Willows: the keystone plant group in the north
Willows (Salix spp.) are arguably the most important group of plants in northern ecosystems. Their role as a keystone taxon follows from a combination of wide distribution, high abundance, high number of species, and extreme ecological diversity. The smallest willow species inhabit arctic tundra and never exceed a few centimetres in height, while the largest species grow to become canopy trees in boreal forests. Most species are, however, small to medium-sized shrubs that are particularly abundant in wet environments such as bogs and shorelines, but that can be found in practically every habitat across the Holarctic region.
In addition to overall size, willow species differ markedly in their morphology, phenology, and defensive chemistry. Therefore, willows present a ubiquitous but heterogeneous resource base for numerous animals, including plant-feeding mammals, birds, and insects. Especially in the spring, willow flowers constitute a vitally important source of energy and nutrition for early-flying flies, bumblebees, and moths, as well as other pollinating insects. The various herbivores and pollinators, in turn, support a diverse community of vertebrate and invertebrate predators and insect parasitoids. In essence, willows form the basis of food webs spanning several trophic levels, with extreme diversity of species and connections across all levels. How these networks function, and how they have formed through time, constitute major questions and challenges for modern eco-evolutionary research.
Globally, there are over 400 willow species. Unlike most plant and animal taxa, willows exhibit a reversed latitudinal gradient of diversity, so that Salix species richness increases towards the north, with diversity peaking in the northern boreal zone. In Europe, circa 65 species are found. This web page focuses on the circa 25 willow species that are found in the northern parts of Norway, Sweden, and Finland, and in northeastern Russia.
The genus Salix has tradionally been divided into three subgenera (Salix, Vetrix, and Chamaetia). As a general rule, species in sg. Salix are large and tree-like, while the species-rich sg. Vetrix includes small to medium-sized northern shrubs and trees, and s.g Chamaetia encompasses the small, creeping arctic–alpine lineages. However, recent phylogenetic analyses based on genetic data have demonstrated that while the genus Salix itself is monophyletic (i.e., all willow species descend from a single common ancestor), the subgenera do not constitute natural (monophyletic) groups. This is true especially for the dwarf tundra willows, which have evolved several times convergently from subarctic shrubs within sg. Vetrix. The subgeneric classification of species within Salix is therefore still in an uncertain state, and will not be used here.
About this website
This website is an attempt at increasing knowledge on Salix diversity, and the ecological role of willows in northern environments. Species determination of willows is often considered difficult, but the difficulties largely stem from the fact that otherwise excellent printed identification guides and floras include only one or a few drawings or illustrations per species, which is not enough for describing the variation found within species and within individual plants through the growing season. Searching the web for willow photographs can be used to supplement descriptions in printed botanical guides, but unfortunately the internet is littered with images of wrongly identified willows.
The main aim of this website is therefore to smoothen the steep learning curve that early-stage willow enthusiasts currently face by providing guidance to informative traits and, more importantly, numerous photographs of reliably identified willow individudals representing both sexes in different habitats and lighting conditions. Nevertheless, it is still advisable to consult good botanical guides, which include detailed measurements of some floral and leaf structures in the species illustrated on this website.
Overall, our hope is that facilitating willow identification will over time stimulate research on Salix species and Salix-based food webs and pollinator networks especially in subarctic and arctic ecosystems. Deepening our knowledge on willow ecology is crucially important in the changing climate of today – because of their fast growth rates and efficient dispersal, willows respond very quickly to environmental changes. In the arctic, willows are part of environmental change in both good and evil: one one hand willows support a high diversity of associated animals, on the other hand climatically driven willow shrub expansion presents a clear threat to plants and animals living in open tundra environments.
The construction of this website has been funded by The Norwegian Biodiversity Information Centre (Artsdatabanken), via its support to the Insects of the Forest–Tundra Ecotone Project (ForTunE) in operation at the Svanhovd Research Station of the Norwegian Institute of Bioeconomy Research (NIBIO).