Factors limiting primary productivity in terrestrial communities

Sunlight, carbon dioxide (CO2), water and soil nutrients are the resources required for primary production on land, while temperature, a condition, has a strong influence on the rate

NPP : B ratios tend to decrease during successions

Table 17.3 Above-ground net primary productivity (ANPP) for forest age sequences in contrasting biomes. (After Cower et al., 1996.)

Biome/species

Location

Range of stand ages, in years (no. of stands shown in brackets)

Peak

Oldest

% change

Boreal Larix gmelinii Picea abies

Cold temperate Abies baisamea Pinus contorta Pinus densiflora Populus tremuloides Populus grandidentata Pseudotsuga menziesii

Warm temperate Pinus elliottii Pinus radiata

Tropical Pinus caribaea Pinus kesiya Tropical rainforest

Yakutsk, Siberia Russia

New York, USA Colorado, USA Mt Mino, Japan Wisconsin, USA Michigan, USA Washington, USA

Afaka, Nigeria Meghalaya, India Amazonia

0-60 (6) 40-245 (3) 16-390 (7) 8-83 (5) 10 -70 22-73 (4)

31.1

19.2

30.1

13.2

Forest Different Ages
Figure 17.7 Annual above-ground net primary productivity (ANPP) (Mg dry matter ha-1 year-1) in stands of different ages in a subalpine coniferous forest in Montana, USA: early successional whitebark pine, late successional subalpine fir, and total ANPP. (After Callaway et al., 2000.)

of photosynthesis. CO2 is normally present at a level of around 0.03% of atmospheric gases. Turbulent mixing and diffusion prevent the CO2 concentration from varying much from place to place, except in the immediate neighborhood of a leaf, and

CO2 probably plays little role in determining differences between the productivities of different communities (although global increases in CO2 concentration are expected to have profound effects (e.g. DeLucia et al., 1999). On the other hand, the quality and quantity of light, the availability of water and nutrients, and temperature all vary dramatically from place to place. They are all candidates for the role of limiting factor. Which of them actually sets the limit to primary productivity?

17.3.1 Inefficient use of solar energy

terrestrial communities use radiation inefficiently

Depending on location, something between 0 and 5 joules of solar energy strikes each square meter of the earth's surface every minute. If all this were converted by photosynthesis to plant biomass (that is, if photo-synthetic efficiency were 100%) there would be a prodigious generation of plant material, one or two orders of magnitude greater than recorded values. However, much of this solar energy is unavailable for use by plants. In particular, only about 44% of incident shortwave radiation occurs at wavelengths suitable for photosynthesis. Even when this is taken into account, though, productivity still falls well below the maximum possible. Photosynthetic efficiency has two components - the efficiency with which light is intercepted by leaves and the efficiency with which intercepted light is converted by photosynthesis to new biomass (Stenberg et al., 2001). Figure 17.8 shows the range in overall net photosynthetic efficiencies (percentage of incoming photosyn-thetically active radiation (PAR) incorporated into above-ground NPP) in seven coniferous forests, seven deciduous forests and eight desert communities studied as part of the International Biological Programme (see Section 17.1). The conifer communities had the highest efficiencies, but these were only between 1 and 3%. For a similar level of incoming radiation, deciduous forests achieved 0.5-1%, and, despite their greater energy income, deserts were able to convert only 0.01-0.2% of PAR to biomass.

However, the fact that radiation is not used efficiently does not in itself imply that it does not limit community productivity. We would need to know whether at increased intensities of radiation the productivity increased or remained unchanged. Some of the evidence given in Chapter 3 shows that the intensity of light during part of the day is below the optimum for canopy photosynthesis. Moreover, at peak light intensities, most canopies still have their lower leaves in relative gloom, and would almost certainly photosynthesize faster if the light intensity were higher. For C4 plants a saturating intensity of radiation never seems to be reached, and the implication is that productivity may in fact be limited by a shortage of PAR even under the brightest natural radiation.

There is no doubt, however, that what radiation is available would be used more efficiently if other resources were in abundant supply. The much higher values of community productivity recorded from agricultural systems bear witness to this.

productivity may still be limited by a shortage of PAR

0.02

0.01

0.02

CO

C

C C C

D D D

D D

C

Conifer forest

D

Deciduous forest

De

Desert

De nDe De De De

1,000,000

2,000,000

3,000,000 4,000,000

Photosynthetically active radiation reaching the community (kJ m-2 yr-1)

Figure 17.8 Photosynthetic efficiency (percentage of incoming photosynthetically active radiation converted to above-ground net primary productivity) for three sets of terrestrial communities in the USA. (After Webb et al., 1983.)

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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Responses

  • jasper
    What factors limit productivity in an ecosystem?
    1 year ago
  • florian
    How light affect the productivity of terrestial biomes?
    10 months ago

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