In this article we will discuss about:- 1. Definition of Seed Orchards 2. Types of Seed Orchards 3. Design 4. Location, Establishment and Management.
Definition of Seed Orchards:
A seed orchard is a plantation of genetically superior trees, isolated to reduce pollination from genetically inferior outside sources, and intensively managed to produce frequent, abundant, easily harvested seed crops. It is established by setting out clones (as grafts or cuttings) or seedling progeny of trees selected for desired characteristics.
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Advantages of Seed Orchards:
i. Isolation from surrounding wild populations to avoid pollination by outside pollen.
ii. Selection of locally favourable site with respect to climate, soil and application of management.
iii. Promotion of early flowering by grafting of mature scions.
iv. Avoidance of inbreeding by randomizations of clones or genotypes.
v. Maximizing seed yield through cultural practices such as fertilization, maintaining wide spacing, bending or girdling of stems, root pruning, irrigation and control of weeds and animals.
Types of Seed Orchards:
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Seed orchards are of two types based on propagation material:
i. Clonal seed orchards
ii. Seedling seed orchards.
i. The Clonal Seed Orchards (CSO):
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Vegetative material (grafts, cuttings or plantlets derived from tissue culture) of selected phenotypes (plus trees) are planted in areas with good isolation, under conditions favouring flowering and fertilization and managed for production of maximum amount of seed. The identity of each ramet (member of a clone) is carefully maintained by tags and maps.
It is a characteristic of the clonal orchard that main emphasis is given to crown development for maximum seed production by use of wide spacing. Therefore silvicultural characteristics such as fine stem form, clean bole and fine branches, which are important in production forests or plantations and are valid criteria for selecting clones for inclusion in the seed orchard and are not valid criteria for seed orchard management.
The establishment of a clonal seed orchard should be preceded, or at least closely followed, by progeny tests. CSOs established by grafting are most commonly used. The greatest disadvantage of grafted clonal orchards is the laborious work involved with scion collection and grafting, maintaining healthy ramets in nursery before planting and last but not least, incompatibility between stock and scion, which may occur even several years after establishment.
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To avoid narrowing of genetic base, at least 25-30 clones should be planted. The ramets of clones should be distributed in such a way that inbreeding is avoided. Adequate measures should be taken to keep the contamination of outside pollen below 20 per cent.
ii. The Seedling Seed Orchards (SSO):
Progenies from open or controlled pollination of selected phenotypes are planted at normal plantation spacing. Isolation and other conditions should be as for clonal orchards. The identity of families is maintained in order to allow for thinning among families and individuals within families based on their phenotypic performance. This thinning must be done before abundant seed production starts.
The design of the orchard is of great importance for the later thinning. Seedling seed orchards permit selection and rouging within as well as between families. One disadvantage of SSO is that their formation and future quality is based on early selection, at a time when the juvenile/ mature correlation may still be low. SSO also generally take longer to start flowering and to reach seed productivity.
Seed Orchard Design:
The general requirement in the orchard design is:
i. Minimize selfing
ii. Maximize outcrossing and mating of all genotypes
iii. Simple and easy establishment and management
iv. Allowing any number of clones or families to fit into the design
CSO:
Planting distance of between 2 meters and 16 meters in both directions have been reported, however, an initial spacing of five to six meters is used. There is also trend towards wider initial spacing since it is now generally accepted as being more economic in the longer term to accept lower early seed yields than to systematically thin expensively raised grafts.
SSO:
Planting distances are less than clonal seed orchards. They range between 0.6 to 6 m in both directions with majority at 0.5 to 2.0 m. It is a common practice to use large number of individuals/ family plot thus providing better opportunity for within plot rouging at later stages.
Planting Pattern:
Either square or rectangular arrangement of plots is used. A rectangular arrangement provides more access to machinery and full advantage of sun.
Number of Clones /Family:
Most orchard designs are based on 20-50 clones keeping in view broad genetic base.
Seed Orchard Designs:
Number of seed orchard designs is in vogue: pure row, chess board, completely random, randomized complete block, fixed block, rotating block, unbalanced and balanced incomplete block, cyclic balanced incomplete block, balanced lattice, permuted neighborhood, alpha design etc. Each design has its advantage and disadvantages.
The two most frequently used designs are:
a. Randomized complete block design
b. Incomplete block design
a. Randomized Complete Block Design:
The area is first divided into equal blocks, each sufficient in size to contain one ramet of each clone or one member of each family. The ramet position in each block is completely randomized and then manipulated to avoid similar ramets in adjacent planting positions. These design help in accounting for variation across different blocks in the experiment site. They are however, not very effective in accommodating large number of genotypes as their will be much heterogeneity within the large blocks.
b. Incomplete Block Design:
To accommodate large number of genotypes (200 or more), incomplete block designs are increasingly used. Here the blocks are smaller than a complete replication and the genotypes are planted in plots within a block and groups of incomplete blocks constitute a replication.
Location, Establishment and Management of Seed Orchards
i. Local Climate and Site Requirement:
Above average soil fertility is essential for good orchard tree development. Shelter from strong winds and good air and soil drainage are also basic requirements. Wherever basic information on site selection is lacking, it should be possible to identify potentially good sites from observation of flowering and seed production in local stands.
ii. Isolation from Undesirable Pollen Sources:
Good isolation from contaminating pollen sources is a necessity. Complete isolation in wind pollinated species is however very difficult to obtain, unless orchards are established outside the region in which they are found in plantations or natural stands.
Ideally seed orchard should be established on sites where contamination of pollen from same or related species is negligible. If this is not possible then pollen dilution zone should be maintained to avoid outside pollen. There should be a gap of 300 to 500 m between plantation and seed orchard and orchard should be located on windward side of plantation.
iii. Establishment of Seed Orchard:
The cost of producing planting stock from seed orchard is quite high; therefore it is essential to achieve high standard of general site preparation and planting. Site preparation should start well in advance. Need for fertilization should be determined on the bases of soil analysis.
Protection against wild animals should be provided by adequate fencing before planting. Take utmost precautions while marking plant positions on the ground. The position should be marked with stakes bearing an appropriate label for identification. Labels of plants from nursery should be checked against those on stakes before planting.
iv. Management of Seed Orchard:
Weeds should be kept under strict control within half meter radius of the plant until it is well established and growing vigorously. Subsequently entire orchard area should be kept weed free. For proper growth and flowering, fertilizers containing nitrogen, phosphorous and potassium should be used.