Picture of a continuous cover silviculture forest
Kriittinen tila

A naturally developed forest is bountiful – Continuous cover structure is a solution for forestry

Lukuaika: 13 min.

Forests, together with land and waterways, are an essential part of our living environment. Peatland drainage, clear-cutting, and tillage are among the worst environmental offenders in forests. They all have similar detrimental effects. Not only do they play havoc both above and below the ground, but they can also spoil drinking water and pollute lakes traditionally used for activities such as fishing and outdoor swimming. Forest drainage can lead to increased algal growth in water systems from ponds and lakes to local sea areas. Clear-cutting sites are not only an eyesore in the landscape, but more importantly, clear-cutting hamper berry and mushrooms picking, fishing and many other healthy outdoor activities, that would relieve the stress and strain of modern life. The situation is similar all over the world.

Forests are full of life

Forests are national treasures everywhere. The best way to make sure that their numerous inhabitants, birds, mammals, insects, plants, fungi, lichens, and all other forms of life, including us humans, are doing well is to keep forests diverse. Naturally developed forests are teeming with trees from saplings through bigger trees all the way to old-growth, with a suitable mixture of hardwoods and conifers, all vying for space. In these forests there are no solitary stands of certain species in certain age, but the forest is a mixture of species of a different age that are suitable for certain habitat. In this text the term natural forest is used as a comparison to the forests that have been treated with conventional forestry in Finland.

Natural forests in Finland also grow bushes and shrubs such as bilberries, crowberries, and lingonberries on heaths, arctic bramble and bog whortleberry on moist lands, and cloudberries and cranberries on wet peatlands. All forest dwellers need protection and nourishment. Bilberry is a versatile example. It provides food for the grouse family, Siberian jay, and many other species, not forgetting us humans. Bears and cranes too, are partial to it. Clear-cutting destroys bilberries and other berries’ production. Small trees and shrubs are vital to numerous forest inhabitants.

Many forest species rely on rotting wood, snags and decaying fallen trees to house and feed them, others live among trees for the mutual benefit of both parties. Many birds, for instance, keep down insect populations which could become real pests in a forest. Likewise, without mycorrhizal fungi in the soil, forests would not flourish. Test results worldwide show that in a natural forest, trees and other vegetation live and interact in mutually beneficial partnerships. Forest soil is full of mycelium, fungal hyphae that lives in symbiosis with trees and other plants, and through their network the divergent forest species communicate, exchange water, nutrients, and photosynthetic products. Clear-cutting and tilling tears up and destroys this fine-tuned, mycelial ”neural network”. Negative human impacts can put an end to co-operation in forests and turn it into a brutal combat where species struggle for survival.

Forests are becoming less and less accessible also for outdoor pursuits such as foraging for berries and mushrooms, recreation, ecotourism, and reindeer herding in the north. Exercise in forests, which has numerous health-related benefits, becomes difficult in present day monoculture stands.

Clear-cutting diminishes biodiversity

How we treat our forests and forest soils will determine what kind of environment we live in, and which forest species will survive and flourish. In the past, forests were managed by taking out the bigger trees, large enough to make good saw timber. This practice had next to no impact on forest soil. It was called high thinning, and it gave good economic returns. When forests did not lose all their big trees at once, they retained their diverse structure and genuine identity.

The worst culprit in the Finnish forestry regime is clear-felling combined with tilling which destroys forest ecosystems. Working against all received wisdom of past generations, forestry has turned natural, multifaceted, multi-layered forests into monotonous, same age, same-size tree fields, abhorrent not only to traditional forestry but also to nature herself.

Under the current forestry regime small trees, much of the undergrowth and hardwoods have been hacked, poisoned, uprooted, and cleared away. This has, impoverished soils and their biome changed forest microclimates and led to a massive loss of good, usable raw material. At its worst, every scrap of timber from the forest, including stumps, have been removed.

Clear-felled, tilled sites and plantations are havens for forest pests like voles and deer. A few clumps of shrubbery, the occasional tree, or a rotting snag left lying on the forest floor are a poor substitute for natural-like forest habitat which has rich biodiversity. The random trees left on the clear-cutting site won’t replace the habitat but rather encourage scavengers and predators to the site.

Trees are assumed to be at the end of their age around the age of one hundred-years old. In reality, conifers are barely middle-aged then. In forest terms, age is in fact a misleading and absurd concept. It refers to the biggest trees that, after all, are only a small, even if the most eye-catching, element in a forest ecosystem. It does not represent the entire life span of the forest. A natural forest contains everything from newly hatched insects to centuries-old trees, little bushes, shrubs, snags and everything in between.

Bilberry, together with many other indicator species, will disappear for years to come

The present practice of low thinning, i.e. targeting smaller trees and undergrowth, makes no use of cost-effective natural regeneration. Clear-cutting and tilling, particularly if carried out in spring and summer, will rob many species of natural shelter and destroy their nests, even their young sometimes. Bilberry, together with many other indicator species, will disappear for years to come.

Current silvicultural practises have wreaked havoc among the many forest species with some even going extinct. Others yet have become rare or endangered. For them the prognosis does not look good, and there are over 800 forest species on the endangered list already only in Finland.

Forest soils contain a lot of carbon, organic matter, and heavy metals

Forest soils, especially in peatlands, contain much more organic matter than that tied up in trees. After clear-cutting and tilling, this organic matter starts to decompose leading to a rapid increase in carbon emissions. As organic matter such as peat breaks up, carbon dioxide and other gases like poisonous methane and sulphur compounds are released into the atmosphere often to the detriment of plants that are unable to deal with these pollutants.

Small trees sequester a miniscule amount in comparison to the carbon emissions they cause. This imbalance can persist for decades, even centuries, with these tree fields adding to carbon emissions rather than removing them from the air. With short rotation times, the imbalance has no chance of correcting itself and the bulk of emissions will remain permanently in the atmosphere.

Tilling tears up the ground and brings to the surface a soil layer laced with aluminium, mercury, and heavy metals. Combined with acid rain, these pollutants become water-soluble and poisonous to plants and other organisms. It takes millennia for soil, especially the podzol layers typical to coniferous zone, to reform.

Organic matter, mainly in the form of colloidal particles, leaches from tilled and clear-felled areas and drained peatlands into natural water systems. It colours them brown, only coagulating when it reaches brackish sea water. As the drains age, the rate at which peat decomposes speeds up, further burdening water systems all the way to the sea. This leaching process will last as long as there is organic matter, in other words, humus and peat left to travel, and it may take hundreds of years. Meltwater and increasingly frequent violent downpours will lead to flooding that dumps more nutrients and organic debris into surrounding water systems. Sludge hoppers and basins are only a partial solution.

Climate change impacts and clear-cutting

Low thinning and clear-cutting opens forests for winds to enter and leaves them vulnerable to severe storm damage. The trouble this causes in neighbouring forests and under power lines can be costly and is often done with impunity. On the edges of clear-felled areas, trees grow cross-grained and to be of poor quality. Climate change is having an impact on snowfall, making it wet and heavy, which in turn will lead to increases in damaging snow loads, particularly in densely-crowned plantations. In mountainous regions worldwide avalanches are increasing.

To a point, emissions like nitrogen and carbon dioxide, even acid rain, will act as fertilizers bulking up timber. However, these, and other harmful emissions, when coupled with ozone and its impact on upper atmosphere and UV radiation, play havoc among forest trees, particularly their leaves and needles. Birch and aspen leaves, for instance, are showing symptoms soon after they are formed. In early June, they can already start browning, curling, twisting and rolling up.

The biggest loser from air pollution seems in the Northern countries to be spruce: serious damage from changing weather patterns, insect and fungal attacks are posing the most imminent threat

Conifers will thin out, prematurely losing needles. Their annual needle production has fallen by about a half in the past few decades, even down to one third in places. Our comparative studies using electron microscopes have shown how widespread pollution damage was from as far back as the 1980s. As capturing sulphur dioxide became more cost-effective than releasing it, there has been a reduction in sulphur dioxide emissions. The biggest loser from air pollution seems in the Northern countries to be spruce: serious damage from changing weather patterns, insect and fungal attacks are posing the most imminent threat. Fast-growing trees are increasingly prone to such damage.

Global warming has already increased the heat summation and duration of growing seasons, thus making individual trees grow faster. Increased drainage has raised tree growth in peatland forests. Increases in production ought to be a good thing, but the factors described before that are hastening climate change will, in the long-term, take their toll.

Forest management and the utilisation of timber play an increasingly important role in mitigating climate change. Largely focused on energy production, tree fields are responsible for increasing carbon emissions. The key factor regarding our future is the amount of carbon in the atmosphere and how the carbon is stored. Where forests lose their diversity and tree fields begin to dominate, forestry becomes nothing but a mass producer of bulk pulp wood.

A better alternative: Continuous Tree-cover Silviculture

There is a workable alternative available. It is called Continuous Tree-cover Silviculture (CTS) and it allows woodlands to keep their tree cover despite repeated harvesting. For over 30 years our research team has studied and developed alternatives to clear-cutting, such as a model for continuous tree-cover management that keeps forests as close to their natural state as possible. It is applicable anywhere in the world. It works in other ecosystems too, such as waterways. Our scientific findings in the field of continuous tree-cover silviculture together with their associated teaching materials have been available for decades. Our earliest textbook is from 1985.

Continuous tree-cover is maintained utilising felling methods such as high and selection thinning, and by the creation of small holes to a forest stand where no more than 0.01 ha of canopy is removed. An optimal stand is a continuum of tree sizes, where smaller trees dominate and big ones are in minority. Considering all the different ways a forest may be used, the advice should be to not remove all big trees but to leave some, oddly-shaped ones for instance, to create variety and scenic focal points.

Continuous cover forest before cutting Photo: Timo Pukkala

Because clear-felling removes all freely regenerated underwood it becomes an expensive operation; removing small trees is costly and time consuming. These escalating costs also include undergrowth clearance, an unnecessary extravagance in continuous tree-cover terms. Felling costs are often lower in the latter anyway, and certainly lower than those incurred during intermediate cutting in monoculture regimes. The financial calculations for a monoculture site should start with the initial costs of clear-cutting, followed by those of planting and low thinning, and end when the final felling figures become available.

Under continuous tree-cover management, wood production increases, especially that of good quality and financially valuable saw timber. Overall stand quality also improves. After clear-cutting and tilling operations, the ensuing tree fields waste the sun’s energy on grasses and herbs instead of growing trees. Low thinning as practised today gives next to no advantage to bigger trees. High thinning as practised under continuous tree-cover management will give smaller trees a considerable boost.

In addition to yielding high quality timber, continuous tree-cover silviculture also provides huge amounts of pulp wood and other bio-based products, such as sawing waste from big trees. They can be used in the pulp industry, or turned into innovative plastic substitutes, textiles or scents like any other biotic material.

Continuous cover forest after cutting Photo: Timo Pukkala

For decades, less has been taken out of forests in Finland than they would naturally yield. The difference between allowable cut and outturn has inflated the figures for both the number of trees and their annual increment. Huge sums of money have been spent on foreign imports amounting to tens of millions of cubic metres, while home-grown hardwoods have been left rotting on the ground. There has been an increase both in conifer numbers and their yields, but the figures are misleading for the simple reason that the majority of bigger trees are out of the system and planted trees show strong growth early on in their development, which pushes up their increment percentage.

Timber production becomes more profitable under continuous tree-cover management

One by one, our research has debunked all arguments that monoculture proponents have put forward. For the sake of economy, it would make sense to abandon the present drive for pulpwood and instead put the emphasis on good quality saw timber and its associated bioproducts. The use of timber as a building material should become a growing trend. It would efficiently capture and store carbon long into the future. Concrete and steel as building materials are detrimental to both human health and the environment.

The large-scale roll-out of continuous tree-cover practices could make positive changes to forest structure and biodiversity. It would also facilitate the multiple usage of forests. Timber production becomes more profitable under continuous tree-cover management, and the state subsidies, at present wasted on bulk cellulose pulp, could be terminated altogether.

Continuous Tree-cover Forestry protects the environment, society and economy

Continuous tree-cover silviculture would reduce the negative impacts clear-cutting and tilling have on biodiversity and scenery, not to mention their effects on climate change. Where the ground is neither tilled nor drained and it is allowed to maintain its tree cover with its associated vegetation, huge amounts of carbon remain stored, instead of being released to pollute the atmosphere.

The continuous tree-cover adage states: Cut saw timber, let smaller trees and saplings rise to replace them, and you won’t spoil our shared environment. Continuous tree-cover management never seeks to remove undergrowth or destroy hardwoods. The forest ground and water systems are neither spoiled with tilling nor polluted through draining and its need for aftercare, in the form of drain clearing and supplementary drainage. Continuous tree-cover forests, enriched with hardwoods, will maintain balanced water relations without the need for tilling or draining. Under continuous cover trees, as well as bushes and shrubs, will flourish. Continuous tree-cover is what makes a wooded area a natural forest where one can feel refreshed and healthy.

Mature, high-grade retention trees will act as future seedbanks. In reindeer herding areas, mature retention trees will provide and spread beard moss, which is vital to reindeer diets. Continuous tree-cover silviculture can be adapted to any site with no need for a transitional period when moving over from rotational regimes for instance. Research has shown that species like bilberry find their optimum under the same structural conditions that yield the best results in continuous tree-cover forests. The same would appear to apply for many other forest species.

Even-aged (left) and uneven-aged (right) forest

Under continuous tree-cover management, the forest is resting on a solid foundation, meaning that the symbiosis between mycorrhizal fungi and tree roots is kept healthy and functioning. The regime will also consider other, culturally and socially important aspects. The impact forestry has on climate change will slow down considerably, thus improving our environment and getting us out to enjoy it more.

Worldwide, research shows that people can live with forests in mutual harmony and co-operation. Urban dwellers, always in a hurry and under pressure, will find a visit to a natural forest invigorating. Those with hypertension may find their blood pressure improved, and the same goes for many other ailments. We calm down in a forest. The high-grade timber grown in such forests will build better, healthier houses, a much better alternative to the present pulpy, mass-produced wood prone to mould damage when encased in concrete. It has been shown that people with severe sensitivities to environmental pollutants can find solace in log-built houses. The bare wood surface will eliminate microbes.

In both ecological and financial terms, the continuous tree-cover management is a better alternative to the current forestry system. The difference is not a matter of a percent or two but their multiples. If our decision makers had had the foresight to take and apply the advice that research and logical thinking have made available decades ago, our economy and environment would now be on a much better footing. Climate change would be a lesser threat than it is at the moment. We could change that immediately.


Article picture: Kari Antikainen

Some references

Laiho, O., Lähde, E. & Pukkala, T. 2011. Uneven- vs even-aged management in Finnish boreal forests. Forestry 84(5): 547–556.

Laiho, O., Pukkala, T. & Lähde, E. 2014. Height increment of understorey Norway spruces under different tree canopies. For. Ecos. 1(4): 1-8.

Lähde, E. 1969. Biological activity in some natural and drained peat soils with special reference to oxidation-reduction conditions. Acta For. Fenn. 94. 69 pp.

Lähde, E., Eskelinen, T. & Väänänen, A. 2002. Growth and diversity effects of silvicultural alternatives on an old-growth forest in Finland. Forestry 75(4): 395-400.

Lähde, E., Laiho, O. & Norokorpi, Y. 1999. Diversity-oriented silviculture in the Boreal zone of Europe. For. Ecol. Manage. 118: 223–243.

Lähde, E., Laiho, O. & Norokorpi, Y. 2000. Hyvän metsänhoidon opas. (Guides for close to nature silviculture). Ekometsätalouden Liitto and Rakennusalan kustantajat RAK. 56 pp. Guidebook.

Lähde, E., Laiho, O., Norokorpi, Y. & Saksa, T. 1991. The structure of advanced virgin forests in Finland. Scand. J. For. Res. 6: 527-537.

Lähde, E., Laiho, O., Norokorpi, Y. & Saksa, T. 1999. Stand structure as a basis of diversity index. For. Ecol. Manage 115:213-220.

Lähde, E., Laiho, O., Norokorpi, Y. & Saksa, T. 2002. Development of Norway spruce dominated stands after single-tree selection and low thinning. Can. J. For. Res. 32: 1577–1584.

Lähde, E., Norokorpi, Y. & Oikarinen, M. 1985. Mikkelin ekoläänin metsien vaihtoehtoiset käsittelymallit. (The alternative forest treatments for Mikkeli ecocounty). Research Notes. The Finnish Forest Research Institute. 180.

O´Hara, K. L., Lähde, E., Laiho, O., Norokorpi, Y. & Saksa, T. 2001. Leaf area allocation as a quide to stocking control in multi-aged, mixed-conifer forests in southern Finland. Forestry 74(2): 171-185.

Pukkala, T., Laiho, O. & Lähde, E. 2016. Continuous cover management reduces wind damage. For. Ecol. Manage 372: 120–127.

Pukkala, T., Lähde, E. & Laiho, O. 2009. Growth and yield models for uneven-sized forest stands in Finland. For. Ecol. Manage. 258: 207-216.

Pukkala, T., Lähde, E. & Laiho, O. 2011. Metsän jatkuva kasvatus. (Continuous Cover Forestry). Joen For. Program Consult. Bookwell, Porvoo. Guidebook, 229 pp.

Pukkala, T., Lähde, E. & Laiho, O. 2015. Which trees should be removed in thinning treatments? For. Ecos. 2(1): 1-12.

Pukkala, T., Sulkava, R., Jaakkola, L. & Lähde, E. 2012. Relationships between economic profitability and habitat quality of Siberian jay in uneven-aged Norway spruce forest. For. Ecol. Manage 276: 224- 230.

Valkonen, S., Lähde, E., Lappalainen, S., Laiho, O. & Saksa, T. 2017. Tree and stand recovery after heavy diameter-limit cutting in Norway spruce stands. For. Ecol. Manage. 389: 68-75.

Zenner, E., Lähde, E. & Laiho, O. 2011. Contrasting structural dynamics of even-sized and uneven-sized Picea abies dominated stands over 15 years. Can. J. For. Res.41: 289–299

Kirjoittajan kuva

Erkki Lähde

Erkki Lähde is professor emeritus in Forest Soil Science and in Silviculture of The Finnish Forest Research Institute and still continues his research work. He has developed the Continuous tree-cover silviculture method.