Resource Allocation

Numerous attempts have been made to rationalize the behaviour of plants with respect to their allocation of resources. The concept of plant functional types (see Table 3.2) provides a basis for grouping species or populations that share comparable responses to a particular stress. It is a debatable point as to whether plants that share a similar marginal habitat with regard to the availability of light exhibit the same properties in relation to their allocation of resources. Late succes-sional shade-tolerant trees might be expected to share some common attributes as to how they manage to survive as seedlings on the shaded forest floor. A study of seedlings of five such species (Fagus sylvatica, Acer pseudoplatanus, Quercus robur, Taxus baccata, Abies alba) grown in a natural forest understorey in very low and low light microsites and exposed to varying CO2 concentrations demonstrated that no single species combined all the characteristics traditionally considered as adaptive to low light conditions and increasing carbon dioxide. At very low light intensities, only F. sylvatica and T. baccata responded to CO2 stimulation of seedling biomass with increased leaf-area ratios and decreased leaf dark respiration. At slightly higher light levels, interspecific differences in the biomass response to elevated CO2 were reversed and correlated best with leaf photosynthesis (Hattenschwiler, 2001). Although the seedlings in these species may all be shade tolerant, this tolerance is achieved in different ways and does not strictly represent a common functional type.

The short growing seasons and low thermal input into arctic habitats requires plants that live in these areas to control carefully the allocation of their carbon resources. In addition to the short period normally available for photosynthesis, the Arctic has also a very variable climate. In cold years the accumulated temperatures in day degrees can be so low as to result in no net photosynthetic gain being made by the plants. In Alaska a fall of just over one degree can be sufficient to result in a reduction of net photosynthesis by 27% (Chapin et al., 1980). Therefore, in maintaining an adequate partitioning of their carbon resources between growth and carbohydrate reserves arctic populations ensure their energy supplies are adequate to last not just from one year to another but for more than one missed growing season. This conservation of carbohydrate may be achieved in several ways depending on the relationship between growth and productivity in the species concerned. In arctic species a high proportion of the carbon dioxide that they fix is allocated to the maintenance of underground organs.

The large root systems of many northern and arctic plants with their considerable carbohydrate reserves, while enabling these plants to survive long periods with little photosynthetic gain, require nevertheless a considerable portion of the available energy input just for their maintenance. The bog rosemary (Andromeda polifolia) growing in the Swedish tundra translocates 75% of its fixed carbon below ground (see Fig. 9.17). Similarly in an overall estimation for grass species in Alaska 59% of the carbon dioxide fixed was found to be translocated below ground where half was used just for maintenance respiration and the other half contributed to new biomass (Chapin et al., 1980). By comparison, prairie grasses translocate 57% of the carbon fixed below ground but use 85% of this portion to contribute to new growth and only 15% for maintenance. The strategy of the northern plants in maintaining adequate reserves to combat the thermal uncertainties of their environment places them at a disadvantage in terms of growth potential with less well-insured species. Thus, it is not just in their photosynthetic capacity that arctic plants differ from those further south but also in the manner in which they allocate their reserves.

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