High elevation habitats

Such metabolic alterations may have serious consequences for the herbivore populations and their increased impact on these plants (for an example of increased feeding with phenolics content see Ikonen, 2002). Thus, the future prospects for plants that respond metabolically in this manner, if the present trends of atmospheric chemistry (ozone depletion and CO2 rapid increments) continue, should be of concern, especially in high elevation habitats where these changes are assumed to be greatest. However, not all plants respond to enhanced levels of UV-B with increased levels of metabolites from the phenyl alanine lyase pathway (e.g. Levizou and Manetas, 2001 Kypeirissis et al., 2001)... 

In spite of the overall low productivity of high altitude habitats, mountain plants possess a highly efficient carbon fixation capacity in order to complete their life cycles in the short growing season of temperate climates. In fact, many alpine species are of the stress tolerant type (Jones Maberly, 2003) (see section 5.4). As reviewed above, the equivalent thermal restriction in the tropical high elevations would be the few hours of moderate temperature in each circadian cycle. Low temperature, high irradiance and low CO2 partial pressure (PCO2) -see below- all contribute to reduce carbon assimilation. 

This contention was tested in soils beneath high elevation willows (Salix brachycarpa) in the Colorado Rockies (3500 m asl) (Schmidt et al., 2000), whose bark accumulates 280 pg g of salicylate in average. As a control, Kobresia myosuroides that yields much less salicylate was used. Both tree species were dominant in the area. Differences in salicylate-mineralizing microbes from under both species were very significant (<1 pg biomass C g" soil in the control vs 4.6 -10 pg biomass C g" soil under the Salix trees). Therefore, soil microbe populations may depend strongly on secondary plant chemistry, and as a result plant community structure may govern soil performance in these elevated habitats and others as well. 

Water birds have not been shown to be directly affected by acidification. However, the prey of waterbirds may be of concern as these lower food-chain organisms may have elevated levels of toxic metals related to acidification of their habitat. Moreover, most water birds rely on some component of the aquatic food-chain for their high protein diet. Invertebrates that normally supply caJcium to egg-laying birds or their growing chicks are among the first to disappear as lakes acidify. As these food sources are reduced or eliminated due to acidification, bird habitat is reduced and reproductive rate of the birds is affected. The Common Loon is able to raise fewer chicks, or none at all, on acidic lakes where fish populations are reduced 37 and 5S). However, in some isolated cases, food supplies can be increased when competitive species are eliminated (e.g.. Common Goldeneye ducks can better exploit insects as food when competition from fish is eliminated). The collective influences of acidification are difficult to quantify on a specific area basis but for species that rely on a healthy aquatic ecosystem to breed, acidification remains a continuing threat in thousands of lakes across eastern North America 14). 

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