Thinning
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Thinning is called a silvicultural maintenance measure in which a large number of trees are deliberately removed from a tree population . Depending on the goal of the thinning, only dead, additionally weak and incorrectly grown trees or trees that are already ready for action are felled. The general objective of thinning is the stability of a tree population, the control of the tree species composition and the control of the tree growth ( timber yield ).
Preliminary stages of thinning are cultivation and clearing . A maintenance measure is only called thinning when usable wood (cf. timber , solid wood ) is produced.
The term thinning was first introduced into literature by Georg Ludwig Hartig (1791, instructions on wood cultivation for foresters ).
Types of thinning
The following types of thinning are available: (The distinction between types of thinning is based on tree class systems.)
Thinning
Interventions in the predominant and dominant tree layer. The dominant layer ( tree class 2 according to Kraft ) is the stand-forming layer. The consequence is a formative effect on the existing building, especially with regard to the increase in value and stability. This type of thinning (mostly) represents the current thinning procedure.
Thinning
Interventions in the lower strata of the stand (especially Kraft tree classes 3 and 4). Formerly the most common thinning method. However, the effect on the stand is rather negative because it creates collective stability rather than individual tree stability. Furthermore, due to the high density in the ruling area, growth losses are the result, which is why this method is only used in exceptional cases today.
Low thinnings are divided into different intensities (degree of thinning - no longer in use today):
- A grade, poor thinning: removal of dead and dead trunks, as well as bent poles (tree class 5) and diseased trunks
- B-grade, moderate thinning: removal of dead, dead trunks, bent, suppressed trunks, whipers, overgrowths (tree classes 4.5 and parts of 2)
- C-grade, strong thinning: Gradual removal of all trunks of tree classes 2–5, as well as some of the class 1
Procedure
Selection thinning (after Walter Schädelin )
Positive elite thinning
Here the readout trees (cf. Z-tree ) are first determined in the stock . A readout tree is selected according to the following criteria:
- Vitality (characteristic: crown length),
- Quality (characteristic: shaft shape, knottiness),
- spatial distribution
The trees are selected either schematically (e.g. a selection tree every 5–7 meters) or depending on what is available on the area (e.g. 250 selection trees per hectare). After the selection trees have been determined, about 1–2 trees that are in direct competition for resources (light, water, nutrients, standing space) with the selection trees are removed . These trees are known as oppressors .
Negative readout thinning
In doing so, unwanted trees are eliminated. Badly shaped, coarse-branched, twiggy or loose trees are undesirable. This process is more costly than positive selection.
The selection of the readout trees is made "new", ie dynamically, with each thinning. A selection tree of the thinning in year X can possibly be a threat to the next selection thinning five years later (see Z-tree concept).
Systematics: High thinning → selective thinning
Z-tree concept (according to Altherr)
In the Z-tree concept, trees of the future ( Z-trees ) are selected at the age of sticks (with an upper height of approx. 15-20 m) , which should remain the value carriers for the life of the stand. The selection is static and can be made schematically or based on a total number per hectare. This number depends on the respective tree species (e.g. beech 60–100 (120) Z-trees per hectare, spruce 120–200 (220) per hectare). The criteria for the Z-tree selection are the same as for selection thinning:
- Vitality (characteristic: crown length),
- Quality (characteristic: shaft shape, knottiness),
- spatial distribution.
The Z-tree concept was developed in Baden-Württemberg and is mainly used there.
Systematics: Thinning → Z-tree concept
Group crawl (after Klein)
This thinning essentially represents a selective thinning taking into account natural group structures. In the above thinning, natural groups are not taken into account. Here it is preferred not to promote individual readout trees, but rather groups of readout trees. Theoretically, there is no maximum value for group members, but in practice no more than seven trees are grouped together. There is strong intervention around the selected groups, so that weaknesses arise in the stand structure. This will lead to early rejuvenation and the groups will also be stabilized, as larger crowns can be developed. Once the groups have been selected, they must also be harvested together, as the groups above all have a collective stability. Groups are dynamic: There is no permanent group membership. Falling trees are specifically removed.
The high growth with horizontal unevenness is remarkable.
Systematics: High thinning → selection thinning taking into account group structures
Forest conversion process
The following thinning procedures are not actually thinnings, but measures in the context of forest conversion , more precisely the transfer of forest stands.
Structural thinning (according to Reiniger, von der Goltz)
This instrument is used to transfer single-layer age-class forests (mostly pure spruce stands) into permanent forests . First of all, Z1 trees are selected in the upper class, which, after clearing the ground, grow until they have reached the target diameter. Then they are harvested individually .
Subsequently, halfway well-crowned intermediate and lower trees are selected and designated as Z2. Its purpose is to stabilize the upper layer after the first target diameter harvests (= opening of the canopy) and to hold on to the development of the stand until they take over their role after the complete harvest of the Z1 trees. The result should be a tiered inventory through the use of individual logs.
- according to cleaner: 300 Z1 trees, 300 Z2 trees (each per hectare)
- According to von der Goltz: 150–200 Z1 trees, 150–200 Z2 trees (each per hectare)
Plenter thinning
Bernard Borggreve became internationally known for his research on plenter thinning.
Quality of thinning
You can see the level of training and understanding of nature conservation concerns from a thinning crew at the thinning. Trained foresters also pay attention to what has grown naturally and are ready to promote such, be it to increase the resilience of the stand ( h / d ratio , biodiversity , etc.) or for other ecological and economic reasons.
A basic rule for thinning, coined by Carl Justus Heyer , is: “Early, often, moderately.” An early start means an early control towards the silvicultural goal (tree species composition, stability, growth control) - depending on the goal, however, it can also take place too early. Frequent, moderate thinning (according to the forestry timescale) gives the trees the opportunity to adapt to the changed stand structure. In a decade, a single intervention with a certain amount of usage carries a higher risk than three interventions with 1/3 usage each. The deteriorating ratio of income to costs makes compliance with this basic rule increasingly impossible.
When thinning stands that have not yet been thinned or negatively thinned (and thus - often unintentionally trimmed to clear felling due to excessive caution ), there is an increasing need to continue the previous management form with increasing stand age.
Motto: "A good thinning requires a clear goal, a sharp ax and a cold heart" - depending on the given situation.
history
The first written reports about thinning come from forest regulations of the 15th century. In his book Von Forstlicher Überherrligigkeit vnd Gerechtigkeit , the legal scholar Noe Meurer emphasized the beneficial influence of thinning measures on the growth of the remaining trees.
The term thinning itself was first introduced into literature by Georg Ludwig Hartig in 1791 in instructions on wood cultivation for foresters and given rules for its execution. However, Hartig did not present any new findings, but only summarized and systematically processed the existing experience.