Water and temperature as critical factors

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shortage of water may be a critical factor

The relationship between the NPP of a wide range of ecosystems on the Tibetan Plateau and both precipitation and temperature is illustrated in Figure 17.9. Water is an essential resource both as a constituent of cells and for photosynthesis. Large quantities of water are lost in transpiration - particularly because the stomata need to be open for much of the time for CO2 to enter. It is not surprising that the rainfall of a region is quite closely correlated with its productivity. In arid regions, there is an approximately linear increase in NPP with increase in precipitation, but in the more humid forest climates there is a plateau beyond which productivity does not continue to rise. Note that a large amount of precipitation is not necessarily equivalent to a large amount of water available for plants; all water in excess of field capacity will drain away if it can. A positive relationship between productivity and mean annual temperature can also be seen in Figure 17.9. However, the pattern can be expected to be complex because, for example, higher temperatures are associated with rapid water loss through evapotranspiration; water shortage may then become limiting more quickly.

To unravel the relationships between productivity, rainfall and temperature, it is more instructive to concentrate on a single ecosystem interaction of temperature and precipitation

Ecosystem Water Productivity Interaction

type. Above-ground NPP was estimated for a number of grassland sites along two west-to-east precipitation gradients in the Argentinian pampas. One of these gradients was in mountainous country and the other in the lowlands. Figure 17.10 shows the relationship between an index of above-ground NPP (ANPP) and precipitation and temperature for the two sets of sites. There are strong positive relationships between ANPP and precipitation but

Figure 17.9 Relationship between total net primary productivity (Mg dry matter ha-1 year-1) and annual precipitation and temperature for ecosystems on the Tibetan Plateau. The ecosystems include forests, woodlands, shrublands, grasslands and desert. (After Luo et al., 2002.)

the slopes of the relationships differed between the two environmental gradients (Figure 17.10a).

The relationships between ANPP and temperature are similar for two further environmental gradients (both north-to-south elevation transects) in Figure 17.10b - both show a hump-shaped pattern. This probably results from the overlap of two effects of increasing temperature: a positive effect on the length of the

Productivity Hump Shape Elevation

Figure 17.10 Annual above-ground net primary productivity (ANPP) of grasslands along two precipitation gradients in the Argentinian pampas. NPP is shown as an index based on satellite radiometric measurements with a known relationship to absorbed photosynthetically active radiation in plant canopies. (a) NPP in relation to annual precipitation. (b) NPP in relation to annual mean temperature. Open circles and diamonds represent sites along precipitation gradients in the lowland and mountainous regions respectively. Closed circles and triangles represent sites along two elevation transects. (After Jobbagy et al., 2002.)

Figure 17.10 Annual above-ground net primary productivity (ANPP) of grasslands along two precipitation gradients in the Argentinian pampas. NPP is shown as an index based on satellite radiometric measurements with a known relationship to absorbed photosynthetically active radiation in plant canopies. (a) NPP in relation to annual precipitation. (b) NPP in relation to annual mean temperature. Open circles and diamonds represent sites along precipitation gradients in the lowland and mountainous regions respectively. Closed circles and triangles represent sites along two elevation transects. (After Jobbagy et al., 2002.)

growing season and a negative effect through increased évapotranspiration at higher temperatures. Because temperature is the main constraint on productivity at the cool end of the gradients, an increase in NPP is observed as we move from the coolest to warmer sites. However, there is a temperature value above which the growing season does not lengthen and the dominating effect of increasing temperature is now to increase evapotranspiration, thus reducing water availability and curtailing NPP (Epstein et al., 1997).

Water shortage has direct effects productivity and on the rate of plant growth but also the structure of leads to the development of less dense the canopy vegetation. Vegetation that is sparse intercepts less light (much of which falls on bare ground). This wastage of solar radiation is the main cause of the low productivity in many arid areas, rather than the reduced photosynthetic rate of drought-affected plants. This point is made by comparing the productivity per unit weight of leafbiomass instead of per unit area of ground for the studies shown in Figure 17.8. Coniferous forest produced 1.64 g g-1 year-1, deciduous forest 2.22 g g-1 year-1 and desert 2.33 g g-1 year-1.

S 300

ra E

250 500 750

Growing season degree-years

1000

Figure 17.11 Above-ground biomass accumulation (a rough index of NPP) expressed as megagrams (= 106 g) per hectare in relation to accumulated growing season degree-days in broadleaf forest stands growing on sandy or nonsandy soils. o, nonsandy soils; •, sandy soils. (After Johnson et al., 2000.)

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Responses

  • hugo
    Why water and temperature are critical factors for NPP?
    4 years ago
  • travis
    Why water and temperature are critical factor for NPP?
    3 years ago

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