Production

Net annual primary production (NPP) is lower in deserts than in most major biomes. However, when taking into account that deserts typically are also characterized by

Table 2 Phytomass and primary production of deserts in comparison to some other major biomes of the worlda

Plant formation

Phytomass of mature stands (thai

Net annual primary production (tha-1 yr-1

Relative primary production

Tropical forests

60-800

10-50

0.004-0.05

Deciduous forest

370-450

12-20

0.03-0.06

Boreal forest

60-400

2-20

0.03-0.05

Savanna

20-150

2-20

0.1-0.14

Temperate

20-50

1.5-15

0.08-0.3

grassland

Tundra

1-30

0.7-4

0.09-0.1

Deserts

1-4.5

0.5-1.5

0.33-0.5

aModified from Evenari M, Schulze E-D, Lange O, Kappen L, and Buschbom U (1976) Plant production in arid and semiarid areas. In: Lange OL, Kappen L, and Schulze E-D (eds.) Water and Plant Life - Problems and Modern Approaches, pp. 439-451: Berlin: Springer; and other sources.

aModified from Evenari M, Schulze E-D, Lange O, Kappen L, and Buschbom U (1976) Plant production in arid and semiarid areas. In: Lange OL, Kappen L, and Schulze E-D (eds.) Water and Plant Life - Problems and Modern Approaches, pp. 439-451: Berlin: Springer; and other sources.

low amounts of permanent plant mass (standing phytomass), relative primary production (the ratio of NPP/ standing phytomass) is among the highest worldwide (see Table 2). As rainfall fluctuates strongly within and between years, it is no wonder that there is a tremendous spatiotemporal variation in the amount of primary production. However, due to the lack of responsive vegetation structure and typically low levels of soil fertility, deserts are somewhat limited in their biological potential to react to extremely wet years. Semiarid grasslands, rich in very plastic perennial plant structures and therefore exhibiting high potential growth rates, show much larger fluctuations in response to changing water availability (Figure 13). Also water-use efficiency (NPP divided by annual water loss) in deserts is lower than it is in dry grasslands (0.1-0.3 g per 1000 g water in deserts compared to up to 0.7 g in dry grasslands and 1.8 g in forests).

During brief periods when water is available in excess, the typically short supply of nitrogen (and other plant macronutrients) is limiting. Even though nitrogen is limiting in almost all terrestrial ecosystems, deserts are typically more limited due to four reasons: (1) plant growth is triggered by available water faster than nutrients can be replenished by decomposition; (2) desert soils typically have little nutrient-holding capacities; (3) the nutrient-rich organic matter is located in the upper layers of soils, a layer that is typically too dry for root growth to occur, rendering the nutrients inaccessible; and (4) detritus and other organic material is deposited and accumulated unevenly across the desert surface. Plant debris typically accumulates passively under the canopy of shrubs or is concentrated in nests of animals such as harvester ants and termites. Thus the desert is an 'infertile sea' with interspersed islands of fertility.

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