Approaches of Continuous Cover Silviculture in Sitka Spruce Pioneer Plantations

Hans-Jürgen Otto

1. Pioneer forest

In natural forest dynamics without human influence a successional pioneer stage means the new beginning of forest life after a large scale disturbance, for instance a storm throw. This is said to be a secondary succession, because a forest or some other vegetation has existed before. In nature many strategies of resettling open areas exist. They are in most cases very effective and at the beginning very quick. A cover of ground vegetation may precede the real pioneer stage with tree species. This stage is the "non-forest-stage". Within this stage one will meet different phases of plant species, following a phase pattern from lichens via mushrooms and mosses to grass species, herbs, ferns, and shrubs. The succession of the earliest phases may be run through in less than one year. So, in the ecological view, their are no "weeds" in nature, but strategies to cicatrices "wounded" open areas in a short period.

The pioneer stage, in which tree species begin to settle, can also be subdivided in different phases, in which different species will dominate. The main common character of all pioneer tree species is: light demanding or at least tolerating full light. Effective seed production, that means regular seed crops, rich crops, very good long distance dispersal of seeds, successful germination of large numbers of seeds, well developed capacities to germinate in different ground vegetation layers and other difficult circumstances. Pioneer species are also sprinters: Their youth growth is high. So in a short period they become dominant in competition with non-tree species, pushing them more or less out of the ecosystems. Later on, successions move to intermediate and subclimax stages, which may also have varying phases, until the climax forest is reached. This final forest is the composition of trees in a certain forest association, which is best adapted to the surrounding environmental growth features: climate, soils, and to the impact of special disturbance patterns.

Sitka spruce plantations can be considered to be a forest pioneer vegetation, although they are not natural. But they are set into open areas, covered by non-tree vegetation. The Irish and British forestry have accomplished an outstanding operation of land cultivation, which is in some basic achievements similar to North German reforestations of heathland, which began 200 years ago: Soils have been covered, wind and water erosion have been stopped. Valuable timber production was started. A humus layer began to be forced, and some first modest improvements of increasing biodiversity could be observed. Sometimes, a former excess of seepage water on special sites was diminished, when the water-pump of trees began to consume more water. These are positive results which can hardly be overemphasized.

However Sitka spruce and Norway spruce are not considered to be typical pioneer species. Despite this both have comparable effective regeneration strategies, so relatively good and frequent crops, satisfying seed dispersal strategies, good germination properties and fast growth. And both species can tolerate the full light situation of open areas.

2. Advantages of conventional forestry

Traditional Sitka and Norway spruce plantations have been realised by planting on open areas, with or without preceding soil preparation, with or without preceding weed control. Different assortments of plants, coming from nurseries, have been planted with different numbers of plants and normally with an accurate regular spacing. The plantations have been tended, thinned, and finally harvested in a calculated commercial period, the rotation time. Final harvesting method was clearfell, often on larger areas. This silvicultural method, imitating agricultural methods of cereal-fields, was successful during several generations of stands. And it cannot be excluded, that on some stable sites this might again continue for some time. Its main advantages are: Easy to plan, easy to execute, easy to calculate, easy to control, not needing sophisticated techniques. In fact, everybody can do it.

3. Disadvantages and risks of conventional forestry

However, in German Norway spruce forests some disadvantages of this traditional forestry were revealed, the longer the above mentioned methods were practiced and the more exactly the same operations were executed in many generations of succeeding stands on the same sites with the same conifer monocultures, the more evident these disadvantages have become. These disadvantages are:

· Poor stability against the impact of storm disturbances. In German Norway spruce plantations, the former regular age class distribution, which was almost maintained during the 19th century, has been increasingly disturbed, where the same plantations were repeated on the same soils. From the beginning, five age classes were planned, each covering a planning period of 20 years, with a total rotation time of 100 years. In theory the five age classes should be have with the same volumes. But this is no longer true. They became "stairs", with steeper and steeper steps. The 5th age class (age 81 to 100 years) has today a volume of 1/5 of what should be there. The rest was destroyed by storms and insects before becoming mature. The explanation for this phenomenon is the following: Old trees and stands bring a heavy weight on the soils. We measured in a 60 years old Norway spruce stand a total weight of about 600 t/ha. In Sitka spruce it will probably be higher. When spruces produce a shallow root system, they will stamp the soils when swinging in a permanently blowing wind. The result has been modification of soils by increasing compactness, minimising the aeration, with this forcing the trees to make even shallower rooting. In Germany we are quite sure, that spruces are deteriorating soils, because they have a "lazy" rooting, and more than one or two generations of spruce without stabilising mixtures are considered to be riskful.

· Spruces produce acid humus. The turnover and recycling of nutrients in the humus layers is not perfect, often it is seriously inhibited. There is no sustainable soil protection, no guarantee for soil fertility on the same level with repeated generations of spruce monocultures.

· Spruces are not really good for biodiversity. Ground vegetation is often poor or absent.

· The high risk of storm hazard produces uncertain economies.

· The clearfelling systems in spruce plantations do not provide a regular income.

· The system produces too large amounts of poor timber, too low proportions of valuable and of large timber.

· The regular spacing of planting, tending and thinning is one of the reasons of instability.

· The system is very intensive considering the impact of energy, man­power and money. From the plantations until the final clear- felling, one intensity has as a consequence the next one.

4. Possibilities for changing silvicultural methods in Sitka spruce forest

What can be done to evolve the traditional silvicultural technics in order to obtain safer, more biodiverse forests, providing a sustained regular income at the least costs to the forest owner, and minimising natural and market risks?

The possibilities can be subdivided in

• measures, where Sitka spruce silviculture shall be continued;

• measures, where Sitka spruce shall be replaced by other species.

4.1 Site Mapping

A correct analysis of what are the potentialities of species choice under the locally given environmental frame conditions needs site maps with recommendations. Site mapping is a basic requirement. It provides: Which species can be recommended; which species should not be chosen; definition and map delimitation of non stable soils; information over site-adapted soil preparation methods; site-adapted recommendations for fertilising. Site mapping methods are: a climatical analysis (macro-climate and micro­climate in relation to reliefs, aspects and exposition o.s. 1.). Soil analysis with chemical lab analysis; definition of "site types"; delimitation of these site types by mapping the fields; presentation in a site map 1:10 000; description of each site type with the conclusion of the above cited recommendations. No forest enterprise in our land is working without site mapping. The basic investment costs per ha are not low, but as sites do not change so quickly, this investment must not be repeated.

4.2 Possibilities in existing Sitka spruce stands

In the existing stands, one can see three approaches:

• methods increasing the physical stability in non-mixed stands;

• methods of natural regeneration, making profit from pioneer situations;

• methods of creating mixed stands.

4.2.1 Physical stability of Sitka spruce stands

All natural forests without human interventions have two patterns of ongoing stand growth: segregation and aggregation processes. Segregation or individualisation of tree individuals occurres through competition. The more vigorous trees suppress and eliminate the less vigorous ones. Thus, all forests move from high stem numbers to low ones. Foresters are used to sustain this evolution by tending and thinning. But in natural forest, the result of this segregation process is never a stand with regular distances between trees. Denser and clearer parts subdivide these forests in a mosaic of different densities.

Some trees stay not only close together, but they begin to merge their roots. In such biogroups there is not only a mutual exchange of nutrients, but the trees hold each other against the impact of wind. In this way, natural forests are subdivided in a mosaic of more and less stable parts. This means; that all risks will be diffused and so minimised. As a matter of fact, large scale wind-throws areas are an exception in virgin forests. The normal feature is a structure of smaller and larger gaps, denser and clearer parts.

In our traditional intervention methods, thinning operations have destroyed biogroups, when spacing "future trees" in equal distances and eliminating their competitors. As a consequence stand stability against storm impact was changed for the worse.

Sitka spruce and Norway spruce are well grouping species. Respecting biogroups can be a contribution to better stand stability. "The practical intervention methods should be;

· Identification of the existing biogroups before thinning.

· Where there are none, thinning as usual, favouring the formation of large crowns with green branches on 1/3 to 1/2 of the trees height thus bringing down the centre of gravity in the stems. This should be done early in the stands life.

· Conservation of biogroups. Groups should be considered as 'thinning units". This means, that thinning should liberate the aggregation of the crowns of a group, taking off competing trees in the neighbourhood of this aggregation, as one is used to do in favour of single trees.

· When site mapping has shown sites (often these are only micro-sites measurable in square meters) with lower stand stability, spruces should no longer be planted, because here normally is the starting point of wind-throw even on more stable parts of the whole stand. These gaps should be planted with alder.

4.22 Stream valleys

In order to introduce some landscape structure into large Sitka spruce stands and in order to increase the stability of a large conifer area, it is useful to avoid spruce plantations in stream valleys. These are very often moist areas and can be the starting point of storm hazards. The shadow given by spruces and their needle-fall is a disaster for the life forms of streams and their interactions. A strip of 20 m both sides of the stream or even more should be planted with alder (A. glutinosa only), and on better sites, with ash and sycamore, in the Harz mountains (Norway spruce) we even took away the existing trees near streams, an easy operation with large positive effects and with no visible increase of stability risks.

4.2.3 The handling of natural disturbances for natural regeneration

Storms, when they are not too strong, are good site mappers. They throw first those trees, which are located on less stable parts of the soils. Perhaps the remaining trees after a storm-throw have been luckier than their fallen neighbours. But a more consistent factor is the stability of micro-sites, which allow a deeper rooting. So it can be said, that the artificial enlarging of wind- throw gaps is a mistake. In a way, one can say, that the repeated impact of storms is a permanent process of increasing relative stability, which should not be influenced by excessive human activities. The borders of such gaps can eventually be the new borders of stability. The next point is, that wind-throw gaps are the starting point of natural regeneration. The formation of gaps in natural forest is a continuous process of stand openings in time and space. In managed forest, no human intelligence until now found an effective method to avoid this dynamic process. So you have to live with it and make the best of it. The result will be a pattern of irregular regenerations with different heights and sizes. This is welcome, because the mosaic pattern of different densities and openings, tree ages and heights is an element of stability and the capacity of reaction after disturbance events (= elasticity) of the whole forest unit. The larger a homogeneous situation, the larger will be the scale of disturbances. Small scale variations in the stand composition will disperse the impact of gales.

4.2.4 Plantations on clear - felled areas

Open fields do not have good microclimate. This situation can easily inhibit the germination of natural regeneration and young saplings can dry out in a few dry summer days. This often happens in thick raw humus layers, which easily dry in hot summer periods. Young tree plants on open areas may also suffer from the competition of weeds, the impact of sea salt spray and from high stagnant seepage water after the destruction of the old stands water pump, and from late frost. The following measures can help:

· Artificial draining of moist mineral soils and peatland, until the water pump of the new stand begins to work;

· Liming and fertilising;

· Soil preparation in order to open the mineral soil. Wood debris and the humus should be conserved as much as possible, and micro-structures on the areas surface should not be graded. They provide hidden micro-niches, where wind impact is lower, the relative humidity is higher and late frost lower;

· Planting "little shelters": alder 3x3m or 4x3m (or birch, ash, pine species, larch). The plantation should be done 2 years before planting Sitka spruce or together with it. All alders are good: A. ghitinosa, A. rubra and A. incana. They minimise evaporation losses, create a new micro-climate, lower late frost risks and bring a better deep rooting, so an increasing stand stability. These little shelters can be taken off later with thinnings, disappear by themselves, or they can be used as commercial species. In Germany, alder of about 40 cm dbh or thicker gets 250 to 600 Euio/m3.

4.2.5 Commercial mixture species

Soils don't have the same properties. So again the categorical imperative of Silviculture is to respect the site by selecting conforming plantations. Tree species have developed different site adaptations. In consequence the main objective must be to put the right tree on the right soil; the same has to be said for mixtures.

The creation of mixed stands follows long-term experiences: site conform species choice and well adapted mixtures are an element of better stand stability, of better ecosystem elasticity after disturbances, of an improved economic income and of higher biodiversity, and often they provide better recreational values.

Which tree species could perform these objectives in Sitka spruce stands? The question is not easy to answer for Irish growth conditions. Some long-term experiences from the German North sea coast might be allowed to be brought into discussion. This coast is also a windy coast (Mean wind speed 6 to 8 m/sec/y. The following proposals can be articulated:

· Exclude all non stable sites (high seepage water levels in soils, heavy clays, and others) from Sitka spruce plantations.

· The following mixture species can be recommended as mixture trees in Sitka spruce stands:

a) Ash (Fraxinus excelsior): A most interesting species in real mixtures with Sitka spruce could be ash. It is an extremely deep-rooting tree, has a very stiff crown, which is not harmed by permanently blowing winds and by sea salts, it has a comparable growth rhythm with Sitka spruce and can produce very valuable timber. With these characteristics it is present near all coasts of temperate Western Europe, even replacing beech near the coast lines. The main condition is that ash should be chosen on soils of more than ph 5.0. And as a matter of fact it cannot follow Sitka spruce on peatland.

b) Sycamore (Acer pseudoplatanus): The same characteristics are valid for sycamore, but the risk of grey squirrel damage seems to be higher. Both, alder and sycamore as well have good rooting and are stabilising species.

c) Alder (Alnus glutinosa, A. rubra): Alders can be commercial species with good perspectives in future markets, perhaps a bit lower in value than ash and sycamore. Their deep rooting, independent of soil aeration, can provide a natural nitrogen supply to soils, and their very good humus decomposition, the light and warmth which are brought into the stands are tremendous advantages.

Normally, and mainly with ash, the mixture form should be a group, not a single tree mixture, which would be more difficult to manage. But alder can be mixed as single tree or on alternating lines with Sitka.

d) A stabilising conifer species could be Abies grandis. But there are no known examples of such mixtures. Its deep rooting even on peatland is certainly a stabilising factor. Its market value however is low.

4.3 Species change: Recommendations near sea coasts

The potentials of different species choices are summarised in the annexe table (Broadleaves; Table: 1 & Conifers; Table: 2). Its base is the situation near the North sea coast in Germany, as we experienced it since the first plantations 200 years ago. This experience must be checked for Irish conditions and eventually modified.