Nutrient Retention In Marginal Areas

Bryophytes and in particular the various species of Sphagnum that are common in wetland and oceanic regions have long been recognized as chelators of mineral ions. The rapid drop in soil pH after the colonization of wetlands by Sphagnum spp. is well known, Similarly, in many nutrient-deficient landscapes abandoned human settlements frequently stand out for the verdure of their immediate surroundings.

The fifteenth-century abandoned Norse settlements in Greenland (c. 1450), Bronze Age and Neolithic homesteads in Scotland (Fig. 3.36), and winter hunting camps in the Arctic can still be recognized from a distance from their vegetation. Particularly striking in the High Arctic are the winter encampments of the Pomors, Russian hunters who crossed the sea (Russian po more, on the sea) from the Kola Peninsula to hunt for furs in Spitsbergen in the seventeenth and eighteenth centuries or possibly earlier. Most of the encampments fell into disuse two or three centuries ago but are still marked by the lush bryophyte communities (Fig. 3.38).

Despite these many well-documented examples of nutrient retention in unproductive habitats it was an astonishment to the scientific community when the Russian nuclear reactor at the Ukrainian site at Chernobyl exploded in 1986 that the resulting pollution with radiocaesium (137Cs) that spread westward was retained for so long in the upland oceanic grazing lands of the British Isles. As was pointed out at a subsequent government enquiry (Grime, 2001), the predictions made at the time to allay the fears of British farmers and the general public were made on productive and intensively managed lowland pastures and no account was taken of the upland situation where the 'slow dynamics' of unproductive land would inevitably lead to sequestration and slow release of radiocaesium from both living and dead components. The impoverished upland soils of oceanic lands experience high levels of precipitation combined with mild wet winters, which had led to a false assumption that constant leaching of mineral ions would be inevitable in these conditions. The erroneous belief that this leaching would rapidly

Fig. 3.37 Cluster roots developing on Lupinus albus. The plants were grown in a regime of low internal phosphate and iron. Inset shows a close-up of a cluster root nearing its full development. (Photos by courtesy of Dr K. R. Skene.)
Fig. 3.38 Site of long-abandoned high arctic hunting station at Krossfjorden, Spitsbergen. This site was used by Russian trappers (Pomors - see text) probably in the early eighteenth century. The encampment with its residual wooden posts is clearly distinguished by the lush bryophyte growth.

cleanse the soil of radioactive pollution was made in ignorance of the capacity of oligotrophic vegetation for nutrient retention in these marginal sites.

Nutrient retention in upland vegetation depends on both physical and physiological properties. Over large upland areas of the western regions of the British Isles from Shetland to Wales the soils are predominantly thin or else covered with layers of peat of variable depth. In shallow mineral soils the entire soil profile is usually occupied from top to bottom by a dense mat of roots. This total occupation of the soil profile maximizes nutrient retention and minimizes leaching. When the soils are organic the upper layer of peat which becomes aerated during the growing season (the acrotelm) is also densely occupied by living roots which maximize nutrient retention. The lower region (the catotelm) is composed of dense anaerobic peat which both impedes drainage and decomposes extremely slowly and will therefore retain any nutrients that escape from the acrotelm.

Localized areas that have higher nutrient status than the surrounding area as a result of past settlements, or from herding together of domestic animals at certain times of the year, still attract intense grazing.

Although the grazing intensity over the area as a whole may be light, the greater proportion of verdant palatable herbage, particularly in winter and early spring, in these more fertile areas attracts many animals. The manuring that results from this behaviour facilitates nutrient cycling in these patches and perpetuates this vegetation pattern in a manner that reflects the past usage of the land even though it may have been abandoned for centuries or even millennia (Fig. 3.38).

Physiologically the vegetation in many arctic and subarctic deciduous species demonstrates a strong facility for retaining nutrients from one season to another. In a study of mineral nutrient economy in shrub tundra in northern Sweden, dwarf birch (Betula nana) showed the greatest capacity for the reabsorption of N, P and K from the leaves into the body of the plant before leaf fall (Jonasson, 1992). Similarly, in cotton grass (Eriophorum vaginatum) 90% of the phosphorus can be withdrawn from single leaves relative to their total nutrient content so that only about 10% of the annual demand to maintain the biomass needs to be absorbed annually from the soil. Similar patterns for nitrogen reabsorption have been found in Carex bigelo-wii (Jonsdottir & Callaghan, 1990) and for phosphorus in Lycopodium annotinum (Headley et al., 1985). Other more general features of the vegetation of infertile areas also aid the retention of nutrients in upland areas and include the presence of living foliage (including lichens and bryophytes) and active roots throughout the year, as well as longevity, slow turnover rate and a close association with mycorrhizal fungi (Grime, 2001).

Experimental addition of nutrients in arctic sites makes it possible to provide some quantification of the capacity for nutrient retention. In a prolonged study documented for 3-10 years and carried out at 13 sites in Alaska on Eriophorum vaginatum, E. angustifolium and Carex aquatilis the effects of fertilizer on flowering were in some cases still significant after six years (Shaver & Chapin, 1995). These three species belong to the ecological group often referred to as graminoids, which in comparison with some other species show the greatest capacity to respond to additional nutrients. They are also less sensitive to temperature in their absorption of phosphate and ammonium. All three species examined, namely Carex aquatilis, Dupontia fischeri and Eriophorum angustifolium, can absorb phosphate at 1 °C at 20-60% of the rate at 20 °C (Kielland & Chapin, 1992). As already mentioned, graminoid species are also able to absorb free amino acids which occur in the high soluble nitrogen pool of many tundra soils. Thus, despite the slow rates of nitrogen mineralization in tundra soils and low concentrations of inorganic N these soils have considerable concentrations of both structural and soluble organic nitrogen.

The addition of nutrients can fundamentally alter the effects of competition between species. In a nine-year study of environmental perturbations at Toolik Lake (Alaska) nutrient addition increased biomass and production of deciduous shrubs but reduced growth of evergreen shrubs and non-vascular plants (Chapin et al., 1995). Certain species (e.g. Dryas octopetala) when growth is stimulated by the artificial addition of nutrients can become frost sensitive and as a result decline in presence. Similarly, in the Arctic when palatable vegetation is stimulated to grow with extra nutrients it can be selectively grazed out of existence by unrelenting visits from reindeer and lemmings. If flowering is promoted this can also attract floral herbivory from reindeer with adverse effects on regeneration (Cooper & Wookey, 2003).

In a sense many manipulation or perturbation experiments run the risk of not being realistic as they can produce short-term changes in the vegetation that would be unlikely to take place under natural conditions. Short-term manipulation experiments such as adding water or nutrients and placing transparent shelters around experimental plots produce an artificial forcing of phenotypic plasticity. The long-term natural response to environmental change would be through genotypic alteration which could select for a totally different set of environmental adaptations with greater Darwinian fitness.

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