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For plants with bulbil reproduction (vivipary: entire spikelet except the glumes transformed into a single bulbil), see Poa arctica / pratensis viviparous plants.
Mat-forming graminoid herb growing in often extensive stands due to horizontal, branched rhizome, typically with rhizome branches of 2–6 cm between aerial shoots. Aerial shoots ascending from rhizome, at base with several prophylls (reduced leaves without or with a short blade). Culms 5–40 cm (occasionally taller in fertilized sites), erect, smooth or minutely scabrous just below the panicle. Base of aerial shoots with a few withered leaves forming a loose sheath. Leaves and culms glabrous.
Leaves keeled, flat or weakly folded, smooth. Basal leaves (3)5–10(15) cm long, narrow, 0.8–1.2 mm broad, tapering at the apex. Culm leaves usually 2–3 (above the withered basal sheaths), similar to basal leaves and 1.5–3 cm long, slightly broader, up to 2 mm broad. Flag leaf blade attached above the middle of the culm. Ligula 1–2(3) mm, subacute or acute.
INFLORESCENCE AND FLOWER
The units of the inflorescence of Poaceae are the spikelets, nearly always numerous in a panicle or spike-like inflorescence. Spikelets are composed of 2 glumes (bracts for the spikelet) and one or more flowers (the term used below) or rather floral units often named ‘florets’ because we do not know what is the exact flower. A flower or floret is composed of a lemma with 1 mid vein (probably the floral bract), a palea with 2 mid veins (either 2 fused bracteoles or perhaps 2 fused perianth leaves), 3 small organs called ‘lodiculae’ and essential in the opening of the flower at anthesis (possibly transformed perianth leaves or transformed stamens), 3 stamens (mostly), and a gynoecium of 2 fused carpels with 2 feathery stigmas and one seed.
Inflorescence an open, narrowly elongate or more rarely pyramidal panicle (at and after flowering) 4–10(12) cm long, with erect to spreading branches; panicle occupying 1/4–1/5 of culm length. Panicle with 6–12 nodes and with 3–5 branches at each of the lower nodes. Branches 20–30(40) mm long, smooth or very sparsely scabrous, the lower ones with up to 10 spikelets (occasionally even more) along larger parts of the branch length. Spikelets (4)5–6(7) × (1.5)2–2.5(3) mm, usually with 3 flowers. Bracts (glumes and lemmas) with often fairly sharp keels. Glumes 2.5–3 mm, ca. 1/2 as long as spikelet, narrowly or broadly lanceolate, acute, with (1)3 indistinct veins, glabrous, smooth or slightly scabrous on the keel in distal parts, violet or often green at base, without or with a very narrow white hyaline margin. Lemmas (2.5)3–3.5 mm, lanceolate or broadly lanceolate, acute, with 5 more or less distinct veins, with wavy (curly) hairs on the veins and keel but glabrous between the veins, with a large tuft of cottony hairs at the base of the lemma, violet or green at base and with a narrow hyaline margin variegated in bronze yellow and white. Paleas with pubescent veins and shorter than lemmas. Anthers usually shrivelled and not exposed (non-functional).
Fruit an achene (with one seed).
Reproduction by seeds, probably mainly or entirely asexual; efficient local vegetative reproduction by growth by the rhizomes, may form extensive stands that can be fragmented by soil movement (frost action, solifluction, landslides), other disturbance, or simply age. Flowering regularly and probably producing seed.
In some grass genera, Poa among them, there is a fairly good correlation between ploidy levels and modes of reproduction. Diploids (mostly 2n = 14) and tetraploids (mostly 2n = 28) usually have sexual seed-set, whereas higher ploidy levels are often, but not always, asexual with either asexual seed-set (without fertilization, agamospermy) or vegetative propagation by bulbils replacing the flowers in the spikelet (vivipary). Both seed and bulbil reproduction are reported in this subspecies. We are not sure whether bulbil reproduction occurs within the species or races of P. pratensis s. lat. (except for ssp. colpodea) or whether it is a result of hybridization with other species, in this case with P. arctica and perhaps P. alpina (see Comments). We currently suspect the latter and assign the majority of the viviparous plants to a special entry in this flora. The seed reproduction in P. pratensis s. lat. is assumed to be partly sexual, partly asexual (agamospermic), i.e., facultative apomixis. However, no developed stamens have been observed in the herbarium material of P. pratensis ssp. alpigena from Svalbard, and none seen in the field. We therefore assume that the Svalbard strains of this plant are mainly or entirely agamospermic.
The amount of seed production in seminiferous plants may be small in these rhizomatous and potentially very long-lived plants. Remember that if the stands may exist for hundreds or perhaps thousands of years (i.e., the generation length of the genetic individual), the number of efficient seed reproductions during this span of time needed for sustenance of the population is exactly one. Very often the panicles stand with intact spikelets (where no grains have been shed) into the winter and until the next season. Nevertheless, recruitment on fresh ground (e.g., road verges and fresh screes) by seminiferous P. pratensis vouches for some seed reproduction, although ripe seeds were not found in Svalbard (Eurola 1972; Alsos et al. 2013) or NE Greenland (Sørensen 1941).
Fruits (inside florets) are probably dispersed mainly by wind but also possibly by birds.
Poa arctica s. lat. and P. pratensis s. lat. differ from all other Poa species in Svalbard in their extensive, extravaginal rhizomes, evident even in apparently tussocky individuals when dug up. The rhizomes may be short or long but always run some distance horizontally before bending up and producing aerial shoots. These two species or species complexes also differ from the other Svalbard species of Poa (except for P. alpina) in usually larger and more open panicles with longer branches.
The differences between P. arctica s. lat. and P. pratensis s. lat. are more subtle (see Haugen 2000). The key difference lies in the lemmas: in P. arctica the lemmas have veins with more or less straight hairs and there are short hairs or spinules between the veins; in P. pratensis the lemmas have veins with crisped hairs and the lemmas are glabrous and smooth between the veins. Another feature is that the hyaline margin of the lemmas in P. arctica is broad, ca. 1/2 the width of the lemma in the upper part and narrows to less than half the width towards base, in P. pratensis it is more narrow, less that 1/2 the width of the lemma the upper part but narrows less towards the base. The panicle branches are smooth in P. arctica but at least slightly scabrous in P. pratensis. Less reliable but more easily observable differences are found in the shape of the panicle (triangular in outline with spreading to even retrorse branches in P. arctica, more elongated ovate in outline with more ascending branches in P. pratensis), number of branches at the lower nodes in the panicle, 1–2(3) in P. arctica and usually more than 3 in P. pratensis, and the number and position of spikelets on branches (one of very few and only distally in P. arctica, several and along the branches in P. pratensis).
The differences between the five subspecies of P. pratensis we accept to be present in Svalbard may be artificial. Formally, we recognize the short-grown, very slender, strongly red-pigmented rhizomatous plants with a narrow panicle and reproducing by bulbils as ssp. colpodea. There are other viviparous plants approaching ssp. alpigena in many characters, and they are the more common ones, but they differ from ssp. colpodea in the following characters reported by Haugen (2000): In ssp. colpodea culm length ca. 9–15 cm, panicle length 2.4–3.2 cm, panicle branches 1.3–7.0 mm, erect and sometimes sparsely scabrous, lower glume usually 3-veined, nearly as long as upper glume 2.6–3.5 mm, upper glume keel often scabrous, lemmas 0.7–0.9 mm wide, never hairy between veins, tuft (tomentum) at base of lemma moderately developed, and anthers usually missing; in the other subspecies: culm length 12–20 cm, panicle length 3.3–5.2 cm, panicle branches 6.1–14.1 mm, ascending or more or less spreading and smooth, lower glume often 1-veined, shorter than upper glume 3.3–3.8 mm, upper glume keel smooth, lemmas 0.9–1.0 mm wide, sometimes with some hairs between veins, tuft (tomentum) at base of lemma well-developed, and anthers often present but shrivelled. The viviparous plants, except for ssp. colpodea are treated by us under P. arctica/pratensis viviparous plants.
The seed-reproducing plants in Svalbard divide on four rather different groups which we here assign to four subspecies: the native ssp. alpigena and the introduced ssp. angustifolia, ssp. irrigata and ssp. pratensis. Plants assigned to ssp. pratensis are characterized by broad and long leaves (longer than in the other subspecies), panicles with very numerous spikelets (often more than 50), spikelets comparatively small and not strongly violet pigmented, glumes with very strongly marked veins, almost like narrow ribs (veins on glumes often indistinct in the three other subspecies), and veins of lemmas similarly raised but with comparatively little hairs (slightly raised and much more pubescent in the others).
Plants assigned to ssp. angustifolia are similar to ssp. pratensis in most of these aspects but have conspicuously narrow (filiform) basal leaves (even if culm leaves are much broader), more narrow panicles with much shorter branches, and less distinct and more pubescent lemma veins. The plants assigned to ssp. pratensis and ssp. angustifolia also seem to have shorter runners and more tussocky growth than the two others.
Plants assigned to ssp. alpigena have an ovate or oblong panicle with branches not spreading very much, leaves narrow (rarely more than 2 mm) but fairly long, and usually 3–5 branches at the lower nodes in the panicle.
Plants assigned to ssp. subcaerulea have a short, broadly pyramidal panicle with strongly spreading branches, leaves broader (1.5–3 mm) and much shorter than in ssp. alpigena, usually 1–2 branches at the lower nodes in the panicle, and have bluish green leaves and culms (bloom, i.e. wax) and with bluish violet spikelets, colours absent in all the three other subspecies.
Poa pratensis ssp. alpigena is common in closed herb mats and meadow vegetation throughout Svalbard, often forming dense grass swards in meadows in bird cliffs and on southfacing slopes, but is also present as more open stands in flat or gently sloping heaths. This plant is largely indifferent as to soil moisture, ranging from the tussock level in mires and moist sediment plains to dry ridges and screes, but with its main occurrences in moist to moderately dry mossy sites. It is a strong competitor. It is largely indifferent as to soil reaction (pH), present on both acidic and basic substrates.
Poa pratensis ssp. alpigena is found in all zones and sections and in a broad range of site types, probably throughout the islands. It is recorded from the majority of the investigated parts of Svalbard including the outlying islands of Bjørnøya (where it seems to be the only representative of the rhizomatous group as all reports of P. arctica have been refuted), Kong Karls Land and Hopen.
Outside Svalbard, P. pratensis ssp. alpigena is one of the most widespread of arctic plants (competing in range with Festuca rubra ssp. richardsonii), present in almost every main region and also with extensive ranges in northern temperate mountains south to Scandinavia, C Asia, and the southern Rockies.
The European material of Poa pratensis s. lat. is traditionally divided on several major subspecies (e.g., Hylander 1953; Elven in Lid & Lid 2005) or species (e.g., Edmondson 1980; Hämet-Ahti 1988). It is a vast complex with 3–4 modes of reproduction – vegetative by rhizomatous growth and fragmentation, sexual by seed, asexual by seed, and by bulbils – and at numerous ploidy levels from tetraploid (4×) to very high polyploid (20–21×). The species or subspecies accepted within the complex are to a large degree artificial groups of possibly little related biotypes but kept together by single or few characters, perhaps not taxonomically very significant ones. That means that they may be 'grades' rather than 'clades'.
Complexes with asexual reproduction, whether by seeds or by bulbils, often do not fit the ordinary taxonomic and nomenclatorial categories that depend on reproductively conditioned species limits (the Biological Species Concept etc.). The Poa pratensis and the P. arctica complexes are excellent examples of such cases where our traditional taxonomic methods break down and where we at present have no functional alternatives. We try to circumscribe species or races but parts of the variation resist fitting within any framework depending on a model with distinct entities. As someone said about another group, this is "a wide-ranging and obviously polymorphic complex that has not read the literature" (Brooks & Clemants 2000 about the Juncus arcticus group in Flora of North America). Application of the category subspecies to the subsets in such complexes is perhaps not very appropriate but we still here follow the subspecies approach, also because it makes it possible to fit deviant plants and populations at least into the category species, as P. pratensis s. lat.
Five taxa are commonly recognized as native in N Europe, all of them probably relevant for Svalbard: ssp. alpigena (P. alpigena, native), ssp. angustifolia (P. angustifolia, introduced in Pyramiden), ssp. colpodea (native), ssp. irrigata (P. humilis, P. irrigata, P. subcaerulea, introduced in Longyearbyen on Spitsbergen), and ssp. pratensis (P. pratensis, introduced in several places in Svalbard).
As to the problems connected to the majority of the viviparous (bulbil-reproducing) plants, see special entry for P. arctica/pratensis viviparous plants. The bulbil-reproducing P. pratensis ssp. colpodea is given a separate entry.
Brooks, R.E. & Clemants, S.E. 2000. Juncus Linnaeus. – In: Flora of North America Editorial Committee (eds.), Flora of North America north of Mexico. 22. Magnoliophyta: Alismatidae, Arecidae, Commelinidae (in part), and Zingiberidae: 211–255.
Edmondson, J.R. 1980. Poa L. – In: Tutin, T.G. et al. (eds.), Flora Europaea. 5. Alismataceae to Orchidaceae (Monocotyledones): 159–167.
Hämet-Ahti, L., Suominen, J., Ulvinen, T. & Uotila, P. (eds.) 1998. Retkeilykasvio. Ed. 4. – Luonnontieteellinen keskusmuseo, Kasvimuseo, Helsinki.
Haugen, J. 2000. Possible hybrid origins of Poa hartzii and Poa arctica ssp. caespitans (Poaceae) investigated by morphology and isoenzymes. – Cand. scient. Thesis, Univ. Oslo, Oslo.
Hylander, N. 1953. Nordisk Kärlväxtflora, I. – Almqvist & Wiksell, Stockholm.
Lid, J. & Lid, D.T. 2005. Norsk Flora. Ed. 7 by R. Elven. – Det Norske Samlaget, Oslo.