Densities of large living and dead trees in old-growth temperate and boreal forests

Forest Ecology and Management 161 (2002) 189±204

Densities of large living and dead trees in old-growth temperate and boreal forests Sven G. Nilssona,*, Mats Niklassonb, Jonas Hedina, Gillis Aronssonc, Jerzy M. Gutowskid, Per Lindere, HaÊkan Ljungbergf, Grzegorz MikusinÂskig, Thomas Raniush a Department of Ecology, University of Lund, Ecology Building, S-223 62 Lund, Sweden Southern Swedish Forest Research Centre, Swedish University of Agricultural Sciences, P.O. Box 49, S-230 53 Alnarp, Sweden c Stenhagsv. 83B, S-752 60 Uppsala, Sweden d Natural Forests Department, Forest Research Institute, 17-230 Bialowieza, Poland e Department of Forest Vegetation Ecology, Swedish University of Agricultural Sciences, S-901 83 UmeaÊ, Sweden f Department of Geology, University of Lund, Tornav. 13, S-223 63 Lund, Sweden g GrimsoÈ Wildlife Research Station, Department of Conservation Biology, Swedish University of Agricultural Sciences, S-730 91 Riddarhyttan, Sweden h Department of Zoology, University of Lund, Helgonav. 3, S-223 62 Lund, Sweden

b

Received 10 July 2000; received in revised form 30 January 2001; accepted 5 February 2001

Abstract We recorded and reviewed densities and basal areas of large living and dead trees in old-growth forest in Europe. Recorded densities were similar to those reported from old-growth forests in eastern North America, but lower than in northwestern North America. Based on our results we suggest that, 10±20 living trees per ha with dbh > 70 cm may have been typical values for many central European and south Scandinavian virgin forests. In boreal forests, it was probably common with at least 20 living trees per ha with dbh > 40 cm. Basal areas of living trees in mixed old-growth forests in central Europe and southern Sweden were 34±40 m2 per ha on dry ground and about 60 m2 per ha in wet alder±ash±spruce forests. Densities of large trees (dbh > 40 cm) were twice as high in the latter forest type than on dry ground in BiaøowiezÇa forest, Poland. Based on our results, we propose the following generalizations to be further tested in other old-growth temperate and boreal forests: 1. Among all standing trunks (including high stumps) about 10% are dead, but this proportion increases for the largest trees. The proportion of standing trees that are dead seem to be independent of total basal areas. Based on this, we suggest that the volume of dead wood is directly proportional to the productivity of old-growth forests. 2. Standing dead trees (snags) are on average larger than downed dead trees. Trees with dbh > 40 cm often dominate the basal area and volume of standing dead trees and living trees. 3. About 30% (20±40%) of the basal area and volume of dead trees is standing in old-growth forests. This proportion seems to be independent of total volume of dead wood.

* Corresponding author. Tel.: ‡46-462229315; fax: ‡46-462224716. E-mail address: [email protected] (S.G. Nilsson).

0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 1 ) 0 0 4 8 0 - 7

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Large disturbances by ®re, strong winds and insects may temporarily change these proportions considerably in individual stands. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Tree size; Snag densities; Log densities; Coarse woody debris; Reference values; Forest ecology; Old-growth forest

1. Introduction Large trees, both living and dead, have been identi®ed as the most important substrates for threatened species in temperate and boreal forests (e.g. Harding and Rose, 1986; Berg et al., 1994, 1995; Samuelsson et al., 1994; Peterken, 1996; Jonsell et al., 1998; Bunell et al., 1999). Features such as rough bark, trunk hollows, exposed dead wood, sap-¯ows, dead branches and dead tops are often developed in large trees, and these are habitats for numerous species. Both in living trees (Ranius and Jansson, 2000) and dead trees, several saproxylic (wood-living) species are restricted to large dimensions (e.g. Palm, 1959; Siitonen and Saaristo, 2000). Especially among species-rich groups like insects and fungi, many threatened species require large living and large dead trees (review in Nilsson et al., 2001). An increase of these features is therefore one of the most urgent measures which should be taken for biodiversity preservation and restoration. However, there is generally little understanding of what amounts of large trees and dead wood that are required in different types of forests for long-term preservation of high biodiversity and thus a sustainable forestry (Keddy and Drummond, 1996). One way to achieve a better understanding of the requirements of large trees and dead trees for biodiversity preservation is to measure these features in virgin forests. This would reveal what characterized the environment where adaptations of the species living in forests have evolved, and also provide a baseline for comparison with managed forests. However, for temperate forests in Europe, such measurements are problematic, because after closer studies ``virgin forests'' have proved to be more or less in¯uenced by past management (e.g. FalinÂski, 1986; Koop, 1989; Peterken, 1996). Another problem is to decide what is relevant to measure. Often, only the volume of dead trees has been reported, with little information on the size distribution. However, several threatened species only develop in larger trunks (e.g.

Palm, 1959; Ssymank, 1994; Siitonen and Saaristo, 2000), which often are completely absent in managed forests (e.g. Majewski et al., 1995; Kirby et al., 1998; È stlund, 1998). Furthermore, the size Linder and O distribution of dead trees and old living trees may differ between forests with old trees due to differences in the past management (e.g. Nilsson and Baranowski, 1997; Ranius and Nilsson, 1997). Fire suppression during the last 100 years has changed the tree density and diameter distribution in many boreal forests in northern Europe (e.g. Linder, 1998). The aim of this study is to report densities and basal areas of living and dead trees of different dimensions in old-growth forests in Europe. The emphasis is on large trees, both living and dead, since they seem to be crucial in biodiversity preservation (Nilsson et al., 2001). The study sites were selected in order to be as little in¯uenced by past management as possible. 2. Methods 2.1. Study areas All study areas were old-growth forests with many canopy trees more than 200 years old. Many of them were mixed forests of deciduous and coniferous trees (Table 1). To qualify as old-growth, we consider that an old stand have canopy trees with an age exceeding half their maximal life spans. The stands have been almost free from cuttings during the developing time of the present trees. The traces of past management were small, but probably all stands except some boreal forests have been used as grazing land for domestic animals long ago (Nilsson, 1997; Vera, 2000). Dead trees were common in all stands, but we can not exclude the possibility that dead trees could have been removed and used as ®rewood 50 years ago or more (see Section 4). In central-southern Sweden, we studied the nature reserves BjurkaÈrr and Siggaboda. BjurkaÈrr is dominated by beech (Fagus sylvatica), with some oak

S.G. Nilsson et al. / Forest Ecology and Management 161 (2002) 189±204

191

Table 1 Basic data about the old-growth forest plots in Europea Forest

Latitude (8N)

asl

Dominating living trees

Area

Reference

Corsica, Fango France, Fontainebleau France, Fontainebleau Slovakia, Dobrocsky prales Poland, BiaøowiezÇa 314C Poland, BiaøowiezÇa 398 Poland, BiaøowiezÇa 317 Poland, BiaøowiezÇa 317D Poland, BiaøowiezÇa 340C Poland, BiaøowiezÇa 399C Sweden, Siggaboda Sweden, BjurkaÈrr Sweden, Biskopstorp Sweden, HaroÈn Swedish taiga

42 48

360 140

49 52

800 155

56 57 57 60 64

150 140 140 10 340

Holm oak Beech Beech, oak Fir, beech, maples, spruce Alder, ash, spruce Ash, alder, spruce Lime, oak, hornbeam, maple Lime, hornbeam, oak, spruce Spruce, lime, hornbeam, elm Hornbeam, lime, spruce Spruce, beech Beech, oak, pine Beech Spruce, lime Spruce, pine, birches

0.64 2.0 0.75 1.0b 1.0c 1.0c 1.0c 1.0c 1.0c 1.0c 1.0 1.3 4 1.0 1.63

66 62

600 175

Spruce, birches Spruce, pine Spruce, pine Pine

1.0 1.0 1.0 1.0

PanaõÈotis et al. (1997) Koop and Hilgen (1987) Pontailler et al. (1997) This study This study This study This study This study This study This study This study This study Karlsson (1996) This study Linder and Elfving (1996) Linder (1986) Annila in litterature Annila in litterature Annila in litterature

Sweden, KirjesaÊlandet Finland, Seitseminen N.P. Finland, Seitseminen N.P. Finland, Seitseminen N.P. a

Height above sea level (asl) is in (m) and size of the sampled area is in (ha). For dimensions < 40 cm dbh and all down trees only 0.1 ha. c For dimensions < 40 cm dbh only 0.1 ha. b

(Quercus robur), pine (Pinus sylvestris), birch (Betula spp.), spruce (Picea abies) and several other minor species. The oldest tree generation with beech, oak and pine is 220±280 years old. Some cuttings probably occurred about 130 years ago and of some oaks 60 years ago (Niklasson, unpublished data). Siggaboda is dominated by spruce and beech, with the oldest generation being 200±280 years old (BjoÈrkman and Bradshaw, 1996; Niklasson et al., in preparation). Pollen analysis has revealed an unbroken continuity of tree occurrence over more than 2000 years, but the composition of tree species has changed dramatically (BjoÈrkman and Bradshaw, 1996). After a ®re 1000 years ago, beech established in a mixed forest of mainly oak Quercus sp., lime (Tilia cordata), birch and hazel (Corylus avellana). Several low intensity ®res 250±350 years ago resulted in pine regeneration, but these pines were cut about 200 years ago (Niklasson et al., in preparation). After this last ®re episode, beech strongly increased (BjoÈrkman and Bradshaw, 1996), and spruce immigrated about 200 years ago. Thus, the oldest spruces that now dominate the stand represent the ®rst generation of this species.

On several islands surrounded by rapids at the lower part of River DalaÈlven, eastern Sweden, there seem to be stands of virgin forest (Palm, 1942, own observations). One of the larger of these islands, HaroÈn, is covered by a spruce-dominated stand with some lime, oak, alder, ash, elm and maple in the tree layer and abundant hazel in the shrub layer. Many trees are more than 200 years old, and no cut stumps were found in the study plot. In central Slovakia we studied the central part of Dobrocsky prales (prales ˆ virgin forest), which is situated within Polana biosphere reserve. This stand is dominated by silver ®r (Abies alba), beech, maples (Acer pseudoplatanus and Acer platanoides) and spruce, and the highest ®rs and spruces are more than 50 m high. In eastern Poland we studied six 1 ha plots in the BiaøowiezÇa forest within the area that has been strictly protected since 1921. We selected plots with mixed forests of many tree species, on dry ground mainly lime, oak (Q. robur), hornbeam (Carpinus betulus), maple (A. platanoides) and spruce and on wet ground ash (Fraxinus excelsior), alder (Alnus glutinosa)

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and spruce. These two forest types, which dominate in the strictly protected part of BiaøowiezÇa forest, are described in detail in FalinÂski (1986), Koop (1989) and Peterken (1996). In 1892±1915 there was a high density of wild and domestic grazers and browsers and that strongly favoured the establishment of spruce, and almost prevented regeneration of several other species, especially lime and hornbeam (FalinÂski, 1986; Peterken, 1996). Strong wind storms in 1983 and 1986 resulted in many wind-fallen trees, especially spruces (Gutowski and Kubisz, 1995). Comparable data were also collected from other studies reporting on old-growth forests in Europe (Table 1). Central and northern Europe is divided in the boreal, boreonemoral and nemoral forest regions (SjoÈrs, 1963). The boreal region extends south to the northern distribution limit of oaks, while the nemoral region extends north to the southern natural distribution limit of spruce. The transition zone in between is the boreonemoral (or hemiboreal) region. 2.2. Field surveys In BjurkaÈrr, we randomly selected 13 study plots and in Siggaboda 10, each being a circle with an area of 0.1 ha. Only the least disturbed core areas on dry ground, about 20 and 5 ha, respectively, were included. All trunks (living, dead and stumps at least 1.3 m high) were measured at 1.3 m height (dbh) in 1994. Fallen trunks that originated from the plots were measured 1.3 m from the roots. At HaroÈn one plot of 100 m  100 m in the drier northwestern part was sampled in April 2000. In Dobrocsky prales and BiaøowiezÇa we selected study plots in spring 1998, with sizes of 100 m  100 m, in stands that seemed unaffected by cuttings. In BiaøowiezÇa two plots were selected in wet swamp forests and four plots in drier mixed forests dominated by lime, hornbeam and several other tree species. In the oldest, strictly protected part of the National Park, the area of the latter forest type is about twice the area of swamp forest (FalinÂski, 1986). We measured the dbh of all trunks with dbh > 40 cm, both living and dead, and recorded the tree species. Both standing and fallen trunks were measured, except in Dobrocsky prales where only standing trees were recorded. The height of standing

dead trees, both those with broken trunks and intact tops, were measured. Among fallen trunks only those that originated from the plots and could be clearly identi®ed were measured (at 1.3 m from the roots). Their diameters were measured with the bark included if present. All trunks with dbh between 10 and 40 cm, both standing and fallen, were counted in a circular plot, with an area of 0.1 ha, randomly selected within each of the 1 ha plots. 2.3. Calculations For standing dead trees and snags (high stumps) the function for a parallel cut cone was used to estimate standing dead tree volumes (V ) V ˆ 13 L…br ‡ bt ‡ …br  bt††0:5 where L is the length of the section; br the basal area at root end; bt the basal area at top end. Since the taper of a trunk is considerably different below and above 1.3 m above ground, it was divided into two cut cones, one above 1.3 m and one below 1.3 m from the ground. The variables in the function are the length of the cut cone and upper and lower basal area. Since in our case only dbh is known, the diameters at ground and at the height where the trunk is broken must be estimated to calculate the areas. This was done (for all tree species) with a taper function for spruce P. abies (Anon, 1962). The total volume of the dead tree or snag was given from the sum of the volumes of both cut cones. 3. Results 3.1. Living trees The density of living trees in beech-dominated forests in central Europe was about 30 large trees (dbh > 70 cm) per ha (Table 2). In BiaøowiezÇa forest and one of the beech-dominated forests in southern Sweden, the density was about half of that. In central Europe, densities of living trees with dbh > 80 cm were 10±17 ha 1 in most plots, with the highest values in beech-dominated stands. In boreal forests large trees (dbh > 70 cm) were rare. In these forests, only pine and spruce occasionally reached such dimensions.

S.G. Nilsson et al. / Forest Ecology and Management 161 (2002) 189±204

193

Table 2 Densities (number per ha) of living trees of different minimum sizes (dbh in cm) in old-growth forests in Europea Minimum tree sizes

Fango Fontainebleau Fontainebleau Dobrocsky prales BiaøowiezÇa 314C BiaøowiezÇa 398 BiaøowiezÇa 317 BiaøowiezÇa 317D BiaøowiezÇa 340C BiaøowiezÇa 399C Siggaboda BjurkaÈrr HaroÈn Swedish taiga KirjesaÊlandet Seitseminen N.P. Seitseminen N.P. Seitseminen N.P. a

10

20

30

40

50

60

70

80

268 263 224 173 471 460 401 704 236 334 402 445 335 1081 501 1158 926 597

250 133 175 123 301 310 181 284 186 284 268 255 195 455 182 232 462 366

95 72 132 123 241 240 101 144 126 154 183 151 175 190 60 120 181 109

48 48 88 93 191 140 61 34 86 94 113 92 125 64 18 74 64 49

38 37 64 77 130 86 44 14 50 50 39 43 65 15 3 37 32 12

22 33 44 58 71 48 32 7 31 22 13 23 24 1 1 0 8 3

6 24 28 34 30 18 22 5 13 13 4 12 6 0 1 0 4 0

2 12 19 16 17 10 17 5 9 10 1 5 2 0 0 0 0 0

Note that values are cumulative from right to the left. The forest plots are listed in the same order as in Table 1.

If living trees with a dbh > 40 cm are considered, densities were mostly 50±100 ha 1 both in boreal, boreonemoral and nemoral forests on dry ground (Table 2). However, the two swamp forest plots in BiaøowiezÇa had densities that were twice as high. A spruce-dominated forest near the Scandinavian mountains had only 18 such large trees, while another spruce-dominated forest near sea level had 125 such trees. These differences are most likely related to productivity. For lower trunk size limits, i.e. dbh > 20 or >30 cm, densities were approximately equal in nemoral and boreal forests. For trees with dbh < 20 cm, densities were higher in boreal forests than in nemoral and boreonemoral forests (Table 2). 3.2. Dead trees Very large dead trees (dbh > 70 cm) had higher densities in nemoral and boreonemoral forests while smaller dead trees (dbh < 20 cm) had higher densities in boreal forests (Table 3). In boreal forests, there were few standing dead trees with a dbh > 70 cm, while further south there were on average 2±3 such trees per ha. As much as 8 standing dead trees with dbh > 70 cm, including 5 silver ®rs, were found in

Dobrocsky prales. For standing dead trees with a dbh > 40 cm, average densities were 10±20 ha 1 in all types of old-growth forests, but in boreal forests the variability was high. Table 3 Densities (number per ha) of standing dead trees of different minimum sizes (dbh in cm) in old-growth forests in Europea Minimum tree sizes

Dobrocsky prales BiaøowiezÇa 314C BiaøowiezÇa 398 BiaøowiezÇa 317 BiaøowiezÇa 317D BiaøowiezÇa 340C BiaøowiezÇa 399C Siggaboda BjurkaÈrr HaroÈn Swedish taiga KirjesaÊlandet Seitseminen N.P. Seitseminen N.P. Seitseminen N.P.

10

20

30

40

50

70

34 55 42 31 45 54 52 39 31 29 187 39 93 94 58

24 35 42 11 5 24 32 29 21 19 89 10 93 51 18

24 15 22 11 5 24 2 24 17 19 53 0 84 39 15

14 15 22 11 5 14 2 14 13 19 19 0 28 27 6

12 13 14 9 4 9 1 8 8 8 4 0 0 11 3

8 2 3 5 2 3 0 2 2 1 0 0 0 0 0

a Note that values are cumulative from right to the left. The forest plots are listed in the same order as in Table 1.

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Table 4 Densities (number per ha) of downed dead trees of different minimum sizes (dbh in cm) in old-growth forests in Europea Minimum tree sizes

Dobrocsky prales BiaøowiezÇa 314C BiaøowiezÇa 398 BiaøowiezÇa 317 BiaøowiezÇa 317D BiaøowiezÇa 340C BiaøowiezÇa 399C Siggaboda BjurkaÈrr HaroÈn Swedish taiga Seitseminen N.P. Seitseminen N.P. Seitseminen N.P.

10

20

30

40

50

70

110 214 183 63 392 135 114 41 33 72 211 148 232 101

90 174 113 43 212 95 64 19 21 52 97 55 83 55

50 114 43 43 132 65 44 9 18 32 32 27 24 24

40 24 13 23 32 15 14 5 12 22 6 9 4 12

30 7 2 13 11 7 5 3 5 14 0 0 4 3

10 2 0 4 1 2 1 1 1 0 0 0 0 0

a Note that values are cumulative from right to the left. The forest plots are listed in the same order as in Table 1.

The densities of downed dead trees were more variable between plots than the densities of standing trees (Table 4). Total densities of downed dead trees with a dbh > 10 cm seem to be 100±200 ha 1 both in old-growth boreal and boreonemoral forests. In Siggaboda and BjurkaÈrr these ®gures were considerably lower. Densities of larger logs were higher in temperate forests with 10±30 downed logs with dbh > 40 cm and 1±2 downed logs with dbh > 70 cm per ha. 3.3. Basal areas of living trees In southern Sweden and on dry ground in central Europe, the basal areas of living trees were similar at 34±40 m2 per ha (Table 5). The wet swamp forests in BiaøowiezÇa had higher basal areas at about 60 m2 per ha. Along two transects in the primary stands in BiaøowiezÇa, that were dominated by hornbeam, spruce, lime, and oak, Falinski et al. (1988) found basal areas of 34 and 37 m2 per ha. That study also included trees with dbh < 10 cm, which means that our values are not directly comparable. However, in old-growth forests these small stems contribute little to the total basal areas. Our data from Siggaboda and BjurkaÈrr show that 3.7% (S:D: ˆ 1:22, N ˆ 10) and

Table 5 Basal areas (m2 per ha) of living and dead trees in some mixed boreonemoral old-growth forests (see Table 1)a Stand

Living

Standing dead

Downed dead

Dobrocsky pralesb BiaøowiezÇa 314Cb BiaøowiezÇa 398b BiaøowiezÇa 317b BiaøowiezÇa 317Db BiaøowiezÇa 340Cb BiaøowiezÇa 399Cb Siggaboda (1 S.D.) BjurkaÈrr (1 S.D.)

37.0 64.6 52.0 33.8 33.6 33.7 37.3 37.4 (5.77) 39.8 (8.00)

9.9 5.0 6.0 4.2 3.7 6.2 2.3 4.7 (3.67) 3.6 (2.94)

± 16.4 9.4 8.4 18.2 9.2 7.2 2.7 (3.23) 3.6 (1.67)

a Sample plots of 1 ha except in Siggaboda and BjurkaÈrr where 10 and 13 random plots of 0.1 ha were studied. b The lower trunk size limit is 10 cm dbh.

2.0% (S:D: ˆ 1:42, N ˆ 13) of the total basal areas were due to trees with dbh < 10 cm. 3.4. Proportions of trees standing dead In most of our study plots the proportion of dead trunks of all standing trees are highest among the largest trees, but there is a high variability between plots (Table 6). The largest variation was between nearby plots within BiaøowiezÇa forest. If all trunks > 10 or Table 6 Dead trunks as proportion (%) of all standing trees by different minimum size classes (including snags and stumps higher than 1.3 m)a Minimum diameters (cm) 10

20

30

40

50

70

Dobrocsky prales BiaøowiezÇa 314C BiaøowiezÇa 398 BiaøowiezÇa 317 BiaøowiezÇa 317D BiaøowiezÇa 340C BiaøowiezÇa 399C Siggaboda BjurkaÈrr

16 10 8 7 6 19 13 9 7

16 10 12 6 2 11 10 10 8

16 6 8 10 3 16 1 12 10

13 7 14 15 13 14 2 11 12

13 9 14 17 22 15 2 17 16

19 6 14 19 29 19 0 33 14

Arithmetic mean (N ˆ 9)

10.6

S.D. a

9.4

9.1 11.2 13.9 17.0

4.50 3.91 5.23 4.18 5.67 10.2 Note that smaller size classes also include all larger classes.

S.G. Nilsson et al. / Forest Ecology and Management 161 (2002) 189±204

> 20 cm dbh are included, about 10% of the standing trunks were dead. This is also the median value for studies of old-growth forests both in Europe (range 7±16%) and North America (range 8±14%; data in Table 7). If basal area is considered instead of trunk numbers, the proportion of standing dead trees were 8±12% in all forests studied except two (Tables 5 and 7). At Biskopstorp, in a stand of almost pure beech, where most canopy trees are 250±300 years old (Karlsson, 1996), as much as 32% of the standing trees were dead. In Dobrocsky prales, the studied plot was dominated by large silver ®rs, and 24% of the standing

195

trunks and 31% of the standing basal area of this species was dead. Thus, in both these stands, the high proportion of standing dead trees is due to a high mortality rate of a single tree species. Proportions of the trunks standing dead based on basal areas were about the same as proportions based on trunk densities both in our study plots and in most other old-growth forests (Table 7). The proportions of standing trees that are dead seem to be independent of trunk densities in old-growth forests (Table 7). The same pattern is valid for basal areas (Fig. 1). In some studies, tree volumes were presented instead of basal area, but for calculation of the proportion standing

Table 7 Total amounts of standing trees including dead trunks (trunk densities, basal areas and standing volumes) and proportions (%) of dead trunks among all standing in old-growth boreal and temperate forests in Europe and North Americaa Minimum dbh

Trunks per ha

%

m2 per ha

%

m3 per ha

%

Reference

Europe SL, Dobrocsky prales PL, BiaøowiezÇa, alder-ash PL, BiaøowiezÇa, lime-hornbeam S, Siggagoda S, BjurkaÈrr S, Biskopstorp S, HaroÈn Fi, Seitseminen, spruce-pine Fi, Seitseminen, pine S, KirjesaÊlandet S, Muddus ‡ Jelka ‡ Reivo S, KaÊtaberget S, VaÈnsjaÈrv Russia, Pechora-Ilych, pine Russia, Pechora-Ilych, spruce Fi, N. Finland, pine Fi, N. Finland, spruce Fi, N. Finland, mixed

10 10 10 10b 10b 2 10 10 10 10 5 10 4 10 1 1 1 1

207 514 464 441 476 458 364 1136 655 540 961 418 1679 465 2462 ± ± ±

16 9.4 11.3 8.8 6.5 10.3 8.0 8.2 8.9 7.8 10.7 8.4 13.0 15 10.7 ± ± ±

46.5 63.8 38.7 42.1 43.5 14.3 ± ± ± ± 17.1 ± ± ± ± ± ± ±

21 8.6 10.6 11.2 8.4 32 ± ± ± ± 11.7 ± ± ± ± ± ± ±

± ± ± ± ± ± ± ± ± ± ± ± 87 ± 268 88 91 165

± ± ± ± ± ± ± ± ± ± ± ± 8.4 ± 10.4 8.3 6.7 7.7

This study This study This study This study This study Karlsson (1996) This study This study This study Linder (1986) Dettki and Esseen (1998) Jonsson (1997) NihleÂn and Uebel (1993) Majewski et al. (1995) Kuuluvainen et al. (1998) Sippola et al. (1998) Sippola et al. (1998) Sippola et al. (1998)

North America USA, Ohio USA, Wisconsin ‡ Michigan USA, Adirondack USA, Midwestern states USA, Minnesota, maple-bassw. USA, Minnesota, oak USA, Colorado USA, Washington CA, NE Alberta

10 10 10 10 10 10 10 5c 10

405 396 452 372 372 413 1048 52c 601

9.9 13.3 13.2 8.9 9.1 8.0 14 29c 11.0

32.8 43.6 42.3 31 ± ± ± 90 ±

6.1 18.3 20 9.7 ± ± ± 19 ±

± ± ± ± ± ± 345 ± ±

± ± ± ± ± ± 22 ± ±

Goebel and Hix (1996) Goodburn and Lorimer (1998) McGee et al. (1999) Spetich et al. (1999) Hale et al. (1999) Hale et al. (1999) Arthur and Fahey (1990) North et al. (1999) Lee et al. (1997)

Forest

a

Minimum dbh is the lower limit of trunk diameter included and measured at 1.3 or 1.4 m above ground. For basal areas, the lower dbh limit is 0. c For trunk density only trunks >50 cm dbh. b

196

S.G. Nilsson et al. / Forest Ecology and Management 161 (2002) 189±204

3.6. Volume of standing dead trees In Dobrocsky prales, the volume of standing dead wood was estimated at 53 m3 per ha. In BialjowiezÇa, the volumes of standing dead wood varied between 16 and 37 m3 per ha, with a median of 28 (Table 10). The variation between plots is too large to reveal any possible difference between forest types. 3.7. Dominance of large trees

Fig. 1. Proportion (%) of the standing basal area consisting of dead trees in relation to total standing basal area in old-growth forests in Europe and North America (r Spearman ˆ 0:061, N ˆ 12, P > 0:1).

dead, volume and basal area produce similar values (Table 7). 3.5. Proportions of all dead trunks standing Among the dead trunks with dbh > 10 cm, both standing and downed, the densities were similar in BiaøowiezÇa and boreal forests, but in Siggaboda and BjurkaÈrr the densities were lower (Table 8). These latter two stands also had a higher proportion of trunks standing of all dead trunks, 48±49%, while other stands had 20±34% of all dead trunks standing. If basal areas instead of trunk densities are considered, these proportions are higher (Table 8). Proportion of total volume of dead trees that are standing is in most cases within the range, 20±40% (Table 8). However, in KirjesaÊlandet, a sprucedominated forest near the Scandinavian mountains, only about 9% of the volume of dead trees were standing. The proportion of the total volume of dead trees that is standing seems to be independent of total volume of dead wood (Table 8, Fig. 2). On the other hand, the proportion of dead trees that are standing increases with dimensions (Table 9). Thus, our data (Tables 8 and 9) indicate that the standing dead trees are on average larger than the downed dead trees.

Large trees (dbh > 40 cm) dominated the basal area both for living and standing dead trees with medians of 68% for living and 82% for standing dead trees (Table 11). For downed dead trees, such large trees were not as dominating but still very prominent with a median of 34%. Thus, the proportion of the basal areas consisting of trees with dbh > 40 cm was higher for standing dead than for downed dead trees in 7 out of 8 samples (Table 11). When considering the volume of standing dead trees, large trees were also dominating with a median of 81% of total volumes. In reality, this value ought to be a few percentage lower, since dead trees thinner than 10 cm were not included. 4. Discussion 4.1. Densities of large living trees A problem when comparing our results with others is that there are many opinions about what constitutes a large tree. In the literature we have found limits from 30 to 100 cm dbh as minimum size. Therefore, to make more comparisons possible in the future, here we have presented densities with many different minimum sizes. In the Great Lake region of USA, densities of large living trees in hemlock-hardwood forests increase strongly with stand age even above 200 years (Tyrell and Crow, 1984a). However, in forests with an age exceeding 300 years there is no further increase. In those forests, densities of trees larger than 50 and 70 cm dbh are similar to the values we found in nemoral and boreonemoral old-growth forests in Europe. Densities of large trees several times higher than we found occur in old-growth forests in northwestern North America. For example, in forests dominated by

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197

Table 8 Total amounts (trunk densities, basal areas and volumes) of dead trees and the proportion (%) standing in old-growth forests in boreal and temperate forests in Europe and North Americaa Minimum dbh

Trunks per ha

%

m2 per ha

%

m3 per ha

%

Reference

Europe F, Fontainebleau F, Vosges UK, Lady Park Wood PL, BiaøowiezÇa, alder-ash PL, BiaøowiezÇa,lime-hornbeam S, Siggagoda S, BjurkaÈrr S, HaroÈn Fi, Seitseminen, spruce-pine Fi, Seitseminen, pine Fi, SW Finland, spruce S, KirjesaÊlandet S, VaÈnsjaÈrv S, Granlandet, spruce Russia, Pechora-Ilych, pine Russia, Pechora-Ilych, spruce Fi, N. Finland, pine Fi, N. Finland, spruce Fi, N. Finland, mixed

9.5 12 5 10 10 10b 10b 10 10 10 5 5 4 15 10 1 1 1 1

± ± ± 247 222 80 64 101 284 169 354 405 ± 125 238 ± ± ± ±

± ± ± 20 26 49 48 29 33 34 ± 26 ± 27 27 ± ± ± ±

± ± ± 18.4 14.9 7.4 7.2 ± ± ± ± ± ± ± ± ± ± ± ±

± ± ± 30 28 64 51 ± ± ± ± ± ± ± ± ± ± ± ±

155 91 104 ± ± ± ± ± ± ± 111 79 87 39 ± 145 19 19 60

23 27 36 ± ± ± ± ± ± ± 37 8.5 24 16 ± 19 39 31 21

Koop and Hilgen (1987) Schnitzler and Borlea (1998) Green and Peterken (1997) This study This study This study This study This study This study This study Siitonen et al. (2000) Linder (1986) NihleÂn and Uebel (1993) Jonsson (2000) Majewski et al. (1995) Kuuluvainen et al. (1998) Sippola et al. (1998) Sippola et al. (1998) Sippola et al. (1998)

North America USA, Ohio USA, Wisconsin ‡ Michigan USA, Midwestern states USA, Minnesota, maple-bassw. USA, Minnesota, oak USA, Colorado USA, Oregon ‡ Washington USA, Washington

10 10 10 10 10 10 10 5

165 ± ± ± ± ± ± ±

24 ± ± ± ± ± ± ±

± ± ± ± ± ± ± ±

± ± ± ± ± ± ± ±

48 137 82 88 75 243 534 552

38 28 26 31 36 22 41 30

Goebel and Hix (1996) Goodburn and Lorimer (1998) Spetich et al. (1999) Hale et al. (1999) Hale et al. (1999) Arthur and Fahey (1990) Spies et al. (1988) North et al. (1999)

Forest

a b

Minimum dbh is the lower limit of trunk diameter included and measured at 1.3 or 1.4 m above ground. For basal areas the lower dbh limit is 0.

coniferous trees, North et al. (1999) found 30±40 trees per ha with dbh > 80 cm. For living trees with dbh > 40 cm, Siitonen et al. (2000) found about 60 trees per ha in sprucedominated old-growth forests in southwestern Finland, similar to our results from boreal forests. However, these values may be higher than in virgin landscapes with a natural ®re regime (see 4.8). In pinedominated virgin forest with a natural ®re regime in the Pechora-Ilych Reserve, Komi Republic in Russia, Majewski et al. (1995) found 40 trees per ha with dbh > 40 cm. In hemlock-hardwood forests in the Great Lake region of USA, which are more than 300 years old, the

densities of living trees with dbh > 10 cm (Tyrell and Crow, 1984a) are similar to the values we found in temperate Europe. A comparison of other old-growth forests in eastern USA with our results for living and dead trees also shows many structural similarities between the continents (Tables 7 and 8). 4.2. Densities of dead trees Densities of standing dead trees with dbh > 10 cm were 20±80 ha 1 in different studies in eastern USA (McGee et al., 1999; Spetich et al., 1999) which are similar to our results. For trunks with larger diameters few comparable data are available. In old-growth

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S.G. Nilsson et al. / Forest Ecology and Management 161 (2002) 189±204 Table 10 Volumes (m3 per ha) of standing dead trees with dbh > 10 cm

Fig. 2. Proportion (%) of total dead wood volume that is standing in old-growth forests in Europe and North America (rSpearman ˆ 0:012, N ˆ 19, P > 0:1).

forests of southwestern Finland, Siitonen et al. (2000) found about 15 dead trees with dbh > 40 cm per ha, which is about half of the density we found. In spruce-dominated forest in northernmost Sweden, Jonsson (2000) found 56 and 106 dead trees per ha with minimum dbh of 30 and 20 cm, respectively. These density values included 20 and 31 standing dead trees, respectively, and the rest downed. These values for standing dead trees are within the lower range of our results. In old-growth mesic forests of eastern USA, densities of 15±18 dead trees (dbh > 25 cm) per ha (Runkel, 2000) are also lower than in most of our plots. On the other hand, in coniferous forests of the northwestern USA, densities of large snags (dbh > 50 cm) were about 15 ha 1 (North et al., 1999), almost twice as high as the average in our plots south of the boreal forests (Table 3). Table 9 Proportion (%) standing among medium sized and large dead trees Forest

BiaøowiezÇa Siggaboda BjurkaÈrr HaroÈn Swedish taiga Seitseminen N.P.

dbh (cm) 10±39

>40

18 41 46 17 45 29

36 74 52 46 76 71

Stand

Volume

Dobrocsky prales BiaøowiezÇa 314C BiaøowiezÇa 398 BiaøowiezÇa 317 BiaøowiezÇa 317D BiaøowiezÇa 340C BiaøowiezÇa 399C

53.4 37.2 28.2 27.3 16.1 36.2 21.4

For dead trees on the ground, few studies report the number of individual trees as we have done, and therefore comparisons are dif®cult. In most other studies, the number of pieces of downed wood are reported. However, the number of dead trees on the ground is more directly related to the population ecology of trees and also to the number of standing dead trees. In old-growth forests of southwestern Finland, Siitonen et al. (2000) found about 233 dead trees with dbh > 10 cm per ha, including both standing and downed. This value is similar to our data from other sites in Finland and northern Sweden. Furthermore, the densities of downed dead trees of different dimensions reported by Jonsson (2000) are also similar to our results from boreal forests. Obviously, more studies of the densities of trees, especially large living and dead, are needed to Table 11 Proportion (%) of basal areas and volumes consisting of trees with a dbh > 40 cma Stand

Dobrocsky pralesb BiaøowiezÇa 314Cb BiaøowiezÇa 398b BiaøowiezÇa 317b BiaøowiezÇa 317Db BiaøowiezÇa 340Cb BiaøowiezÇa 399Cb Siggaboda (1 S.D.) BjurkaÈrr (1 S.D.) a

Basal areas Living

Standing dead

Downed dead

Volume standing dead

90 83 71 68 25 79 65 62 (11.4) 62 (21.9)

87 72 87 95 89 82 26 53 (41.1) 66 (39.3)

± 31 22 73 34 40 34 22 (35.8) 56 (42.0)

81 59 82 96 81 64 19 ± ±

Sample plots of 1 ha except in Siggaboda and BjurkaÈrr where 10 and 13 random plots of 0.1 ha plots were studied. b The lower dbh limit is 10 cm.

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establish baselines in different types of forests. We recommend larger study plots than usually have been used, to get more reliable density values for the largest trees. 4.3. Dead trunk proportions among standing trees We found rather consistently about 10% dead trunks among the standing trees. To investigate whether this is a general rule, we surveyed the literature and found similar values in most studies (Table 7). We found only one other study that examined the proportion of dead trunks to standing over different trunk diameter size classes (Spetich et al., 1999). In that study, about the same proportions of trunks were standing dead in different diameter classes. However, very few large trunks were examined by Spetich et al. (1999) which precludes any conclusion about the largest trees. Another study examined a larger number of standing large trees with dbh > 50 cm, and among these trees 29% were dead (North et al., 1999), a much higher value than in any other study including also thinner trunks. This pattern with a higher value for larger trunks can have different causes. These large trees may approach their maximum age with increased mortality as a consequence. In some forests, the mortality rate increases with tree size (Runkel, 2000) while in others mortality rate is independent of tree size when dbh > 10 cm (Parker et al., 1985; Pontailler et al., 1997; Runkel, 2000; Woods, 2000). Hurricanes may temporarily increase the mortality rate of large trees more than of smaller trees (Batista et al., 1998). Another possibility why there may be a high proportion of dead trees standing among the largest trees is that large trees will remain standing over a longer time after death than smaller trees (e.g. Cline et al., 1980; Raphael and White, 1984). 4.4. Proportions of standing and downed dead wood Based on basal areas of dead trees we found that in BialjowiezÇa 28±30% consisted of standing trees, but if volume is considered this proportion would be slightly lower. This is because the trunks of many of the standing dead trees were broken, and thus treetops from some standing dead trees were on the ground and added to the volume of downed wood (cf. Tyrell and

199

Crow, 1984b). However, in many cases, the tops were decomposed, while the lower part of the trunks of dead trees were still standing. The volume of downed dead wood in BiaøowiezÇa has previously been estimated at 71±94 m3 per ha (Falinski, 1978; Kirby et al., 1991). However, more recent and extensive estimates of downed dead wood in this forest indicate about 120 m3 per ha (Bobiec, in literature). Our estimate of 28 m3 standing dead wood per ha means that about 20% of the total volume of dead wood was standing in this forest. Other studies of old-growth forests, both in Europe and North America, indicate that generally 20±40% of the volume of dead wood is standing (Table 8). Both the lowest and highest values are from boreal coniferous forests. There is no clear pattern that coniferous forests in general have higher proportions standing dead wood as suggested by Harmon and Chen Hua (1991) and Peterken (1996). The differences recorded by Harmon and Chen Hua (1991) may re¯ect the different ages of the compared stands more than tree species composition. However, tree species composition may also affect the proportion standing dead, with spruce having a lower proportion standing among the dead trees than other tree species in the same forest (Linder, 1986; Siitonen et al., 2000). The exceptionally high proportion of the dead trees that are standing in Siggaboda and BjurkaÈrr may be because dead downed trees have been removed in the past, but we have no evidence for that. Both stands have only recently been strictly protected. Another possibility is that small scale cuttings, that occurred long ago (Niklasson et al., in preparation), reduced the tree mortality rate and consequently the number of dead trees. If so, the amount of dead wood would increase in the future and the proportion of the dead trees standing would decrease. 4.5. Total amounts of dead wood Densities and volumes of dead trees are of the same magnitude in old-growth forests of temperate Europe and eastern USA, whereas the volumes are much larger in the northwest of North America (Tables 7 and 8). We suggest that these patterns re¯ect site productivity, with more productive sites supporting higher densities of large trees. This hypothesis can also explain the lower densities and volumes of large

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living and dead trees in boreal forests in Europe compared to nemoral forests further south. Since the proportion of dead wood that is standing did not appear to vary with productivity, we may be able to predict the average volume of dead trees in oldgrowth stands from the volume of living trees. The lowest values of total dead wood volume in Europe, about 20 m3 per ha, have been measured near the timberline in northern Finland (Sippola et al., 1998). The highest values may occur in virgin mixed beech, silver ®r and spruce forests of eastern and central Europe, where Mauve (1931) and Korpel (1987) reported timber quantities of 500±1000 m3 per ha. Similarly, in a beech-dominated old-growth forest in Denmark, the volume of living trees was 700 m3 per ha (Emborg et al., 2000). Based on these values, and our results that about 10% of standing trunks are dead and 20±40% of dead trunks are standing, we predict that there are about 50 m3 per ha of standing dead trees and totally about 200 m3 of dead trees in the most productive old-growth forests in Europe. The volume of dead wood in European old-growth forests may thus vary one order of magnitude (20±200 m3) depending on site productivity. At many sites, 30±50 m3 standing and about 100 m3 downed dead wood were probably commonly occurring amounts before the forests of Europe were exploited by man. Amounts around these values have also been measured in some of the few remaining old-growth sites on more productive soils (e.g. Korpel, 1987; Koop and Hilgen, 1987; Kirby et al., 1991; Martikainen et al., 1999, this study). The in¯uence of ®re on dead wood volumes, especially downed dead trees, is at present uncertain (see below). At some sites, ®res and large windfalls may temporarily increase the volumes of dead trees considerably, but if many canopy trees die, the volumes of dead trees will decrease when the large boles have decayed 50±100 years later (Duvall and Grigal, 1999). Thus, the generalizations we propose are not relevant in single stands, but only over a larger area. 4.6. Problems with the selection of reference stands Densities and volumes of dead trees are very much dependent on the time since a catastrophic disturbance such as a hurricane or a crown ®re. We intentionally avoided sites with such events in the recent past, since

it is self evident that extremely high values of dead wood will be recorded then. Instead, we wanted to show conditions that are more representative of the entire landscape. One problem that we have not dealt with explicitly is the possible role of pollution in affecting densities of large living and dead trees. We can not exclude the possibility that the high proportions of dead silver ®rs at Dobrocsky prales and beeches at Biskopstorp are effects of air pollution. Air pollution levels are known to be high in these areas. It is of critical importance to be very strict when selecting reference stands with which to compare managed forests. About 100 years without management may be enough to hide the traces of past cuttings, even though the structure of the forest may still be in¯uenced by past management. Detailed examination of the old-growth stand at Siggaboda, believed to be an almost virgin stand, has revealed cutting of large pine trees that occurred about 200 years before our sampling (Niklasson et al., in preparation). The stumps of the cut pines were preserved below the moss cover because they were impregnated by tar due to forest ®res 250±350 years ago. At the time of the cutting, other large trees with less durable stumps may also have been removed, but there is no evidence for that. The lower density of large trees (dbh > 70 cm), compared to the other similar forest studied in the region (BjurkaÈrr), could be due to these cuttings. If these pines had not been cut they would have been about 300 years old at present. Similarly, the comparatively low density of downed trees in Siggaboda and BjurkaÈrr may be because of unrecorded legal or illegal removal of dead trees for ®rewood more than 50 years ago. These two stands have otherwise survived devasting cuttings due to the stony ground and the low human population density of the region. Tree sizes are correlated with soil productivity. As far as we can judge, site productivity in our stands does not deviate much from the regional averages. Otherwise, uncut stands are often left at the most unproductive sites. For example, large landscapes with virgin forests occur near the timberline in the Scandinavian mountains, while in southern Sweden, with more productive soils, it is extremely dif®cult to ®nd stands without past cuttings. Therefore, BiaøowiezÇa forest in eastern Poland, with its similar climate, may be a better reference forest for southern Sweden

S.G. Nilsson et al. / Forest Ecology and Management 161 (2002) 189±204

than Siggaboda and BjurkaÈrr as their management history is less well documented. However, several studies show that management measures have affected BiaøowiezÇa too. Most signi®cant may be that the high density of large mammalian herbivores in 1892±1915 resulted in a gap in the trunk diameter distribution of lime and hornbeam and also an increased proportion of spruce (Pigott, 1975; FalinÂski, 1986). How this may affect the structures we have studied are uncertain. 4.7. Fire and tree density A general problem for European and temperate North American studies of old-growth forests is to evaluate the effect of ®re suppression during the last century (Swetnam, 1993; Swetnam et al., 1999; Niklasson and GranstroÈm, 2000). What will the effect of ®re suppression be on densities of large trees and dead trees? For example, ®re occurred 130 years ago in Seitseminen National Park (E. Annila, personal communication) and a high density of old pines testify of its past frequency (see below). During the last 100 years, this forest has been invaded by numerous spruces, as is a general pattern in boreal forests in northern Europe (Linder et al., 1997). Most likely, the densities of smaller trees that ®res mainly will kill (KolstroÈm and KellomaÈki, 1993; Linder et al., 1998; Wirth et al., 1999), are much higher now than previously. An increase of the timber volume and the density of trees, mainly spruce, has also been documented for some old-growth stands in northern Sweden (Linder, 1998). Pine-dominated forests with a ®re interval of 20± 50 years develop into a sparse forest with many very old pine trees (Kohh, 1975). Fires kill most of the young trees and reduce tree density, which increases the survival rate of the large pines (Wirth et al., 1999). Therefore, it is possible that the density of very large living pines, but not spruces, is higher in old-growth boreal forests than when ®re is prevented. Forest surveys from large areas with pine-dominated forests at Hamra and Orsa in central Sweden, collected more than 100 years ago, recorded at least 14 and 20 pine È stlund, trees per ha with dbh > 40 cm (Linder and O 1998). These forests were considered to be almost virgin forests with a rather natural ®re regime. Therefore, the densities recorded for large trees in these forests may be true reference values. The present

201

average density of large living trees in the area, which now constitute intensively managed forests, is only 1 È stlund, tree with dbh > 40 cm per ha (Linder and O 1998). In both Hamra and Orsa, the volumes of standing dead trees were about 10% of the volume of living trees more than 100 years ago (Linder and È stlund, 1998). This is the same value as has been O recorded in the preserved old-growth forests (Table 7). In old-growth forests in southwestern Finland, at the present time dominated by spruce, there are many old ®re-scarred pines with a density of 27 ha 1 living and several dead pines per ha with a dbh > 40 cm (Siitonen et al., 2000). There are also a high density of large birches and aspen in these forests. The original density of large trees in old-growth boreal forests could very well have been lower than at present (>60 trees per ha) when a natural ®re regime prevailed. This is supported by data from virgin pine-dominated forests with frequent ®res in the Pechora-Ilych Reserve, Komi Republic of Russia, where Majewski et al. (1995) found 40 living trees per ha with dbh > 40 cm. However, the relevance of this comparison is uncertain, since the productivity of the different forests is unknown. 5. Conclusions The similar densities of large living and large dead trees in different types of old-growth encourage us in proposing reference values for these features. About 10 to 20 living trees per ha with dbh > 70 cm may have been typical values for many virgin forests in central Europe and southern Scandinavia. Further north, in boreal forests, at least 20 living trees with dbh > 40 cm were probably common. These densities are expected to be higher at the most productive sites and lower in unproductive sites. The densities of trees that are standing dead of the same dimensions are expected to have been 10±20% of the densities of living trees. Based on our results, we propose the following generalizations to be further tested in other old-growth temperate and boreal forests: 1. Among all standing trunks (including high stumps) about 10% are dead, but this proportion increases for the largest trees. The proportion of

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standing trees that are dead seem to be independent of total basal areas. Based on this, we suggest that the volume of dead wood is directly proportional to the productivity of old-growth forests. 2. Standing dead trees (snags) are on average larger than downed dead trees. Trees with dbh > 40 cm often dominate the basal area and volume of standing dead trees and living trees. 3. About 30% (20±40%) of the basal area and volume of dead trees is standing in old-growth forests. This proportion seems to be independent of total volume of dead wood. Large disturbances by ®re, strong winds and insects may temporarily change these proportions considerably in individual stands. Acknowledgements This study was supported by SUFOR, Sustainable Forestry in Southern Sweden. We thank Errki Annila for providing data from Seitseminen N.P. Lisa Henkow and Mikael Svensson helped us with the tree inventories in Poland and Slovakia. Andrzej Bobiec supplied valuable unpublished data. References Anon, 1962. Praktisk skogshandbok. Sveriges SkogsvaÊrdsfoÈrbund. (in Swedish). Arthur, M.A., Fahey, T.J., 1990. Mass and nutrient content of decaying boles in an Engelmann spruce-subalpine ®r forest, Rocky Mountain National Park. Colorado. Can. J. For. Res. 20, 730±737. Batista, W.B., Platt, W.J., Macchiavelli, R.E., 1998. Demography of a shade-tolerant tree (Fagus grandifolia) in a hurricanedisturbed forest. Ecology 79, 38±53. Ê ., EhnstroÈm, B., Gustafsson, L., HallingbaÈck, T., Jonsell, Berg, A M., Weslien, J., 1994. Threatened plant, animal, and fungus species in Swedish forests: distribution and habitat associations. Conserv. Biol. 8, 718±731. Ê ., EhnstroÈm, B., Gustafsson, L., HallingbaÈck, T., Jonsell, Berg, A M., Wesligen, J., 1995. Threat levels and threat to red-listed species in Swedish forests. Conserv. Biol. 9, 1629±1633. BjoÈrkman, L., Bradshaw, R.H.W., 1996. The immigration of Fagus sylvatica L. and Picea abies (L.) Karst. into a natural forest stand in southern Sweden during the last 2000 years. J. Biogeogr. 23, 235±244.

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