Norway spruce is one of the tree species expected to be challenged by climate change, and at the same time it is among the most common conifers in Danish forests. Especially in heathland plantations, monocultures of Norway spruce dominate. Therefore, this type of stand is the most relevant to study concerning conversion to close-to-nature forest management
>> Also read: What is Close-To-Nature Forestry?:
When converting a plantation of Norway spruce to close-to-nature forest management parts of the stand is cut down. Some of the larger trees are left as shelterwood – among other reasons to preserve the forest climate in the harsh microclimate of the heathland plantation. Some studies have examined how the under story grows in these situations, but the growth conditions for the canopy are still unknown. We have no knowledge of how the canopy grows when it suddenly in an advantaged age gets exposed to more light after having stood densely all its life.
Norway spruce is interesting as it has traits both as a pioneer tree and as a climax-tree. The climax species as silver fir will in general respond better to stand cuttings in an advanced age.
Prejudice on shelter-wood to accelerate growth
For a long time, it has been believed the believe, that shelterwood of Norway spruce accelerate their growth, when they suddenly get more light. The logic being that “even if they won’t grow taller, then they might still growth in diameter”.
To examine this IGN's, Ditlev Reventlow have looked into the growth of Norway spruce plantations of different ages. In short, the conclusion states that:
The older the trees are when converting, the more difficult it is for them to increase the growth in diameter and in old stands increase is not possible at all.
This leads, with increasing age, to a progressively greater loss in the stand’s overall growth.
A robust data foundation spanning app. 20 years
Two stands in respectively Klosterheden and Gludsted Plantation, are cases for the study. These heath plantations consist of large and homogenous stands, presenting opportunities for parts of a stand to be converted into close-to-nature management, while the rest is traditionally managed as conifers plantations. This gives a perfect frame of reference.
Stands | Age at release (years) | Hight at release (m) | Age at final measurement (years) |
Klosterheden | 91 years | 22 m | 109 years |
Gludsted | 61 years | 17 m | 78 years |
Stand with no shelterwood thinning. Stand with shelterwood thinning in 2005
Old stands in Klosterheden after final conversion. The fotos are taken at the same time both in approximately 110 year old stands in Klosterheden..
(To the left, dense plantation. To the right a more open stand with shelterwood thinning).
A large set of data
The data have been collected as a combination of drill samples and long-term data respectively since 2005 and 2000, furthermore the cutting is monitored in order to calculate the accumulated stand increment. The increment of the 100 thickest trees pr. hectare (also known as the dominant trees) are compared, in order to compare across stand with different numbers of stems.
In Klosterheden, shelterwood thinning was made on a 91-year-old Norway spruce stand, and the 100 thickest trees on 7 sample plots were measured drilling cores samples.
>> Read also: Drill samples gather knowledge about trees from the upper storey
Old trees come to a halt
There was no difference in diameter growth between the shelterwood and the trees in the reference stand during the 18 years between the measurements. The annual diameter increment is less than 0.5 cm for both stands, so there is no growth benefit from releasing the stand at that age.
When the stand is cut down and the shelterwood are left standing, the basal areas (x-axis) decreased, but there is no significant change in the diameter growth of the trees, contrary to what was previously believed. Klosterheden.
Difference in basal areas and volume
The basal areal is the acreage that the trees cover. Meaning, as if you cut a slice of each tree and measure the collective area in m2.
Volume is the wood density. Meaning, that if you cut down all the trees and measure the combined material density in m3.
For most people, the increment measured in volume (m3) is both easier to understand and more relevant than measured in m2, as it shows how much more wood is actually growing. The numbers show – not surprisingly – that the fewer trees in the area, the less increment in volume in the stand (some of the basal areas differ from the graph above, due to the sample plots not being entirely identical).
The middle-aged stands respond to a larger extend
The middle aged Norway spruces turn out to grow more than the older trees. The Gludsted stands were cut in their 60ies, and there is a clearer response in diameter growth in the shelterwood after 17 years.
The results for the increment of the dominant trees in volume – the 100 thickest.
The results show that the increment immediately after the cutting (64-69 years) is not as distinctive as later on (69-78 years), and it make sense that the trees need some years to utilize the space and growth larger canopies.
The stand growth is still declining, but it declines less than in the stands at Klosterheden.
The conclusion of the two experiments it, that Norway spruce does not seem to be able to respond to cutting at an advanced age.
The experiments generate better models
Several foresters use the tool Vidar in planning the forest management. Vidar is a simple and flexible PC-based module, which can project a specific forest’s condition based on stand inventory data.
The new data on shelterwood’s growth will be used to for more precise modeling of how different late interventions affect the increment in the Norway spruce plantations. Vidar has some challenges in precisely modeling the increment of shelterwood – especially if the canopy is very thin.
The next step is to examine the same on other species of conifers in particular Douglas fir, which is highly interesting in regards to close-to-nature forest management in our local regions with conifers.
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Is it profitable?
As a forest owner, the financial aspects of a conversion to close-to-nature forest management are of course of interest, where the next step is calculating the contribution margin. Clear-cutting must be compared with conversion in order to provide some indications of how the economy will turn out by conversion of Norway spruce.
>> Read also: Does Close-To-Nature Forest Management Pay?