Nursery Techniques - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 46

lateral roots are pruned away, and the tap root is cut off at 10-15 cm below the collar.
The stump is then planted in a polypot or in a bed. In about 6 months time, it should
be ready for outplanting. Tall plants can also be raised if the stump is allowed to
grow for a full year. If stumps are required to be transported to a distant site, the
same precautions as in case of branch cuttings should be followed. Use of rooting
hormones can also be considered.
Tissue culture technique can be considered in case of those species where seed
germination is difficult and cuttings are either not available or do not root easily. In
tissue culture, a small portion of the shoot of an adult tree is cultured in a labora-
tory and multiplied to produce propagules. Such propagules are planted in polypots
and grown into plants of desired size. Considering that the facilities for tissue cul-
ture for large-scale multiplication of plants are not widely available, this method of
propagation perhaps has limited scope outside of industrial-commercial plantations.

2.16 Culling, Selection, and Grading

Culling is desirable to prevent inferior specimens from being raised and sent to
field. To avoid unproductive expenditure, it is desirable to cull the poor specimens
at an earlier age. Given the uniform quality of potting mix and irrigation in a bed of
polypots, it should be possible within a couple of months of sowing to identify the
laggards. These should be pricked out of polypots and destroyed. Degree of culling
depends upon how stringent one is about quality control. However, on the whole, a
20 % culling seems to be generally beneficial, and it should not be less than 10 %
in most cases. If up to 50 % of the plants are culled, then the need for grading and
selection will be obviated, and quality will be assured. If high standards of quality are
to be maintained and costs are any consideration, then culling is a better alternative
compared with selection.
Selection occurs when plants are chosen to meet predefined criteria of quality.
The fraction meeting the criteria is retained and used in outplanting, whereas the
remaining plants may be used for some other purpose, or may be destroyed. The latter
option means a higher cost in raising of each plant that goes into field. Assurance of
genetic superiority is better established in selection than in culling, because all the
plants have had a full chance to compete and the winners only are retained. Extent
of selectivity is a matter of degree of quality control—it can be as stringent as 10 %
or as relaxed as 75 %. To implement, say a 45 % selection, all the plants in a bed
are arranged in order of size and health. The top 45 % plants from the best end are
retained and the rest are graded out. If the plants graded out are destroyed, then they
can be said to be culls.
Grading of plants is a matter of planned use of resources according to priorities.
All the plants of a particular species should be divided into four to ten brackets, in
order of quality. Plants belonging to a particular quality bracket can be assigned to a
particular site. For example, a poor quality site can be compensated by assigning to
it the highest quality of plant material, and vice versa. Again, the last two brackets
may be destroyed for ensuring stringent quality of plants.