A review of the characteristics of black alder (Alnus glutinosa (L.) Gaertn.) and their implications for silvicultural practices

© Institute of Chartered Foresters, 2010. All rights reserved. Forestry, Vol. 83, No. 2, 2010. doi:10.1093/forestry/cpp038For Permissions, please email: [email protected] Advance Access publication date 29 January 2010

Introduction

Black alder (Alnus glutinosa (L.) Gaertn.) is naturally widespread across all of Europe (Figure 1), from mid-Scandinavia to the Mediterranean countries, including northern Morocco and Algeria (Meusel et al., 1965; Jalas and Suominen, 1976; Kajba and Gracan, 2003).

It is typically a component of mixed broadleaved forest and represents less than 1 per cent of the forest cover in most countries (Turok et al., 1996) because most of the suitable sites have been converted to agriculture (Noir-falise, 1984). Also, former silvicultural practices favoured oak and ash over alder. However, in North Central Europe (Netherlands, Northern Germany, Poland and the Baltic States) and South Central Europe (the plains of Slovenia and Croatia), black alder represents ~5 per cent of the for-est area and forms large highly productive stands (Glavac, 1972; Roisin and Thill, 1972; Turok et al., 1996).

Despite its relative scarcity in forests, black alder has a good potential for timber production. Nevertheless, the production of high-quality black alder timber is only pos-sible on the sites that correspond closely to its autecologi-cal optimum, as with other valuable broadleaves (Thill and Mathy, 1980; Claessens et al., 2002). In these conditions, it

grows as fast as ash, maple or cherry and its wood charac-teristics are such that high prices can be paid for timber.

Moreover, the species contributes particularly to riverine ecosystems and to the services they provide (Claessens, 2003, 2005). It contributes to biodiversity by providing habitats for a specific flora and fauna both on the tree itself and in the flooded root system (Dussart, 1999), it assists with water filtration and purification in waterlogged soils (Peterjohn and Correll, 1984; Pinay and Labroue, 1986; Schnitzler and Carbiener, 1993), and the root system helps to control floods and stabilize riverbanks (Piégay et al., 2003).

There is therefore considerable interest in using black alder in alluvial and marshy ecosystems where nature conserva-tion and watershed management are at least as important as wood production (Claessens, 2005). It is also a suitable species in the restoration of functional alluvial ecosystems (Schäfer and Joosten, 2005), for example in transforming spruce and poplar plantations to more natural stands.

Outside the forest, black alder is important in open land-scapes, especially in linear stands along stream and river margins. In Belgium, a comparison between forest (Lecomte et al., 2003) and riverbanks inventories (Debruxelles et al., 2008) has shown that alders are equally represented in wood-land and in linear features along watercourses. They also

A review of the characteristics of black alder (Alnus glutinosa (L.) Gaertn.) and their implications for silvicultural practicesHuGuES CLAESSENS1*, ANNE OOSTERBAAN2, PETER SAVILL3 and JACquES RONDEux1

1 unit of Forest and Nature Management, Gembloux Agro-Bio Tech, university of Liege, 2, Passage des Déportés, 5030 Gembloux, Liege, Belgium2 Alterra, Wageningen uR, Wageningen, The Netherlands3 Oxford Forestry Institute, university of Oxford, Oxford, uK*Corresponding author. E-mail: [email protected]

Summary

Black alder is a scattered, widespread and short-lived species that thrives in low-lying damp and riparian places. It has a use in flood control, stabilization of riverbanks and in functioning of the river ecosystems. To thrive, precipitation must exceed 1500 mm if access to groundwater is not possible. Alders are unusual among European trees in that they fix nitrogen. To regenerate naturally, alder requires high levels of both light and moisture, usually achievable only on disturbed sites. Growth rates up to ages 7–10 are very fast but then slow rapidly. Sixty to seventy years is the maximum rotation for growing timber if heart rot is to be avoided. Maximum mean annual increments range from 4 to 14 m3 ha21 year21. Alder wood is used for energy, as fibre for paper and particle board and, most profitably, in joinery as solid wood or veneer. Logs must be at least 3 m long and ideally 50–60 cm diameter. Aspects of plantation silviculture are discussed with emphasis on thinning, which needs to be started early and to be heavy and frequent around selected final crop trees to achieve marketable timber before heart rot sets in.

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form the ‘alder-line-netscape’ of the Netherlands (~100 000 km of corridor plantations) (De Boer and Oosterbaan, 2005) where alders protect banks from eroding and play a part in moderating water temperature and nutrient status.

The objective of this review was to synthesize existing knowledge about the silviculture of black alder on suitable sites in relation to the production of high-quality wood. The first section presents a comprehensive review of the site requirements, growth patterns and stand dynamics of black alder, as foundations for silvicultural prescriptions. The second section considers optimal silviculture for the production of valuable timber.

Characteristics of black alder that influence silviculture

Site requirements

TemperatureThe natural distribution of black alder (Figure 1) indicates that the species is adapted to a wide range of tempera-tures. According to Mac Vean (1953), the species does not extend into regions where the mean daily temperature is above freezing for less than 6 months of the year. The criti-cal winter temperature is around −49°C (Mac Vean, 1953) to −54°C (Groszman and Melzer, 1993), but Dewald and Steiner (1986) have found important differences of winter frost resistance of 2-year-old seedlings between Northern and Southern European populations, with critical tem-perature of −39 to −43°C and −30 to −34°C, respectively. Black alder is also reported to be relatively tolerant to late autumnal and early spring frosts (Coté and Camiré, 1984; Johansson, 1999; Vares et al., 2004; Brus, 2005), but

white frosts in the spring affect negatively radial growth increments (Laganis et al., 2008). Flowers are not affected by temperatures of −6°C (Mac Vean, 1953). Eschenbach (1995) did not show injuries of leaves exposed to tempera-tures up to 44°C in summer.

The tree grows well in the continental climates of both Northern (e.g. the Baltic States and Northern Germany) and Southern Europe (e.g. the Danube plain). Glavac (1972) has proposed the existence of an ecotype adapted to long days that maintains an acceptable rate of growth in Finland, but the studies of Prat et al. (1992) and Gömöry and Paule (2002) did not find clear geographical trends in the genetic structure of alder populations within the major part of its European range.

WaterThe occurrence of black alder is closely linked to the avail-ability and abundance of water. Atmospheric humidity must remain high during all phases of its reproductive cycle:

• atmospheric humidity favours the development of the pollen tube; some alders produce pollen that germinates only after a humid treatment (Bensimon, 1985);

• owing to two cork appendages, alder seeds can float on water for 1 year without decrease of the germination capacity (Mac Vean, 1953);

• a low hygrometric level (less than 50 per cent) inhibits the seed germination, even in humid soils (Mac Vean, 1953; Dethioux, 1974) and to allow seedling develop-ment, this level must be maintained at least 1 month after germination (Martin, 1985).

Consequently, the range of the species is limited in the East by aridity (where annual rainfall is below 500 mm). At the drier limits of its range, it finds refuge in the humid

Figure 1. Distribution of Alnus glutinosa (Kajba and Gracan, 2003). It is also present in Northern Turkey, Caucasus and in some valleys of the Atlas Mountains (Meusel et al., 1965).

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microclimates of valleys, for example in the Atlas Mountains (Roisin, 1986), and along rivers such as the Volga, Don and Dniepr in Eastern Europe (Jalas and Suominen, 1976; Kajba and Gracan, 2003).

Black alder is a typical water-demanding species because its leaves have no mechanism for controlling transpiration (Braun, 1974). In Germany, Herbst et al. (1999) demon-strated that evapotranspiration in a black alder stand is equal to total annual rainfall. This means that the tree can suffer from water deficits during dry and warm periods in summer. In consequence if growth is to be satisfactory, and access to groundwater is not possible, annual precipitation must be high. Based on their local observations, Fremstad (1983) for Norway, Lhote (1985) for the Atlantic Pyrénées in France and Mac Vean (1953) for Wales have fixed this minimum at 1500 mm. However, on the large majority of sites where alder grows, roots have access to a groundwater table where a supply is assured.

The root system is adapted to very wet soils. Many strong, vertically growing, sinker roots anchor the tree on riverbanks, and they are able to penetrate deeply into wet and anaerobic soils (Mac Vean, 1956; Schmidt-Vogt, 1971). Kostler (1968) observed roots reaching almost 5 m deep. under anaerobic conditions, an oxygen sup-ply for the roots comes from the aerial parts of the tree via enlarged lenticels on the stem (Gill, 1975) connected to well-developed aerenchyma cells, especially in roots where the wood density is ~0.3 g cm23 (Liepe, 1990). These also provide a route for the removal of toxic gases (e.g. acetone) that are produced under anaerobic condi-tions (Crawford, 1992). In addition, the metabolism of alder is adapted to reduce the production of toxic sub-stances under anaerobic conditions (Crawford, 1992). In the case of prolonged flooding, loss of function in the root system can induce the development of adventitious roots from lenticels (Gill, 1970, 1975).

NutrientsBlack alder can grow on a wide range of soils of varying nutrient status, and equally well on acid and basic soils, with a large range of pH values between 4.2 and 7.5 (Mac Vean, 1953; Noirfalise, 1984; Pavle et al., 1996; Claessens, 1999).

The tree is able to fix atmospheric nitrogen in symbi-otic root nodules (Figure 2) with bacteria in the genus Frankia (Bond et al., 1954). Nodulation can occur in all soil conditions where alders grow but is the best in the pH range 5.5–7.2 (Griffiths and McCormick, 1984) and is least pronounced in nitrogen-rich soils (Bond et al., 1954). The increase in wood production due to Frankia has been estimated to be 25 to 33 per cent (Hendrick-son et al., 1993). Alder leaves are nitrogen rich and the contribution of nitrogen to the soil at leaf fall has been estimated at 30–130 kgN ha21 year21 in forest condi-tions (Moiroud, 1991). It also improves the availability of phosphorus (Giardina et al., 1995) and various biva-lents cations (e.g. magnesium, manganese and zinc) (Ho-mann et al., 1994). Alder has been used as a component of mixed plantations to increase the production of trees

Figure 2. Root nodules of black alder (copyright H. Claessens).

such as walnut, Douglas fir or poplar on infertile sites (Schlesinger and Williams, 1984; Giardina et al., 1995; Vares et al., 2004).

Site typesThree main site types where black alder grows can be iden-tified (Dethioux, 1974; Claessens, 2003) in relation to the type of water supply:

• Marshy sites that have waterlogged subsoil throughout the year. On these, black alder forms pure stands owing to its physiological adaptations that allow it to grow in anaerobic soils. This is the Alnetum community accord-ing to the habitat classification of Noirfalise (1984).

• Riverside sites in which the soil in the rooting zone is well aerated during the growing season. On these, black alder is often mixed with ash, elm, maple, willow and other species that are adapted to alluvial sites (Alno-Padion community).

• Plateau sites with high soil moisture contents, for example deep loamy soils, generally with a temporary water-table and where the stand is dominated by other species (e.g. ash, maple and oaks) that are more competitive (Carpin-ion community).

Growth pattern

Height growth and site index curvesSeveral authors have constructed height growth curves (Sopp, 1974; Lockow, 1995; Mitscherlich, 1945, in Schober, 1995; Johansson, 1999; Giurgiu and Draghiciu, 2004; Thi-baut et al., 2004; Table 1). Despite the different methods and approaches to construction, the shape of curves is al-ways very similar (Figure 3). A comparison of the curves that go through 24 m height at age 50 shows only small differences in shape despite the fact that site index ranges differ between countries (Figure 4). It seems from them that alder grows faster in southeastern Europe (Hungary and Romania) when young (Figure 3).

All models show that black alder height growth pattern is characterized by a very high growth rates when young (Martin, 1985, Lockow, 1994, and Thibaut et al., 2004

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Table 1: Height growth studies of black alder in Europe (chronological order)

Authors (date) Region Definition of height Data for model

Mitscherlich (1945) in Schober (1995) Germany Mean height Periodic measurements*Glavac (1972) Germany, Finland, Yugoslavia Top height Periodic measurementsSopp (1974) Hungary Mean height Periodic measurementsLockow (1995) Germany Top height† Stem analysisJohansson (1999) Sweden Top height Stem analysisGiurgiu and Draghiciu (2004) Romania Top height Stem analysisThibaut et al. (2004) Southern Belgium Top height Stem analysis

* Periodic measurements of mean (or top) height and age on permanent sample plots. † Top height is the mean height of the 100 largest diameter trees per hectare in the stand.

Figure 3. Height growth curves for black alder in Europe – comparison of shapes for site index = 24 m of dominant height at age 50 (complete references in Table 1).

mention annual shoot elongation of up to 1.5 m between ages 4 and 10 in suitable sites), but with an early decrease. By age 25, black alder reaches half the height it will achieve at maturity. Considering all the species together on a suit-able site, the height growth of black alder is similar to that of ash and birch up to age 40.

Height or diameter reached at a reference age (i.e. the site index) is often used to estimate potential productivity in a stand. Published site index curves range from 10.5 m (class 5 in Romania) to 28.5 m (class 1 in Spreewald, Ger-many) at age 50 (Figure 4). In the sets of site index curves, the differences are evident by age 20 (5.1–18.8 m) and are confirmed in old age (13.4–31.3 m at age 80). On the best sites in Northern Germany, Slovenia and Southern France, records exist of black alder having a height of 35 m at ma-turity (Glavac, 1972; Lhote, 1985).

In Belgium, when modelling height growth, Thibaut et al. (2004) have found slightly different shapes of curves, justi-fied by different shape parameters, according to two main site types. The genetic origin can also have a very marked influence on the pattern of growth. In The Netherlands,

Verweij (1983) has shown the existence of a lowland peat ecotype that grows fast for the first 10–15 years but then slows down very considerably. This ecotype has been grown widely in shelterbelts but is not suitable for timber production. Its use was thought by Schmidt-Vogt (1971), Liepe (1990), Franke (1994), Cech and Hendry (2003) to explain the alder decline observed during the early part of the twentieth century in German plantations.

Forecasting site index and productionGlavac (1972), Stortelder et al. (1998), Claessens (1999) and Gaudin et al. (1999) used the site index method to clas-sify alder sites types according to their levels of productiv-ity. All authors have found that productivity is greatest on riverside sites followed by plateau sites with high levels of soil moisture. These are followed by marshy sites and pla-teau sites that suffer from periods of soil drought. Stortelder et al. (1998) and Claessens (1999) related site index to volume production according to the yield tables published by Mitscherlich (in Schober, 1995) and concluded that only the first two site types (riversides and plateaux with ample soil moisture) are suitable for high-quality wood production. Despite the fact that alder is the only tree that will grow in marshes, production is often slow and the wood of low qual-ity in these conditions.

For alder stands in Southern Belgium, Claessens (1999) developed two models for forecasting site index. One uses abiotic parameters, including elevation, type of humus and ecoregion, and the other uses vegetation types. These explain 75 and 66 per cent of site index variation, respectively.

Volume growth and yieldYield tables for black alder in Europe have been pro-duced by Sopp (1974) for Hungary, Mitscherlich (1945) in Schober (1995) and Lockow (1995) for Germany. The oldest one is that of Schwappach (1919) for Germany.

All these tables show that average current annual volume increment reaches a maximum at ~20 years, with maxi-mum values of up to 13–18 m3 ha21 year21 according to yield tables. Mean annual volume increment (MAVI) is at a maximum between ages 30 and 50, with values of 14.6–4.5 m3 ha21 year21 according to yield class (Table 2).

In Belgium, Claessens et al. (2002) have confirmed Mitsche-lich’s yield values with periodic measurements in 33 alder

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stands. In the Netherlands, Stortelder et al. (1998) found MAVIs ranged between 2.4 and 14.9 m3 ha21 year21.

The productivity and pattern of volume increment of black alder are comparable to those of other broadleaves, such as ash and sycamore as indicated by different Euro-pean yield tables. On sites suitable for timber production, the expected MAVIs range between 6 and 12 m3 ha21 year21 at age 80 (the maximum age for harvesting because older trees always suffer from heart rot). At this time, cu-mulative volume production is between 500 and 1000 m3 ha21.

These values show that black alder has a real productive potential, especially when young. In the early 1980s, many species in the genus Alnus were tested in short rotation biomass production crops, largely because of the N-fixing capabilities of their roots. The best results were achieved by crosses between Alnus rubra × A. glutinosa and Alnus cordata × Alnus incana (Martin, 1985). However, Alnus is always rated well behind Eucalyptus, Populus and Salix as a biomass producer (Weisgerber and Heige, 1988). One of the reasons for this is that the genus has a poor tolerance of moisture stress, and consequently, species are difficult to establish except on the most favoured sites (Hall, 1990).

Diameter growthThe potential diameter growth at breast height (d.b.h.) is a key parameter in devising silvicultural prescriptions. However, at the level of single trees, this information about growth is sparse. Values indicated by yield tables are based on silvicultural practices of the early twentieth century when black alder was not considered a potential timber species and small dimension wood was valuable in local markets. According to yield tables produced by Sopp (1974), Mitscherlich (1945) in Schober (1995) and Lockow (1995), 100 years is required to produce alder with a d.b.h. of 40 cm, even in the highest yield class. However, Sopp’s and Lockow’s first yield tables show that diameter growth is highest during the first 15 years of life, at more than 1 cm year21.

In Belgium, Claessens et al. (2002) found large variations in d.b.h. increment of dominant trees growing in various condi-tions of competition. In the same third yield class, d.b.h. at age 60 can vary from 25 to 45 cm (Figure 5), corresponding to a mean annual diameter increment of 0.4–0.7 cm. Moreover, in 10-

to 20-year-old stands, measurement of dominant d.b.h. in experimental plots varied from 0.8 to 1.6 cm year21 for the first yield class.

The d.b.h. increments have been explained (r2 = 0. 7) by a growth model based on the d.b.h./crown diameter ratio, using age, crown width and potential crown width (Four-bisseur et al., 2002). In this relationship, potential crown width was established from a set of trees of different ages growing without any competition on open land (r2 = 0.97). This growth model shows that the crop trees can reach a diameter of 50–60 cm at age 60 in the second and first yield classes, respectively.

LongevityBlack alder is a relatively short-lived tree. Its maximum lifespan varies from 100 to 160 years depending on phy-togeographic region (Glavac, 1972; Martin, 1985; Longauer and Hoffmann, 1996). The oldest stands are found in Central Europe where trees can reach 35 m tall and 1 m d.b.h. at maturity but maximum dimensions decrease towards the north (Finland) and west (Atlantic parts of Europe) (Glavac, 1972). However, wood quality declines after 60–70 years because of heart rot (Claes-sens, 2005). From a study in Belgium (Thibaut et al., 1998), all trees had started to develop heart rot at age 70. On average sites in Bavaria (Southern Germany), Im-mler (2004) placed the beginning of the phenomenon at about age 50. According to Lockow and Chizon (1996), the frequency of heart rot differs with site but diagnosis from external characteristics is unreliable. The authors advised that increased space should be given to future crop trees early in the rotation to accelerate diameter growth to achieve usable sizes sooner.

Wood characteristics and products

The aesthetic and mechanical characteristics of alder wood make it suitable for many different uses (Table 3), includ-ing energy (Podge, 1980), fibre for paper (Lethonen et al., 1978; Vurdu and Bensend, 1979; Haarlaa and Karkkainen, 1982) though it has the problem of a red colouration, and fibre and particle boarding (Crave, 1990), but it is not suit-able for uses where strength is required (Wagenführ and

Figure 4. Range of site index (height at 50 years) of black alder for different countries in Europe (complete references in Table 1).

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Scheiber, 1985) as in constructional wood. The most lu-crative use is in joinery, as solid wood or veneer (Bohme et al., 1983) because both the grain and the colour (alder is also easily dyed) are appreciated in joinery (Crave, 1990; Vacher, 1991). As with any species, only the best quality boles that are acceptable for joinery and minimum dimen-sions must be 3 m long and 40 cm diameter though if d.b.h.

are greater they achieve higher prices (Thill and Mathy, 1980; Gaudin et al., 1999; FNEF, 2007).

The timber of black alder is very durable under water. Some of the piles in Amsterdam and Venice are alder (Fontnoire, 1974; Abrami et al., 2001). In this respect, it is comparable with the best species: oaks, larch and black locust (Krauss and Racskowski, 1985).

Table 2: Summary of yield tables of Hungary (Sopp, 1974, three representative yield class) and Germany (Mitscherlich in Schober, 1995, extreme yield class; Lockow, 1995, mean yield class)

Yield table and yield class Age

Main cropVolume increments (m3 ha21 year21)

Mean height (m) Number of stemsStanding volume m3 ha21 year21 CAVI MAVI

Hungary (Sopp, 1974) I 5 8.0 2900 50 – 12.420 18.8 960 210 15.4 14.430 22.5 685 307 14.8 14.640 25.3 530 390 13.4 14.450 27.5 440 462 12.0 14.060 29.2 385 525 10.2 13.480 31.3 325 619 6.8 12.0

100 32.3 290 674 4.4 10.7

III 5 5.4 4100 24 – 9.420 13.7 1420 118 12.0 11.130 17.2 980 180 11.6 11.340 19.8 735 236 11.0 11.350 21.7 580 284 9.8 11.060 23.2 480 324 8.4 10.780 25.1 370 381 5.6 9.7

100 26.1 320 415 3.6 8.7

V 5 3.6 5400 9 – 2.220 10.0 2010 64 5.4 4.130 13.1 1360 103 5.4 4.640 15.4 1010 138 5.4 4.850 17.1 790 169 5.2 4.960 18.5 635 196 4.8 4.980 20.2 435 234 2.8 4.6

100 21.1 383 256 1.4 4.0

Germany (Schober, 1995) I 25 17.3 1224 154 13.4 7.130 18.5 972 184 11.2 7.840 20.8 665 228 9.8 8.450 22.9 479 260 8.8 8.560 24.7 363 283 8.0 8.580 27.7 229 313 6.8 8.2

III 25 10.2 1969 61 6.2 2.830 11.5 1529 81 6.4 3.440 13.5 989 113 6.0 4.150 15.2 703 138 5.6 4.460 16.7 523 156 5.1 4.580 19.2 331 179 4.2 4.5

Germany (Lockow, 1995) II 15 12.2 1488 60 11.0 –20 14.9 1105 105 12.9 7.130 19.0 748 187 11.6 8.840 21.8 581 251 9.4 9.150 23.8 487 300 7.4 8.960 25.2 427 335 5.8 8.480 27.0 361 381 3.2 7.4

100 28.0 330 405 1.6 6.4

CAVI = current annual volume increment.

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Figure 5. Mean of dominant diameter of black alder stands at different levels of competition for yield class 3 in Belgium (H50 = 20 m). The rectangle highlights different diameters in a same age class in relation with different competition levels (Claessens et al., 2002).

Table 3: Wood characteristics and uses of black alder (Claessens, 2005)

Characteristics Consequences Corresponding uses

Homogeneous wood with diffuse pores, small vessels with thin walls, numerous small medullary rays

Grain fine and regular Veneer (slicing, rotary cutting)

The wood does not form tyloses during drying

Very permeable wood Impregnation easy (dyeing, preservation)

Mass = 540 kg m23 (12% moisture content); hardness = 35 MPa; shrinkage = 13.4%

Semi-hard and fairly stable wood Turnery, modelling (carving); machining and screwing easy

Low strength properties unsuitable where strength is required

Wood hardness remains when immersed in water Hydraulic works

Alder wood burns quickly with a steady flame and gives off little smoke and leaves little residue

Energy wood

Stand dynamics

RegenerationMature alders, at age from 3 to 30 according to the eco-types and the stand conditions (Liepe, 1990), produce plentiful seed every 3 or 4 years. Germination rates are highly variable, ranging from 10 to 90 per cent accord-ing to the crop year and the stand (Dethioux, 1974). A mature tree produces ~4000 cones containing ~60 keys (Suska et al., 1994). Seedling survival demands an un-derstory light level of at least 20 per cent of the above-canopy light, although this does not enable the seedlings to grow, and a high level of moisture over a period of 1 month (Dethioux, 1974). In consequence, natural regen-eration of black alder is not possible under the canopy of a mature stand, except in openings of more than 1000

m2. For the same reason, herbaceous competitors (Fil-ipendula ulmaria, Carex sp., tall Poaceae that are abun-dant in the alder sites) can prevent seedling development (Mac Vean, 1956). In natural conditions, alder regenera-tion is dependent on soil disturbance (e.g. by deer, flood-ing or forest harvesting) or changes in the forest cover caused by diseases or clearfelling.

In the past, extensive flooding was the main stimulant to alder regeneration in valleys (Noirfalise, 1984). New conservation areas are now being created on old pastures by removing the top layer of the soil and/or raising the groundwater level artificially in The Netherlands (Ooster-baan, 2000).

Natural pruning, tree form and competition

Natural pruning. unlike many species, black alder does not produce shade leaves. Within the crown, shaded leaves are less active in assimilation, but their respiration rates remain at the same level as leaves grown in full light (Es-chenbach, 1995). In consequence, shaded branches die rap-idly. This explains the strictly light-demanding nature of alder, its growth pattern, good natural pruning and poor competitive abilities.

In forest conditions, natural pruning begins before age 10 and progresses into the crown faster than the total height increases (Claessens, 2005). Branches never reach large dimensions, and after they die, they quickly decay, leading to very good natural pruning.

Because of this, live crown size decreases and this leads to a fall in diameter increment. The trees progressively be-come unbalanced, with a tall stem but proportionately, an increasingly small crown. In dense stands, the crown ratio in terms of height (crown length/total height) can decrease to below 25 per cent and the ratio of total height /d.b.h. can reach up to 140, even for dominant trees (Claessens, 1990). After several decades, the d.b.h. increment of such

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trees reduces to less than 0.3 cm year21 and the stems often produce epicormic shoots (Claessens, 2005).

Low competition resistance. In forests where alder is mixed with ash or maple, its height growth pattern and its intolerance to shading prevent it from dominating a stand, except on marshy sites where other species do not thrive.

Alder diseaseSince the 1990s, a new disease caused by Phytophthora alni (Brasier et al., 1995) has been spreading through European alder populations, especially those on riverbanks (Gibbs, 1994; Gibbs et al., 2003). Symptoms are tar-coloured spots at the base of the trunk and small yellowish leaves. After a few years, branches, and in some cases, entire trees die. The spread of the disease depends on the presence of free water that allows the development and transport of the fungus (Chandelier et al., 2006). Alder disease represents a severe risk to alder trees, especially on riverbanks and when flooding occurs. In Belgium and Britain, monitoring has shown that the disease affects 15–30 per cent of the trees on riverbanks (Webber et al., 2004; Debruxelles et al., 2007). The proportion is lower in forests (~5 per cent of diseased trees in Belgium) that have no contact with a river (Claessens, 2005).

Silviculture for high-quality timber production

Production objectives

The objective that potentially gives the most valuable re-turn from growing black alder is to grow high-quality tim-ber of 50–60 cm d.b.h. and a bole length of ~6 m. The rotation required to produce such trees will be between 50 and 70 years.

Traditional silvicultural prescriptions, such as those of Roisin and Thill (1972), and yield tables (Mitscherlich, 1945, in Schober, 1995; Sopp, 1974) that were developed in the socio-economic context of the mid-twentieth century may no longer be appropriate. By following these models, it would take 100 years to achieve a d.b.h. of 40 cm by which time heart rot will have developed. New silvicul-tural prescriptions were suggested in the 1990s by Girault (1988), Poulain (1991), Vaast and Billac (1996), Gaudin

et al. (1999), Claessens et al. (2002) and Rupp and Hafemann (2003) (Table 4). They all say that on sites where black alder will grow well, target d.b.h. of ~40–50 (55) cm can be obtained with rotations of (30) 40–65 years, thus avoid-ing heart rot. Rotations of 80 years have been achieved on the very best sites in the German forest of Spreewald where trees of 60–65 cm d.b.h. have been grown (Rupp and Hafe-mann, 2003). Indications of numbers of future crop trees are shown in Table 4. They depend mainly on the different target diameters suggested by the authors.

Gaudin et al. (1999) and Claessens (2005) also proposed target values for less suitable sites (Table 4) where the production of high-quality valuable wood cannot be guar-anteed. In these conditions, they stress the importance of alder for biodiversity conservation. This is sometimes the main objective of forest management in such areas.

Empirical foundations for new silvicultural prescriptions

Taking into account the fast growth of black alder, some authors (e.g. Poulain, 1991; Claessens and Thibaut, 1994; Vaast and Billac, 1996) have devised new empiri-cal prescriptions based on those of other fast-growing and light-demanding trees, such as ash and cherry. They have also tested the prescriptions in experimental plots in order to validate them. In the context of future crop tree silviculture, Claessens (2005) has proposed thinning prescriptions based on the growth model of Fourbisseur et al. (2002) presented in the section on diameter growth. In this way, they established an ‘intensive’ silvicultural regime that takes into account the biological characteris-tics and production objectives. The main features of the regime and its justification are summarized in Table 5. Claessens (2005) considers it to be the only system that will work for the production of valuable timber. Other scenarios involving, for example, light thinning or de-layed first thinning result in a too small d.b.h. before the critical time when heart rot sets in. Moreover, in the case of late thinning, when the ratio of crown depth to tree height is less than 25 per cent, alder produces epicormic shoots. Claessens (1990) provided critical values for a slenderness index (tree height/d.b.h. ratio) above which the risk of production of epicormics shoots is high. These values decrease from 110 at age 20 to 70 at age 65.

Table 4: Objectives of black alder production according to different authors

Authors (date) Region Final d.b.h. (cm)Final number (trees ha21)

Corresponding age (years)

Poulain (1991) Northern France 40 – 40Vaast and Billac (1996) SW France (Landes) 40 300 30Girault (1988) France 40 200 40Gaudin et al. (1999) Eastern France 50–55* –

30–40† –Claessens et al. (2002) Wallonia (Southern Belgium) 50 80 50–65*

80–100†

Rupp and Hafemann (2003) Central Germany 60–65 – 80

* For sites with a high potential for black alder growth. † For alder on low potential sites.

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Main silvicultural prescriptions

Natural regenerationSuitable conditions for natural regeneration of black alder occur rarely in the anthropogenic landscape of Europe. They sometimes occur in small areas after clearcutting (e.g. of spruce stands along rivers).

under an alder stand, light levels are inadequate and herbaceous vegetation prevents the development of new seedlings. Claessens (2005) reported some successful re-generation after heavy shelterwood cuttings in Germany (basal area of 10 m2 ha21 after thinning) and after gap regeneration in Belgium (gaps of ~1000 m2).

Plantation silviculture

Plants. The genetic origin of black alder must be consid-ered because both form and growth rate vary across Europe (Krstinic et al., 2006). The most frequent commercial prov-enances come from marshes in the North Atlantic biogeo-graphic region and are not suitable for timber production (Verweij, 1983), even in the Atlantic region from which they originate. In Europe, some provenances are available, nota-bly in Belgium (Anonymus, 2005) and Germany, but their suitability for wider use has not been tested.

In most cases, bare rooted plants are used. Container-grown stock is sometimes produced, but the extra expense of producing it is unnecessary because transplantation stress is generally very low in the humid soil conditions found in black alder sites.

The desirable age and height of planting stock will de-pend on the vegetation on the planting site. In tall herba-ceous conditions such as sites with Filipendula ulmaria, 2-year-old plants that are more than 120 cm tall are de-sirable. With such plants, weeding is rarely necessary or beneficial, unless the herbaceous vegetation is very tall (e.g. Phragmites or Calystegia) (Claessens, 2005).

Spacing and density. Proposed planting densities vary from 800 to 1600 stems ha21 according to different authors (Gi-rault, 1988; Vacher, 1991; Vaast and Billac, 1996; Claes-sens, 2005). Lower values (400, 600) are also suggested when planting with certified stock of selected genetic origin or if natural regeneration can supplement the planted trees. According to Claessens (2005), on many sites, weeding

and pruning are unnecessary if densities exceed 1000 stems ha21 and 120 cm tall plants are used.

Use of coppice shootsTraditionally, black alder has been managed as coppice stands. In Belgium, Regional forest inventories show that coppicing was the main treatment during the twen-tieth century (Claessens, 2005). Despite the rapid height growth of coppice shoots (Glavac, 1972), the quality of the boles grown from coppice is not good enough for the valuable timber market because of their vulnerability to heart rot (Immler, 2004). Alder coppice is better suited to biomass production.

ThinningRoisin and Thill (1972) described the silviculture of black alder in Slovenia as being characterized by very light thin-nings, leading to high basal areas (>40 m2 at age 60). Con-sidering the light-demanding nature of the species, this is incompatible with the production of boles of 50–60 cm of d.b.h. With the exceptions of the approaches of Lockow (1995) and Immler (2004), new proposals (e.g. Girault, 1988; Poulain, 1991; Vaast and Billac, 1996; Claessens, 2005) suggest intensive and especially early thinning re-gimes beginning at age 10–15. Two models are commonly used:

• A homogeneous thinning of the whole stand, favouring more or less equally all the trees remaining after thin-ning (referred to as ‘stand thinning’ and characterized by a decrease in stand stem number with age).

• A crown thinning that concentrates on removing com-petitors to a small number of selected trees of high po-tential quality (due to, e.g., form, growth and health) that will form the final stand (referred to as ‘future crop-tree thinning’) and characterized by creating space around the crown of the future crop trees.

Stand thinning. Some recently produced thinning prescrip-tions are summarized in Figure 6 (Girault, 1988; Vaast and Billac, 1996; Claessens, 2005). They all propose heavy early thinning (beginning at age 10–15) aimed at reducing stem numbers to 200–300 ha21 by age 20–30. This is in marked contrast to traditional silviculture (the old Slove-nian model described by Roisin and Thill, 1972, and that

Table 5: Main silvicultural guidelines imposed by biological characteristics of black alder (adapted from Claessens, 2005)

Black alder characteristics Adapted silvicultural guidelines

Maximum height growth ~10 years (half of target top height reached ~20 years); maximum CAVI of 10–15 m3 ha21 year21 at 20 years

Early first thinning (at about age 10)Short thinning cycle to start with (around every 3 years)

Light-demanding species, very affected by competition Crown thinningHeavy thinning

Early slowing up of growth and onset of wood decay from age 50–70

Longer thinning cycle from age 30Short rotation (~50–80 years)

CAVI = current annual volume increment.

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of Lockow, 1995) that maintained numbers at ~800–1200 ha21 at the same age, which is close to the stand density at which self-thinning occurs (Claessens, 2005). Of the recent recommendations, the most contrasting prescriptions (e.g. Girault, 1988; Vaast and Billac, 1996; Claessens, 2005) (see Table 4) proposed respectively basal areas of 36, 25 and 15 m2 ha21 before harvesting.

Future crop trees thinning. Claessens (2004) has proposed a thinning guide for stands issued from natural regenera-tions or dense plantations (more than 1000 stems ha21) based on the growth model of dominant trees of Fourbis-seur et al. (2002). It is useful for silvicultural interventions from 10 to 18 m height, after a phase of natural pruning and an early selection of the future crop trees. It is pre-sented in the form of the minimum distances that must be maintained between a crop tree and its nearest neighbours (from trunk to trunk) as a function of the height and d.b.h. of the trees (Table 6). The guide aims to prevent completely

Figure 6. Different silvicultural models in terms of decreases in stem numbers, and comparison with the maximum density stand.

Table 6: Minimum distance between a future crop tree and its nearest neighbour (from trunk to trunk) as a function of total height and d.b.h. of the future crop tree – all values in metres (Claessens, 2004)

Total height of the future crop tree (m)

Minimum distance between future crop tree and its nearest neighbour (m)

9 30 × d.b.h.12 27 × d.b.h.15 24 × d.b.h.18 21 × d.b.h.

any contact between the crowns of future crop trees and their neighbours.

Distances between final crop trees should be 10–12 m (i.e. ~70–100 final crop trees ha21). At the first thinning (at 8–10 m height), 2–3 times more trees can be selected than will be needed in the final stand at spacings of ~6–8 m. After two or three thinnings around the selected trees, an overall basal area of ~15 m2 ha21 is achieved and this needs to be maintained by thinning homogeneously throughout the stand (‘stand thinning’). Continuing to thin round the selected trees would lead to the removal of too many trees at a whole-stand scale.

PruningBlack alder generally grows straight, without forks or large branches. On suitable sites and in plantations with den-sities that exceed 1000 stems ha21, formative pruning is seldom required.

As already stated, natural pruning of black alder is good. At plantation densities of more than 1000 stems ha21, no ar-tificial high pruning is necessary (Claessens, 2005). Branch-less bole lengths of 8 m can easily be achieved (Immler, 2004). However, for crop tree silviculture that involves heavy crown thinning, some complementary artificial prun-ing of the selected trees may be necessary up to 6 or 8 m (Claessens, 2005). Where planting densities are lower than 1000 stems ha21, artificial pruning will be needed.

Maturation phase and regenerationSilvicultural trials in alder stands are few and recent. Ac-tual knowledge and experience cannot yet provide validated guidelines for the dynamic treatment of alder over the age of 40 years. For the same reason, natural regeneration of mature stands is not well known, except for plantation sil-viculture. Only some local trials are running, which are not yet reported in the scientific literature.

Conclusions

Black alder is well adapted to growing in damp ecosys-tems, especially along watercourses where it contributes particularly to riverine ecosystems and to the services they provide.

It is fast growing when young and can be as productive as birch and ash, but it does not withstand competition from neighbours. This means that enough space must be created around the best trees by the time they are 10 m tall if development is to be adequate for commercial timber production. Early thinnings must be heavy and frequent. Competition can best be controlled by thin-ning around the crowns of the trees selected for the final stand.

The short lifespan of black alder and the risk of epicormic shoots developing if crowns are constricted by delayed or light thinning (after 15 m tall) will result in trees being too small (i.e. less than 50 cm d.b.h.) to have any significant value before the onset of heart rot at around age 70. There is therefore no alternative but

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to adopt an interventionist form of silviculture for alder, especially on less productive sites where growth is slow.

In plantation conditions, many options are available in terms of spacing and plant choice. Stems planted at densities of 1000 stems ha21 or fewer have to be artifi-cially high pruned, but formative pruning is not gener-ally necessary. Weeding is also rarely either necessary or beneficial. A critical recommendation is that certified seed sources suitable for timber production are used.

Funding

This review paper has been made possible through travel grants from the Eu-funded project COST Action E 42 ‘Growing Valu-able Broadleaved Tree Species’ and is largely based on the results of a research programme funded by the Forest and Nature De-partment of the Walloon Region (Belgium).

Conflict of Interest Statement

None declared.

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A review of the characteristics of black alder (Alnus glutinosa (L.) Gaertn.) and their implications for silvicultural practices