Long-term experiments in forest research. The thinning experiments maintained by Luke, monitored for decades, and the data collected from them, form an extensive, geographically representative dataset spanning from the late 1920s to the present day. This data is utilized in clarifying the development of tree stands, undergrowth, and soil. Based on the results obtained from long-term experiments, existing models are developed and their reliability in predicting forest development is improved. Behind the boards and at the Finland 100 anniversary forest sites, you can explore two of Luke's oldest thinning experiments.
Extended rotation period in forest cultivation. An extended rotation period refers to growing an even-aged forest to be slightly older and larger in diameter than customary before regeneration felling. The extended rotation period as a forest management method is described on the backside of the boards.
Research on forest growth
It is important to understand the long-term effects of forest management methods on forest growth, mortality, and ecosystem function when the goal is to maintain or increase forest carbon sinks. The thinning experiments maintained by Luke, monitored for decades, and the data collected from them, form an extensive, geographically representative dataset spanning from the late 1920s to the present day. This data is utilized in clarifying the development of tree stands, undergrowth, and soil. The experiments cover the main tree species in various growing sites and in stands thinned in different ways. The experiments include control plots that have not been treated for decades. These can be used to determine carbon sequestration and storage, as well as the quantity and quality of deadwood (tree species, decay stage) in untreated stands. The experiments also investigate the effects of extending the rotation period in stands treated in various ways.
Based on the results obtained from the long-term experiments, existing models are developed and their reliability in predicting forest development is improved. Based on the results, it is possible to choose the forest management chain, the intensity of treatments, and their timing to reconcile timber production, carbon sequestration and storage, the maintenance of forest biodiversity, and other ecosystem services.
The results obtained from the experiments have been utilized, for example, in the preparation of forest management recommendations.
Increasing thinning intensity has many different effects on forest development and the profitability of forest cultivation. Certain features or factors increase (+) and others decrease (-) with thinning intensity.
Target information
Spruce thinning experiment (Pu041)
All sample plots
61.81698 and 29.32317
Altitude above sea level 83 m
One of the longest-running long-term experiments is experiment Pu041, located in Punkaharju.
The stand on a herb-rich heath forest (OMT) was established in 1938 by planting 2+2 year old seedlings. The planting density was 1.8 m x 1.8 m, or approximately 3100 seedlings/ha. The origin of the seeds was Parikkala. The deciduous shelterwood was removed in 1947.
The experiment was established in 1964. There are 4 sample plots. The stands have been thinned by basal area, so that plot 1's level is 100% (unthinned), plot 2 approx. 50%, plot 3 approx. 90%, and plot 4 approx. 70%.
The stand has already passed the recommended final felling time according to forest management guidelines and illustrates the effects of extending the rotation period on stand development.
Growing stock data from the latest measurement in autumn 2023:
Development of growing stock volume with different thinning intensities from 1964 to 2023.
Sample plot map from the 1960s.
Extended rotation period in forest cultivation
An extended rotation period refers to growing an even-aged forest to be slightly older and larger in diameter than customary before regeneration felling.
In managed commercial forests, the recommended regeneration time is determined by the stand's stem size, rather than primarily by the age of the trees. Measured in years, regeneration size is reached faster the higher the growth rate of the stand. By applying recommended rotation periods, forest owners generally achieve the best economic return. However, in healthy pine and spruce stands, tree growth continues vigorously for several years beyond this point. Therefore, for the production of sawlogs, growing larger diameter logs, and forest carbon sequestration, cultivating trees to be slightly larger in diameter would be a better option. In healthy and fast-growing cultivated spruce stands, growing trees to be slightly larger is also more economically viable.
However, rotation periods should not be extended for decades. The goal is to grow trees so that their stem thickness increases by 2-4 centimeters before regeneration felling. Often, a delay of 5-10 years is sufficient, but the exact time depends on the trees' growth rate, which is most influenced by site fertility and stand density.
Extending the rotation period is only suitable for vigorous and healthy pine- and spruce-dominated forests. It is safest to extend the rotation period in pine stands, where damage risks are generally lower than in spruce stands. It is also suitable for spruce stands if the trees are healthy and there is no immediate risk of storm, root rot, or bark beetle damage. If these occur, or the risk is high, then immediate regeneration is a better option. Birch stands and other common deciduous tree species in Finland are naturally shorter-lived than pine and spruce. Their growth begins to decline earlier with age than conifers, and extending the rotation period is not justified, at least from the perspective of timber production or the profitability of forest cultivation.
Figure 1. Age-dependent volume development of a planted spruce stand on a fresh heath forest and a sown pine stand on a dryish heath forest with different rotation lengths. The development is faster in the spruce stand because it typically grows on more nutrient-rich sites than pine. In a pine stand, growing the trees to a larger diameter before regeneration felling takes longer than in a spruce stand.
Figure 2. Average annual production of sawlogs and pulpwood in spruce and pine stands with different rotation lengths.
In a healthy spruce stand, growing the trees slightly larger than recommended increases timber production. In a pine stand, the average yield gradually decreases as the rotation period increases because the increase in diameter is slower than in a spruce stand.
Figure 3. The bare land value (BLV) illustrating the profitability of spruce and pine stand cultivation. It represents all net income per hectare obtained from timber production, discounted to the time of regeneration using a three percent discount rate, assuming the same forest management is applied permanently in the future.
In a spruce stand, a moderate extension of the rotation period can improve the profitability of forest cultivation. In a pine stand, profitability generally decreases when cultivated longer than current recommendations, as tree growth and stem diameter increment begin to slow down.
Figure 4. The average carbon stock of growing stock in spruce and pine stands with different rotation lengths.
Extending the rotation period significantly increases the carbon stock of growing stock in both spruce and pine stands for several years, provided the trees remain healthy and no damage occurs.
Target information
Spruce thinning experiment (Pu042)
All sample plots
61.81034 and 29.32029
Altitude above sea level 89 m
The stand on a herb-rich heath forest (OMT) was established by clear-cutting an area previously growing grey alder and planting 2+2 year old spruce seedlings in May 1928. The planting density was 1.6 x 1.6 m, or approximately 3900 seedlings/ha. The origin of the seeds was Mustila, Elimäki. The alder shelterwood was thinned in 1934, 1936, 1943, 1946, and finally removed in 1949.
The experiment was established in 1964. There are 4 sample plots. The stands have been thinned by basal area, so that sample plot 2's level is 100% (unthinned), plot 1 approx. 60%, plot 3 approx. 90%, and plot 4 approx. 50%.
The stand has already passed the recommended final felling time according to forest management guidelines and illustrates the effects of extending the rotation period on stand development.
Growing stock data from the latest measurement in autumn 2023:
Development of growing stock volume with different thinning intensities from 1964 to 2023.
Sample plot map from the 1960s.
