Seed collection and preparation

Despite advances in natural regeneration, world forestry still relies heavily on seedlings raised in a nursery. Seed collection, treatment, storage and germination were formerly undertaken by local craftsmen to great effect. Due to the large scale now involved, much greater efficiency and consistently higher standards are required; bulk handling of tree seed is highly scientific and can only satisfactorily be undertaken by organizations with the appropriate machinery and well-equipped cold stores. The aim is both to reduce needless losses and to improve the quality of the seedlings produced. Particular care has to be taken with the seeds of forest broadleaves if losses are to be avoided; much is known about the maturation and harvest of fruits and seeds, transportation of seeds, temporary storage, cleaning and grading (Suska et al., 1996). Upon collection, seeds have to be dried and stored under conditions that reduce losses caused by excessive drying and by disease. These need particular consideration in such species as silver maple Acer saccharinum, sycamore Acer pseudoplatanus and the oaks, the viability of whose recalcitrant seeds is damaged if dried below a certain moisture level (40-48% of fresh weight in the case of acorns). Orthodox seeds by contrast are relatively long-lived and able to withstand dehydration to very low moisture contents without losing viability. The seeds of beech Fagus sylvatica, which crops irregularly, are well understood; they can now be stored for long periods and sown after the elimination of dormancy. Figure 10.11 demonstrates the critical importance of moisture content upon germination capacity of seeds stored for very long periods. Above a moisture content of 9% germination capacity decreases progressively, none at 12.9% germinating after 6 years. When moisture contents are reduced to 5.7% or lower, germination rates can drop dramatically shortly after drying.

Hardwoods such as ash and beech possess hardy seeds which can be stored for considerable periods; others give more difficulty. In Britain, trees of oaks (Quercus robur and Q. petraea), sweet chestnut Castanea sativa, horse chestnut Aesculus hippocastanum and sycamore Acer pseudoplatanus are very frequently grown from seed. All have extremely perishable fruits that are shed at high

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Years

Figure 10.11 Influence of seed moisture content on percentage germination of beech Fagus sylvatica during long-term storage at —5 °C. After 10 years germination was highest (87%) in seeds stored at 9.0% moisture and lower (69%) at 6.2% moisture. Seeds remained viable for 5 years maximum when stored at 12.9% moisture. (Redrawn from Suska et al., 1996. Seeds of Forest Broadleaves: from Harvest to Sowing. INRA.)

Years

Figure 10.11 Influence of seed moisture content on percentage germination of beech Fagus sylvatica during long-term storage at —5 °C. After 10 years germination was highest (87%) in seeds stored at 9.0% moisture and lower (69%) at 6.2% moisture. Seeds remained viable for 5 years maximum when stored at 12.9% moisture. (Redrawn from Suska et al., 1996. Seeds of Forest Broadleaves: from Harvest to Sowing. INRA.)

moisture contents, are frequently infected by fungi, and are killed by very little drying. They cannot be stored in sealed containers as they need sufficient oxygen for respiration, and die at temperatures below —3 °C. Their germination rates drop sharply during the winter; only half the acorns planted in the spring will germinate, despite having a germination rate of 90% when collected in the autumn.

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