Olavi Laiho

Diversity-oriented silviculture in the Boreal Zone of Europe

Abstract This article presents the concepts and models of diversity-oriented silviculture in the Boreal Zone of Europe based on literature, Finnish national forest inventories and field experiment data. The basic idea following the natural processes of forest ecosystems in silvicultural practices has been known since the last century but appropriate comparison materials for modelling have been scanty previously. Stand structure classification used in this study is based on the range and shape of stem distribution. The two main groups are even-sized and uneven-sized. A subclass of the latter, regularly all-sized, with dbh distribution resembling a reversed J, forms the primary basis for treatment models. This kind of structure, following the development and dynamics of natural mixed stands, fulfils the requirements for biodiversity according to international resolutions. Single tree selection and group selection are primarily the main treatments and seed tree and shelterwood cuttings are secondary treatments in diversity-oriented silviculture. In addition, regimes aiming to diversify even-sized stands in different development stages are discussed.

The Structure of Advanced Virgin Forests in Finland

The study deals with the structure of advanced virgin forests in Finland based on the national forest inventory carried out in the 1920s. At that time, virgin forests covered 2/3 of the forested area of Finland. Stand structure was described in terms of the d.b.h. distribution. The study material consisting of 10 x 50 m sample plots was grouped according to geographical region, site type and the age class of the overstorey. A stand was classified to be advanced if the age of the overstorey was at least half of the currently applied rotation age and if the stem volume exceeded 40 m3/nectare. About 60% of all virgin forests (i.e. 932 sample plots) were classified to be advanced. In the 1920s. the majority of advanced virgin forests were mixed stands with an all‐sized structure. Trees of small diameter prevailed and the stem number diminished rather steadily with increasing d.b.h. Even‐sized stands with a d.b.h. range of 15 or 20 cm and a normally distributed diameter frequency accounted for only 10 sample...

Species Interactions in the Dynamics of Even- and Uneven-Aged Boreal Forests

Many boreal tree stands are neither clearly even-aged nor clearly uneven-aged. The stands may undergo a series of stages, during which an even-aged stand is transformed into two-storied mixed stand, and finally to multistoried or uneven-aged stand structure. The species composition often changes during the succession of stand stages. This study developed models for stand dynamics that can be used in different stand structures and species compositions. The model set consists of species-specific individual-tree diameter increment and survival models, and models for ingrowth. Separate models were developed for Scots pine, Norway spruce, and hardwood species. The models were used in a growth simulator, to give illustrative examples on species influences and stand dynamics. Methods to simulate residual variation around diameter increment and ingrowth models are also presented. The results suggest that mixed stands are more productive than one-species stands. Spruce in particular benefits from an admixture of other species. Mixed species improve diameter increment, decrease mortality, and increase ingrowth. Pine is a more beneficial admixture than birch. Simulations showed that uneven-aged management of spruce forests is sustainable and productive, and even-aged conifer stands growing on medium sites can be converted into uneven-aged mixed stands by a series of strong high thinnings.

Optimizing any-aged management of mixed boreal forest under residual basal area constraints

The current trend of forest management in many countries is reduced use of clear-felling and planting, and increased use of continuous cover management. In Finland, the new forest act of 2014 made all types of cuttings equally allowable on the condition that if the post-cutting residual stand basal area is too low, the stand must be regenerated within certain time frame. Forest landowner can freely choose between even- and uneven-aged management. This study developed a method for optimizing the timing and type of cuttings without the need to categorize the management system as either even-aged or uneven-aged. A management system that does not set any requirements on the sequence of post-cutting diameter distributions is called any-aged management. Planting or sowing was used when stand basal area fell below the required minimum basal area and the amount of advance regeneration was less than required in the regulations. When the cuttings of 200 stands managed earlier with even-aged silviculture were optimized with the developed system, final felling followed by artificial regeneration was selected for almost 50% of stands. Reduction of the minimum basal area limit greatly decreased the use of artificial regeneration but improved profitability, suggesting that the truly optimal management would be to use natural regeneration in financially mature stands. The optimal type of thinning was high thinning in 97–99 % of cases. It was calculated that the minimum basal area requirement reduced the mean net present value of the stands by 12–16 % when discount rate was 3–5 %.

Silvicultural alternatives in an uneven-sized forest dominated by Picea abies

In this article, we report on the increments in basal area and tree diameter as well as the structural development observed in variously thinned plots that underwent either uneven- or even-sized treatment. The experimental forest was originally an uneven-sized mixed stand dominated by Picea abies. Twenty-eight randomized sample plots underwent each treatment, and the trees were monitored for 15 growing seasons after thinning. The uneven-sized plots retained a reverse J-shaped diameter distribution, but this was changed into a bell shape by low thinning in the even-sized plots. Absolute basal area increment was positively correlated with basal area in the even-sized treatment but not in the uneven-sized treatment. In the latter, all of the plots grew almost equally well, and only the basal area of broadleaves explained slightly positively the increment variation. Relative basal area increment was negatively correlated with basal area in both treatments. Additionally, the basal area of Scots pine was a positive explanatory variable in the relative increment variation in the even-sized treatment. For the dominant Norway spruce trees, diameter increment was negatively correlated with basal area in both treatments and, conversely, heavy removal increased the diameter increment. Relative basal area increment averaged 5% annually in uneven-sized plots representing the “target selection.” This was more than double the increment observed for the even-sized plots that represented the “prevailing practice.” Likewise, the diameter increment of Norway spruce trees was 48% greater in the uneven-sized compared to the even-sized plots.

Stand management optimization - the role of simplifications

BackgroundStudies on optimal stand management often make simplifications or restrict the choice of treatments. Examples of simplifications are neglecting natural regeneration that appears on a plantation site, omitting advance regeneration in simulations, or restricting thinning treatments to low thinning (thinning from below).MethodsThis study analyzed the impacts of simplifications on the optimization results for Fennoscandian boreal forests. Management of pine and spruce plantations was optimized by gradually reducing the number of simplifying assumptions.ResultsForced low thinning, cleaning the plantation from the natural regeneration of mixed species and ignoring advance regeneration all had a major impact on optimization results. High thinning (thinning from above) resulted in higher NPV and longer rotation length than thinning from below. It was profitable to leave a mixed stand in the tending treatment of young plantation. When advance regeneration was taken into account, it was profitable to increase the number of thinnings and postpone final felling. In the optimal management, both pine and spruce plantation was gradually converted into uneven-aged mixture of spruce and birch.ConclusionsThe results suggest that, with the current management costs and timber price level, it may be profitable to switch to continuous cover management on medium growing sites of Fennoscandian boreal forests.

Using optimization for fitting individual-tree growth models for uneven-aged stands

A convenient model type for simulating the dynamics of uneven-aged and uneven-sized stands of Finland is individual-tree model. This is because the stand structures are complex due to the presence of several tree species and irregular size distributions of trees. The required minimum set of models in this approach consists of species-specific individual-tree diameter increment models, individual-tree survival models, and ingrowth models. The development of these models needs data in which the diameter and survival of each tree of the sample plots is known for at least two time points. For this, the trees need to be numbered, which is tedious in uneven-aged forests due to the great number of small trees and the continuous ingrowth process. This study proposes a modelling approach that fits the above models but requires only the diameter distributions of the plots in the beginning and at the end of the measurement interval. The method uses non-linear optimization to derive such values for model parameters that, when the models are applied to the initial diameter distribution, the simulated stand development results in a diameter distribution which agrees with the measured ending distribution. The study showed that the method produces similar models and model parameters as regression analysis. Since the method is less demanding in terms of modelling data, it brings new data sets available for modelling the dynamics of uneven-aged stands and reduces the cost of collecting new data. The models fitted by the proposed optimization method were rather similar to the models developed earlier for Finnish uneven-aged forests.

Continuous Cover Forestry in Finland – Recent Research Results

The current official silvicultural instructions of Finland recommend even-aged rotation forest management (RFM) combined with low thinning and artificial regeneration. However, the direction is gradually changing towards increasing freedom and flexibility in forest management. The first sign of a new silvicultural era was the gradual approval of high thinning, which is now accepted although it was strictly forbidden in the past. As a further step, uneven-aged management and other forms of continuous cover forestry (CCF) are now gaining popularity and acceptance. The harmful impacts of clear felling and plantation forestry on the recreational value and biodiversity of forests have been increasingly emphasized. In addition, encouraging results on the yield and profitability of CCF have been obtained recently. This chapter reviews those results. The review shows that CCF is often more profitable than even-aged management. The superiority of CCF increases if non-wood benefits are included in the analysis. The growth rates of most Finnish forests are too low to warrant the high stand establishment and management costs of even-aged plantation forestry. If the forest landowner wants to practice forestry that is profitable (without state subsidies) also with high discounting rates, on poor growing sites and in the northern parts of Finland, her only option is continuous cover management.

Height increment of understorey Norway spruces under different tree canopies

BackgroundStands having advance regeneration of spruce are logical places to start continuous cover forestry (CCF) in fertile and mesic boreal forests. However, the development of advance regeneration is poorly known.MethodsThis study used regression analysis to model the height increment of spruce understorey as a function of seedling height, site characteristics and canopy structure.ResultsAn admixture of pine and birch in the main canopy improves the height increment of understorey. When the stand basal area is 20 m2ha-1 height increment is twice as fast under pine and birch canopies, as compared to spruce. Height increment of understorey spruce increases with increasing seedling height. Between-stand and within-stand residual variation in the height increment of understorey spruces is high. The increment of 1/6 fastest-growing seedlings is at least 50% greater than the average.ConclusionsThe results of this study help forest managers to regulate the density and species composition of the stand, so as to obtain a sufficient height development of the understorey. In pure and almost pure spruce stands, the stand basal area should be low for a good height increment of the understorey.