Anoxia Tolerant

Saxifraga caespitosa S. oppositifolia S. foliosa Ranunculus sulphureus Cardamine nymanii Eriophorum scheuchzeri Juncus biglumis Carex misandra Luzula arctica Dryas octopetala Puccinellia vahliana Deschampsia alpina Alopecurus borealis Anaerobe Jar

ANOXIA-INTOLERANT Saxifraga hieracifolia S. cernua

Ranunculus pygmaeus Oxyria digyna Pedicularls hirsuta Cochlearia groenlandica

Polygonum viviparum

Draba oxycarpa

Fig. 3.28 Experimental testing of anoxia tolerance in some High Arctic perennial species. The experiments were carried out at Ny Alesund (Spitsbergen, 79° N). Intact plants were placed in anaerobe jars (as shown above) and examined for their ability to survive 7 days of total anoxia in the dark at ambient arctic temperatures. In all the anoxia-tolerant species the entire plant including leaves and flowers survived undamaged except for Salix polaris, which lost its leaves but otherwise remained viable (Crawford et al, 1994).

encasement in ice yet emerge and become metabolically highly active with high rates of photosynthesis when the ice and snow eventually melt. When examined experimentally many arctic species show a remarkable ability to withstand total anoxia (Fig. 3.28), with the whole plant including leaves and sometimes flowers surviving an anaerobic incubation in the dark at 20 °C for over a week (Crawford et al., 1994).

The ecological consequences of deprivation indifference can be considered in relation to the processes that create renewal gaps in plant communities. The similarity of the resources required by plants has prompted the suggestion that differential use of resources is unlikely to be an important ecological discriminator between plants because all species have the same basic needs. Instead, it is the ability of species to regenerate in gaps, where individuals have died, that is viewed as the ultimate cause of coexistence (Grubb, 1977). The causes of the gaps will be associated frequently with adverse periods which take place irregularly; consequently the presence or absence of a species in any particular community will be governed by competition that takes place only at irregular intervals, as for example when regeneration of the community takes place either after a natural catastrophe or when dominant individual plants die. It follows, therefore, that as gaps occur, as opposed to total devastation, not all species are equally affected. This ability to occupy an open site and thus pre-empt space can be described as pre-emptive competition or founder control (Werner, 1979). Species which can survive resource deprivation are therefore in a position of being able to maintain their presence in these areas despite the risks that arise from flood, fire, drought or storm and will be in a favourable position to pre-empt spaces from other species.

3.6.4 Avoiders and tolerators

A further useful ecological concept in relation to resource deprivation is that of avoiders and tolerators, a view that has been developed particularly in relation to stress physiology in plants (Levitt, 1980). The adaptive responses of being able to either avoid rather than

Fig. 3.29 Sea club-rush (Bolboschoenus maritimus) growing on the shore of the island of Gotland (Baltic Sea — reduced salinity). (Inset) Extension growth of dormant bud while kept in the dark under total anoxia for 12 days.

tolerate stress appears to be a more flexible attribute in terms of plant physiology than the concept of competitor or tolerator.

In evolutionary terms every adaptation carries a certain negative element in that it increases habitat specialization. Species that manage to avoid exposure to the stress do not have to resort to the more specialized adaptations that are needed to confer an ability to survive. Stresses that impinge at a cellular level, as in tissue desiccation, or oxygen deprivation, require specialized physiological adaptations which can involve a metabolic cost and consequently may reduce the competitive status of the species in the unstressed condition. Plants avoid stress most commonly either by reducing demand or by not being active during the adverse season. Thus drought avoiders either reduce the rate of water loss, or adjust their phenology to grow at times when water is available. The commonest mechanism for plants avoiding low-oxygen stress is to have aeration mechanisms that supply oxygen to their roots when they are flooded.

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