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Managing Boreal Forest Landscapes for Flying SquirrelsAuthor(s): Pasi Reunanen, Mikko Mönkkönen, Ari NikulaSource: Conservation Biology, Vol. 14, No. 1 (Feb., 2000), pp. 218-226Published by: Blackwell Publishing for Society for Conservation BiologyStable URL: http://www.jstor.org/stable/2641921Accessed: 23/05/2010 12:51

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Managing Boreal Forest Landscapes for

Flying Squirrels

PASI REUNANEN,*t MIKKO MONKKONEN,* AND ARI NIKULAt *Department of Biology, University of Oulu, POB 3000, FIN-90401 Oulu, Finland tFinnish Forest Research Institute, Rovaniemi Research Station, POB 16, FIN-96301 Rovaniemi, Finland

Abstract: Flying squirrel (Pteromys volans) populations have declined severely during the past few decades, and the species has beconme afocal species in forest management and the conservation debate in Finland. We compared land.scape structure around known flying squirrel home ranges with randomly chosen forest sites to determine which landscape patterns characterize the areas occupied by the species in northern Finland. We sought to identify the key characteristics of the landscape that support the remaining flying squirrel popu- lations. We analyzed landscape structure within circular areas with 1- and 3-knm radii around 63 forest sites occupied by flying sqtuirrels, and arotund 96 random sites. We applied stepwise analysis of the landscape structure where landscapes were built ul) step-by-step by adding patch types in order of their suitability for the flying squirret The land-use and forest-resource data for the analysis were derived from multisource na- tional forest inventory and imported to a geographical information systenm. Landscape patch types were di- vided into three suitability categories. breeding habitat (mixed spruce-deci.duous forests); dispersal habitat (pine atd young forests); and unsuitable habitat (young sapling stands, open habitats, water). Flying squir- rel landscapes contained more suitable breeding habitat patches and were better connected by dispersal hab- itats than random1 landscapes. Our results suggest that for the persistence of the flying squirrel, forest manag- ers should 1) maintain a deciduous mixtutre, particularly in spruce-dominatedforests; 2) maintain physical connectivity between optimal breeding habitats; and 3) impose coarse-grained structures on northeastern Finnish landscapes at cturrent levels of habitat availability.

Manejo de Paisajes en Bosques Boreales para Ardillas Voladoras

Resumen: Las poblaciones de ardilla volador (Pteromys volans) han declinado severamnente durante las tilti- mnas decadas y la especie se ha convertido en twla especie focal en el manejo de bosques y el debate sobre conservacion en Finlandia. Comparanios la estructura del paisaje alrededor de zonas conocidas de ardillas voladoras con sitios de bosque seleccionados al azar para determinar cuales patrones del paisaje caracteri- zan las areas ociupadas por la especie en el norte de Finlanlida. Intentamos identificar las caracteristicas clave del paisaje que soportan las poblaciones remanentes de ardillas zvol,doras. Analizamos la estruictura del paisaje con areas circulares de 1 y 3 kin de radio alrededor de 63 sitios d.e bosque ocupados por ardillas voladorasy alrededor de 96 sitios seleccionados al azar. Aplicamos tin analisisporpasos de la estructura del paisaje, dond.e los paisajesftueron construidos paso a paso al agregar tipos de parche en base a su idoneidad para las ardillas volad.oras. Los datos de uso del suelo y recuirsos forestales para el andlisis fiteron derivados de un inventario forestal nacional de recursos mtiltiples e importados a un Sistema de Informaci6n Geogra- fica. Los tipos de parches de paisaje fueron divididos en tres categorfas de idoneidad. habitat para reprodtwc- cfon (bosque mixto de abeto-decidtqo); habitat disperso (pino y bosquesj6venes) y babitat indeseable (cirbo- les j6venes, habitats abiertos, agua). Los paisajes para ardilla voladora contenian mcis parches de habitat id6neo para reproduccion y estuvieron mejor conectados por habitats dispersos que los pai.sajes al azar. Nuestros restultados sugiereni que para la persistencia de la ardilla voladora el manejo de bosques debera: 1)

t emtail pasi. reina-nen @ollufi Paper stibm nittedJtune 30, 1998;revised inlanulscript accepted June 9, 1999.

218

Conservation Biology, Pages 218-226 Volumiie 14, No. 1. Febiarvry 2000

Rewtanen et al BorealForest Matiagemetitfor Flyilg Squirrels 219

mantener una mezcla decidua particularmente en bosques donuinados por abetos; 2) mantener la conectiv- idadfisica entre habitats optimnos de reproducci6n; y 3) imponer estructuras de textura gruesa a los niveles actuales de habitat viable en los paisajes nortenos de Finlandia.

Introduction

Landscape structure affects several ecological processes in heterogeneous environments and can be divided into three components: composition, configuration, and con- nectivity (Taylor et al. 1993; Merriam 1995). Composi- tion refers to the relative proportions of different habitat types within the landscape, whereas configuration de- scribes the physical layout of these habitat patches in space (Dunning et al. 1992). A decline in the amount of original habitat is likely to have an adverse effect on pop- ulation numbers in the landscape (Andren 1994, 1996; Fahrig 1997; Bender et al. 1998). The relationship be- tween the amount of original habitat and population abundance, however, depends on species' habitat re- quirements and on other life-history traits (With & Crist 1995, Pearson et al. 1996; Andren et al. 1997). General- ist species are able to compensate for habitat loss by ac- cepting novel habitats and using them in the same man- ner as the original. Species showing specialization to original habitat are likely to be affected by habitat loss and changes in landscape configuration. After a certain threshold the decline of population numbers will be more rapid than expected for habitat loss alone (Andren 1994, 1996). This critical threshold, varies between spe- cies, however, and is affected by landscape struLcture (Bender et al. 1998) and landscape context (Monkk6nen & Reunanen 1999).

Landscape connectivity measures the probability that individuals are capable of moving across a landscape and colonizing suitable habitat patches within their dispersal range (Merriam 1991). Connectivity is not solely a func- tion of landscape structural characteristics, but also de- pends on species-specific life-history traits. Characteristics such as habitat affinity and dispersal behavior, for exam- ple, are essential to landscape connectivity (Merriam 1984; Baudry & Merriam 1988; Roitberg & Mangel 1997; With et al. 1997). Thus, landscapes can be connected because suitable patches are physically close to each other or because the dispersal potential of the species is able to link patches even if unsuitable habitat patches separate them.

Since the 1950s, modern forest practices have been applied in northern boreal forests (Esseen et al. 1997). As a consequence, in northern Finland the proportion of old forests (>100 years old) has decreased by 29% (from 58% to 29%) in 40 years. During the same period, the area of spruce-dominated forests has declined signifi- cantly (from 27% to 12%). In regeneration stands, conif-

erous trees were preferred over deciduous trees, which has led to a considerable decrease in the proportion of de- ciduous trees (Finnish Forest Research Institute 1997). Prior to the 1950s, forests in northern Finland were sub- jected to selective cuttings, but since then, present-day old-growth forests have not been managed.

In the boreal Fennoscandia, modern forestry has cre- ated completely new landscape patterns over a relatively short time (Hansson 1992). Modern forestry has turned the former continuous forest landscapes into managed forest landscapes where natural forests are separated by clear-cut areas and young forests. This change in the landscape pattern has had an influence on all important landscape structural components: the proportion of orig- inal habitat has declined, after the remaining original hab- itat patches have become isolated by large areas of man- aged forests, and the degree of connectivity in boreal forest landscapes has diminished even from the perspec- tive of species that show a moderate affinity to the origi- nal habitat (e.g., Kurki 1997).

The flying squirrel is a nocturnal, forest-dwelling spe- cies distributed across Eurasia. In Finland it occurs in the northwestern-most verge of its geographical distribution (Corbet & Hill 1991). The flying squirrel favors spruce- dominated forests with a marked deciduous component, mainly aspen (Populus tremula), birch (B3etula sp.), and alder (Alnus sp.). In northern Finland, the species oc- curs almost entirely in mixed forests dominated by ma- ture spruce. Within the spruce forest areas, suitable mixed spruce-deciduous habitat patches are relatively small and scattered in the forest matrix. The size of the core home range of the flying squirrel is usually a few hectares; but the entire area exploited by individuals, es- pecially males, may be as large as a few square kilome- ters (Hanski et al. 1999; P. Reunanen et al., unpublished data). Dispersal of the species has not yet been studied in detail, but observations of ear- and radio-tagged indi- viduals suggest that the dispersal distance can be up to a few kilometers (Makela 1996; personal observation).

Populations of the flying squirrel have declined dramati- cally since the 1950s (Hokkanen et al. 1982). Changes in landscape structure (i.e., habitat loss and fragmentation of natural forest areas) are considered the main cause of the population decline (Anonymous 1996). It is therefore im- portant to identify key characteristics of the landscapes that support the remaining flying squirrel populations. The flying squirrel has recently become a focal species in forest management and the conservation debate in Finland. Thus, it is of public interest and practical impor-

Conservationi Biology Volunme 14, No. 1, February, 2000

220 Boreal Forest Alanagement for Flying Squirrels Reunanen. et al.

tance to determine management practices allowing the persistence of viable flying squirrel populations.

We compared the landscape structure around known flying squirrel home ranges with randomly chosen forest sites to determine which landscape patterns character- ize areas occupied by the species in northern Finland. We focused on the role of landscape connectivity distin- guishing occupied landscapes from average (random) landscapes. The definition of landscape connectivity im- plies the importance of adopting an organism-centered view when connectivity in landscapes is estimated (sensu Merriam 1984; With et al. 1997). We therefore applied a stepwise analysis of the landscape structure in which landscapes were built up step by step by addition of patch types in order of their suitability for the flying squirrel. Because of the species-specific aspect of con- nectivity, it is difficult to measure landscape connectiv- ity directly (Keitt et al. 1997). We assessed connectivity indirectly by using indices related to landscape configu- ration. Finally, we considered the implications for man- aging boreal forest landscape for viable populations of the flying squirrel.

Methods

Study Area

The study was conducted in Koillismaa, northern Fin- land (lat 65035' N, long 28010' E). Phytogeographically the area is situated in the northern boreal zone (Ahti et al. 1968) and is covered mainly by pine- and spruce-dom- inated forests of various ages (>65%). Wetland areas (bogs and fens) are numerous and form a distinctive landscape element (approximately 25%). The rest of the area consists mostly of lake and river systems. Cultivated areas are sparse. Three-quarters of the forest in the area is pine-dominated, whereas the proportion of spruce- dominated forests remains near 20%. Most of the forests in the study area are managed for commercial timber production. Old-growth forest in Koillismaa is mainly spruce-dominated and can vary from a few square kilo- meters to more than 100 km2. Topography in the study area varies conspicuously, and elevation ranges from 100 to 360 m.

Flying Squirrel Locations and Random Sites

The occurrence of the flying squirrel and the locations of home ranges were determined in the field. We deter- mined the presence of individuals by the presence of fe- cal pellets around old aspens and conspicuous large spruces. In summer, discarded leaves of deciduous trees left on the ground provide a clear indication of an occu- pied forest site. These methods were reliable for deter- mining the presence or absence of the species (Anony-

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Volulme 14, No. 1, Februlary 2000

mous 1993), but to some extent were inaccurate for locating the precise core areas of home ranges (Hanski 1998).

We first located 150 random forest sites by using a ran- dom nutmber generator. We analyzed these random sites and selected only sites that contained >80% forest land within an 80-in radius. Because we wanted to eliminate completely irrelevant random sites from the analysis, we excluded randomly chosen sites on watercourses, in wetland areas (bogs and fens), in extremely small forest patches (a few pixels in size), and on boundary zones between mineral soils and peatland areas. Thus, random sites were all situated strictly on forest land and com- pletely within the boundaries of the study area. This re- sulted in 96 random sites. Random sites represent the av- erage forest landscape structure in Koillismaa.

Landscape Data

The land-use and forest-resource data for the analysis were derived from the multisource national forest inventory (NFI; Tomppo 1991, 1993, 1996). In Finland NFI exploits Landsat TM 5 satellite images and ground study plots to get geographically explicit, up-to-date information about forest resources. The original pixel size of 30 x 30 m is re- sampled to 25 X 25 m during the operation. The result is a rectified multichannel image that can be imported to a geographic information system (GIS). Image analysis enables estimates of stand age and growing stock vol- ume of pine, spruce, birch, and other tree species for each pixel. We calculated the total timber volume for each pixel by summing the species-specific values. Digi- tal masks for roads, settlements, and agricultural land were also imported to the GIS. We assigned each pixel to one of the following timber volutme classes: 0, 1-20, >20-50, >50-100, >100-150, and >150 mr3/ha. If the proportion of pine or spruce of the total timber volume in a pixel exceeded 80%, we considered it pure pine or spruce forest. For deciduous trees the corresponding limit was set at 60%. A pixel was classified as mixed de- ciduous-conifer forest if the proportion of deciduous trees ranged from 30-60% of the total volutme. The re- maining forest pixels were composed of mixed pine- spruce forest class. Watercourses and open areas (fields, clear cuts, and fens) were classified as habitat classes of their own.

Stepwise Landscape Analysis

Landscape structure was analyzed within circular areas of 1- and 3-km radii around 63 forest sites occupied by flying squirrels and around 96 random sites. The selection of ra- dii was based on observations of ear-tagged juveniles (Makela 1996) and on the behavior of radio-collared indi- viduals (Hanski et al. 1999; P.R. et al., unpublished data). The 1-km radius represents the average dispersal

Reunanen et al. Boreal Forest Alanagementfor Flying Squirrels 221

distances of juvenile squirrels and the average ranging distances of adult males, whereas the 3-knm radius repre- sents maximum distances.

Landscape structure was classified into 10 landscape classes. To provide a species-centered view, these classes were arranged in order of decreasing suitability for the flying squirrel (Table 1). The arrangement of landscape classes was based on our earlier knowledge of the habi- tat associations of the species (Monkkonen et al. 1997) and on the survey of the structural characteristics of for- est sites that are occupied by the species (P. Reunanen et al., unpublished data). Moreover, we analyzed a small plot (r = 200 m) around the flying squirrel site to define the general pattern of landscape classes within the home range of individuals and used this information to validate our arrangement. The 10 habitat classes were further as- signed into three broad categories according to their suitability for the flying squirrel: (1) habitat the species uses for breeding and for daily activities (landscape classes 1 and 2), (2) habitat used for dispersal and tem- porary stays (classes 3-6), and (3) unsuitable habitat (classes 7-10).

In the stepwise procedure, we built up the landscape by adding landscape classes one by one, in order of their suitability. For each step, the included habitat classes to- gether formed the "habitat," and the rest of the habitat classes made up the "matrix." After two steps, for exam- ple, spruce and mixed deciduous-spruce forests with more than 100 m3 timber composed the habitat, and the patterns in landscape structure were compared between flying squirrel and random landscapes. We used Frag- stats software (McGarigal & Marks 1993) to analyze land- scape patterns. Fragstats computes several variables for

Table 1. Types of forest patches arranged in order of their suitability for the flying squirrel in northern Finland.

Percent Patcb tylpe Sttitabilitya land"

Spruce and mixed spruce-deciduous forests >150 m3/ha breeding 0.58

Spruce and mixed spruce-deciduous forests 100-150 m3/ha breeding 0.74

Pine and pine-spruce forests >150 m3/ha dispersal 5.79

Pine-spruce forests 100-150 m3/ha dispersal 7.53 Pine forests 100-150 m3/ha, spruce,

spruce-deciduous, and pine-spruce forests 50-100 m3/ha dispersal 16.11

Pine forests 50-100 m3/ha dispersal 6.01 Deciduous sapling stands unsuitable 8.76 Conifer sapling stands and pine bogs unsuitable 38.8 Open areas (clear-cuts, open fens,

fields) unsuitable 8.24 Water unsuitable 7.44

"Patcb types are also classified according to tbeir suitability for breeding and dispersing. bPercent land refers to the proportion of t[e patcb type in random

ldSCalpeS in a 3-kmw r adiues.

each land-use class and for the landscape as a whole. We selected the following landscape variables, which were calculated after each step for both radii: percent land (proportion of habitat of the total area, 314 ha and 2827 ha for 1- and 3-km radii, respectively); mean patch size (MPS, ha); and patch density (PD, number of patches/ 100 ha). Percent land refers directly to the amount of fo- cal habitat type in the landscape (composition), whereas MPS and PD refer to the spatial arrangement of these patches and the degree of fragmentation in the land- scape (configuration). In general, relatively high values of percent land and MPS characterize continuous land- scapes. Patch density can be insightftilly interpreted only in relation to other landscape indices: for a given amount of habitat (percent land) in the landscape, PD tends to increase with increasing fragmentation.

In the univariate analysis, we compared landscape variables after every step between flying squirrel and random sites using the Mann-Whitney U test. We used principal component analysis (PCA) to extract orthogo- nal multivariate landscape axes based on landscape vari- ables after two steps (all breeding habitat included) and after six steps (all breeding and dispersal habitat in- cluded). For the PCA, percent land was arcsine-square- root-transformed, and MPS and PD were log-transformed. All statistical analyses were conducted with SPSS for Win- dows (version 6.1.4.).

Results

Univariate analysis revealed significant differences in habitat availability between flying squirrel and random landscapes. There was almost three times more suitable breeding habitat after two steps in flying squirrel land- scapes than in random landscapes within a 1-km radius (4%0 and 1.5%, respectively; Fig. la) and two times more within a 3-km radius (2.7% and 1.3%; Fig. lb). The flying squirrel landscapes also had more dispersal habitat (after six steps) than did random landscapes (1-km, 52% vs. 42%; 3-km, 41.9% vs. 36.7%; Fig. 1). After six steps, signif- icant differences started to disappear, and the land- scapes on average became much alike when sapling stands, bogs, open areas, and watercourses were in- cluded. The largest increase in percent land occurred when coniferous sapling stands and pine bogs were in- cluded (step 8). The coverage of this landscape class was on average 30% in flying squirrel landscapes and 39% in random landscapes within a 1-km radius, but 24.8% and 26.0%, respectively, within a 3-km radius.

Flying squirrel landscapes were physically more con- nected than the average landscapes in the study area. (Results for 1- and 3-km radii were so similar that we show results only for 1-km radius). First, mean patch sizes were consistently higher in flying squirrel than in random landscapes in regard to breeding and dispersal

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222 Bo eal Forest MlawagenientJfr Fling Squirrels Reunanen, et al.

100 - a

80 - Flying squirrel Random

Z 60 -

2.

20

00

1 2 3 4 5 6 7 8 9

100 -b

80

CZ 60

-J

(D 40 -

20-

0 1 2 3 4 5 6 7 8 9

Step number

Figure 1. The proportion of total landscape area rep- resented by flying squirrel habitat within (a) 1- and (b) 3-kin radii aroutnd flying squirrel sites and ran- domly selected sites at each step of the landscape pro- cedure. After two steps all breeding patch types and after six steps allpatch types sutitable for dispersal are included in habitat.

habitat classes. The average size of breeding and dis- persal habitat patches were often more than twice as large in forest areas occupied by flying squirrels as in random forest sites (Fig. 2). Second, breeding habitat patch density was higher in flying squirrel than in ran- dom landscapes. There were 2.5-3 times more breeding- habitat patches per 100 ha in the flying squirrel land- scapes. By contrast, random landscapes contained a higher density of dispersal patches (Fig. 3). This suggests that breeding patches, in addition to being more exten- sive, were also more numerous in flying squirrel land- scapes, whereas the smaller area of dispersal habitat in random landscapes was subdivided into a larger number of patches (i.e., was more fragmented; Figs. 4 and 5).

The relative roles of breeding and dispersal habitat can be analyzed further by picking a sample of flying squirrel sites so that there is exactly the same amount of dis-

8

7 Flying squirrel Random

co5

N

5)

0

1 2 3 4 5 6 7

Step number

Figure 2. Mean patch size (ha) in landscapes witvtn a 1-kmn radius around flying squirrel sites and ran- domnly selected sites. Results for only the Jirst seven steps of thwe landscape procedure are given. Mean patch sizes after eigbt steps are more than 100 ha and after ninle steps more than 250 ha in bothw fly ing squir- rel and randoms landscapes.

persal habitat after six steps, within a 1-km1 radius, as in random landscapes (42%o). In this sample of 39 flying squirrel sites, the proportion, mean patch size, and patch density of breeding habitat (after two steps) in the landscape were significantly higher (percent land= 3.35, MPS = 0.17 ha, PD =19.0 patches/100 ha) thaan around random sites (percent land = 1.49, z =5.10, p K

0.000; MPS =0.12, z =4.44,p K 0.001; PD =10.8, z =

4.91, p < 0.00 1). This fturther indicates the importance of breeding-habitat availability. Whaen we picked a sam-

30

Flying squirrel

25- T randon ' RandomlT

c 15 I ti L o 3

1 2 3 4 5 6 7 8 9

Step number

Figure 3. Patch density (patches/100 ha) in land- scapes within a 1-km radius arouhnd flying squirrel sites and randomly selected sites at each step of the landscape analysis.

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Volume 14, No. 1, FebuLary 2000

Reunanen et al. Boreal Forest Management for Flying Squirrels 223

_ l 1 ~4% 0 l 52.4 %e ElMatrix Mti

1Pteromys

- i' !l fl1.5 % j A * " - ' 41.5%

* Random _I

Figure 4. Habitat patches (dark) and their physical layout in a 1-km radius after the sec- ond step (left panels, breeding habitat only) and the sixth step (right panels, breeding and dis- persal habitat) in the average flying squirrel landscape and random landscape. The propor- tion of breeding and/or dis- persal habitat in the landscapes is indicated in both cases. Matrix refers to all habitat classes not included with breed- ing and/or dispersal habitat.

ple of flying squirrel sites so that all differences in the availability of breeding habitat with random sites disap- peared (percent land after two steps was 1.5 in both cases), there were no differences between these 16 fly- ing squirrel sites and random sites in dispersal habitat availability or layout (percent land = 41 and 41, PD = 15.5 and 17.2, MPS = 3.78 and 4.41 in flying squirrel and random landscapes, respectively). This suggests ei-

ther that the "poorest" flying squirrel sites are relicts from times when landscapes were more continuous or that the availability of dispersal habitat is not an impor- tant factor.

Principal component analysis produced three inde- pendent landscape axes (Table 2). The first principal component (PCI), which explained more than 50% of variation in landscape structure, in practice summarizes

.. ............... 207 4 41 .9 % nMat*x Matrix

Fteromys

.;.:,.<lU 1.3 % ~ ~Q ~ * 36.7 % A11 * Matrix ... f OMatrix

1'IRandom

* wx.....'.

Figure 5. Physical layout of hab- itat patches (dark) in a 3-km ra- dius after the second step (left panels, breeding habitat only) and the sixth step (right panels, breeding and dispersal habitat) in the averageflying squirrel and random landscapes. The proportion of breeding and/or dispersal habitat in the land- scapes is indicated in both cases. Matrix refers to all habitat classes not included into breed- ing and/or dispersal habitat.

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224 Boreal Forest Manageinenitfor Flying Squirrels Reunanen et a

Table 2. The results of the principal component analysis for the landscape data consisting of percent land, mean patch size, and patch density after two and six steps of the stepwise analysis; average score of flying squirrel and random landscapes on PC-axes; and t test and two-tailed significance for difference in average scores.a

Variable, radius, step no. b PC] PC2 PC3

Percent land 1,2 0.72 0.55 0.30 1,6 0.81 -0.40 0.25 3,2 0.77 0.50 -0.30 3,6 0.81 -0.34 -0.22

Mean patch size 1,2 0.50 0.61 0.29 1,6 0.79 -0.50 0.29 3,2 0.50 0.61 -0.27 3,6 0.83 -0.42 -0.29

Patch density 1,2 0.66 0.46 0.38 1,6 -0.69 0.53 -0.30 3,2 0.73 0.45 -0.30 3,6 -0.73 0.44 0.33

Eigenvaltie 6.18 2.88 1.05 Percent of variance explained 51.5 24.0 8.7 Average flying squirrel landscapes 0.60 0.34 0.20 Random landscapes -0.41 -0.24 -0.14 Test 7.12 3.68 2.15 p <0.001 <0.001 0.033

"Only principal components with eigenvalue >1 are shownl. Load- ings of the landscape variables, eigenvalutes, and the percenitage of varianice explainedfor PI-PC3 are given. b The first niumlber r-efer-s to the radius (I or 3 km) of the landscape procedure; the second niumiber denotes the step number (2 or 6).

the results of univariate analyses. By combining informa- tion for 1- and 3-km radii, PCI arranged landscapes from fragmented to continuous because it was positively re- lated to the availability and patch sizes of breeding and dispersal habitat and to the density of breeding patches but was negatively related to dispersal patch density. The PC2 distinguished breeding and dispersal habitat classes. Landscapes that contained large areas of breed- ing habitat in large and dense patches but that tended to have relatively fragmented dispersal habitat, were lo- cated on the positive end of PC2. The PC3 described the ratio of landscape structure between 1- and 3-km ra- dii, so landscapes scoring high on PC3 were character- ized by continuous landscapes at a 1-km radius but frag- mented at a 3-km radius.

Flying squirrel landscapes scored significantly higher, on average, on all three PC axes than did random land- scapes (Table 2). There were only six flying squirrel sites for which landscapes scored negatively both on PCI and PC2, and they therefore could be considered located in relatively fragmented landscapes with relatively little breeding habitat (Fig. 6). The average score of these six sites on PC3 was 0.86, much higher than the average score for flying squirrel sites (Table 2). In other words,

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3

2

0 C 13 -2g -1 o 1 2

C\J ~ 00 000 0 *De0

-1~ ~~~~~C

0 0 ~ C\j 0 00 0 QD0 c~~~~~~~~~

-2 0 0 *D0 *0

0 O

-3 ~~~~0 0

-3 -2 - 0 1 2

Figure 6. Location offlying squirrel landscapes (filled symbols) and random landscapes (open symnbols) on the first two principal components.

even though these sites were located in fragmented land- scapes, fragmentation was more pronounced on a 3-km scale than on a 1-km scale.

Discussion

It seems that the availability of both breeding and dis- persal habitat and their layout in the landscape were im- portant influences on the existence of flying squirrels. Compared to random landscapes, flying squirrel land- scapes contained more suitable breeding habitat and were better connected by habitats that could be used for dispersal. Random sites did not cover the entire princi- pal component space, and a large part of the occupied sites were situated in landscapes outside the range of random variation in landscapes. This suggests that land- scape structure is an important factor in determining which areas can support viable populations of the flying squirrel. As shown by random sites, the forest landscape in Koillismaa is relatively fragmented (i.e., the average patch size of the original habitat is reduced and the total number of patches has increased; Bender et al. 1998).

The flying squirrel can be considered a habitat special- ist rather than generalist in northern Finland. All known home ranges in our study area contain mixed spruce- deciduous forest. These deciduous forest patches are temporary, and during the forest succession they gradu- ally disappear if not maintained by reculrrent distur-

Reunanen et at, Boreal Forest Managementfor Flying Squirrels 225

bances (Pickett & White 1985). The species that inhabit these transitional habitats should have adapted to envi- ronmental conditions by developing a life history in which dispersal plays an important role (Tiebout & Anderson 1997).

In landscape models, connectivity is an essential fac- tor for viable population dynamics (Lefkovitch & Fahrig 1985). Connectivity is a good predictor of population persistence, especially for species that are narrowly adapted to certain habitat (Henein et al. 1998). Our re- sult, that landscapes around flying squirrel sites con- tained more dispersal habitat than random sites, indi- cates the importance of landscape connectivity for local persistence.

It has been shown that, below a 50% threshold of hab- itat availability, fine-scale fragmentation poses a greater risk to landscape connectivity than the same amount of habitat reduction arranged in a coarser pattern (Rolstad & Wegge 1987; Pearson et al. 1996). From the point of view of the flying squirrel, the forest landscape in our study area is coarse-grained, because on a local scale the remaining old-forest blocks are large and continuous but on larger scales the landscape is heterogeneous and frag- mented by managed forest stands. We found that flying squirrel landscapes on average contained more than 50% of dispersal habitat within a 1-km radius, whereas ran- dom landscapes and flying squirrel landscapes on a 3-km scale contained only 40%. This means that a high pro- portion of the flying squirrel sites we found were lo- cated in areas where coarse-graininess is obviously a necessary condition for local population persistence.

Furthermore, according to the principal component analysis the most fragmented sites occupied by flying squirrels (six sites scoring negatively both on PCI and PC2) were less fragmented on a 1-km scale than on a 3-km scale. This suggests that local-scale fragmentation is more harmful to flying squirrels than fragmentation on larger scales. On the other hand, all six of these sites were located in relatively recently fragmented land- scapes, as suggested by a somewhat higher than average amount of open habitat within a 1-km radius (8.7% vs. 7.2% in random landscapes). These sites may therefore represent extinction debt (Tilman et al. 1994) in that sooner or later they are going to be deserted.

Because of the coarse graininess, most of the occupied areas were well connected on a local scale and no major barriers for movements within areas existed. On this scale, connectivity can be seen as a consequence of landscape structural characteristics. Movements between separate old-forest areas, however, depend mainly on dispersal abil- ity (i.e., ftinctional connectivity; Monkkonen 1999). For successftil long-distance dispersal, individuals must to some extent traverse unsuitable managed forests where they have limited possibilities of finding suitable breeding habitat patches. Because we do not know the dispersal be- havior of the species sufficiently well, however, it is still

unclear how the present landscape patterning affects the population dynamics of the flying squirrel.

Based on our results, forest management recommenda- tions can be made to ensure flying squirrel persistence in northern Finland. First, it is important to maintain a decid- uous mixture, particularly in spruce-dominated forests. Earlier studies have shown that aspen is likely the most important tree in this respect, providing both food (Hanski et al. 1999) and shelter in terms of woodpecker cavities. Promoting deciduous trees would require their presence in all successional stages. Wh-en forest harvest- ing in spruce-dominated, old-forest areas is planned, the sites with a marked deciduous component should be treated as key habitats and set aside. Within preserved old-forest areas, this would require providing small-scale openings to encourage deciduous regeneration. Other- wise, natural succession would lead to a diminishing pro- portion of deciduous trees.

Second, our results imply the importance of physical connectivity between optimal breeding habitats. Boreal forest landscapes are characterized by continuous change over a period of time. Little of the forest area is perma- nently cleared and turned into some other landscape ele- ment. Natural disturbances, forest management, and for- est succession create an ever-changing mosaic of habitat patches of various ages, sizes, and shapes (Bormann & Likens 1979; Pickett & White 1985; Pearson et al. 1996). Consequently, landscape connectivity also varies over time. In managed landscapes, it is possible to include con- nectivity in the management planning process so that changes in landscape structure can be predicted. In the long run, maintenance of population processes, by retain- ing landscapes structurally connected, is a challenge for forest landscape management. At the current level of hab- itat availability, it is important to impose coarse-grained structures on the landscape-to preserve the remaining large old-forest patches.

The results of the principal component analysis can be used to predict which occupied sites are the most prone to extinction and which random sites are most likely to be occupied by flying squirrels. Testing these predic- tions would ultimately validate our conclusion of the im- portance of landscape structure to the flying squirrel. Refining the predictive tools and testing the predictions derived is a challenge for future studies.

Acknowledgments

We are grateful to the Academy of Finland, the Finnish Forests and Parks Service (Pohjanmaa region), and the Maj and Tor Nessling Foundation for financial support. D. Lindenmayer and an anonymous referee made several useftil comments on an earlier version of the paper. R. Cooper kindly edited the language. This study is a part of the Finnish Biodiversity Research Programme.

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226 Boreal Forsest Management for Flying Squirrels Reunann et al

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