Carbon dioxide

the rise in global levels

The CO2 used in photosynthesis is obtained almost entirely from the atmosphere, where its concentration has risen from approximately 280 ||l l-1 in 1750 to about 370 ||l l-1 today and is still increasing by 0.4-0.5% year-1 (see Figure 18.22). In a terrestrial community, the flux of CO2 at night is upwards, from the soil and vegetation to the atmosphere; on sunny days above a photosynthesizing canopy, there is a downward flux.

Above a vegetation canopy, the air variations beneath a becomes rapidly mixed. However, the canopy situation is quite different within and beneath canopies. Changes in CO2 concentration in the air within a mixed deciduous forest in New England were measured at various heights above ground level during the year (Figure 3.11a) (Bazzaz & Williams, 1991). Highest concentrations, up to around 1800 |ll l-1, were measured near the surface of the ground, tapering off to around 400 |ll l-1 at 1m above the ground. These high values near ground level were achieved in the summer when high temperatures allowed the rapid decomposition of litter and soil organic matter. At greater heights within the forest, the CO2 concentrations scarcely ever (even in winter) reached the value of 370 |ll l-1 which is the atmospheric concentration of bulk air measured at the Mauna Loa laboratory in Hawaii (see Figure 18.22). During the winter months, concentrations remained virtually constant through the day and night at all heights. But in the summer, major diurnal cycles of concentration developed that reflected the interaction between the production of CO2 by decomposition and its consumption in photosynthesis (Figure 3.11b).

That CO2 concentrations vary so widely within vegetation means that plants growing in different parts of a forest will experience quite different CO2 environments. Indeed the lower leaves on a forest shrub will usually experience higher CO2 concentrations than its upper leaves, and seedlings will live in environments richer in CO2 than mature trees.

In aquatic environments, variations in CO2 concentration can be just as variations in aquatic striking, especially when water mixing habitats... is limited, for example during the summer 'stratification' of lakes, with layers of warm water towards the surface and colder layers beneath (Figure 3.12).

Figure 3.11 (a) CO2 concentrations in a mixed deciduous forest (Harvard Forest, Massachusetts, USA) at various times of year at five heights above ground: A, 0.05 m; □, 0.20 m; ■, 3.00 m; o, 6.00 m; •, 12.00 m. Data from the Mauna Loa CO2 observatory (A) are given on the same axis for comparison. (b) CO2 concentrations for each hour of the day (averaged over 3-7-day periods) on November 21 and July 4. (After Bazzaz & Williams, 1991.)

Figure 3.11 (a) CO2 concentrations in a mixed deciduous forest (Harvard Forest, Massachusetts, USA) at various times of year at five heights above ground: A, 0.05 m; □, 0.20 m; ■, 3.00 m; o, 6.00 m; •, 12.00 m. Data from the Mauna Loa CO2 observatory (A) are given on the same axis for comparison. (b) CO2 concentrations for each hour of the day (averaged over 3-7-day periods) on November 21 and July 4. (After Bazzaz & Williams, 1991.)

Figure 3.12 Variation in CO2 concentration with depth in Lake Crane Langs0, Denmark in early July and again in late August after the lake becomes stratified with little mixing between the warm water at the surface and the colder water beneath. (After Riis & Sand-Jensen, 1997.)

Figure 3.13 The increase (to a plateau) in photosynthetic rate with artificially manipulated CO2 concentrations in moss, Sphagnum subsecundum, taken from depths of 9.5 m (•) and 0.7 m (o) in Lake Crane Langs0, Denmark, in early July. These concentrations - and hence the rates of photosynthesis - are much higher than those occurring naturally (see Figure 3.12). (After Riis & Sand-Jensen, 1997.)

Figure 3.12 Variation in CO2 concentration with depth in Lake Crane Langs0, Denmark in early July and again in late August after the lake becomes stratified with little mixing between the warm water at the surface and the colder water beneath. (After Riis & Sand-Jensen, 1997.)

... setting a limit on photosynthetic rates

Also, in aquatic habitats, dissolved CO2 tends to react with water to form carbonic acid, which in turn ionizes, and these tendencies increase with pH, such that 50% or more of inorganic carbon in water may be in the form of bicarbonate ions. Many aquatic plants can utilize carbon in this form, but since it must ultimately be reconverted to CO2 for photosynthesis, this is likely to be less useful as a source of inorganic carbon, and in practice, many plants will be limited in their photosynthetic rate by the availability of CO2. Figure 3.13, for example, shows the response of the moss, Sphagnum subse-cundum, taken from two depths in a Danish lake, to increases in CO2 concentration. At the time they were sampled (July 1995), the natural concentrations in the waters from which they were taken (Figure 3.12) were 5-10 times less than those eliciting maximum rates of photosynthesis. Even the much higher concentrations that occurred at the lower depths during summer stratification would not have maximized photosynthetic rate.

One might expect a process as fundamental to life on earth as carbon fixation in photosynthesis to be underpinned by a single unique biochemical pathway. In fact, there are three such pathways (and variants within them): the C3 pathway (the most common), the C4 pathway and CAM (crassulacean acid metabolism). The

Figure 3.13 The increase (to a plateau) in photosynthetic rate with artificially manipulated CO2 concentrations in moss, Sphagnum subsecundum, taken from depths of 9.5 m (•) and 0.7 m (o) in Lake Crane Langs0, Denmark, in early July. These concentrations - and hence the rates of photosynthesis - are much higher than those occurring naturally (see Figure 3.12). (After Riis & Sand-Jensen, 1997.)

ecological consequences of the different pathways are profound, especially as they affect the reconciliation of photosynthetic activity and controlled water loss (see Section 3.2.4). Even in aquatic plants, where water conservation is not normally an issue, and most plants use the C3 pathway, there are many CO2-concentrating mechanisms that serve to enhance the effectiveness of CO2 utilization (Badger et al., 1997).

Was this article helpful?

0 0
Lawn Care

Lawn Care

The Secret of A Great Lawn Without Needing a Professional You Can Do It And I Can Show You How! A Great Looking Lawn Doesnt Have To Cost Hundreds Of Dollars Or Require The Use Of A Professional Lawn Care Service. All You Need Is This Incredible Book!

Get My Free Ebook


Post a comment