Seventy-year changes in tree species composition ...

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Seventy-year changes in tree species composition and tree ages in state-owned forests in Latvia a

b

Aivars Tērauds , Guntis Brūmelis & Oļģerts Nikodemus

a

a

Faculty of Geography and Earth Sciences, University of Latvia, Riga, Latvia

b

Faculty of Biology, University of Latvia, Riga, Latvia

Available online: 02 Jun 2011

To cite this article: Aivars Tērauds, Guntis Brūmelis & Oļģerts Nikodemus (2011): Seventy-year changes in tree species composition and tree ages in state-owned forests in Latvia, Scandinavian Journal of Forest Research, 26:5, 446-456 To link to this article: http://dx.doi.org/10.1080/02827581.2011.586647

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Scandinavian Journal of Forest Research, 2011; 26: 446456

ORIGINAL ARTICLE

Seventy-year changes in tree species composition and tree ages in state-owned forests in Latvia

¯ RAUDS1, GUNTIS BRU ¯ MELIS2 & OL¸G AIVARS TE ¸ ERTS NIKODEMUS1

Downloaded by [University of Eastern Finland] at 05:10 23 January 2012

1

Faculty of Geography and Earth Sciences, University of Latvia, Riga, Latvia, and 2Faculty of Biology, University of Latvia, Riga, Latvia

Abstract During the last 100 years, forest management in Latvia has gradually become more and more focused on industrial logging, which can be expected to have affected the tree species composition and age distribution across the landscape. These changes need to be considered in forest management and conservation of biological diversity. The aim of the study was to use forest records to reconstruct the tree species composition and age distribution in the North Vidzeme Biosphere Reserve in northern Latvia for the period, 19291941. These data were compared to a data-set from 2008, to determine the changes that transpired during a period of intensification of forest management. The silvicultural methods in the 19201941 period were based mostly on natural regeneration of logged stands. This led to succession to birch on harvested spruce stands and can explain the relatively large cover of mature birch stands today. After 1960, forest management was directed on conifer monocultures, and thus spruce stands today predominantly have age B50 years. An age class 150 years is almost absent for all species, and conservation should be directed to adding a portion of old forests to the protection network, and in maintaining the spatial continuity of overmature deciduous stands.

Keywords: Coniferous monocultures, fragmentation, landscape, protection network, tree species composition, young stands, forest history.

Introduction During the last 200 years, logging has caused largescale changes in boreal forest ecosystems (Boucher ¨ stlund, 1998). The natural et al., 2009; Linder & O structure in forests at different spatial scales has become more homogenous by decreasing tree species number, densities of large old trees and dead wood, particularly coarse wood debris, and in the variability created by natural disturbance of different severity (Kuuluvainen, 2002; Lilja & Kuuluvainen, 2005; Lilja-Rothsten et al., 2008; Rouvinen & Kuuluvainen, 2005; Ylla¨sja¨rvi & Kuuluvainen, 2009). Forest fire has historically been the main natural disturbance in forests of the European boreal region, and it is thought that fire suppression has caused an increased component of spruce (Bradshaw, 1993; Gromtsev, 2002; Wallenius et al., 2005; Zackrisson, 1977). The effect of human disturbance on landscape-level changes in tree species composition and age distribution has been described in northern Europe (Axelsson

¨ stlund, 2001; Axelsson et al., 2002; Linder & & O ¨ stlund, 1998) and North America (Rhemtulla et al., O 2009). These historical changes in the forest landscape need to be considered in forest management and in conservation of biodiversity (Axelsson & ¨ stlund, 2001; Axelsson et al., 2002; Lazdinis & O Angelstam, 2005; Lo¨fman & Kouki, 2003; Wallenius et al., 2007). However, there is a lack of available literature on the changes that have transpired in the boreal forest landscape of the eastern Baltic countries. In Latvia, slash and burn agriculture caused major fragmentation of forests (Dumpe, 1999), and in a relatively short period of time, between 1861 and 1914, the forest area diminished from 42% to 23.9%, in some areas to as low as 15% (Vasil¸evskis, 2007a). The cut and burnt forest land was utilised as crop fields for mostly up to four years and then allowed to return naturally to forest on low crop yields (Dumpe, 1999). Fertile soils with deciduous forest were favoured for slash and burn agriculture,

Correspondence: Guntis Bru¯melis, Faculty of Biology, University of Latvia, Kronvalda bulvaris 4, Riga LV-1586, Latvia. E-mail: [email protected]

(Received 14 January 2011; accepted 4 May 2011) ISSN 0282-7581 print/ISSN 1651-1891 online # 2011 Taylor & Francis DOI: 10.1080/02827581.2011.586647


Seventy-year forest history of Latvia 447 which caused an almost complete loss of pedunculate oak (Quercus robur L.) woodland (Zunde, 1999). Forest management plans before 1900 did exist for estates and various regions, with planning and inventory conducted under the influence of the past Russian, German and Swedish regimes (Vasil¸evskis, 2007a; Zunde, 1999), but this did little to control the slash and burn destruction of forests by peasants (Dumpe, 1999). Forest management policy with complete forest inventory was rapidly initiated with the nationalisation of forests after 1918, at a time when forest cover in Latvia had reached a minimum (Vasil¸evskis, 2007a). In 1921 the forest area was 28%, but agrarian reform caused a decline to 25% by 1925 (Kronı¯tis, 1991; Vasil¸evskis, 2007b). About 85% of the forests were owned by the state. The forest area increased to 27% by 1940, due to afforestation of wetlands and fields within state forest land and repurchase of non-productive land that had been parcelled to landowners (Upı¯tis, 1940). Pasture use of forest, although disfavoured in forest and agricultural policy, did continue to some degree up until the 1940s (Upı¯tis, 1940). During the Soviet occupation starting in the 1940s, a large part of the previous privately owned agricultural land, which was then incorporated into collective farms, was left to overgrow naturally to forest. This increased the forest area to 41% by 1983 (Matı¯ss, 1987). With return of land to private owners upon re-independence of Latvia in 1992, more meadows, pasture and crop fields were left to overgrow, or were planted, and forest area in 2007 reached 54.7% (Latvijas statistika, 2010). The forest area today is about the same (but likely with different structure) as that at the end of the 1700s, when it was estimated at about 50% (Zunde, 1999). Thus, a period of fragmentation of forest area was followed by defragmentation. The political changes went hand-in-hand with new aims and methods in forest management, which also would be expected to have caused changes in the age distribution and tree species composition of forests. The aim of this study was to describe landscapelevel changes in the age distribution and tree species composition of forests in the North Vidzeme Biosphere Reserve (northern Latvia) since the 1920s. To achieve this aim, inventory data was used to group stands by age class of the dominant tree species for the periods 19291941 and 2008. The study was restricted to state forests, as historical inventory data were not available for private forests. The proportion of forest protected for conservation of biological diversity was also determined for forests dominated by each tree species to attempt

to link present conservation values with past forest management.

Materials and methods Study area Latvia is located in the boreo-nemoral region, where coniferous forests are mixed with broad-leaved species (Hytteborn et al., 2005). The dominating species are Scots pine (Pinus sylvestris L.), birch (Betula L. spp.) and Norway spruce [Picea abies (L.) Karst.], which contribute 43%, 28% and 15% of the total growing wood volume, respectively (State Forest Service, 2008). The climate is moderate. Annual precipitation is 700800 mm, of which about 500 mm falls in the growing season. Mean annual temperature is 5.58C. Moraine relief with sandy clays and clay sands dominates. Scots pine forests are common on glaciofluvial sediments with Arenosol (Haplic and Albic) soils; birch and Norway spruce on Albeluvisols (Haplic, Gleyic and Stagnic), Gleysols and Stagnosols; and black alder [Alnus glutinosa (L.) Gaertn.] on Histosols (Nikodemus et al., 2008). The investigated state forests are located in the North Vidzeme Biosphere Reserve (terrestrial area 457,600 ha). Within the reserve, four forest landscape areas were chosen (Figure 1), based on dominance of state forests (95% of forest area). State forests were chosen for study, since forest inventory was not conducted in private forests before 1940. The total sampled area was about 30,000 ha, which amounted to 7% of the terrestrial Biosphere reserve area (13% of the forest area). In the study, 37% of the forest area was on dry soils, 27% on wet mineral soil, 14% on peat and the remaining 22% was on wet soil that had been drained. Inventory records and analysis Historical forest inventory data recorded by the Forest Department of Latvia were obtained from archives housed at the Latvian State Forest Research Institute Silava. The selected forest landscapes were inventoried during the period 19291941, and thus the data obtained ranged over a 12-year time span. The archived forest plans were scanned (or photographed), rectified to the LKS-92 coordinate system and stand boundaries were digitised in a GIS database using ArcGIS ArcInfo (Ormsby et al., 2001). A digitised layer of forest stand boundaries in 2008 was obtained as public information from the State Forest Service. A forest stand, defined here as the smallest forest management unit in forest management plans, can be considered to be relatively


448 A. Te¯rauds et al.

Figure 1. Location of studied forest landscape areas in the North-Vidzeme Biosphere Reserve. Numbers refer to the landscape areas in Table I.

homogenous in growth condition, tree species composition and age. In the selected forest landscapes, the area that was not state-owned forest in 2008, including agricultural land, wetlands and private forests, was filtered out and excluded from the analysis. After filtering out non-state forests in 2008, inventory records were missing for 0.05% of the total forest area in 1929 1941, which were also removed. The inventory data were entered as layers in the database. The inventory data provide information on forest type, dominant tree species, age and relative volume by species, as well as land-use (forest, wetland, shrubs, crop field etc.). Part of the area with forests today had a different land-use in 19291941, such as different types of agricultural lands and wetlands. Crop fields, meadows, pastures and gardens were grouped in an agricultural class. Bogs and fens were classified as wetlands. Other land-use types, such as roads, gravel extraction sites, loading areas, forester homes, etc., were grouped together. For each forest stand, wood volume for different species is recorded in the Latvian inventory on a relative scale, with a total volume of 10 units divided among the species in the stand. Age is also estimated for each species. If a species in a stand has two or more cohorts with different ages, then the 10-unit volume is further divided among the different aged layers. Volume is not recorded for species/layers having B 10% of the stand volume. In the pre1940 period, the total wood volume in stands was often not recorded, as this was not required for coniferous trees with age in up to 60 years and

deciduous trees with age B 30 years (Matı¯ss, 2007). Therefore, absolute volumes for each species were not calculated. For each delineated stand in the two inventory periods, the dominant tree species was defined as that with the largest relative volume. Stand age was defined as the age of the dominant tree species. In some cases a tree species with a minor proportion of the volume had an older age, but we based our classification of stands by dominant species, and therefore also its age. If a range of ages from minimum to maximum was given for the dominant species, then the median age was used. The total forest area for each tree species in the 19291941 and 2008 periods was determined. For each tree species, the proportion of the area under protection (% of total species area) was also determined using information in the State Forest Register. Also, area distributions of stand age (further called age distributions) were produced for species in 10-year age classes for both periods. In cases of clear-cuts, if the regenerating species was not mentioned, then this information was not used to produce age distributions (13% of total forest area for the two periods, respectively). European ash (Fraxinum excelsior L.) and pedunculate oak (Quercus robur L.) were combined in a broad-leaved forest class, due to common co-occurrence and low area of each species. However, in other cases, mixed stands were not delineated. To determine the main directions of stand-level species change, for the area of each dominant tree species and land-use type in 19291941, the

Seventy-year forest history of Latvia 449 proportion of area of each of the succeeding species in 2008 was determined. Forested area in 2008 that had other land-use in 19291941 was included in the analysis. The delineation of stand boundaries often differed between the periods; stands tended to be smaller in 2008. The mean stand areas in 1929 1941 and 2008 were 2.3 and 1.9 ha, respectively. In these cases the larger stands were split to confer to the finer division.

Results


Dominant species The total investigated area was slightly larger in 2008 (29,875 ha) than in the 19291941 period (29,591 ha). However, the forested area had increased from about 80% to 90% (Table I). Part of the increase in forest area was due to afforestation of land that previously had other land-use (Table II). The forest landscape in 19291941 was dominated by spruce and pine with a smaller area of birch. By 2008 the forest area covered by birch had increased considerably to 36%, compared to 22% in 19291941. The area of pine slightly decreased, while spruce area decreased considerably from 31% to 22% (Figure 2). A total of 3456 ha (13%) of the forested territory in 2008 had a different land-use in the pre-1941 period, or were clear-cuts. Of this area, most was recorded as wetlands (40%), followed by clear-cuts (30%), agricultural land (26%) and other use (4%). The category ‘‘other use’’, which consisted of diverse land-use types (gravel extraction, roads, etc.) and contributed only 0.5% of the total forest area, is not presented further. The greater part of the forest area dominated by pine (73%) and birch (60%) did not change in the dominant species during the period up to 2008 (Table II). A total of 44% of the area dominated by spruce in the period 19291941 was still dominated by spruce in 2008. For areas dominated by all other species, birch and, to a lesser degree, spruce, were more common as successors Table I. Total area (ha), forest area (ha), and relative forest area (%) in four different landscape areas and in the whole investigated area in two periods, 19291941 and 2008. Total area Forest landscape

1929 1941

1 2 3 4 Total

9306 9145 6492 6547 5175 5534 8618 8649 29,591 29,875

2008

Area forest

19291941

2008

8157 4601 3922 6891 23,571

8759 5253 5057 8098 27,167

% forest 1929 1941 2008 88 71 76 80 80

Note: Number of the forest landscapes (see Figure 1).

96 80 91 94 91

than the original species. However, conversion of the dominant species was common for other species. Birch commonly succeeded spruce (39% of the area in 19291941) and deciduous species (4264%), and also invaded almost half of both the previous agricultural land and clear-cut area. Besides birch, also spruce was, in 2008, growing on areas earlier occupied by several other species (1131%). There was little conversion to aspen, grey alder and broadleaved stands. In 2008 black alder had replaced 10% of the area in 1929 dominated by the two broadleaved species. This species was the most successful of the less common species to invade areas that in 19291941 were dominated by other species. Only 1% of the area of spruce stands had succeeded to a dominant oak or ash forest. However, broad-leaved species were most successful in invading spruce stands, compared to other species, as 58% of the oak plus ash area in 2008 was dominated by spruce in 19291941 (not shown). Pine was successful in invading earlier non-forest land and grew in 2008 on 89% of the former wetland, on 30% of the former clear-cuts and on 27% of the former agricultural land. A total of 6% of the studied forest area was protected (Table III). The form of protection included Nature Parks, Woodland Key Habitats, reserves for protection of ecosystems and microreserves for species and habitat protection (e.g. capercaillie leks, nesting sites of black stork and lesser spotted eagle, protected plants and fragile ecosystems). While black alder and aspen stands had less total area than the coniferous species and birch in the investigated territory, higher proportions of black alder and aspen area (21% and 19%, respectively) were protected (Table III). Spruce and birch stands had the lowest level of protection (4% and 3% of the total species area, respectively). Age distribution In 19291941 the age distribution (all species combined) was more or less even up to the 80-year age class, but older age classes occupied smaller areas (Figure 3). However, in 2008 the age distribution had a broad diffuse peak in the 5080 year classes and fewer stands in both older and younger age classes. The area of forests over 140 years was very low in both periods. The age distributions of the dominating tree species mostly showed peaks, and these peaks differed in age between the studied periods (Figures 4 and 5). The age distributions for the four areas were rather similar, indicated by the standard errors, except for aspen and broad-leaved tree species. For all species, except spruce and broad-leaved trees,

450 A. Te¯rauds et al. Table II. Change in dominant species of stands. Proportion of area (%) in the landscape of a dominant species in 2008, for each species and land-use type in 19291941.


Dominant species in 2008 19291941 Species/land-use

Spruce

Pine

Birch

Aspen

Black alder

Grey alder

Broad-leaved

Clear-cut

Total

Spruce Pine Birch Aspen Black alder Grey alder Broad-leaved Clear-cut Wetland Agricultural land

44 12 22 31 11 31 11 11 1 16

9 73 8 6 2 2 3 30 89 27

39 12 60 42 53 42 64 49 9 47

1 B1 3 17 1 3 B1 2 B1 B1

4 B1 4 2 27 7 10 4

B1 B1 B1 B1 B1 8 B1 B1 B1 3

1 B1 B1 B1 2 2 8 B1

2 1 3 B1 2 4 2 4 B1 B1

100 100 100 100 100 100 100 100 100 100

5

B1

Note: The broad-leaved class includes oak and ash.

these peaks were at higher age classes in 2008 than in 19291941 (Figures 4 and 5). In 19291941, spruce stands over 70 years of age were abundant, but in 2008 there was a peak in the 2150 year age classes, with decreasing area of older forests (Figure 4a). For pine, in 19291941 the age distribution was rather even, but in 2008 there was a peak in the 51110 year classes, with lesser area of older age classes (Figure 4b). Today, the greatest proportion of birch forest area is in the 5080 year class; before 1941, young forests B10 years were the most abundant age class (Figure 4c). European aspen (Populus tremula L.) in both periods showed a bimodal distribution, with a large area of young stands as well as stands over 50 or 60 years (Figure 5a). Black alder had a similar age distribution in both periods, with the exception of greater total area of stands older than 90 years in 2008 (Figure 5b). There was a clear shift from broadleaved stands with age 70120 years to young forests B70 years, although the maximum age had increased by 20 years in 2008 (Figure 5c). Broadleaved stands had an uneven age distribution both in the earlier period and in 2008 with several (three) peaks. These peaks appeared to occur at higher ages in 2008 than in the earlier periods. The area of stands with age B20 years was very low in 2008. However,

the standard errors of relative area covered by age classes were high (Figure 5c), due to differences between the four areas in the age distributions.

Discussion The age distributions of coniferous trees across the landscape indicate the large-scale destruction of oldgrowth forests, which had already occurred by the early 1900s. The cutting age of spruce in forest management policy was 150 years in 1871 (Zviedris, 1960), 120 years at the start of the 1900s (Laivin¸ sˇ , 2005) and 81 years in 2009 (or in some cases less when basal area minimal criteria are used). This indicates that there has been a need to cut younger forests as the older woods had been cut or converted to agricultural land-use. The proportions of area of overmature pine (140 years) and spruce ( 120 years) were less than 2% and 1% in the 19291941 and 2008 periods, respectively (Figure 4). There were no stands that might even approach the maximal ages for pine and spruce, which have been estimated to be at least 200 years (Andersson & ¨ stlund, 2004; Lilja et al., 2006; Fraver et al., 2008; O Siitonen et al., 2000; Wallenius et al., 2005). The area of mature pine forest (101140 years) was

Figure 2. Area of stands by dominant tree species in the periods 19291941 and 2008. Bars represent mean area (ha 10,000 ha 1) calculated for the landscape areas (n 4). Standard errors are shown.

Seventy-year forest history of Latvia 451 Table III. Proportion of protected forest area, calculated as percent of the area occupied by the respective species in all landscapes. Species Spruce Pine Birch Aspen Black alder Broad-leaved Total

Proportion protected (%) 4 9 3 21 19 12 6


Note: The broad-leaved class includes oak and ash.

about 20% for pine in both years, while mature spruce (80120 years) area decreased from 32% to 7%. The proportions of old forests in the landscape of Latvia over the past 70 years have been very low, compared to those in boreal Fennoscandia. In boreal Sweden, it was estimated that 44% of the landscape consisted of forest 150 years in 1915, which ¨ stlund, 1998). declined to 7% in 1990 (Linder & O In a forest landscape in northern Sweden, 23% of pine forest had age over 100 years in 1997 (Axelsson ¨ stlund, 2001), which is comparable to the cover &O of mature plus overmature forests in Latvia today. However, in unmanaged spruce landscape in the northern boreal region, over 40% of the forest area can have age more than 275 years (Wallenius et al., 2005). In contrast to the coniferous species, the relative area of overmature birch ( 71 years) increased from 11% in 19291941 to 30% in 2008 (Figure 4c). Similarly, the relative area of overmature aspen ( 61 years) increased from 54% to 61% (Figure 5a). About half of the birch stand area today had reached an age between 50 and 80 years (Figure 4c). These stands regenerated in the period 19301960. The increase in birch area can partly be explained by

afforestation of previous agricultural land (Table II). In addition, abandonment of livestock grazing in forests (Dumpe, 1999), which was still common in the early 1920s, might also have led to regeneration of pioneer deciduous species. However, the large-scale conversion of spruce and deciduous tree forests to birch stands is likely a legacy of the non-intensive silvicultural practices (Matı¯ss, 1987) used prior to 1960. Also, over half of the broad-leaved (oak and ash) forests originated in previous spruce stands, which likely resulted from release of advance growth after harvest of the coniferous overstorey (Go¨ tmark et al., 2005). Up until 1930, forest policy stressed natural means of post-harvest regeneration, which was sometimes promoted by removal of the turf and cutting mature spruce over a number of years to ensure seed trees (Delle, 1931, 1932). Artificial regeneration methods included sowing of seed and planting. Sowing of seed was used when natural regeneration was not expected, and mostly for pine, followed by spruce (Mangalis, 1991; Zviedris, 1940). Planting was relatively rarely employed (Delle, 1931). Artificial regeneration was hindered by poor seed years and insufficient planting stock (Eihe, 1931). However, the large proportion of pine stand area with age 60100 years and the low amount of conversion of pine stands between the periods (27%) suggest that natural regeneration and seeding were successful for this species. This is not surprising, as a sufficient amount of seed and the appropriate seedbed is well known to allow good ¨ rlander, 2000). pine regeneration (Karlsson & O After 1930 more attention was focused on artificial regeneration in Latvia, and in 1935 the area artificially regenerated was slightly greater than that naturally regenerated (Mangalis, 1991). Between 1923 and 1937 the area of forest thinned annually increased from 400 ha to 6000 ha. In comparison,

Figure 3. Age distribution of forest stands in 10-year age classes for the periods 19291941 and 2008. Bars represent mean area (ha 10,000 ha 1) calculated for the landscape areas (n 4). Standard errors are shown.


452 A. Te¯rauds et al.

Figure 4. Age distribution of stands dominated by (a) spruce, (b) pine and (c) birch in 10-year age classes for the periods 19291941 and 2008. Bars represent the mean relative area [% of the total area of stands dominated by the respective species in the landscape areas (n 4)]. Standard errors are shown.

the area of clear-cut forest during that period ranged from about 9000 ha to 26,000 ha (Mangalis, 1991). Similar to the results obtained in Latvia, in southern boreal forests of North America, conversion of coniferous to deciduous stands in response to logging disturbance has been described (Boucher et al., 2009; Brumelis & Carleton, 1988; Rhemtulla et al., 2009). In Fennoscandia, regeneration of deciduous trees was favoured previously by standreplacing fire, selective logging (Axelsson et al., 2002) and gap disturbances (Lilja et al., 2006). During the twentieth century silvicultural methods (thinning and herbicide) were employed to remove birch in Sweden, which otherwise was common in ¨ stlund, natural succession after fire (Axelsson & O 2001). In Latvia in the pre-1940 period, the area of forest burned annually was B 16% of the area harvested by clear-cut (Kronı¯tis, 1991; Matı¯ss, 1991). Thus, it seems doubtful that fire alone can explain the increased birch regeneration in the pre1940 period.

The post-war Soviet period did not immediately begin with a rapid change in forest management. Until 1960, artificial regeneration continued at similar levels as in the pre-1941 period, and mostly by seeding of pine (Laivin¸ sˇ , 1998). However, in 1960, there was a drastic change to a focus on planting spruce (Laivin¸ sˇ , 1998), and the amount of wood harvested by thinning and sanitary cuts reached about 1/3 of the annual cut (Mangalis, 1991). The high proportion of spruce with age B 50 years in 2008 (Figure 4a) and the large relative area of birch stands aged 50 years compared to stands 5180 years (Figure 4c) clearly are due to the target of high-production spruce monocultures that began in the 1960s (Busˇ s, 1984; Za¯ lı¯tis, 2006). In 1968, 73% of all newly regenerated clearcuts were planted with spruce (Busˇ s et al., 1971). In Latvia, the relative area of young birch stands B20 years in the age distribution for the species dropped from 46% in 1961 to only 10% in 1988 (Kronı¯tis, 1991). A similar decline in the area of birch stands,


Seventy-year forest history of Latvia 453

Figure 5. Age distribution of stands dominated by (a) aspen, (b) black alder and (c) broad-leaved (oak and ash) in 10-year age classes for the periods 19291941 and 2008. Bars represent the mean relative area [% of the total area of stands dominated by the respective species in the landscape areas (n 4)]. Standard errors are shown.

incurred by intensive silvicultural methods aimed at conifers, occurred in northern Sweden during the twentieth century (Hellberg et al., 2009). However, the intensification of forestry occurred about 50 years later in Latvia compared to Fennoscandia. Part of the increase in forest area can be explained by drainage of wetlands, utilised to improve productivity. Between 1929 and 1940, drainage ditches were constructed in 170,330 ha of forest; about 54,000 ha were ditched previous to this time period (Sarma, 1951). In the 1960s drainage plans were developed for 30,000 ha forest yearly (Za¯ lı¯tis & Lazdin¸ sˇ , 1990). A total of 89% of the area of wetlands afforested after 19291941 was dominated by pine in 2008 (Table II), which amounts to 14% of the total pine forest area. Thus, pine was the target species for conversion of wetland. However, in addition, 20% of the spruce stand area B60 years regenerated in ditched areas, which is indicated by a

drained forest type (not shown) in the forest records. A large portion of these were likely planted on clearcuts that were subsequently drained. Thus, the drainage was probably associated with a species change, for example, from black alder to spruce. However, this could not be tested since the inventory data obtained did not specify the time of ditching. The productivity of spruce usually increases after drainage, but after an initial period with a high growth rate, in many drained stands spruce radial growth decreases dramatically (Za¯ lı¯tis & Lı¯biete, 2005). Thus, many of these drained spruce forests have become unproductive, and changes were made in legislation in 2009 to allow cutting of these stands before having reached harvest age (Cabinet of Ministers Regulation No. 1057 on Tree Cutting on Forest Land). The results obtained are to a certain extent biased, since a dominant species was identified for each


454 A. Te¯rauds et al. stand, ignoring the relative contribution of each species in mixed stands. The estimated relative forest area in which the dominant species had changed between the 19291941 and 2008 periods includes also mixed woods that changed in varying degrees in proportions of wood volume (Table II). The effect of this bias could have been lessened by weighted calculation (based on the relative wood volume) of the proportions of area in stands contributed by each species. However, weighted estimates would have caused undue complexity in determining the changes that have occurred in stands between the two periods, as many mixed-wood classes would have been needed. Therefore, we based our analysis on changes in the dominant species (Table II). Cartographic precision also undoubtedly led to an error, particularly in determination of changes that occurred in stands. However, visual examination of GIS layers of stand boundaries in the two periods generally showed good alignment. In cases when a stand, for example, was shown to increase in size between the periods, we assumed that this actually did occur and was not due to mapping error. Clearcuts could have been conducted in part of a stand, or in several adjacent stands, and natural successional processes could have led to new delineated stand boundaries. In any case, there is no reason to believe that the broad changes in dominant species observed were due to an artefact caused by mapping error. The age distributions of target tree species in the economy clearly need to be considered in sustainable forest management. Considering the peaks in the age distributions of the species, harvest in the state forests of the study area might intuitively be expected to shift from a dominance of pine/birch/aspen today to spruce followed by birch/pine/aspen as more stands reach cutting age. A more or less even distribution of stand ages over the landscape in 19291941, for all species considered together, had changed to a unimodal distribution in 2008 (Figure 3). This means that the availability of wood for a given tree species in state forests will change over time. The forest composition in the landscape differs to some degree between regions of Latvia (State Forest Service, 2008) due to abiotic factors. However, the estimated age distribution (Figures 35) and composition (Figure 2) for the species in 2008 are similar to those reported in state statistics for Latvia (State Forest Service, 2008). Therefore, we might expect that the changes described for the studied area occurred also throughout Latvia. Certainly the silvicultural methods used, which led to the unimodal distributions of trees species, were applied throughout the country. From a perspective of conservation of biological diversity targeted on achieving a more natural age distribution, in the study area there is a need to

create a significant 150 year class for conifers by protection of a portion of the overmature forests. As old growth forest is lacking, it can be expected that patches of high biodiversity in Latvia are associated with natural forest structures that have developed over a relatively short period of time. The mean amounts of dead wood in Latvia presently are considerably larger (16.2 m3 ha 1) than in Sweden (6.1 m3 ha 1) and Finland (5.7 m3 ha 1), according to MCPFE statistics (2007). The high proportions of aspen (21%), black alder (19%) and broad-leaved (12%) forest area under protection suggest that structural elements such as dead wood can develop in these woods in a relatively shorter period of time. About 5% of the black alder stands had age over 110 years, which is around the maximal age of this species, estimated at 120 years (McVean, 1953). Riparian ecotones in Lithuania support a greater proportion of overmature forests compared to Sweden, due to the previous protection of forests along watercourses in the former (Lazdinis & Angelstam, 2005). As the same form of protection existed in Latvia, this might also explain the higher relative proportion of, at least, black alder in 2008. Undisturbed successional deciduous woods with age 6095 years are an important habitat for whitebacked woodpecker in Latvia (Angelstam et al., 2004). Similarly, aspen, oak and ash are important substrates for cryptogams (Ju¨ raido et al., 2003, Mezaka et al., 2008). Thus, the mature deciduous forests that developed prior to the industrial forestry started in 1960 are an important component of the biodiverisity in Latvia, and there is also a need to maintain their spatial continuity. In Latvia, the sustainability and targets of forest management with regard to biological diversity concerns are difficult to assess, since there is no National Forest Programme, as envisioned by the Ministerial Conference on the Protection of Forests in Europe (MCPFE, 2007). Further, during the period after 2008, the total annual clear-cut area in state forests has doubled (data from the State Forest Service), legislative changes to lower the minimal harvest age for aspen to 31 years were submitted to parliament in 2011, and the FSC forest management certificate was suspended, mainly due to insufficient protection of EU-protected forest habitats and concentration of clear-cuts in particular areas (Rainforest Alliance, 2010). This current period of logging intensification will without doubt bring new unpredictable changes in the forest landscape structure. Acknowledgements Funding for the research was made possible by funding from the European Social. Fund

Seventy-year forest history of Latvia 455 supported Activity Programme Supplement 1.1.2.1.2. sub-activity ‘‘Support for Doctoral Studies implementation’’ Project ‘‘Support for Doctoral Studies at University of Latvia’’. Appreciation is given to Juris Zarin¸ sˇ for aid in processing forest inventory data. The comments of Bengt-Gunnar.Jonsson, Eeva Korpilahti and anonymous reviewers helped to improve the manuscript.


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