THE ECOLOGY OF ARCTIC AND ALPINE PLANTS

Summary ‘How are plants adapted to the low temperatures and other stresses of arctic and alpine environments ?’ At present it is not possible to answer this question completely. Much work remains to be done, particularly on low-temperature metabolism, frost resistance, and the environmental cues and requirements for flowering, dormancy, regrowth, and germination. However, in brief, we can say that plants are adapted to these severe environments by employing combinations of the following general characteristics: 1. Life form: perennial herb, prostrate shrub, or lichen. Perennial herbs have greatest part of biomass underground. 2. Seed dormancy: generally controlled by environment; seeds can remain dormant for long periods of time at low temperatures since they require temperatures well above freezing for germination. 3. Seedling establishment: rare and very slow; it is often several years before a seedling is safely established. 4. Chlorophyll content: in both alpine and arctic ecosystems not greatly different on a land-area basis from that in temperate herbaceous communities. Within a single species there is more chlorophyll in leaves of arctic populations than in those of alpine populations. 5. Photosynthesis and respiration: (a) These are at high rates for only a few weeks when temperatures and light are favourable. (b) Optimum photosynthesis rates are at lower temperatures than for ordinary plants; rates are both genetically and environmentally controlled with phenotypic plasticity very marked. (c) Dark respiration is higher at all temperatures than for ordinary plants; rate is both genetically and environmentally controlled, with phenotypic plasticity very pronounced, i.e. low-temperature environment increases the rate at all temperatures. (d) Alpine plants have higher light-saturation values in photosynthesis than do arctic or lowland plants; light saturation closely tied to temperature. (e) There is some evidence that alpine plants can carry on photosynthesis at lower carbon dioxide concentrations than can other plants. (f) Annual productivity is low, but daily productivity during growing season can be as high as that of most temperate herbaceous vegetation. Productivity can be increased by temperature, nutrients, or water. 6. Drought resistance: most drought stress in winter in exposed sites is due to frozen soils and dry winds. It is met by decreased water potentials, higher concentrations of soluble carbohydrates, and closed stomates. Little drought resistance in snowbank plants. Alpine plants adapted to summer drought stress can carry on photosynthesis at low water potentials; alpine or arctic plants of moist sites cannot do this. 7. Breaking of dormancy: controlled by mean temperatures near or above 0° C., and in some cases by photoperiod also. 8. Growth: very rapid even at low positive temperatures. Respiration greatly exceeds photosynthesis in early re-growth of perennials. Internal photosynthesis may occur in hollow stems of larger plants during early growth. Nitrogen and phosphorus often limiting in cold soil. 9. Food storage: characteristic of all alpine and arctic plants except annuals. Carbohydrates mostly stored underground in herbaceous perennials. Lipids in old leaves and stems of prostrate evergreen shrubs. Depleted in early growth, and usually restored after flowering. 10. Winter survival: survival and frost resistance are excellent after hardening. Cold resistance closely tied to content of soluble carbohydrates, particularly raffinose. 11. Flowering: flower buds are pre-formed the year before. Complete development and anthesis dependent upon temperature of the flowering year and also, in some cases, upon photoperiod. 12. Pollination: mostly insect-pollinated in alpine regions and even in Arctic, but to a lesser extent. Wind-pollination increasingly more important with increasing latitude. Diptera more important than bees in the Arctic and in the highest mountains. 13. Seed production: opportunistic, and dependent upon temperature during flowering period and latter half of growing season. 14. Vegetative reproduction: by rhizomes, bulbils, or layering. More common and important in Arctic than in alpine areas. 15. Onset of dormancy: triggered by photoperiod, low temperatures, and drought. Dormant plant extremely resistant to low temperatures.