Panarctic Flora

342614 Poa trivialis subsp. trivialis L.

2n= 14 15 (2x). - Europe, Russia, Far East, Canada (W). - Numerous reports.
Not included: Reports of 2n = 28 (4x) from southwestern Europe (Guinochet 1943), Bulgaria (Kozuharov and Kuzmanov 1970), and Japan (Tateoka 1954). The reports of tetraploids need critical evaluation before acceptance.

Geography: European - Asian (W): ICE NOR RUS* RFE** GRL*.

Notes: The Stenopoa group is another large, partly agamospermous aggregate of Poa that reaches the Arctic with several taxa. It differs from the P. arctica and the P. pratensis aggregates by very little bulbil reproduction (if any) and by abundant indications of part fertility and ongoing hybridization. Section Tichopoa with P. compressa (see Casual species) should be merged with sect. Stenopoa. Poa compressa shares most of its characters with Stenopoa, especially in spikelets, and there is frequent hybridization with all the major species of Stenopoa. Like the other aggregates, the differences between current Russian and current North American and northwestern European treatments are not resolved. A consensus proposal has not been attained for the taxa that reach the Arctic.

The four geographically widespread northern species - P. glauca, P. nemoralis, P. palustris, and the adventive P. compressa - are assumed to be partially agamospermous and predominantly polyploids. However, they must have a significant degree of sexuality as they (from morphological evidence) are assumed to hybridize freely. The putative hybrids are often seed-producing and often occupy site types intermediate between those typical of the species. In northern Europe, intermediates between those species that meet frequently are widespread and often locally common, and all possible two-species combinations are known. The frequency and fertility of the putative hybrids and the morphological features suggest that combinations involving more than two species may be present. In some cases, the putative hybrids occur as a morphological continuum between the parents: between P. glauca and P. nemoralis in mountain and subarctic valleys and in low-alpine and low-arctic thickets, between P. nemoralis and P. palustris in flood valleys and northern lowlands, between P. compressa and P. nemoralis on shallow, dry soil in forest margins, and between P. compressa and P. palustris in dry, gravelly and often ruderal sites. Transitions are even found between the thermophilous P. compressa and the usually alpine P. glauca, in interior dry valleys. In northwestern Europe and North America, where the same four species are accepted (e.g., Edmondson 1980; Elven 1994; Soreng et al. 2003; Mossberg and Stenberg 2003; Elven et al. 2005; Soreng 2007), these transitions are considered hybrid swarms and rarely assigned rank and names. In a study of the P. nemoralis-P. glauca transition, Pálsson (1986) demonstrated a close connection between the occurrence of morphological traits of P. nemoralis and P. glauca in the transitional populations, low to high ploidy levels, and occurrence at southern to northern latitudes.

Similar extensive hybridization has not been reported from Russia in the sources we have consulted. What is reported is rather a high number of species (Tzvelev 1964b, 1976, PAF proposal; Probatova 1985), mostly without comments on sexuality or agamospermy and possible taxonomical implications. Our suspicion is that these different approaches address the same biological situation. It makes it difficult to align the Russian taxa with those outside Russia. In an arctic context, this concerns the plants named as P. filiculmis, P. magadanensis, P. pekulnejensis, P. tanfiljewii, and P. urssulensis, all with some hybrid hypothesis suggested by Tzvelev at one time or another, but also the following species which may be more readily acceptable according to North American and European viewpoints: P. arctostepporum, P. botryoides, and P. ochotensis. One or more of these are probably present in Beringian America, without being fully recognized there yet, as the currently accepted variation within P. glauca s. lat. is especially large in that region.

We cannot apply one set of criteria for inclusion of the Russian plants in the Checklist and another for the North American and northwestern European ones. As long as an effective comparison and evaluation has not been undertaken, the disputed plants on the Russian side are entered reluctantly and provisionally.