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Understorey cleaning before thinnings

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When thinning a stand for the first time after regeneration, understorey cleaning is usually done if the number if the the number of understorey layer is so dense that the harvesing will be hindered. As default, understorey refers to trees with a diameter dbh < 8 cm.

To control when and how understorey cleaning is done in Heureka, a number of rules are used which are entered in the TreatmentModel control table, see below. Understorey cleaning is simulated by removing trees immediately before a thinning. It is not considered a separate treatment, but part of a thinning. The removed trees are included in the calculated thinning grade (Treatment.Thinning Grade).

Please note the following:

•	Understorey cleaning is not reported with a specific treatment code, but is considered part of a thinning. However, if understorey cleaning is done, a specific cost is reported (FinancialValue.UndestoreyCleaningCost), and the number of cleaned trees is also reported (Treatment.UnderstoreyCleaned).

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Understorey cleaning is not possible in selection felling, which is the "thinning type" used when choosing Management System = Uneven-aged/Continuous Cover Forestry, since natural ingrowth of new trees is an import part of the system (at least in Heureka where artificial regeneration is only possibly in even-aged management).

•	You can inactivate the possibility to do understorey cleanings by setting the parameter Understorey Proportion Removal to 0 %

•	The number of understorey trees (not cleaned) will affect the thinning harvester cost.

•	You can let the program automatically switch from an ordinary thinning to a biofuel thinning if there are many understorey trees. A control table parameter sets the minimum number of understorey trees that triggers a bio fuel thinning.

The TreatmentModel control table contains settings for when and how understorey cleaning is done.

Parameter	Beskrivning	Standardvärde
Understorey Max Diameter	Diameter limit for understorey trees. A tree will be classified as an understorey tree if its diameter (dbh) is less than this value and the stand is "mature" (i.e. not a plantation or young stand)	8 cm
Understorey Cleaning Stems Threshold	Minimum understorey stem density required for understorey cleaning. If the number of understorey trees is less than this value, no understorey cleaning will be applied.	800 trees/ha
Min Understorey Stems Retained	The minimum number of understorey trees left if understorey cleaning is done. The rationale for this is that usually some understorey are retained, there is simply no reason to cut all such trees.	500 trees/ha
Understorey Proportion Removal	Percentage of understorey trees to be removed. Actual proportion is adjusted if Min Understorey Stems Retained (see row above) is would be violated. Setting this parameter to 0 % mean that no understorey cleaning will be done.	90 %

UnderstoreySettings

Figure 1. Settings for understorey cleaning is found in the TreatmentModel control table.

You can let the program automatically switch from an ordinary thinning and do a biofuel thinning instead if there are many understorey trees. You read read more about this in the Bio fuel thinning chapter.

The cost for understorey cleaning is entered as a fixed cost in the Cost and Revenue control table (parameter "Understorey Cleaning Cost (per ha)").

Cost_Understorey_Cleaning

The time consumption for the thinning harvester is affected by the number of understorey trees that occurs. Choosing Cost Calculation Model = "Skogforsk", means that Brunberg (1997) is used, but that model does not include occurrence of understorey as a hindrance. An addition has therefore been made based on an earlier version (Brunberg 1989). This mean that the time consumption increases with 0.4 cmin per harvested tree, for each thousand small trees ("Harvester Time Increase from Understorey Trees"). Example: Assume there are 5000 understorey trees that are not cleaned. This will result in a 2 cmin additional time per thinned tree (0.4 x 5 thousand). if you thin 1000 trees and the harvester cost is 1200 SEK per hour, the total time consumption will increase by 1000 x 2 cmin = 2000 cmin = 20 minutes, i.e. 400 SEK per hectare.

Cost Calculation Model = "Indelningspaketet" inkluderar småstammar som en variabel.

Understorey cleaning is not reported with a specific treatment code, but is considered part of a thinning. However, if understorey cleaning is done, a specific cost is reported (FinancialValue.UndestoreyCleaningCost), and is of course included in the Total Cost variable. The number of cleaned trees also reported as Treatment.UnderstoreyCleaned in result group Treatment. ForestData.Understorey Stems (in result group ForestData) is a state variable that represents the number of understorey trees. This is only calculated in established stands, not in young stands.

ResultVar_UnderstoreyCleaningCost ResultVar_UnderstoreyCleaned

Referenser

1) Brunberg T, 1997. Underlag för prestationsnorm för engreppsskördare i gallring. Redogörelse nr 8, Skogforsk.

2) Brunberg T, Thelin A, & Westerling, S. 1989. Underlag för prestationsnormer för engreppsskördare i gallring. Redogörelse nr 3, Forskningsstiftelsen Skogsarbeten.