Issued by the Co-ordination Committee for Forest Tree Breeding and Genetics, in accordance with a proposal of Börje Häggström, State Forest Service, Sweden.
In Sweden the selection of plus trees is based on assessments of phenotypic characters, particularly growth rate, stem form, branching habit and wood quality. For plus trees growing on sites at high elevation, ability to flower and seed quality are also considered.
A plus tree is one that is distinctly superior both to certain neighboring trees and to the average of the stand. The trees with which the plus tree is compared are the four largest trees of the same species in the immediate neighborhood, that is, within 25 to 50 meters (23 to 46 yards) from the plus tree. The comparison trees should all be dominants.
Directions for selecting plus trees of Pinus sylvestris and Picea aides
The plus tree and the trees with which it is compared should be of the same age as estimated at breast height. The difference in age must normally not exceed 10 years. However, should the difference in age between the plus tree and the 4 neighboring dominants exceed 10 years, the plus trees should have dimensions markedly larger than normal for the age and site conditions.
Site conditions, such as the fertility and moisture status of the soil, the slope, and the density of the stand should be apparently similar for all trees.
The plus trees must be healthy and free from disease and insect attack. They must be considerably taller and have distinctly larger diameter than the comparison trees. The stems of plus trees must be of good quality with flattened or only slightly acute branch angles.
A point scoring system is used in judging a plus tree and this system is used to calculate both a volume score and a quality score. The volume is estimated under bark. The volume of the comparison trees is adjusted to the stage of development corresponding to the age of the plus tree. The volume score is derived from the ratio of the volume of the plus tree to the average volume of the comparison trees. Every tenth part in excess of the value 1.0 receives one volume point.
The calculation of the quality score is based on estimates of the coarseness of the branches, branch angle and wood density. The number of branches per whorl is also included. How the quality score is arrived at will be clear from Tables A1 and A2.
The total score for the plus tree is the sum of the volume and the quality scores. These figures are converted into a scale of classes: Bc (not approved), B, Ba, AB, a and A according to the scheme in Table A3.
TABLE A 1. - CLASSES USED FOR RELATIVE BASIC WOOD DENSITY 1
|
Density glass |
Score |
|
|
|
Percent |
|
|
Extremely high |
115.5 |
+ 4.0 |
|
Very high |
107.5-111.4 |
+ 2.5 |
|
High |
103.5-107.4 |
+ 1.0 |
|
Average |
96.5-103.4 |
0.0 |
|
Low |
92.5-96.4 |
- 1.0 |
|
Very low |
88.5-92.4 |
- 2.5 |
|
Extremely low |
88.4 |
- 4.0 |
1 Relative basic density expresses the relation between the observed and calculated basic wood density.
TABLE A2. - CLASSES USED FOR BRANCH THICKNESS AND BRANCH ANGLE
|
Branch thickness |
Branch angle 1 | ||
|
Class |
Score |
Class |
Score |
|
Very thick branches |
4 |
50° |
- 2 |
|
Thick branches |
- 2.5 |
50 to 60° |
- 1 |
|
Moderately thick branches |
- 1 |
60 to 70° |
0 |
|
Normal branches |
0 |
70 to 80° |
+ 1 |
|
Moderately thin branches |
+ 1 |
80 to 90° |
+ 2 |
|
Thin branches |
+ 2.5 |
|
|
|
Very thin branches |
+ 4 |
|
|
1 Used for Scots pine only.
NOTE: The quality score is reduced by 1 or 2 points if the number of branches per whorl is abnormally large.
TABLE A3. - CONVERSION OF TOTAL SCORE TO A CLASSIFICATION OF PLUS TREES
|
Total score |
Classification |
|
- 7.5 to - 1.5 |
Bc 1 |
|
0 to + 6 |
B |
|
7.5 to 13.5 |
Ba |
|
15 to 21 |
AB |
|
22.5 to 28.5 |
a |
|
> 30 |
A |
1 Not approved.
NOTE: For Norway spruce the highest possible quality score is 8, as no points are given for branch angle.
The plus tree and comparison trees are inspected by the scientific leader of the Co-ordination Committee for Forest Tree Breeding and Genetics. During this check, variations in site quality, density of the stand, the influence of neighboring trees and past management of the stand are assessed. This second inspection may result in the original classification being changed.
How different volume and quality scores influences the total score is shown in Table A4.
This preliminary assessment of plus trees is, of course rather crude and unsatisfactory. It is therefore important to define more logical and reliable directives for selecting plus trees, based on the results of progeny tests. These directives for selecting plus trees and mass selection should take account of the heritability of different characters and the genetic and phenotypic correlations between them. Also the relative economic value of different properties must be considered.
TABLE A4. - RELATION BETWEEN VOLUME AND QUALITY SCORES USED IN SELECTING PLUS TREES OF SCOTS PINE AND NORWAY SPRUCE
|
Volume score | |||||
|
20 |
20 |
22.5 |
25 |
27.5 |
20 |
|
15 |
15 |
17.5 |
20 |
22.5 |
25 |
|
10 |
10 |
12.5 |
15 |
17.5 |
20 |
|
5 |
5 |
7.5 |
10 |
12.5 |
15 |
|
0 |
0 |
2.5 |
5 |
7.5 |
10 |
|
Quality score |
0 |
2.5 |
5 |
7.5 |
10 |
