Insects and Plants in the Pollination Ecology of the Boreal Zone

Ecological Research (1993) 8, 247-267 REVIEW Insects and plants in the pollination ecology of the boreal zone P. G. KEVAN,1 E. A. TIKHMENEV2 AND M. Usux1 1 Department of Env, ronmental Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada and 2 Institute for Biological Problems of the North, Academy of Sciences, K. Marx Pr. 24, Magadan, 685000 Russia Pollination systems in the boreal zone range from generalist to specialist, both entomologically and botanically. The relative importance of wind pollination, insect pollination, sexual separation between and within plants, and between flowers, hermaphroditism of flowers, and various breeding systems are related to plant growth form and habitat. The diversity and specializations of anthophilous insects parallel those in other bioge0graphic zones, but seem less developed. We suggest that this reflects the combined effects of evolutionary youth, severity of climate, restriction of symbiont ranges within those of their hosts, and the naturally frequent perturbations by fire or insect outbreaks in the zone, requiring faunal and floral vagility and constraining specialization in mutualism. Modern perturbations by logging and pesticides seem to be well buffered because of the relative openness of the ecosystem (compared to others), although damage has been documented. Insect pollination is as much a keystone process in the boreal forest as elsewhere, despite the immediate counter-impression given by the dominance ofwind-pollinated conifers. Nevertheless, there are few studies, botanical or entomological in situ. The boreal system offers important opportunities in general and applied research in pollination ecology and synecology generally. Key words: anthecology; boreal; floral biology; pollinators; taiga. INTRODUCTION Baldwin (1991). Bonan and Shugart (1989) and Zoladeski and Maycock (1990) explain the dynam- The boreal zone is a circumpolar vegetational belt ics of the boreal ecosystem as regulated by soil (also called the taiga) that stretches from northwest- conditions, including permafrost, weather factors, ern Europe eastward through Siberia, the Russian fire and outbreaks of defoliating insects. Far East, and across North America from Pacific to Although the flora of the taiga is well known, its Atlantic Ocean shores. In places the taiga spans insect fauna is not. There is no compendium of more than 10 ~ of latitude. Although this huge area boreal entomology, such as exists for the arctic is not homogeneous and contains various subdivi- (Danks 1981). It is difficult to typify boreal insects, sions and ecotones, as well as enclaves of treeless except by those that are monophagous on plants communities, its overall physiognomy is quite uni- typical of taiga. Danks (1979, 1981) and Danks form. The dominant plants are species of spruce and Foottit (1989) draw attention to a few examples (Picea), larch (Larix), fir (Abies), pine (Pinus), of insects that show geographical ranges which poplar (Populus) and birch (Betula), which together parallel the limits of the boreal forest in North with shrubs, herbs and bryophytes, comprise the America (e.g. Vespula albida Sladen, Xylotrechus characteristic plant communities of the taiga as undulatus Say). A bee that seems to parallel these described by Moir (1958), La Roi (1967), La Roi limits from Hokkaido Island to Sweden is Lasio- and Stringer (1976), Larsen (1980), Chertov glossum (Evylaeus) boreale Svensson (Svensson ( 1981), Elliott-Fisk (1988), Zoladeski (1989) and et al. 1977). More recently, Danks and Foottit (1989) have attempted to rationalize some aspects Accepted 1 June 1993. of boreal entomology. Nevertheless, too little is 248 P.G. Kevan et al. known to allow much synthesis of the proportions of prevent erosion in otherwise denuded landscapes. the fauna fitting various ecological roles, as has been Further, they provide cover for seedlings of sun- done for the arctic (Danks 1987). It is one of these intolerant trees (e.g. balsam fir, Abies balsamea [L.] roles, pollination, which we discuss here. Mill., and white spruce, Picea glauca [Moench] A. The coniferous (Di-Giovanni & Kevan 1990) and Voss). deciduous trees that dominate the taiga are anemo- The plant communities of early successional stages philous (wind pollinated; Regal 1982; Whitehead (in the first decade) are generally shortlived (Hein- 1983). However to appreciate the importance of selman 1981) but comprise the most active biotic pollination by insects to the boreal ecosystem, one communities of both flora and fauna. One facet of must examine the dynamics of succession leading to the reproductive biolgy of the plants that is vital to the establishment of the forest. the ecological processes of succession and trophic Most stands of boreal trees are young (< 100 relationships is pollination. If the plants are not years old). Spruce and pine stands of more than 250 pollinated then there are no fruits for wildlife years are rare and occur in areas protected from fire consumption nor seeds for dispersal. (Rowe & Scotter 1973). It is probable that before The aims of this review are fourfold. We examine, the coming of Europeans, lightning (and perhaps from a botanical perspective, the pollination and humans) set fires that periodically burned large breeding systems of the flora of the boreal zone and tracts of forest in North America and the Russian attempt to make some generalizations and explana- Far East. Dry conditions, accompanied by a high tions about the differences between trees~ shrubs and incidence of forest fires, occur at about 30 year herbs in different habitats within the boreal zone. intervals, as borne out by historical and palaeonto- We then review the boreal fauna ofanthophiles with logical records (Rowe & Scotter 1983). More re- a view to indicating the importance of their flower cently, logging by clear-cutting and prescribed relations for both plants and themselves. We also burning (McRae 1979) have increased the amount address the issue of pesticide use in boreal forests of temporarily deforested ground over large areas of as it affects both pollinators and pollinated plants. the boreal zone. Finally, we draw some overall conclusions about the The taiga trees are well adapted to rapid re- dynamic interactions of insects and flowers in the invasion of deforested areas because of their wind ecology, biogeography, management and conserva- dispersed seeds. Other plants have seed banks in the tion of the boreal forest. soil. The understorey plants of the mature boreal forest are mostly inconspicuous, even though they sometimes have showy flowers or fruits. However POLLINATION AND BREEDING the underground stems and roots of bear-berry SYSTEMS (Arctostaphylos uva-ursi (L.) Spreng.), cranberries and blueberries (Vaccinium spp.), labrador tea (Le- The boreal forest is home to many conspicuous dum spp.), dwarf birches (Betula spp.), alders insects which are known anthophiles and contains (Alnus spp.), willows (Salix spp.) and other fleshy- many plants with flowers that are typical of ento- fruited shrubs (e.g. Ribes, Rosaceae, Caprifoliaceae, mophily. We shall explore the intricacies of the Cornaceae) often are not damaged by fire. They pollination relationships of boreal plants and insects regenerate rapidly after fire to produce a lush and to place their ecological importance into perspective. fruitful vegetation (see Johnson 1975; Flinn & In general, the Gymnospermae comprise only Wein 1977). Rowe (1983) provides a synthesis of about 2% of the vascular plant flora of the taiga. the strategies plants use to recolonize burnt areas. From an examination of the literature we conclude Herbaceous vegetation may be dispersed into defor- that pollination information is available on only ested areas by wind (e.g. Epilobium spp., Gramin- 10% of the North American Angiospermae and eae, Cyperaceae, Asteraceae, ) or by vertebrates (e.g. that studies in situ make up only half of that Aralia spp., Liliaceae). The soil seed bank is also an proportion. important source of regeneration (Johnson 1975). There is little information on pollination biology The shrubs and herbs provide much of the food in the boreal zone. Silen (1905, 1906) recorded and cover for wildlife. They help retain moisture and many visitors to boreal flowers in Finland from his Pollination ecology of the boreal zone 249 observations between 1895 and 1905 but did not canadensis [Theobald] and moths (e.g. Xanthorhoe comment on the plants' breeding systems. In the munitata [Hbn.]; Thien 1969; Thien & Utech USSR, Vereshchagina (1968), Ponomarev and 1970). Platanthera obtusata (Banks ex Pursh) Lind- Vereshchagina (1973) and Kaigorodova (1976) ley is also pollinated by mosquitoes and moths appear to be the only scientists to have made (Gorham 1976; Voss & Riefner 1983). Listera systematic anthecological studies in the taiga, specif- cordata (L.) R. Br. is pollinated by fungus gnats ically in the Ural area. Vereshchagina (1968) and (Sdatidae and Mycetophilidae) in the western Ponomarev and Vereshchagina (1973) suggested coastal redwood forests of North America (Mesler et that most of the species typical of the taiga that they al. 1980). Coeloglossum viride (L.) Hartman is investigated are autogamous (some being cleistoga- visited by beetles and small wasps which pick up mously so). Thus, it might appear that entomophily pollinia on their heads (Silen 1905, 1906; Hagerup and obligate entomophily are not typical for the 1952; Catling 1983). boreal forest. Data from Helenurm and Barrett (1987), Barrett and

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