TASK 1



Population biology of the trembling aspen (Populus tremula)
and the taxa associated with it in boreal forests




The trembling aspen is a functionally important deciduous tree species in boreal forests. Old aspen individuals are keystone components in preserving biodiversity in boreal forests (Esseen et al. 1992). Aspen is most common in young successional stands, but scattered large trees also occur in spruce-dominated old-growth stands. Large living trees, snags and logs are essential microhabitats for a wide variety of organisms, such as hole-nesting birds, invertebrates (Siitonen 1994), wood-rotting fungi, bryophytes and lichens (Kuusinen 1994). Large numbers of specialist species are entirely dependent on aspen. The nutrient-rich litter of aspen is also a significant resource for a range of soil-inhabiting organisms (Ehnström & Waldén 1986). Mature aspen have been strongly disfavored in forest management, which has caused a marked decline in the populations of specialized aspen-dependant species (Rassi et al. 1992, Siitonen & Martikainen 1994).

The aims of this research task are

  • To quantify the assemblages of species associated with aspen in boreal forests
  • To study the population biology and dynamics of aspen in managed and virgin forests
  • To study the population biology and metapopulation dynamics of selected aspen-associated species
  • To estimate the critical density of aspen at forest stand and landscape levels for the long-term survival of the specialist species.

Studies will be carried out in three areas, in Evo in southern Finland, in Kainuu in northeastern Finland and in Russian Karelia (exact location to be decided at a later stage).

The research group is described in the following table (PI=principal investigator, FFRI=Finnish Forest Research Institute, UH=University of Helsinki)

Name (organization)
Juha Siitonen (FFRI) PI
Mikko Kuusinen (UH)

Reijo Penttilä (UH)
Ilkka Hanski (UH)
Ilik Saccheri (UH)
undergraduates (UH)

salary from this project
no
yes

no
no
no
yes

speciality
biology of aspen, beetles
biology of aspen, epiphytic lichens and bryophytes
wood-rotting fungi
metapopulation modelling
population genetics
biology of aspen, study of associated taxa, etc.

1 Background and present knowledge

Old individuals of aspen are keystone components in the preservation of biodiversity in boreal forests (Esseen et al. 1992). Aspen is essentially a pioneer tree species and is hence most common in young successional stands, but scattered large trees also occur in spruce-dominated old-growth stands. Large living trees, snags and logs are essential habitats for a wide variety of organisms, such as hole-nesting birds, invertebrates (Siitonen 1994), wood-rotting fungi (Kotiranta & Niemelä 1981), bryophytes and lichens (Kuusinen 1994). Large numbers of specialist species are entirely dependent on aspen. The nutrient rich litter of aspen is also a significant resource for a range of soil-inhabiting organisms (Ehnström & Waldén 1986).

Aspen reproduces mainly vegetatively by root suckers. Therefore the distribution of trunks usually reveals a distinctly clumped pattern, and groups of trunks may often consist of only one or a couple of clonal individuals. Aspen is thus a good example of a very patchy resource in boreal forests. The inner part of the trunk of old trees is frequently decayed by the fungus Phellinus tremulae, but in spite of this individual trees may reach the age of up to 200 years in suitable habitats in old-growth forests. Surprisingly little is known about the basic population biology of aspen in Fennoscandia (see Blumenthal 1942, Tikka 1954). For instance, patterns of regeneration and recruitment in different successional stages, maximum age and causes of death remain to be studied.

Mature aspen has been strongly eradicated in managed forests owing to its low economic value in the past, tendency to form thickets of suckers on clear-cut areas and partly because of its status as a host of Melampsora pinitorqua, a serious rust disease of young pine stands. Nonetheless the total volume of aspen has not changed significantly in southern Finland during the last decades (Kuusela & Salminen 1991), because young aspen stands are common on abandoned agricultural land, and aspen clones also commonly occur in clear cut-areas. However, there has probably occurred a shift from large, old trees towards younger age classes over extensive areas in Finland. Moreover, the present density of moose populations in southern Finland may largely prevent the recruitment of aspen in old-growth forests.

Recently Kuusinen (1994, 1995a, b, 1996) studied the epiphytic lichen and bryophyte species composition and diversity on old aspens. The species composition on aspen trunks was strikingly different compared to other tree species and included several specialised species. Siitonen & Martikainen (1994) studied the beetle fauna, particularly the rare and threatened species, associated with aspen in Finnish and Russian Karelia. Several rare aspen-specialised species were found in Karelia but none in Finland despite the similar number and quality of trees studied in both countries. The lower species richness in Finland was attributed to the low density of suitable host trees (see also Hanski & Hammond 1995).

2 Aims of the research

  • To quantify the assemblages of species associated with aspen in boreal forests
  • To study the population biology and dynamics of aspen in managed and virgin forests
  • To study the population biology and dynamics of aspen in managed and virgin forests
  • To study the population biology and metapopulation dynamics of selected aspen-associated species
  • To estimate the critical density of aspen at forest stand and landscape levels for the long-term survival of the specialist species.

3 Study areas and material resources

The study will be carried out in three different areas: (1) in Evo, southern Finland, (2) in Kainuu, in northeastern Finland, and (3) in Russian Karelia (exact location will be decided later on). The study area in Evo belongs to the southern boreal zone and has a long history of forest utilisation with only few scattered patches of old-growth forest left. The study area in Kainuu is located between the middle and northern boreal zones. Here intensive forest management was started later and the proportion of old-growth forests is still much higher than in Evo. A study area with virgin forest stands will be selected in Russian Karelia to serve as a reference for the study areas in Finland. In fact, the three study areas in Evo, Kainuu and Russian Karelia span the entire range of density of old aspen trees in boreal forests.

4 Research tasks


A. Species diversity associated with aspen in boreal forests

The project will be started by gathering all the existing knowledge about the species diversity and composition of various taxa on aspen in Fennoscandia. Empirical work will be carried out in the study areas. Species inventories will concentrate on two species-rich but poorly known groups, dipterans and fungi. Fauna and flora occurring in the canopy of large aspen trees, which is very poorly documented, will be a particular target in the inventories. We have the necessary contacts among taxonomists to identify species in difficult taxa.

Survey of insects specialized on the trembling aspen.

In a pilot study during the autumn 1997, two kinds of areas (a total of 40 ha in size), contrasting in the amount of living and dead aspen trees weresurveyed for the aspen specialist insects (15 species of Coleoptera and 1 species of Diptera). We surveyed 20 one-hectare squares from the most aspen rich areas and 20 squares from an aspen poor region (each containing at least one large dead aspen) within the study area in which the occurrence of aspen had already been surveyed. Within the one-hectare squares all dead aspens were searched for the occurence of any marks (e.g. adults, larvae, fragments of dead adults, emergence holes, larval cavities) of the target species. Signs of three target species were observed in the pilot survey: Xylotrechus rusticus, Saperda perforata (Cerambycidae) and Trypophloeus sp. (Scolytidae). All observations were based on emergence holes. The observed species were approximately equally often observed in aspen rich and aspen poor one-hectare squares.

B. Population biology and dynamics of aspen in managed and virgin forests

Population biology of aspen in old-growth forests

The long-term persistence and regeneration of aspen in old-growth forests is poorly documented in Finland. Spatial distribution and size distribution of living trees will be measured in old-growth forests on large sample plots (10-50 ha). Age distribution and clonal structure of aspen patches will be studied using both morphological and isoenzyme methods. Patterns of dying and causes of death will be described. Establishment of root succers in canopy gaps, browsing by moose and minimum gap size for successful recruitment will be studied. These studies will be conducted in collaboration with Task 3 and Dr. R. Heikkilä from the Finnish Forest Research Institute.

Occurrence of aspen snags and logs in old-growth forests

Amount, spatial distribution, size distribution, decay stages and rate of decay of aspen snags and logs will be measured in old-growth forests on the sample plots (10-50 ha) mentioned above.

Occurrence and pattern of patches of mature aspen in managed and virgin forest landscapes

The distribution of all patches of mature aspen trunks (dbh > 20 cm) will be mapped on an area of 50,000 ha in Kainuu and 10,000 ha in two areas in Lammi (forest tract in Evo and forest-farmland tract in the surroundings of the Lammi Biological Station) and in Russian Karelia. The area in Kainuu corresponds in size to a regional-scale forestry planning area as now practised by the Forest and Park Service. Aerial photographs will be used to locate the possible occurrences of aspen, and all the potential areas will later be surveyed on the ground and located using GPS. Number and quality of trunks in each patch will be measured. In areas with more continuous occurrence of aspen without distinct patches, mapping in one hectare grid will be used.

C. Population biology and metapopulation dynamics of selected aspen-associated species

Associated species at forest stand scale

Spatial distribution of selected species in relation to the spatial pattern of suitable host trees (living trees, snags and logs) will be surveyed at forest stand scale in some old-growth forests in the study areas. The temporal dynamics of these species will be followed for several years. These data will be used in modelling (Section 4.4) the long-term persistence of species at forest stand scale, and in estimating the effort needed to detect different species within a forest stand if present. The spatial distribution of 1,250 standing trunks and recently fallen logs has already been mapped in an area of ca. 25 ha area at Kotinen Forest Reserve in Evo (Kuusinen & Lommi, unpubl.). The occurrence of selected epiphytic lichens and one bryophyte species has also been mapped on these target trees. Completing and analysing this existing data set will be one starting point for this research task.Spatial distribution of selected species in relation to the spatial pattern of suitable host trees (living trees, snags and logs) will be surveyed at forest stand scale in some old-growth forests in the study areas. The temporal dynamics of these species will be followed for several years. These data will be used in modelling (Section 4.4) the long-term persistence of species at forest stand scale, and in estimating the effort needed to detect different species within a forest stand if present. The spatial distribution of 1,250 standing trunks and recently fallen logs has already been mapped in an area of ca. 25 ha area at Kotinen Forest Reserve in Evo (Kuusinen & Lommi, unpubl.). The occurrence of selected epiphytic lichens and one bryophyte species has also been mapped on these target trees. Completing and analysing this existing data set will be one starting point for this research task.

Patterns of occurrence of associated species in aspen patches at landscape scale

This survey can be done efficiently based on the previous mapping of aspen patches over large areas (above). Potential target species include lichens (Lobaria pulmonaria, Nephroma spp., Parmeliella triptophylla), bryophytes (Neckera pennata), polyporous fungi (e.g. Phellinus tremulae, P. tremulicola, Polyporus pseudobetulinus), one Diptera (Xylomya czecanowskii) and about fifteen Coleoptera (e.g. Hylochares cruentatus, Xylotrechus rusticus, Xyleborus cryptographus), which are all relatively easy to observe if present. Some of these species are relatively common and can be expected to be present in most of the suitable patches, whereas most of them are rare and threatened in Finland.

Genetic structure of populations of associated species

We will select five or so representative specialist species that show different patterns of spatial occurrence for a genetic study. Several techniques can be used in the study of these species: enzyme electrophoresis, microsatellite DNA markers, and mtDNA RFLP. Dr. I. Saccheri is currently working in the molecular laboratory at the Department of Ecology and Systematics in Helsinki and he will supervise this work. We will start these studies with well-defined undergraduate projects.

D. Critical density of aspen at forest stand and landscape levels for the long-term survival of the species

This research consists of parameterizing metapopulation models (Section 4.4) for selected aspen specialist species. We will start by using the incidence function model (Hanski 1994a), which we have previously successfully applied to butterflies living in highly fragmented landscapes (Hanski et al. 1995b, 1996, Wahlberg et al. 1996).