communities such as kelp beds accounts for their high rate of NPP (see Table 20.1) In addition, the length of the growing season varies markedly among terrestrial biomes, from year-round in some tropical ecosystems to 100 days or less in tundra. Variation associated with different plant growth forms (e.g., grasses vs. shrubs vs. trees) is also important but contributes less to variation among biomes than do growing season and leaf area index. Variation in NPP among aquatic ecosystems, as we saw in Concept 20.2, is primarily related to variation in inputs of nutrients.
What happens to all of this NPP? In the next section, we will introduce some of the concepts associated with secondary production. We will cover energy flow among organisms and its consequences for population growth, community dynamics, and ecosystem function in Chapter 21.
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