Global Environments and Environmental change http://www.envf.port.ac.uk/geog/teaching/environ/tundra/tsld004.htm The Arctic Tundra: CLIMATE presents one of the most extreme environments on earth for plant and animal life these extreme conditions of existence impose constraints on organic activity radiant energy input which is the crucial external variable controlling both the ecosystem and the denudation system a massive decrease in incident radiant energy received at the surface near the poles because of the low angle of incidence of the Sun at these high latitudes polar latitudes are alternatively pointing towards, and away from the Sun because of the tilt of the Earth's axis pronounced annual periodicity in solar radiation received at the surface and the climate is controlled by this annual cycle of solar radiation results in a net, and continual loss of heat by back radiation to space (a negative energy balance) producing clear cloudless skies and hence a relatively dry and very cold climate with air temperature as low as - 60oC surface water, soil moisture and groundwater as ice is unavailable to organisms during the winter † much of arctic is underlain by permafrost short arctic summer has long sunny days so appreciable amounts ** of radiant energy are received (270KWm-2 - 350KWm-2 day-1) a good deal of this energy is absorbed as the latent heat of the change of state of ice to water air temperatures remain low during short summer with temperatures above freezing occurring for only one or two months in the summer low albedo of plants enables them to absorb more heat than the surrounding air believed to speed up the physiological activity of the organisms, such as photosynthesis & respiration intense physiological drought, (often excentuated by cold dessicating winds) is the most profound limiting factor in the tundra The Arctic Tundra: SOILS soil environment too, is extreme with immature A/C profiles of largely undecayed organic remains overlying unweathered parent material Gelic Regosols Gelic Arenosols Gelic Leptosols horizonation is poorly developed and vertical structure is largely a result of soil frost phenomena in the active layer above permafrost soil frost (ice) phenomena such as involutions, and ice wedges development of the ubiquitous patterned ground phenomena like polygons, and stripes movement of the active layer in summer, as solifluction sheets, lobes and terraces a degree of soil instability unknown in temperature latitudes or for that matter in the lowlands of the humid tropics thawing of the active layer creates saturated and water logged mineral soil profile (Gleysols) aquatic, swamp and bog soil complexes (Histosols) .° both Gelic varients decomposition becomes the rate limiting step peat is sink for energy and nutients boreal peat acts as a buffer damping climatic oscillations immobilising or regulating the flow of CO2 and water low temperatures and the existence of the permafrost inhibit the weathering of minerals The Arctic Tundra: ECOSYSTEM STRUCTURE life form spectra that it is dominated by low, herbaceous and largely perennial plants when present woody plants often grow prostrate along the ground surface conditions at the surface are often very different than just 1 or 2 metres above it micro relief becomes very important soil surface is warmer than the air wherever snow lie provides insulation owing to the soil's higher thermal conductivity temperatures are substantially higher near the soil surface in summer The Arctic Tundra: ECOSYSTEM STRUCTURE the prostrate habit parallel to the ground, either active or passive the rosette habit with the aerial shoots dying back in winter the cushion herb the hemicryptophyte habit of the grasses and sedges geophytes or plants with their perennating bud below the soil surface most of the plants are perennial, a fact of adaptive significance asexual and vegetative reproduction is very common the primary adaptation of the vegetation are to its extreme environment not in response to competition for its space or resources by another species structurally then the tundra is a simple ecosystem: vertically and horizontally unsaturated in the sense that bare areas frequently occur it lacks floral and faunal diversity The Arctic Tundra: ECOSYSTEM STRUCTURE 250 thousand or so flowering plants only about 900 are found in the circumpolar arctic flora tropical Brazil for example the figure would be 40,000 or so dropping to between 4 and 5,000 in temperate North America and Canada then in an equivalent area of the arctic to 3-400 Alaska, 440 spp of which 250 in south dropping to 100 at coast at Point Barrow ---------- 4 blank slides ------------- The Arctic Tundra: VEGETATION DYNAMICS & STATUS continuous oscillations, invasions, and retreats of vegetation and many areas undergo a kind of perpetual readjustment no long term tendency towards equilibrium which would be expected of a climax ecosystem each patch in the vegetation mosaic undergoes a short term succession best way to regard the ecosystem as a whole is as a mixed polyclimax each areas stability being limited in extent both in time and space (particularly by permafrost interactions). ------------------------ above ground primary productivity is low with less than 1 g of dry matter production per m2/day above ground primary productivity from 10 to 900 kg/ha/yr (in contrast to figures of from 28,000 to 100,000 kg/ha/yr quoted for tropical rain forests) much smaller biomass of producer organisms of the tundra and its simpler vertical arrangement also reflects the much shorter growing season of 1- 4 months below ground productivity10- 600 kg/ha/yr Alaska 5:1 - 14:1, Russia 4:1 - 13:1 The Arctic Tundra: ECOSYSTEM FUNCTION the primary production is not sufficient to support animal life if only small areas of tundra are considered large herbivores and carnivores are dependent on the productivity of vast areas of tundra and have adopted a migratory way of life small herbivores feed and live in the vegetation mat eating the roots rhizomes and bulbs their grazing pressure is at least responsible in part for the population fluctuation associated with them, eg. lemming and affect the smaller carnivores dependent on them, such as the arctic fox and snowy owl The Arctic Tundra: ECOSYSTEM FUNCTION productivity of both fresh water and marine aquatic ecosystems has a part to play in supporting the animals of the tundra, especially the carnivores also supports migratory birds and hibernating insects The Arctic Tundra: ECOSYSTEM FUNCTION turning to nutrient cycling: low temperatures mean a very slow rate of decomposition and very slow litter and hence nutrient turnover rates and much less significance for the detrital foodchain the existence of permafrost at depth also renders nutrients in the inorganic subsoil at least temporally unavailable to plant roots though frost heave acts as natural ploughing and brings unweathered soil material and nutrients to the surface The Arctic Tundra: ECOSYSTEM FUNCTION two factors mean that nutrient availability is probably a very important further limit to plant growth and means that fluctuation in the permafrost table have considerable effects on plant performance The Arctic Tundra: CYCLIC OSCILLATIONS these are a characteristic of animal population dynamics and regulation in the tundra the lemming cycle, and that of its predators, snow owl (migrates) and arctic fox (starves) is the best documented. Many other animals show such cycles either on a 3 - 4 year periodicity (lemming, arctic fox, snowy owl, mice and voles), or 9 - 10 year periodicity, better shown in the subarctic (ruffled grouse, snowshoe hare, Canadian lynx). The Arctic Tundra: CYCLIC OSCILLATIONS two theories External factors - food supply predator pressure disease Internal mechanism - psychological stress - exhaustion of adrenopituitary system under high densities The Arctic Tundra: CYCLIC OSCILLATIONS not simple but concerns interactions of vegetation, lemming grazing pressure, nutrient status of food, permafrost fluctuation The Arctic Tundra: CONCLUSION arctic tundra ecosystem is one where the biological structure and diversity of the community is kept simple by powerful limiting factors in the extremely severe arctic environment. this simple community cannot damp down and buffer itself against fluctuation and instability in the external environment nor control by interaction fluctuations in the population of individual organisms a low biological productivity a rather simple pathway of energy transfer, and a slow turnover in nutrients both of which set further limits to the development of organic structure The Arctic Tundra: CONCLUSION inherent instability means that the attainment of a mature stable equilibrium state - the climax by progressive successional stages is impossible and at best it can be regarded as a mixed polyclimax.