Coarse woody debris, forest structure and regeneration in the Valbona Forest Reserve, Paneveggio, Italian Alps

Forest Ecology and Management 235 (2006) 155–163 www.elsevier.com/locate/foreco

Coarse woody debris, forest structure and regeneration in the Valbona Forest Reserve, Paneveggio, Italian Alps Renzo Motta a,*, Roberta Berretti a, Emanuele Lingua a, Pietro Piussi b a

Department of AGROSELVITER, University of Turin, Via Leonardo Da Vinci 44, 10095 Grugliasco, TO, Italy b Department of DISTAF, University of Florence, Via S. Bonaventura 13, 50145 Firenze, Italy Received 13 January 2006; received in revised form 4 June 2006; accepted 1 August 2006

Abstract Stand structure, quality and quantity of coarse woody debris (CWD) and the importance of the stumps for the Norway spruce (Picea abies (L.) Karst.) regeneration were studied in the Valbona Forest Reserve in the eastern Italian Alps. Past history, present structures, quantity and quality of coarse woody debris (CWD) are fundamental steps for increasing our knowledge of the natural forest stand dynamics. This is particularly relevant in the Italian Alps where all forests have been used by humans for millennia. Nevertheless, in the last decades there has been a noticeable reduction of the anthropogenic disturbance and, as a consequence, many forest stands have developed naturally even if their composition and structure still reflect past human activity. The mean volume of CWD in the Valbona Forest Reserve was 23.4 m3 ha1 ranging in the sampling plots between 0.0 and 89.3 m3 ha1. Of the total volume of dead and living trees, CWD comprised 4.9%. Among the CWD, the volume of logs (37.6%) was greater than the volume of snags (32.0%) and stumps (30.4%). There are no significant differences in the quantity of CWD among the structural categories. The decomposition classes of the CWD are different in the three CWD types and are the result of the recent land-use history of the reserve: the stumps are present mainly in the most decomposed stages (III and IV) while the snags and the logs are present mainly in the first and in the second decay class, respectively. The stumps play an important role for the regeneration of the Norway spruce: more than 57% of the present dominant trees established on stumps and the present density of the regeneration on the stumps is five times the density of the regeneration on the ground. Stumps in advanced decay classes (III and IV) are more suitable for regeneration than those in early decay classes. # 2006 Elsevier B.V. All rights reserved. Keywords: Dead wood; Decay; Forest dynamic; Regeneration; Managed forests; Norway spruce; Alps

1. Introduction The ecological importance of the coarse woody debris (CWD) has been relatively recently recognised (Harmon et al., 1986), however, in the last decade, many studies have examinated the ecological value of CWD to biodiversity (Spies and Franklin, 1988; Angelstam et al., 2003; HeilmannClausen and Christensen, 2004), its importance in the functioning and productivity of a forest ecosystem (Janisch and Harmon, 2002; Spears et al., 2003; Laiho and Prescott, 2004) and in the energy flow and carbon storage (Cohen et al., 1996; Harmon et al., 1996, 2004). In natural conditions, recurring disturbances, either small-scale gap perturbations or stand-replacing catastrophic events, continuously replenish and

* Corresponding author. Tel.: +39 011 7905538. E-mail address: [email protected] (R. Motta). 0378-1127/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2006.08.007

create CWD (Sturtevant et al., 1997; Franklin et al., 2002; Pedlar et al., 2002). These processes are difficult to observe in the European Alps where natural disturbance regimes have been replaced by diverse human interventions linked to changing economic and social conditions (Motta and Edouard, 2005) throughout man’s presence in the Alps; besides humans have altered the natural dynamic between dead and live trees and almost completely eliminated the CWD (Bretz Guby and Dobbertin, 1996; Dodelin et al., 2004). In the European Alps, forests have traditionally been exploited in various ways to maximize economic and social benefits: timber production has been combined with other functions such as protection (Motta and Haudemand, 2000; Dorren et al., 2005) and, most important, grazable forest land (Motta and Lingua, 2005). Timber production and grazing have existed side-by-side for centuries and only in recent decades grazing areas have been separated from forests where grazing is

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forbidden (Piussi and Farrell, 2000; Johann, 2002). In the past, the harvesting removed the trees before they reached large diameters and the amount of wood residues left in forest was used as a fuel. Through the thinning, the weakened trees were removed and the mortality process strictly controlled by humans. Sanitation fellings harvested senescent trees, and salvage loggings removed trees from small and large windthrow gaps (Piussi, 1983). In recent decades, socio-economic organization in the valleys and public attitudes towards the forest and forestry have changed dramatically and forestry has been towards a more near-to-nature approach to forest management with the aim of developing forest stands that are comparable to natural ones in so far as structure, composition and regeneration processes are concerned (Wolynski, 1998; Dotta and Motta, 2000; Del Favero, 2004). In this perspective, silvicultural practices may be modified with the intent of creating or maintaining an adequate stock of CWD and of promoting CWD structure similar to that found in unmanaged forests (Franklin et al., 1997; Siitonen et al., 2000; Norde´n et al., 2004; Ranius and Kindvall, 2004). Since the amount and the quality of CWD is specific for each forest type and is affected by factors such as tree species, site fertility, climate, disturbance regime and successional stage (Harmon et al., 1986; Rouvinen and Kuuluvainen, 2001), silvicultural restrictions on CWD removal have been proposed or introduced in many regions (Laiho and Prescott, 2004). This is the case of most forests of the European Alps. In this region, there have been few estimates of how much CWD there is and little knowledge of how it is affected by different forest management practices (McCarthy and Bailey, 1994; Sippola et al., 1998; Siitonen et al., 2000; Karjalainen and Kuuluvainen, 2002). Even if the role of CWD in the regeneration process of many subalpine forests has been recognised (Piussi, 1965, 1986; Ott et al., 1991; Zielonka and Niklasson, 2001; Brang et al., 2003) more detailed descriptions of CWD structure (Stokland, 2001) and dynamics are necessary to understand the quality, the quantity and the role of CWD in ecosystem function and to provide foresters and land managers with information useful in maintaining or increasing biological diversity, ecosystem functioning and productivity (Spies and Franklin, 1988). Regarding the importance of the CWD for the regeneration process the role of ‘‘nurse logs’’ as establishment sites for tree seedlings has been a major theme in forest dynamic and CWD research in old-growth forests (Harmon and Franklin, 1989; Kuuluvainen et al., 2002; Brang et al., 2003; Mitchell et al., 2003). Besides, in the subalpine level, microtopographically elevated sites are particularly suitable for Norway spruce regeneration (Scho¨nenberger, 2001; Kupferschmid and Bugmann, 2005). In managed forests, where the logs are rare or almost absent due to the silvicultural tending, the stumps could be an important site for the tree regeneration. Stumps are taller than the herbaceous layer and can escape the forb competition, have a short length of snow cover and provide, at least in the later stages of decomposition, a favourable seedbed (Gensac,

1990; Nakagawa et al., 2003). In the Paneveggio forest, a previous research has showed more than 53% of the present dominant trees established on a stump (with the typical root system on tramples), 18% on a stone and only 34% on the forest ground (Piussi, 1965). The stumps, and the other elevated sites like the stones covered by mosses, represent in this managed forest the best ‘‘safe site’’ for the spruce regeneration (sensu Harper, 1977). The aim of this study was to examine the amount and the quality of CWD in the Norway spruce (Picea abies (L.) Karst.) dominated subalpine Valbona Forest Reserve. The Valbona reserve represents a broad gradient of decreasing intensity and duration of forest utilisation, ranging from forest stands that have been managed or thinned until the end of 20th century to forest stands which have been unmanaged for decades (Piussi, 1965; Cherubini et al., 1996; Motta et al., 2002). Our objectives are: (a) to characterise the amount and the distribution of CWD in the reserve, (b) to delineate the CWD type and decay stage and (c) to analyse the role of the stumps for the Norway spruce regeneration. 2. Material and methods 2.1. Study area The study area is located in the Paneveggio Forest (latitude 468180 N, longitude 118450 E) in the Travignolo Valley (Trentino, Italy) at an elevation between 1560 and 1900 m a.s.l. Rainfall varies between 1207 mm/year at Paneveggio (1508 m a.s.l.), about 2 km from the study sites, and 1316 mm/ year at Passo Rolle (2002 m a.s.l.), about 3 km from the study sites. The average annual temperature at Passo Rolle is 2.4 8C (Gandolfo and Sulli, 1993). Snow cover persists for around 4–5 months in the forest at 1700 m. The bedrock is porphyry, partially covered by morainic material, and the soils are podsols and rankers. The dominant tree layer mainly consists of Norway spruce (more than 90%) with scattered individuals of larch (Larix decidua Mill.) and Swiss stone pine (Pinus cembra L.). The vegetation in Valbona is typical ‘‘subalpine Norway spruce forest’’ (Odasso, 2002). The reserve (123 ha) was established in 1992 and it is owned by the Province of Trento and is part of the ‘‘Parco Naturale Paneveggio-Pale di S. Martino’’. The reserve was divided into two parts: a silvicultural reserve for experimental research (43 ha) and a strict reserve where the forest has been allowed to evolve without silvicultural intervention (80 ha). The latter is the object of the present study and includes three intensive permanent long-term monitoring plots measuring 1 ha each (Motta et al., 2002). 2.2. History of forest utilisation and land-use of the study area The Forest of Paneveggio was part of the Austro-Hungarian Empire until the end of the first world war. Since 1919, the forest has been the property of the Italian State, though subsequently it was entrusted to the Trentino-Alto Adige

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Region and since the 1970s, it has belonged to the Autonomous Province of Trento. According to the management plans in the 19th century, the forest was subdivided into management sectors distinguished by type of treatment (Motta et al., 1999): Unit A was managed with shelterwood felling in small groups with rotation shifts of 160 years and where, after a seeding felling, the final felling was supposedly carried out after 20 years; Unit B was managed with a selection-high grading system among mature specimens of approximately 200 years of age; the upper part of the reserve used to be grazable forestland until the beginning of the 20th century (Motta et al., 2002). Clearcutting in Paneveggio has been practised at least since the end of the 18th century, though not in the present reserve, but rather in the more fertile areas located at low altitudes outside the Valbona Forest Reserve (Piussi, 1965). The sectors of the reserve formerly submitted to the shelterwood felling today have crowns within a single horizontal layer while the sectors formerly submitted to a selection system and used as a grazable forestland have a multilayered or an irregular structure. The forest was heavily disturbed during the first world war as many trees were damaged and trenches were dug in proximity to the forest reserve. Intensive fellings were carried out from 1909 to 1913 and from 1919 to 1921 (Piussi, 1965); the latter period was a salvage operation following windthrows and Ips typographus outbreak when a total of 200,000 m3 of wood was cut in the whole forest (an amount equivalent to that of 50 years of present prescribed fellings). The lower parts of the reserve were managed until the 1980s (selective logging and thinning), while the upper parts of the reserve have been unmanaged at least from the end of the second world war. 2.3. Amount and quality of CWD in the forest reserve CWD has been grouped into downed logs (defined as piece of stem or branch that have fallen and have at least 7 cm diameter and length >1 m), standing dead trees or snags (dead standing trees, dbh > 7 cm and taller than 1.3 m) and stumps (short, vertical pieces created by cutting or by windthrow, diameter at the top >7 cm and height 1 m). The separation of snags from logs is at a 458 angle. Since CWD is highly variable in space and in time we established a systematic sampling network. In each sampling point, a different protocol was applied to estimate the fraction of CWD laying on the ground (logs) and the fraction of CWD still standing (snags and stumps). 2.3.1. Sampling methods 2.3.1.1. Stand structure. We superimposed a regular 100 m grid to the 1:10,000 raster map (Provincia di Trento) for a total of 68 sampling points. In each sampling point, three type of measurement were applied: (a) 20 m  20 m plot centered on the grid for the classification of each point in a structural typology based on six categories (monolayered, multilayered, multilayered and clustered, two layers, openings, young growth) and 14 types (Berretti et al., 2004), (b) line intersect sampling (LIS) 50 m long for the logs and (c) 50 m  8 m rectangular plot for the stumps and the snags.

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In the 20 m  20 m plot, the following parameters were measured for all trees: dbh to the nearest cm, height to the nearest m, crown length to the nearest m and crown radius projection to the ground in four directions to the nearest 10 cm. In addition, in each sampling point we cored three trees (the nearest to the north-eastern corner) at the breast height. In the laboratory, all the cores were fixed to wooden supports and smoothed with a razor blade or sanded with progressively finer grades of sandpaper until optimal surface resolution allowed annual rings to be measured. Ring width was measured to within 0.01 mm. The cores collected at breast height were checked, corrected for missing rings and cross-dated against already available site chronologies (Motta et al., 1999). To help identify past disturbances that affected the stand we examined the ring pattern of all trees successfully cross-dated for evidence of release (Motta et al., 2002). Volumes for living trees were calculated according to local yield tables from the Trento Forest Administration (Castellani, 1982). 2.3.1.2. Amount of CWD. Volume of logs was estimated by using the LIS (Warren and Olsen, 1964; Van Vagner, 1968) in a 50 m linear transect centred in the sampling point and directed north-east. The measurements consisted of the diameter (to the nearest cm) at each intersection point. The volume of logs was calculated using Van Vagner’s formula (1968):  2 p V¼ S d2 8L where V is the volume per unit area, d the piece diameter at intersection and L is the length of sample line. This equation embodies three assumptions that the pieces are randomly oriented, circular in cross section, and horizontal. The volume of standing dead trees was estimated from Paneveggio Norway spruce yield tables (Castellani, 1982). The volume of the broken snags and of the stumps was estimated as a frustum of a cone from diameter at the top, diameter at the ground level (avoiding roots or irregular shape) and length (estimating a plane cutting of the apex parallel to the base):   So þ St Vss ¼ L 2 where Vss is the volume of the broken snags and of the stumps (m3), St the area of the small end (m2), So the area of the large end (m2) and L is the length of the broken snag or stump (m). Volume of snags and stumps was counted on a 50 m  8 m rectangular plot based on the linear transect used for LIS. Stumps were classified as a natural or cut by man. The area of large end of a stump is difficult to estimate due to the irregular shape and to the slope. We used an average basal diameter not taking into account the lateral adventitious roots resulting in a underestimation of the CWD volume. To test whether the abundance of CWD differed in terms of the function of structural category we compared observed-to-expected frequencies using chi-square (x2) test.

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2.3.2. Decay stage The decay stage of logs was classified according to a five class system (Maser et al., 1979; Sollins, 1982): (1) bark intact, small branches present, shape round, wood texture intact, log elevated on support point; (2) bark intact, no twigs, shape round, log elevated but sagging slightly; (3) trace of bark, no twigs, shape round, wood hard, texture with large pieces, log sagging near the ground; (4) no bark, no twigs, shape round to oval, wood hard, texture with blocky pieces, all of log on the ground; (5) no bark, no twigs, shape oval, wood soft and powdery structure, all of the log on the ground. The decay stage of snags was classified according to a five class system (Thomas et al., 1979; Sollins, 1982): (1) standing dead tree with bark and most of the branches intact, wood hard; (2) dead tree with few branches left and loose bark, wood hard; (3) no bark, no twigs, wood hard; (4) no bark, no twigs, wood hard to soft (soft sapwood < 70%); (5) no bark, no twigs, wood hard to soft (soft sapwood > 70%). The decay stage of stumps was classified according to a four class system: (1) bark intact, wood hard; (2) bark almost completely intact, wood hard in the outermost part and decay in the innermost part of the stump, texture with large pieces; (3) trace of bark, decay spread in most of the stump, texture with blocky pieces; (4) bark absent, wood soft and powdery structure. 2.4. Regeneration on stumps Fig. 1. Structural categories in the Valbona Forest Reserve.

We analysed the role of the CWD for the present regeneration. Even if the quantity of the three types are more or less equivalent only the stumps presently have a large number of individuals that are evenly distributed in the reserve and, at least, in three decay stages.

In each stump we counted the number of the seedlings. The stump surface available for the regeneration was obtained from the stump diameter at the base and, consequently, the density of regeneration, total and in each decay stage, was calculated. In

Table 1 Volume (m3 ha1) of CWD (mean, maximum and minimal values) in the Valbona Forest Reserve and in the main six structural categories Sampling points

1

Monolayered

Multilayered and clustered

Multilayered

Two layers

Openings

Young growth

Total

31

14

12

5

4

2

68

881 2176 526

524 842 330

107 207 0

339 501 177

606 2176 0

Living trees (n ha )

Mean Max Min

647 1983 277

487 903 219

Living trees (m3 ha1)

Mean Max Min

590.6 1121.3 83.7

613.8 1009.2 90.7

284.9 462.8 162.4

377.8 533.5 269.8

87.7 192.2 0

13.6 20.9 6.2

479.2 1121.3 0

CWD (m3 ha1)

Mean Max Min

25.2 89.3 0.1

17.2 42.5 3.5

24.0 76.3 5.7

31.9 80.7 3.2

20.3 52.3 0.0

20.0 25.1 14.8

23.4 89.3 0.0

Logs (m3 ha1)

Mean Max Min

10.2 48.7 0.0

5.1 21.7 0.0

10.2 41.5 0.0

15.0 36.5 0.0

6.8 9.9 0.0

1.4 2.8 0.0

9.0 48.7 0.0

Snags (m3 ha1)

Mean Max Min

8.6 57.0 0.0

5.6 30.2 0.0

7.4 31.3 0.6

4.8 23.0 0.0

13.3 48.8 0.0

0.0 0.0 0.0

7.5 57.0 0.0

Stumps (m3 ha1)

Mean Max Min

6.5 34.8 0.0

6.4 17.1 0.0

6.4 14.4 0.7

12.0 22.2 3.2

0.2 0.7 0.0

28.6 32.1 25.1

6.8 34.8 0.0

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the same year, a seedling survey on the ground (irrespective on the quality of ground and partially including also CWD) in the forest reserve was established. This survey was based on 40 transects, each of them 40 m long and with a square 400 cm2 sampling point each 1 m for a total of 800 sampling points and 32.0 m2 of surface. To test whether the regeneration density on stumps and on the ground differed and whether the regeneration density on stumps differed in terms of the function of decay stage we compared observed-to-expected frequencies using chi-square (x2) test. 3. Results The mono-layered structure is the most widespread, covering more than 45.6% of the sampling points. The clumped and multi-layered structures are also well represented with 20.6 and 17.6% of surface area, respectively. Two layers, openings and young growth stands, are less represented with, respectively, 7.4, 4.9 and 2.9% of the sampling points (Fig. 1). The mean volume of CWD in the Valbona Forest Reserve was 23.4 m3 ha1 ranging between 0.0 and 89.3 m3 ha1 (Table 1). Of the total volume of living trees, CWD represents 4.9%. Among the CWD, the volume of logs (37.6%) was greater than the volume of snags (32.0%) and stumps (30.4%). The highest quantity of CWD is mainly concentrated in the central part of the reserve (Fig. 2). Most of the sampling points have a total volume of CWD < 20 m3 ha1 but there are 14 sampling points with an amount of CWD > 40 m3 ha1 (Fig. 3).

Fig. 3. Amount of CWD in the 68 sampling points. Table 2 Decay class distribution (%) in snags, stumps, logs and in the total CWD in the Valbona Forest Reserve (snags and logs have five decay classes and stumps have four decay classes) Decay class

Snags

Logs

Stumps

1 2 3 4 5

63.4 34.1 2.4 0.1 0.0

1.5 43.8 39.4 15.3 0.0

0.4 25.0 41.9 32.7

Across all the structural categories, there are no significant differences in the CWD amount. In the same way, there are no significant relationships between CWD amount and the age of the oldest tree and the time from the last disturbance recorded by abrupt growth release. There are significant relationships (P < 0.01) between CWD amount and the volume of living tree but if the CWD types are analysed separately there are no significant relationship between CWD amount of stumps and logs and volume of living trees but there are significant relationships between CWD amount of snags and volume of living trees. The snags are almost exclusively concentrated in the first and in the second decay classes with 63.4 and 34.1%, respectively (Table 2). The most frequent decay class for logs is class 2 (43.8%) followed by classes 3 and 4 (39.4 and 15.3%, respectively). The stump distribution in decay classes is completely different because stumps are concentrated in the most decomposed decay classes (42.1% in the third and 31.9% Table 3 The relationship between decay class and occurrence and quantity of Norway spruce regeneration in stumps

Fig. 2. Amount of CWD in the Valbona Forest Reserve.

Decay class

Stumps

1 2 3 4

3 125 233 241

Total

602

% of stumps 0.5 20.7 38.7 40.0 100

%Stump with regeneration

Number of seedlings

0 34.4 51.1 39.8

0 130 577 243

42.8

950

%Seedlings 0 13.7 60.7 25.6 100

160

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of stumps with regeneration, respectively, in the third and fourth decay stage). The regeneration density was significantly higher on the stumps than on the ground (5.8 seedlings m2 versus 1.2 seedlings m2; P < 0.001). Tree seedling density differed significantly among the decay classes (P < 0.01): establishment of Norway spruce regeneration began in the second class and the seedling density reached the highest value in the third decay class (Table 3). 4. Discussion Fig. 4. Frequency size distribution of logs, snags and stumps.

Natural mature and old-growth forests are characterised by large volumes of CWD in different stages of decay (Harmon et al., 1986; Spies and Franklin, 1988). In the previously managed Valbona Forest Reserve, the amount and the quality of the three types of CWD in different decay stages represents an historical archive that tells us the recent disturbance history of the forest and the present forest dynamic and could give us useful information regarding the intensity of the past human disturbances and the present naturalness or hemeroby of the

in the fourth class) and are less frequent (25.1% in class 2) or almost absent (0.4% in class 1) in the less decayed classes. The stump size presents a normal distribution (Fig. 4). Regeneration was present on 42.8% of the stumps (Table 3). The regeneration is non-evenly distributed among the different decay stages (P < 0.01) and the distribution is skewed toward the most decomposed stages (51.1 and 39.8% Table 4 CWD amount and quality in European Norway spruce forests

Stumps (m3 ha1)

Total (m3 ha1)

Forest type

Management

Development phase

Paneveggio (I) Alptal (CH)

Norway spruce Norway spruce

Unmanaged Managed

Various Various

Chironico (CH)

Norway spruce

Managed

Various

Wettersteinwald (DE)

Norway spruce

Unmanaged

–

Les Saisies (F) Polana (SK)

Norway spruce Norway spruce

Managed Unmanaged

Mature Various

24.0 150–450

Kosodrevina (SK)

Norway spruce

Unmanaged

Various

50–200

Babia Gora (PL) Babia Gora (PL)

Norway spruce Norway spruce

Unmanaged Unmanaged

Late seral –

Estonia Estonia All Sweden

Norway spruce Norway spruce Norway spruce

Managed Unmanaged Various

Mature Mature –

Swedish boreal forest (SE) Virgin boreal forest reserves (SE) Va¨stra Kvarnen (SE)

Norway spruce

Unmanaged

Old-growth

Norway spruce

Unmanaged

Old-growth

Norway spruce

Unmanaged

Old-growth

Norway spruce and Scots pine Norway spruce and Scots pine Norway spruce and Scots pine Norway spruce and Scots pine Norway spruce

Managed

Mature

Managed

Southern Finland (FI) Southern Finland (FI) Southern Finland (FI) Pahkavaara (FI) Boreal virgin spruce forest (RU)

Snags (m3 ha1)

Logs (m3 ha1)

Location

7.5 – 16 –

58.5 –

9.0 –

6.8 –

23.4 (0–89.3) 3.9

4.2

–

21.6

–

–

84

72.6 –

– –

131.1 165 9.4 36.4 7.2

0.5–13

17–65

Reference This study Bretz Guby and Dobbertin (1996) Bretz Guby and Dobbertin (1996) Rauh and Schmitt (1991) Dodelin et al. (2004) Saniga and Schu¨tz (2002) Saniga and Schu¨tz (2002) Holeska (2001) Jaworski and Karczmarski (1995) Lo˜hmus et al. (2005) Lo˜hmus et al. (2005) Fridman and Walheim (2000) Jonsson (2000)

89

Linder et al. (1997)

76 2–28

Svensson and Jeglum (2001) Siitonen et al. (2000)

Overmature

7–38

Siitonen et al. (2000)

Unmanaged

Old-growth

70–184

Siitonen et al. (2000)

Unmanaged

Old-growth

Unmanaged

Old-growth

CWD definition and sampling methods are different in each study.

24

10.1

52

66.1

2.3

78.5 31.8–326.2

Rouvinen and Kouki (2002) Shorohova and Shorohov (2001)

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forest stands (Groven et al., 2002; Hill et al., 2002; Rouvinen et al., 2002; C ¸ olak et al., 2003). The highest quantity of CWD is mainly concentrated in the central part of the reserve because the lower part of the reserve (north in the map) has been managed until the 1980s and the upper part of the reserve (south in the map) is much more open and was probably grazed until the beginning of the 20th century (Fig. 2). The present volume of CWD of the Valbona reserve is lower than in the unmanaged old-growth boreal and Carpathian spruce forests (where the CWD can be >300 m3 ha1) but the amount and the quality of the dead wood are remarkable if compared with other European temperate Norway spruce forests (Table 4). As expected there are no significant relationships between structural category and amount of CWD: the dead wood is often very unequally distributed across landscapes (Lo˜hmus et al., 2005) which causes large variations among sampling plot measurements even on the same structural category. Besides the present structure largely depends on the past land-use and silvicultural treatments. In the same way, there are no significant relationships between amount of CWD, tree age and time from the last disturbance. The only significant relationship is between the amount of CWD and the volume of living trees. In the latter case, the significance is due to the amount of snags that, if analysed separately, is the only CWD component amount that presents significant relationships with the volume of living trees. In fact, the stumps represent the past human disturbance that affected all the structural categories and all the development stages (thinning, single tree selection and group selection). On the other hand, logs and snags represent the natural mortality and, in this case, the mortality of standing tree increases with the biomass increment due to the competition. The amount of logs in the Valbona reserve is not correlated with the volume of living trees because some logs result from the last tending and thinning; besides snag standing time lasts some decades (Storaunet and Rolstad, 2004) and, in most case, is longer than the time from the last silvicultural intervention (more than 63% of the snags is in the first decay class). An important component of the Valbona CWD biomass is represented by stumps (>30%). The rarity of stumps in the first decay class is a consequence of the recent land-use change and reserve establishment. On the other hand, most of the stumps are in the second and third decay stages (67%) and only one-third (32%) in the fourth stage according to the time when the last cut or silvicultural intervention was applied. The snag and the log size distribution present a different size distribution approaching a reverse J shape. These components are more related to the natural processes established in the forest reserve in the last decades. Even if the Norway spruce is very sensitive to uprooting (Mazzucchi, 1983), the present mortality process affects mainly standing tree of small and intermediate diameter (dominated and suppressed trees) and only marginally affects large diameter and dominant trees (Franklin et al., 1987). The present mortality processes are mainly endogenous (e.g. competition) than exogenous (e.g. windthrow) because most of the stands are young and have not reached the transition or old-growth development phases where

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the mortality, and subsequent gap formation, could interest the dominant trees (Oliver and Larson, 1996). The distribution of the decay stage in the snags is concentrated in the first (63%) and in the second (34%) decay stages. The decay stage distribution of the logs is different. Most of the logs reach the ground after a first stage as a snag (most tree die when they are standing because of competition) and this explain why there are only a few logs in the first decay class. Norway spruce seems to have a natural regeneration strongly connected with decomposed wood (Gensac, 1990; Hofgaard, 1993; Holeska, 2001; Zielonka and Piatek, 2001; Jezˇek, 2004). In the Valbona reserve, the density of seedling in the CWD is five times higher with respect to the average density on forest ground. The establishment of seedlings on decayed stumps is favoured by advantageous moisture and light conditions while competition of other plants is restricted (Zielonka and Niklasson, 2001; Kuuluvainen and Kalmari, 2003; Jezˇek, 2004). In addition, decayed wood can be rich in nutrients as a result of microbial nitrogen fixation (Brunner and Kimmins, 2003). However, not all the decay classes provide the same favourable environment and the rate of wood decomposition has a great influence on the regeneration process. In the Valbona reserve, the first stage of decay is not suitable for the regeneration that begins to establish in the second one; the third stage seems to be the most favourable for the regeneration while the fourth see a decrement of seedlings probably due to the physical collapse of the stump. Due to the lack of large logs on the forest ground, the stumps, that are present in different stage of decomposition, play an important role for the regeneration of the Norway spruce in Paneveggio. Since most of the stands are young and have been intensively thinned in the past, we expect that the quantity of CWD will increase in the next decades and that the dead wood, logs and snags, will reach the most decomposed decay classes. Large logs in the future will provide the sites for the regeneration instead of the stumps. In the European Alps, considering the long time requested to old-growth characteristics to develop (Franklin, 1993; Peterken, 1996; Frelich and Reich, 2003), it is important to preserve forests that have not been intensively managed in the last decades and have retained and or reestablished some important features of the natural ecosystems. Acknowledgements This study was funded by the ‘‘Parco Naturale PaneveggioPale di S. Martino’’ and by the ‘‘Servizio Foreste e Fauna’’ of the Autonomous Province of Trento. References Angelstam, P.K., Bu¨tler, R., Lazdinis, M., Mikusinski, G., Roberge, J.M., 2003. Habitat thresholds for focal species at multiple scales and forest biodiversity conservation—dead wood as an example. Ann. Zool. Fennici 40, 473–482. Berretti, R., Lingua, E., Motta, R., Piussi, P., 2004. Classificazione strutturale dei popolamenti forestali nella Riserva forestale integrale della Valbona a Paneveggio (TN). Ital. For. Mont. 59, 99–118.

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