Variations in the relationship of productivity to biomass

We can relate the productivity of a community to the standing crop biomass that produces it (the interest rate on the capital). Alternatively, we can think of the standing crop as the biomass that is sustained by the productivity (the capital resource that is sustained by earnings). Overall, there is a dramatic difference in the total biomass that exists on land (800 Pg) compared to the oceans (2 Pg) and fresh water (< 0.1 Pg) (Geider et al., 2001). On an areal basis, biomass on land ranges from 0.2 to 200 kg m-2, in the oceans from less than 0.001 to 6 kg m-2 and in freshwater biomass is generally less than 0.1 kg m-2 (Geider et al., 2001). The average values of net primary productivity (NPP) and standing crop biomass (B) for a range of community types are plotted against each other in Figure 17.6. It is evident that a given value of NPP is produced by a smaller biomass when nonforest terrestrial systems are compared with forests, and the biomass involved is smaller still when aquatic systems are considered. Thus NPP : B ratios (kilograms of dry matter produced per year per kilogram of standing crop) average 0.042 for forests, 0.29 for other terrestrial systems and 17 for aquatic communities. The major reason for this is almost certainly that a large proportion of forest biomass is dead (and has been so for a long time) and also that much of the living support tissue is not photosynthetic. In grassland and scrub, a greater proportion of the biomass is alive and involved in photosynthesis, though half or more of the biomass may be roots. In aquatic communities, particularly where productivity is due mainly to phytoplankton, there is no support tissue, there is no need for roots to absorb water and nutrients, dead cells do not accumulate (they are usually eaten before they die) and the photosynthetic output per kilogram of biomass is thus very high indeed. Another factor that helps to account for high NPP : B ratios in phytoplankton communities is

NPP : B ratios are very low in forests and very high in aquatic communities


Open ocean


Swamp and marsh


Woodland and scrubland


Continental shelf


Tropical rainforest




Upwelling zone


Tropical seasonal forest


Temperate grassland


Algal beds and reefs


Temperate evergreen forest


Tundra and alpine




Temperate deciduous forest


Desert and semi-desert


Freshwater lakes


Boreal forest


Cultivated land

and streams

0.002 0.005 0.01 0.02 0.05 0.1 0.2 0.5 1 2 5 10 20 50

Figure 17.6 The relationship between average net primary productivity and average standing crop biomass for a range of ecosystems. (Based on data in Whittaker, 1975.)

the rapid turnover of biomass (turnover times of biomass in oceans and fresh waters average 0.02-0.06 years, compared to 120 years on land; Geider et al., 2001). The annual NPP shown in the figure is actually produced by a number of overlapping phytoplankton generations, while the standing crop biomass is only the average present at an instant.

Ratios of NPP to biomass tend to decrease during successions. This is because the early successional pioneers are rapidly growing herbaceous species with relatively little support tissue (see Section 16.6). Thus, early in the succession the NPP : B ratio is high. However, the species that come to dominate later are generally slow growing, but eventually achieve a large size and come to monopolize the supply of space and light. Their structure involves considerable investment in nonphotosynthesizing and dead support tissues, and as a consequence their NPP : B ratio is low.

When attention is focused on trees, a common pattern is for above-ground NPP to reach a peak early in succession and then gradually decline by as much as 76%, with a mean reduction of 34% (Table 17.3). The reductions are no doubt partly due to a shift from photosynthesizing to respiring tissues. In addition, nutrient limitation may become more significant later in the succession or the longer branches and taller stems of older trees may increase resistance to the transpiration stream and thus limit photosynthesis (Gower et al., 1996). Trees characteristic of different stages in succession show different patterns of NPP with stand age. In a subalpine coniferous forest, for example, the early successional whitebark pine (Pinus albicaulis) reached a peak above-ground NPP at about 250 years and then declined, whereas the late successional, shade-tolerant subalpine fir (Abies lasiocarpa) continued towards a maximum beyond 400 years (Figure 17.7). The late successional species allocated almost twice as much biomass to leaves as its early successional counterpart, and maintained a high photosynthesis : respiration ratio to a greater age (Callaway et al., 2000).

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