The Seed Bank

The seed bank is one of the most successful means employed by flowering plants to maintain populations in a wide range of habitats. Therophytes (annual plants) are entirely dependent on their seed bank for survival from one growing season to another. Many perennial species also have considerable banks of buried and viable seed which, depending on the species, have different expectations of longevity. Variation in the persistence of viable buried seed has prompted a classification of seed banks into three categories (Thompson et al., 1997):

(1) transient - species with seeds which persist in the soil for less than one year, often much less

(2) short-term persistent - species with seeds which persist in the soil for at least one year, but fewer than five years. Such populations can aid the maintenance of plant populations after periods of poor seed setting due to environmental stress such as occurs after drought or intermittent disturbances

(3) long-term persistent - species with seeds that persist in the soil for at least five years and are capable of contributing to the regeneration of destroyed or degraded plant communities.

This last group of persistent species is the component that has the greatest interest for the study of survival in marginal areas. A number of studies have followed the natural restoration of disturbed communities whether as a result of fire, ploughing, volcanism, or other major disturbances. The above classification, however, has its contradictions, with some investigators finding particular species in categories that are not in agreement with others. There is some evidence that position in the soil may influence longevity. Some of the longest-lived seeds have been recorded from lake bottoms. The Chinese water lotus (Nelumbo nucifera) has frequently been reported as yielding viable seeds after centuries of burial in anaerobic mud. A lotus fruit (China antique) from Xipaozi, Liaoning Province, China, has been reliably carbon dated and is the current holder of the world's record for long-term seed viability at 1300 years (Shen-Miller, 2002). Five offspring of this variety, from 200-500-year-old fruits (C14 dates) collected at Xipaozi, have recently been germinated, and are the first such seedlings to be raised from directly dated fruits. These lotus offspring were found to be phenotypically abnormal. Most of the lotus abnormalities resembled those of chronically irradiated plants exposed to much higher irradiances. It was suggested that the chronic exposure of the old fruits to low-dose gamma radiation may have been responsible in part for the notably weak growth and mutant phenotypes of the seedlings (Shen-Miller et al., 2002).

Survival may also have been aided by the anaerobic conditions under which they lay buried by providing protection from the dangers of membrane oxidation, which is also a limiting factor in seed longevity. In general, however, there is only a limited number of species where longevity in the seed bank exceeds 100 years. In a study of the seed banks of north-west Europe (Thompson et al., 1997) only 14 species were recorded as surviving more than 100 years. In a similar study in North America, W.J. Beal in 1879 initiated the longest-running experiment on seed longevity by burying the seeds of 21 different species in unstoppered bottles in a sandy hilltop near the Michigan Agricultural College in East Lansing in 1879 (Telewski & Zeevaart, 2002). After 100 years, only one species, moth mullein (Verbascum blattaria) remained viable.

Apart from the soil conditions there are certain aspects of the seed that appear to be correlated with longevity in the seed bank. Compact, low-weight seeds are more persistent in the seed bank than seeds which are flattened and elongated. Seeds that weigh less than 3 mg are also usually persistent. Ecologically, the consequences of this differential survival in the seed bank results in pioneer species, with small easily distributed seeds being better represented than the larger seeded species of climax woodlands and shaded habitats.

4.10.1 Polar seed banks

The role of the seed bank in maintaining population diversity in the Arctic has been revised in recent years. Earlier suggestions that the seed bank declined with increasing latitude (Johnson, 1975) have been refuted in a number of studies on tundra vegetation (Bennington et al., 1991; Vavrek et al., 1991). In these later studies, seeds of Carex bigelowii were found not only to remain viable for up to about 200 years but also to show clear post-emergence phenotypic and genotypic differences from modern populations. Seeds of Luzula parviflora, excavated from under a solifluction lobe at Eagle Creek, Alaska, have also been carbon dated from their seed coats and found to be capable of germinating even after 175 years of burial (Bennington et al., 1991). Whole populations can lie dormant for decades, or even centuries, buried in the soil seed bank. A recent study in

Spitsbergen found that 71 of the 161 species indigenous to Spitsbergen had the capability of persisting in the soil seed bank (Cooper et al., 2004).

Similarly, it has recently been found that even in the Antarctic the only two flowering plants that exist there naturally, Colobanthus quitensis and Deschampsia antarctica, have considerable seed banks (107-1648 seeds m~2) which are comparable in size to those of arctic and alpine species (McGraw & Day, 1997). The age of the seed is unknown but as the buried seed density was not correlated with the local above-ground abundance where both species were present, it might be assumed that under the slow growing conditions of the Antarctic that the buried seed was of considerable antiquity. Not only do these studies show the potential for long-term viability in high-latitude seed banks and the variability of the seed populations, they have also demonstrated the latent capacity of Arctic and Antarctic ecosystems to respond to environmental change in that seeds taken from different soil strata were genetically differentiated. Seed banks, through their contribution to biodiversity, have therefore important implications for plant population responses to climatic change at high latitudes.

4.10.2 Warm desert seed banks

Semi-deserts are well known to bloom with fast growing plants from buried seed after periods of rain. How much of this seed is derived from recent seed fall and how much is buried in the soil for several years is, however, not clearly established. In the Mohave Desert there is a very high level of viable seed in the upper layers of the soil (Price & Joyner, 1997). Simultaneous monitoring of the seed bank and 'seed rain' over a 19-month period has demonstrated that the total seed bank averaged approximately 106 000 seeds m~2, which is much higher than values reported for other North American desert sites. These numbers correspond roughly to the seed production of a single year, since daily seed rain averaged 262 seeds m~2. However, as input from the seed rain did not accumulate in the soil and the seed bank decreased by a daily average of 114 seeds m~2 it suggests that virtually all seeds either germinate, die, or are harvested by granivores soon after being dispersed. However, in South American and Australian deserts, lower rates of granivory are found compared with their North American counterparts. Measurement of seed reserves in two habitats of the central Monte Desert (Argentina) showed maximum seed standing crops of 16000 and 23000 seeds m~2 in shrublands and open forests, respectively. Seed banks in other South American semiarid areas showed similar values. Total grass seeds as well as those presumably preferred by ants also seem to be similar in both continents. Hence, in South America and Australia granivory is lower than in North America in spite of the great similarity of seed bank sizes (Marone & Horno, 1997).

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