Introduction
Trees are the most important component for sustaining the farming system in Nepal. Pandey (1982) reported that trees provide about 20% of the total animal feeds. For the management of forest resources and trees in an agroforestry system, some silvicultural operations need to be practiced based on tree species to make tree and crop combination more compatible. It is a prescribed method for manipulating the growth of trees so as to satisfy the objectives of the grower.
The main objective of tree management in an agroforestry system is to maximize the value of tree production without great reduction in agricultural potential of the site. Tree management operations need to be practiced based on tree species. The main and common operations are given below:
- Technique for timber species management
Timber production is also an objective of an agroforestry system, and timber species are those species which produce timber needed for making furniture and for constructing houses etc. These species are Sissoo, Eucalyptus etc. These species in agroforestry areas can be managed using following techniques:
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a. Singling
This is an operation for keeping only one healthy and very good form tree regenerated from the stump by removing other coppicing trees.
b. Pruning
Pruning is a process of removing certain above-ground branches from a plant, in which tree branches up to 50-70% of total tree height should be removed with clean cut (without any knot at the stem).
The main purpose of pruning is to shape the plant by controlling or directing plant growth, to maintain the health of the plant, or to increase timber quality producing clean bole/ stem and to reduce shading effect of trees to under storey agriculture crops. All pruning cuts should be relatively smooth since this will aid in healing.
Pruning to branches
Depending on the weight of the branch, the first cut should be a notch on the underside of the branch about a third to half of the way through. The bulk of the branch should then be removed with a follow-through cut slightly above the first cut, thus leaving a limb stub. The purpose of this is to stop the weight of the branch from tearing the bark of the tree from the underside, which would normally occur if the removal was done with one cut. The limb stub ensures that any cracking of the wood resulting from the branch separation is limited to the portion of the wood to be removed. The branch collar should then be located, and can be identified by the strip of rough bark running down from the topside of the branch at its junction with the stem. The cut for removing the limb stub should be just outside the branch collar, leaving a small bump. The bump and the branch collar should not be removed since this action can reduce healing time, which could result in a major infection.
Time period
Pruning small branches can be done at any time of year. Large branches, with more than 5-10% of the plant’s crown, can be pruned either during dormancy in winter, or, for species where winter frost can harm a recently-pruned plant, in mid-summer just after flowering. Autumn should be avoided, as the spores of disease and decay fungi are abundant at this time of year.
- First pruning should be done after 4-6 years of plantation establishment removing branches of 30-35% of total tree height.
- Second pruning can be carried after 10 years of plantation establishment removing lower branches up to 50% of total tree height.
- Third pruning can be carried out after 14-16 years of plantation establishment removing lower branches up to 70-75% of total tree height).
c. Thinning
It may be defined as the removal of disease, dry and weak trees to open gaps between trees for creating favorable environment to under storey agriculture crops or vegetation.
Agroforestry farmer controls the development of a tree stand by removing injured, diseased, or unwanted individuals. This selective removal of tree stems is referred to as thinning. The trees which remain after thinning should be the best trees. These are the ones which are growing the best, or have the best potential for future growth. These remaining trees are “crop trees.”
Trees grow in response to the amount of competition from other individuals in the stand. As the trees in a stand become more crowded, they all begin to slow their growth. Thinning, because it removes the least productive trees, provides room for the remaining trees to grow more rapidly. If thinning is properly timed, the stand will have a more uniform growth rate through the years.
This will result in greater total wood yields and will shorten the time needed for the trees to become mature. According to an old definition, the object of thinning is to provide the remaining trees with “room to grow but none to waste.” In addition, the main aim of thinning in an agroforestry system is to reduce shading effects of trees to under storey agriculture crops.
Thinning Regimes
Thinning regimes can be customized by a forester to meet landowners’ individual needs. For example, where large trees are desired as soon as possible, a thinning regime that cut the smaller, weak and disease trees and left the minimum number of stems per Ropani would be used. We can carry thinning 3 times depending on tree species and farmers interest to open the field reducing tree numbers.
- First thinning should be carried after 4-6 years of agroforestry plantation establishment.
- Second thinning 10 years of plantation establishment.
- Third thinning which is known as production thinning can be carried out after 15-16 years of plantation establishment.
- Fodder tree management
Following operations should be carried out in fodder trees management for the minimization of the shading effects of trees to under storey agriculture crops.
a. Lopping
Fodder tree branches which are thumb size should be cut without any damage to the remaining portion of cut branches from where lateral branch comes.
b. Coppicing
It should be carried out in Mulberry and other coppicing fodder and firewood species in which plant above 15 cms height should be cut and removed from where new shoots regenerates.
c. Pollarding
Topping of fodder trees at one meter height from where new growth/ branches come, and these branches lopped for fodder harvesting. Pollarding can be practiced successfully in Ipil-Ipil, Mulberry, Tanki, Koiralo etc.
A tree pollard could also be defined as a tree which has had its crown structure removed to leave only a trunk and perhaps a few stubs with the aim to generate new fresh shoots and ultimately a new branch structure.