Notes on Moehringia trinervia

Dimorphism in Moehringia trinervia (L.) Clairv. (Caryophyllaceae): two kinds of plants occur, partly quite separately but sometimes (like here) in adjacent pure stands. Single plants of one morph may also occur within the stand of the other. The yellowish-green  plant on the right is fertile and has almost sessile leaves; the individual plants produce flowers and seeds almost through the growing season. The blue-green plants have long-petiolate leaves and are sterile (Finland, Lohja, Vanhakylä, 2000). – Photo: Harri Harmaja.
    I have observed the life cycle and population development of M. trinervia during several growing seasons. The dark green plants that appear as if representing another species are in fact the first year's shoots of this predominantly biennial species. They have usually got rise in the spring when most seeds germinate, grow vigorously during the following season, developing erect shoots with distinct nodes. After winter these sterile shoots are in good condition but fall somewhat down along the ground to form the decumbent and in time the more and more inconspicuous basal parts of the second year plants. In the spring, the morphologically differing second year growth, to consist the fertile portion of the shoot, rapidly emerges from the terminal and axillary buds of this overwintered part. The first year's growth is at first well discernible during the second year (in some floras, this is recognized by describing that "the lower leaves are petiolate"), but in the course of summer the old leaves wither gradually, leaving their dead petioles attached as last remains.
    M. trinervia seems to be mainly a long-lived biennial in Finland. A minor proportion of the seeds germinate in late autumn: on a few occasions, I have observed very recently born seedlings then, and also small plants in early spring which no doubt only got rise at the end of the preceding season. It remains to be clarified whether these plants get fertile in their second season (the plants would then be short-lived biennial) or if their maturity is retarded till the third season (the plants would then be triennial). This striking shoot dimorphism of M. trinervia has mostly been ignored in the descriptions of the floras; however, the recent "Flora Nordica" is an exception.
    M. trinervia possesses a considerable soil seed bank, comprising seeds obviously ready to germinate almost instantly after having experienced some stimulus. Climatic circumstances that are specifically favourable for the species naturally provoke germinating, and obviously also e.g. when the soil surface has been disturbed in the winter due to constructing forest tracks, lumbering, or house-building, an enormous swarm of seedlings may emerge in the spot in the following spring. The abundance of plants in their known habitats varies quite much in different seasons. For instance, in the growing seasons of 2002 and 2003, I observed the first year's growth to be exceptionally plentiful in southern Finland. In 2007, most species of rock outcrops were enormously numerous in southern Finland, as were the flowering plants of M. trinervia on the basal portions of rocks.
    M. trinervia may also display some genetically determined variability. E.g., robust first year's plants with 5-nerved leaves and fairly narrowly leaved flowering plants have been met with, but they may represent phenotypic variability as well.