Resource Acquisition In Marginal Areas

Strategy theories are devised to explain different modes of plant life in a manner which aids human understanding. Convenient and useful as these theories can be (see Chapter 2) they merely reflect human perceptions and inevitably discard information in the desire to produce conformity with a set ecological philosophy. One such dangerous simplification can be seen in the concept of environmental stress which is usually conceived as a permanent feature of particular habitats. Thus plants that inhabit areas that are considered chronically unproductive are classified as stress tolerant (Grime, 2001). There is, however, an increasing realization that resources are not constantly available to plants and that there are instead pulse periods when resources are high and most growth and resource accumulation occurs, as well as interpulse periods when resources are too low to be taken up and when resource-deficit mortality is most likely to take place (Goldberg & Novoplansky, 1997). Even when resources are available, conditions may be such that they are temporarily inaccessible. Prolonged flooding, drought and low temperatures frequently induce lengthy, quiescent periods with regard to resource acquisition during which competition for resources is minimal and survival depends on the possession of stress-tolerance adaptations. These quiescent periods can then be followed by periods of rapid growth when competition for resources comes into play as a factor in determining survival.

3.6.1 Competition for resources in marginal areas

Acquiring resources demands more from plants than just being in the right place at the right time. There is also the need to be able to acquire resources in the face of competition both from other species (interspecific competition) and from other individuals of the same species (intraspecific competition). When natural phenomena are discussed in terms of concepts such as competition there is always the possibility of arriving at conflicting conclusions depending on how the concepts are interpreted. Resource acquisition has stimulated considerable debate as to whether competition is more intense in areas that are rich in resources as compared with areas where resources are scarce. There has been therefore much research on whether the intensity of competition changes along environmental gradients with changing productivity. Many studies demonstrate that competition is more intense in productive habitats (Grime, 2001; La Peyre et al., 2001). Nevertheless, there are situations in marginal areas where despite very reduced productivity and low biomass accumulation competition may still be a factor in determining the species composition of plant communities. The biased sex ratio in favour of female willows that is frequently found in both arctic and subarctic areas appears to be linked to the ability of the female plants to outcompete the males for space in favourable habitats (Crawford & Balfour, 1990). It is possible therefore to question the frequent assumption that as resources become constrained all plants are similarly restricted in their supply and the factors that control plant distribution become physical rather than biological. In marginal areas such as deserts and bogs as well as arctic and montane regions, environmental fluctuations, some seasonal, others at longer intervals, are demographic-ally decisive and determine how competition through the presence or absence of neighbours will affect recruitment, growth and mortality. As will be discussed in the following sections of this chapter, the nature of competition, particularly in marginal areas, will vary between the adverse and favourable periods with different populations and species within any one area profiting or losing depending on their relative (competitive) susceptibilities to environmental oscillations.

Stress-tolerant species are usually described in terms of the particular resource (the stressor), which is in limited supply and yet does not adversely affect their long-term survival. Plants that are characteristically found in shade or in arid conditions are described respectively as shade or drought tolerators. However, such classifications, even with the recognition of intermediates, do not take into account the fact that plants can have opposing strategies at different stages in their life

Fig. 3.24 The grass state of the southern pine (Pinus palustris) at the edge of a forest in South Carolina, USA. The plant will remain in this dwarf stage until exposed to the heat of a forest fire.

cycle and at different seasons of the year. An example is Glyceria maxima which has a high tolerance of anoxia early in the growing season but by early summer is markedly less tolerant due to a reduction in carbohydrate reserves (Braendle & Crawford, 1999). Pinus palustris seedlings (Fig. 3.24) remain in a genetically controlled dwarf condition — the 'grass state' — with a deep taproot, which permits seedlings to lose foliage and survive low intensity fires (an adaptation also found in various other pines, notably P. engelmannii in Arizona) as part of the understorey shade-tolerant plant community; they maintain this state until exposed to the heat of a forest fire when the seedlings elongate to become part of the forest canopy (Nelson et al., 2003).

Resource acquisition and resource deprivation are not just positive and negative aspects of the same phenomenon. The ability to exist for an ecologically meaningful period without a certain resource is a vital characteristic, genetically controlled, and with substantial survival importance. Surviving without a specific resource for a certain period of time does not preclude the eventual demand and active use of this resource when it becomes available. One of the most powerful competitive strategies available to plants in marginal areas is to be able to dispense temporarily with a resource that is in continual demand by other species. This ability for the adapted species to be able to endure deprivation more easily than a potential competitor has been termed deprivation indifference (Crawford etal, 1989).

3.6.2 Deprivation indifference

In natural situations most plants exist under suboptimal conditions due to the constant and ubiquitous effects of competition, predation and climatic oscillations. Survival in such situations depends on a combination of relative tolerance to extreme conditions and the extent of fluctuations in resource availability. The severity and duration of these periods of adversity are highly variable and plants differ greatly in their ability to withstand periods of deprivation. In addition, an ability to survive deprivation of a particular resource, whether it be light, oxygen or nutrients, alters with the seasons and the stage of development of the plant and is not necessarily a fixed characteristic. The ability to endure temporary resource limitations has been described as deprivation indifference (Crawford et al, 1989).

The ability of plants to endure resource deprivation is seen in those species that can endure long periods of drought, darkness, and even in some cases total oxygen deprivation. Adaptation to these conditions has created the specialized floras that have settled in many marginal areas such as deserts, lakes, marshes and periglacial polar regions. Consequently, long periods of drought or flooding rarely extinguish the natural vegetation of marginal areas even though they may be disastrous for agriculture. Desert plants can lie dormant in the seed bank for decades yet spring to life with rapid growth after periods of rain. There are even species of flowering plants that can live where it never rains and survive by extracting enough water to complete their life cycles from fog and dew (Section 2.6, Fig. 2.23). In the Arctic, long winters and ultra-short growing seasons provide striking examples of just how little in the way of resources are necessary to sustain plant life. Some arctic species can even survive total encasement in ice without access to oxygen for many months and still emerge and flower (Section 3.6.3).

This approach to resource limitations differs from the more classical studies where plants are classified as shade, drought or flood tolerant as permanent characteristics. A disparity between species in their ability to withstand long periods of deprivation, particularly in adverse seasons when resource capture is usually minimal, can determine the eventual outcome of competition rather than resource capture during a non-stressful period. By definition, a resource is something that is consumed (Table 1.1). Therefore oxygen, as it is consumed by plants, should also be considered as a resource. Plants that can withstand long periods of flooding, particularly those that overwinter under mud and are therefore cut off from the usual aeration mechanisms, may have to survive without oxygen for months. The degree to which they can endure deprivation of this particular resource can determine the eventual outcome of competition. No plant can endure a permanent absence of oxygen, but the ability to do without this essential resource longer than a competitor may eventually determine the outcome of competition in specific habitats. Such is the range of deprivation indifference and ecological tolerance shown by many species offlowering plants that it can be argued that it is the ability to survive under suboptimal conditions that lies at the base of species diversity.

Examples of deprivation indifference to a number of resources, including water, light, carbon dioxide and oxygen, can be found in many species. As mentioned above, saplings of many deciduous forest trees can survive near the light compensation point for years. Once a gap occurs the long-suppressed saplings can grow rapidly in response to the greater availability of light. Typical examples are found in oak seedlings, and in shade-tolerant evergreen species which minimize biomass losses through long life spans and reduced respiration rates coupled with low leaf/mass ratios (Walters & Reich, 1999). Different tree species can share the same species of mycorrhizal fungi and thus be connected to one another by a common mycelium which permits transfer of carbon and mineral nutrients. Field studies on whether or not these movements of resources are bidirectional are limited. However, laboratory experiments using isotope tracers have shown that the magnitude of a one-way transfer can be influenced by shading of'receiver' plants and fertilization of'donor' plants, indicating that movement may be governed by source-sink relationships (the internal movement of resources within plants from where they are acquired to where they are consumed). Shading has been found to have an important influence and the transfer of carbon by shaded seedlings of Douglas fir (Pseudotsuga menziesii) from paper birch (Betula papyrifera) accounted on average for 6% of carbon isotope uptake through photosynthesis. This magnitude of net transfer is influenced by shading of Pseudotsuga menziesii and strongly suggests that such source-sink relationships regulate carbon transfer under field conditions and contribute significantly to the shade tolerance of the understorey trees while in the suppressed condition (Simard et al., 1997).

Carbon dioxide is a resource that can be recycled during periods of drought. Orchids and cacti rarely wilt and during extreme drought continue to photo-synthesize by recycling the carbon dioxide released from respiration and tissue and protein breakdown. Plants with C4 and CAM improve their access to limited supplies of carbon dioxide by using the initial non-light-dependent CO2-fixing enzyme phospho-enol-pyruvate carboxylase. A number of drought-tolerant species have large reserves of stored water and this together with morphological adaptations to reduce water loss enables the plant to recycle internally respired carbon dioxide.

Bromeliad phreatophytes are also highly efficient CAM plants, particularly those that are obligate epiphytes such as Tillandsia usneoides and T. brachycaulos (Fig. 3.25). This latter species is capable of making a net diurnal gain of carbon even after a continuous drought of 30 days (Graham & Andrade, 2004).

3.6.3 Deprivation indifference through anoxia tolerance

Flood-tolerant plants which can survive without oxygen longer than their competitors are able to maintain their

Tillandsia Peten

Fig. 3.25 The obligate epiphytic bromeliad Tillandsia brachycaulos, a native of Mexico and central America. In common with most other epiphytic bromeliads the leaf rosette acts as a well for trapping water. This species can continue to fix carbon dioxide even after 30 days of continuous drought (Graham & Andrade, 2004).

Fig. 3.25 The obligate epiphytic bromeliad Tillandsia brachycaulos, a native of Mexico and central America. In common with most other epiphytic bromeliads the leaf rosette acts as a well for trapping water. This species can continue to fix carbon dioxide even after 30 days of continuous drought (Graham & Andrade, 2004).

Fig. 3.26 Use of an anaerobic incubator to test the anoxia tolerance of pseudo-viviparous plantlets of alpine meadow grass (Poa alpina var. vivipara). (Left) Anaerobic incubator where plants are held in the dark under an atmosphere of90% oxygen-free nitrogen and 10% hydrogen continually circulated over a platinum catalyst. (Right) The plantlets in the glass vessel have just been taken from the incubator and found to have suffered no injury even though they had been kept under total anoxia for three weeks at 20 °C in total darkness.

Fig. 3.26 Use of an anaerobic incubator to test the anoxia tolerance of pseudo-viviparous plantlets of alpine meadow grass (Poa alpina var. vivipara). (Left) Anaerobic incubator where plants are held in the dark under an atmosphere of90% oxygen-free nitrogen and 10% hydrogen continually circulated over a platinum catalyst. (Right) The plantlets in the glass vessel have just been taken from the incubator and found to have suffered no injury even though they had been kept under total anoxia for three weeks at 20 °C in total darkness.

presence in an extreme habitat as they can endure a stress that has eliminated those species that require an uninterrupted supply of oxygen. Rhizomes of wetland species such as the bulrush (Typha latifolia) and the common club-rush (Schoenoplectus lacustris) can survive for months under total anoxia and even produce new shoots without having to wait for access to oxygen. Mature green overwintering leaves of Acorus calamus and Iris pseudacorus have been shown to survive up to 75 days and 60 days respectively of anoxia in the dark. During this period of anaerobic existence there is a down-regulation of metabolism and carbohydrate reserves are conserved (Figs. 3.26-3.27). On return to air A. calamus leaves rapidly recover full photosynthetic activity. This well-developed ability to endure prolonged periods of oxygen deprivation in both these species is associated with a down-regulation in metabolic activity in response to the imposition of anaerobiosis (Schluter & Crawford, 2001) for other examples of anoxia tolerance (see Figs. 3.28-3.29 and Crawford, 1992).

Many arctic species can in a sense dispense with time and survive being deprived of one or even more entire growing seasons. Diminutive forms of species such as Polygonum viviparum, Salix polaris, Ranunculus

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