Rsum of factors limiting terrestrial productivity

The ultimate limit on the productivity of a community is determined by the amount of incident radiation that it receives - without this, no photosynthesis can occur.

the crucial importance of nutrient availability

Nov 20

S Oct 30

Oct 10

100 200 300 400

Mean annual precipitation (mm)

Nov 20

Oct 30

Oct 10

Sep 20

Mean annual temperature (°C)

May 20

Apr 30

Apr 10

100 200 300 400

Mean annual precipitation (mm)

May 20

Apr 30

Apr 10

J_I_I_I_I_I_I_i* i_I_I_I

Figure 17.13 (a) Start and (b) end dates of the growing season for Argentinian pampas communities described in Section 17.3.2. Circles represent sites along the precipitation gradient in the mountainous region and triangles represent sites along the lowland gradient. (After Jobbagy et al., 2002.)

Incident radiation is used inefficiently by all communities. The causes of this inefficiency can be traced to: (i) shortage of water restricting the rate of photosynthesis; (ii) shortage of essential mineral nutrients, which slows down the rate of production of photosynthetic tissue and its effectiveness in photosynthesis; (iii) temperatures that are lethal or too low for growth; (iv) an insufficient depth of soil; (v) incomplete canopy cover, so that much of the incident radiation lands on the ground instead of on foliage (this may be because of seasonality in leaf production and leaf shedding or because of defoliation by grazing animals, pests and diseases); and (vi) the low efficiency with which leaves photosynthesize - under ideal conditions, efficiencies of more than 10% (of PAR) are hard to achieve even in the most productive agricultural systems. However, most of the variation in primary productivity of world vegetation is due to factors (i) to (v), and relatively little is accounted for by intrinsic differences between the photosynthetic efficiencies of the leaves of the different species.

In the course of a year, the productivity of a community may (and probably usually will) be limited by a succession of the factors (i) to (v). In a grassland community, for instance, the primary productivity may be far below the theoretical maximum because the winters are too cold and light intensity is low, the summers are too dry, the rate of nitrogen mobilization is too slow, and for periods grazing animals may reduce the standing crop to a level at which much incident light falls on bare ground.

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