Although Minnesotans pride themselves on their Scandinavian heritage, the biological connections between Minnesota and Scandinavia precede European immigration of humans by many centuries. One of the intriguing features of the boreal forest is that its occupants are circumpolar. Thus, moose, wolves, and lynx are found not only in Minnesota, Canada and Alaska, but also in Scandinavia and Siberia. Curiously, while the tree species are different on each continent, many of the major wildflower species are found both here and in northern Scandinavia. I had the pleasure for many summers doing research on moose and reindeer in Sweden, teaching a course, and otherwise poking around in the wonderful forests. Often when I looked down, I could instantly pick out many species that can be found throughout northern Minnesota.
Twinflower (Linnaea borealis)
One of my favorite flowers both here and in northern Sweden is the twinflower, or in its Latin scientific name, Linnaea borealis. I was delighted to learn from my Swedish colleagues that this was also the favorite of Carl Linnaeus, the father of modern botanical research and the inventor of the universally used system of classification with Latin names indicating affinities between different species. Linnaeus named the genus Linnaea after himself (modesty was not his strongest virtue), and emphasized its uniqueness by placing only a single species in it. Although Linnaeus himself did not believe in evolution, his system of classification forced scientists to come to grips with the question of why we should be able to group species together in hierarchical categories. This eventually led Darwin to propose his theory of evolution from common ancestors by natural selection some 200 years after Linnaeus lived.
The twinflower is aptly named for the exquisite twin, fragrant, rose-pink bellflowers that punctuate the ends of graceful flowering shoots. In large masses, these bellflowers form a pink foam floating above the moss carpet of the floor of coniferous forests. The American Heritage Dictionary defines exquisite as ‟of such beauty or delicacy as to arouse delight”, and the person who would not be so aroused by twinflower is sadly devoid of poetry in his or her life. When the pink foam laps up against a grey boulder or weathered grey-brown stump, the effect is breathtaking.
Spencer Barrett and colleagues at the University of Toronto have made extensive studies of the flowering biology of twinflower and its companion plants in boreal forests. The average lifetime of a flower is less than a week, usually in late June or early July. This is a shorter flowering time than most of the other plants with which twinflower shares space, such as Clintonia borealis (blue-bead lily), Chimaphila umbellata (Pipsissewa), Trientalis borealis (starflower), and Maianthemum canadense (wild lily-of-the-valley). Each flower also produces far less pollen than the flowers of these other companions (approximately 3500 pollen grains per flower compared with ten times that for the flowers of Chimaphila). The major pollinators of twinflower noticed by Barrett and colleagues include solitary bees and syrphid flies. The seeds have tiny, sticky hairs, which may enable them to cling to the coats of small mammals passing by, such as voles and hares, thereby bumming a free ride to new places. In fact, twinflower seedlings are often associated with vole runways in Swedish forests. The seeds are tiny, weighing in at much less than 0.1 mg apiece. Small seeds have very limited carbohydrate reserves, giving them a very low survival rate unless suitable seedbed conditions are found soon after dispersal. The seeds of twinflower are no exception in that virtually none survive winter.
Population dynamics of twinflower
If the flowers are so unsuccessful at producing viable seeds, then how does twinflower survive, to say nothing of dominating large patches of the forest floor? The answer, discovered by Swedish botanist Ove Eriksson, is through a complex life history of branching and extension of the leafy shoots. Twinflower is actually a creeping shrub with runners called main shoots trailing and branching off into lateral shoots across the moss carpet on the floor of coniferous forests. Small, twin round leaves emerge from opposite sides of the shoot. The leaves are attached to the shoots by very short, almost nonexistent, petioles, which are the little stems at the base of leaves. The leaves remain green over winter and generally last about two years. The spot where the leaf attaches to the shoot is called an axil in the trade. Each axil often contains a bud from which lateral or flowering shoots emerge in the second year of the life of the leaf.
Erikkson unraveled the population dynamics of these different shoots by simply marking them with colored tags and periodically making detailed maps of each shoot and associated leaves at least three times each summer for several years. He found that about 60% of the axillary buds produce no shoots, while 30% produce short lateral shoots, another 8% produce flowering shoots, and only 2% produce the long main shoots. Clusters of roots form at the terminal node of each year’s growth of each type of non-flowering shoot, anchoring them to the soil and enabling them to obtain water and nutrients. The main shoots have a 60% chance of survival each year, resulting in 1% of main shoots surviving for 10 years.
The population of relatively fast growing main shoots is replenished by transforming about 5% of the short, slow growing lateral shoots into main shoots by a biochemical process that remains at present unknown. This happens at just the rate needed to maintain the population, or even allow it to expand slowly. Each main shoot can produce an average of 6 but as many as 10 leaf pairs each year. If one-third of these produce axillary buds that become lateral shoots, then each main shoot will produce at least 2 lateral shoots each year and as many as 20 lateral shoots over its ten-year maximum lifetime. If 5% of these lateral shoots turn into main shoots during the main shoot’s ten-year lifetime, then the twinflower will have produced 1 main shoot to replace each older main shoot when the latter dies.
The population will just break even with these rates of production of lateral shoots and their conversion into main shoots to balance the death of older main shoots. If all goes well over a ten-year period there may even be two or even three new main shoots produced from lateral shoots to replace the older main shoot, allowing the population to grow. Such a delicate balancing act between different portions of the populations of shoots is as mathematically exquisite as the twin bellflowers are visually exquisite.
A population of twinflowers therefore begins from one shoot, spreading out across the forest floor somewhat like an amoeba, extending here, dying there, engulfing space here, retreating there. This habit of sending many flowering shoots skyward and lateral shoots across the ground is known as clonal growth, and is a common feature of other plants of the boreal forest, most notably aspen and the clubmosses. Consequently, most, if not all, of the flowering shoots in a patch of twinflower are from the same plant and are genetically identical. Since the timing of flowering is under genetic control, the phenology of flowering is therefore synchronized. Given the short growing season of boreal forests and the small likelihood of a seed landing in a suitable open spot in the crowded and moss covered forest floor, it is the clonal growth habit rather than the showy flowers that enables a population of twinflower to expand and survive. Because the growth rate of main shoots is less than 10 cm per year, the large patches of twinflower clones I saw in several forests in Sweden may have been over 100 years or even several centuries old. Old growth does not only apply to trees; these twinflower patches are themselves old growth.
Such a delicate balancing act between the growth, transformation, and death of different shoots also means that the population could just as easily decline under unfavorable conditions as remain stable or grow under favorable ones. Decline is more likely with small populations than large ones. For example, given a 1% chance of survival to 10 years, a minimum population of 100 main shoots is needed to ensure that at least 1 survives to produce at least one lateral shoot to become a replacement main shoot. Eriksson estimates that the minimum sustainable population consists of at least 250 main shoots.
Conservation of twinflower
Twinflower does not do well when the forest is disturbed by fires or timber harvesting. Eriksson found that too much light (such as when the overstory is removed) is detrimental to the plants, perhaps by causing drought stress and shutting down photosynthesis. Although large patches of twinflower are not common, the species is by no means endangered. Nonetheless, one wonders what timber harvesting here and in Sweden will do to population of these plants. It may not cause the extinction of twinflower, but Eriksson’s painstaking work suggests that it certainly would not help the population either.
If we wish to preserve the beauty of our forest wildflowers, we will have to find some other way of sustaining our timber economy besides clearcutting the forests every forty or fifty years. Otherwise, species like twinflower will be found only in wilderness preserves. While this will probably be enough to save the species, our lives will be impoverished by the removal of such a charming plant from most of the rest of our landscape.
For Further Reading
Barrett, S. and K. Helenurm. 1987. The reproductive biology of boreal forest herbs. I. Breeding systems and pollination. Canadian Journal of Botany 65: 2036-2046.
Eriksson, O. 1988. Variation in growth rate in shoot populations of the clonal dwarf shrub Linnaea borealis. Holarctic Ecology 11: 259-266.
Eriksson, O. 1992. Population structure and dynamics of the clonal dwarf shrub Linnaea borealis. Journal of Vegetation Science 3: 61-68.