[Pruning Guidebook Table of Contents]
Ranking stands for optional pruning
The stand selection process must consider both the biological factors associated with pruning and the operability factors. The following stand selection guidelines are for even-aged conifer stands. Consider these guidelines when determining if pruning should be included in the stand management prescription. These guidelines have been written to:
- produce high-value sawlogs containing clear wood
- minimize pruning costs
- maximize return on investment.
Appendix 1 lists the ranking criteria used by the B.C. Forest Service to meet these objectives. These criteria were developed for 3 metre, first lift pruning costs of a maximum of $1000/ha for the interior and $1500/ha for the coast. This ranking criteria must be modified if the objectives of the treatment differ from those shown above. An example of an alternative objective for pruning would be the reduction of fire risk by pruning uneven aged-stands of Ponderosa pine or Douglas-fir.
Consider the biological and economic principles of pruning as stated below, regardless of the treatment objectives.
Artificial pruning removes the lower dead and living branches to hasten the formation of clear wood. Clear wood is produced naturally, but slowly, under the right conditions. Natural pruning occurs in second growth forests as crowns close and lower branches are shaded. This cycle of shading, death, and loss of branches is not completed until late in a tree’s life cycle. The completion of branch loss by natural means is usually later than projected harvest ages. The degree and speed of natural pruning vary with stand density and species. To produce clear wood in rotations of less than 100 years, artificial pruning is necessary.
The death of lower tree branches is a normal consequence of inter-tree competition and self shading. Dead branches remain attached until they break off or rot. If dead branches are pruned, the tree produces clearwood decades sooner than is possible without pruning. As the tree grows after pruning, the pruning scar is covered over. The total time to heal the wound depends on the rate of diameter growth of the tree, branch diameter and other factors. Long branch stubs heal over more slowly. Live knots generally heal over sooner than dead knots.
When pruning, it is important not to cut the branch collar. Minor nicking of the branch collar is acceptable in order to get a close parallel cut. The branch collar is the site of the trees natural healing tissues.
Pruning effects wood quality by influencing the production of knots and hastening production of mature wood. Knots are produced when a tree stem grows over branches. Dead branches lead to the production of loose or dead knots while live branches form tight or live knots. Wood produced within the crown is juvenile wood in addition to being knotty. Juvenile wood tends to have lower density than mature wood.
Removing branches also removes part of the trees photosynthetic capacity. Severe pruning will therefore decrease the trees ability to produce carbohydrates and decrease growth. Pruning according to this guidebook will cause some growth loss for 1–2 years. Do not prune severely and slow long-term tree growth. Young trees are in a stage of rapid growth and high nutrient demand. Pruning of only the lowest branches on a tree will have little effect on tree growth since these branches produce few carbohydrates.
Pruning is normally done to obtain a higher price for the pruned logs because of their higher proportion of clearwood. The size and shape of the log, plus the size of the knotty core will impact the log’s value. Timely pruning produces a relatively small knotty core, and a thick knot-free shell with high-value clear lumber. Late pruning will create a wider knotty core and less clear lumber. Larger, higher-value clear wood boards can be cut out of logs with large shells of clear wood. The value of clear lumber produced from a pruning treatment can be three to five times greater than comparable knotty boards.
The net present value (NPV), measured in dollars per hectare, of a pruned stand is lower for lower site qualities. There is an optimum stand density to obtain maximum NPV for a properly pruned stand. Above this density you are sacrificing the size of the clear wood shell. Below this density overall stand volume has decreased too much.
Seven biological factors are associated with stand selection for pruning.
Select species with consideration of future market trends and products. Preference should be given to stands where the desired species makes up more than 60% of the stand.
Some species may not respond beneficially to pruning. Spruce (particularly Sitka), amabalis fir, western larch, and western redcedar have buried buds under the bark (adventitious buds) which sprout after pruning. This makes it difficult to obtain clear wood through pruning of these species without returning for several years to remove these newly formed branches. Varied local conditions may change the impact of pruning on these species. Small trials only are recommended to determine if pruning is a feasible treatment on these particular species. Pruning of western hemlock in the interior is only recommended on a trial basis since it may speed up decay by Indian paint fungus.
Give preference to stands which have more than 60% of the desired species. Species with adventitious buds are not good candidates for pruning.
Choose rapidly growing stands on good or medium-good sites to maximize the investment returns. The increase in wood value from pruning on low-quality sites may not be sufficient to offset the treatment cost. Pruning is currently economically viable on stands of site index 20 or better. A decrease in pruning costs may make lower site index sites candidates for pruning.
Average stand height
The objectives of the pruning project must be clearly stated before the desired average stand height of crop trees at time of pruning can be determined. Objectives must also be considered when deciding the number of pruning entries. The treatment timing and stand height at the time of entry will vary depending on the number of pruning entries. The B.C. Forest Service suggests that 50% of the tree’s height should be left in live crown. This can, on occasion, be dropped as low as 30% live crown, or three whorls remaining on the tree, whichever is greater length. This should be done to obtain a fixed pruning height throughout the stand while accommodating variation in tree height. This guideline ensures that enough photosynthetic capacity remains to supply the nutrient requirement after pruning.
To obtain a constant pruning height throughout a stand, the shorter trees may be pruned to leave a minimum 30% live crown or three whorls remaining on the tree, whichever is greater.
Based on leaving 50% crown on a tree, the number of planned pruning lifts and the final log length required, the desired average stand height can be determined. The dominant stems in each stand are taller than the average height and will therefore be above optimum height for the lift. This compromise is necessary to ensure that the majority of crop trees in a stand are of a prunable height.
Note: If the stand is to be pruned only once, it is important to ensure that a minimum desired length of clear
wood lumber can be obtained.
Stand height = 2 x (desired lumber length + stump height + trim)*
* This equation is dependent on leaving 50% live crown on the majority of the stems.
Trim is the wasted wood that can come from either end of the log in order to ensure the finished log length.
If two or three pruning lifts are planned, the stand could be pruned at an earlier age (smaller knotty core, smaller stand height) while still obtaining clear sawlog lengths. Multiple lifts maximize wood value and the production of mature wood.
Pruning in multiple lifts allows stands to be pruned early producing a smaller knotty core.
Pruning trees in the 5 m to 7 m height range will normally result in small knotty cores and greater production of clear wood. Diameter at time of treatment is related to the optimum time for financial rotation. Treating small trees will encourage earlier harvest ages. Pruning larger trees means they will require more time to yield the volume of clear wood necessary to recover the pruning cost.
The quality of wood is also affected if pruning is delayed. Dead branches, which occur as the live crown lifts, create loose knots and occlude slower than live branches. Pruning larger diameter branches may also result in the formation of bark and pitch pockets and greater grain distortion around the branch stubs.
Pruning small diameter trees:
- minimizes the knotty core and maximizes the amount of clear wood produced
- ensures branch scars heal quickly and exposure to pathogens is minimized
- reduces the possibility of bark and pitch pockets forming over the branch scar
- reduces grain distortion around the branch stub.
Branch size and number
The preferred tree for pruning will have small diameter branches and a small number of branches in each whorl. Small diameter branches are easier to cut and occlude faster than larger diameter branches. Fewer branches per whorl are easier to cut and the risk of girdling the tree is low.
Insects, disease and wildlife damage
The presence of trees damaged by insects, diseases, wildlife, or abiotic cause may preclude stand management activities. Details on the threshold levels of this kind of damage and related surveys are presented in the Silviculture Surveys Guidebook. The threshold levels that are most relevant to spacing are listed in the Spacing Guidebook. Stands having an incidence of some diseases, particularly stem rusts (e.g., white pine blister rust), may benefit from pruning treatments done to improve the chance of crop tree survival. Refer to the Pine Stem Rust Management in B.C. Guidebook for more information on pruning to reduce the affects of white pine blister rust.
Local wildlife populations should be considered for their potential impact on pruned stands. An analysis should be made of damaging agents in surrounding stands to determine potential impact on the stand to be pruned. Pruned stands may be damaged by several species of wildlife including porcupines, squirrels, hares, large ungulates and bears. (See Pests of Young Stands Guidebook).
Pruning should only be considered in stands that have low current and projected levels of tree damage caused by biotic or abiotic factors. Pruning species such as western white pine can be considered as a preventative treatment.
Stand structure and density
Stands chosen for pruning should preferably have a uniform structure with sufficient stocking over the majority of the area and a narrow range of tree heights.
Inter-tree competition should be low enough, in pruned stands, that each stem grows sufficient clear wood to make the treatment economically viable. Spacing is also a selection process to ensure that the best trees are pruned.
Select post-spacing densities to ensure that growth of pruned trees does not decrease due to inter-tree competition. The return on investment (net present value) is limited if stems are pruned in higher density, slower growing, stands. Prior to harvest, pruned stems should attain a minimum of 10 cm of clear wood on each side of the knotty core at the top end of the pruned section. Figure 1 shows the approximate dimensions of a pruned log at time of harvest. The 3-metre log in the figure shows a knotty core of the maximum desired size and a clear wood shell of the minimum desired size. A primary objective of pruning is to produce as much clear wood as possible in the shortest period of time to maximize future wood values.
To maximize the pruning investment, growth should be maintained at levels which encourage the maximum amount of clear wood production.
Figure 1. Approximate dimensions of a pruned log (one lift) at time of harvest.
Evaluate any additional treatment costs that can be expected for a particular stand. Stands close to communities with good road access are preferred over more distant or inaccessible stands. Consider other factors affecting future logging costs, such as probable harvesting system and size of block, when ranking stands for pruning potential. Due to the high cost of pruning, the treatments should be restricted to those stands where the risk of land alienation is low.
No special measures for fire hazard reduction are required for a pruning treatment.
[Return to top of document]