Briza media L.
2002; Wiley; Volume: 90; Issue: 4 Linguagem: Inglês
10.1046/j.1365-2745.2002.00684.x
ISSN1365-2745
Autores Tópico(s)Plant Diversity and Evolution
ResumoA loosely tufted perennial with short rhizomes from which arise vegetative shoots. Culms 15–60 (−100) cm high, slender, erect, 2–3-noded, solitary, smooth. Leaves with dull glaucous to mid-green adaxial and abaxial surfaces, hairless; blades usually 4–20 (−28) cm long, 2–4 (−7) mm wide, with minute acroscopic teeth on the margins, with a slender boat-shaped tip; ribs numerous, closely packed, less obvious on the abaxial surface; sheaths smooth, entire, soon splitting, dark reddish-brown at base; ligules up to 1.5 mm long, membranous, blunt with irregular margins. Culm leaves 3 or 4, uppermost with long sheaths. Panicles 4–10 (−18) cm long, lax, roughly pyramidal, with up to 20 branches, and up to 60 spikelets, sparsely divided, spreading, with scabrous hair-like pedicels 5–20 (−50) mm long and slightly thickened below the spikelets. Spikelets 4–7 mm long and wide, loosely scattered and drooping, broadly elliptic to broadly ovate, laterally compressed, generally purplish-brown, usually breaking up at maturity beneath the lemmas but many spikelets remaining undispersed on the branches and shed from the panicle intact, 4–9 (−12)-flowered. Glumes slightly unequal, deeply concave, hooded at the apex, boat-shaped, 2.5–3.5 mm long, with 3–5 veins, firmly membranous, dull-purple with white and shiny margins. Lemmas 3.7–4.1 mm, strongly cordate at the base, bluntly keeled in the lower half, closely overlapping, rounded on the back, with 7–9 veins, variegated purple and green, with hyaline margin. Paleas flat, thin, almost as long as the lemma, with two keels, narrowly winged. Lodicules 2, linear-lanceolate, acute. Grain enclosed within the papery lemma and palea, c. 2 mm long, rounded on the back, flattened on the front; mean oven-dry weight 0.53 mg. The root system is a dense mat of whitish fibrous roots. The primary roots are up to 1 mm in diameter and are largely unbranched, but there is a dense network of branching secondary roots. The root mass is fairly vertical with an average root length of c. 40 cm. The species produces pale brown rhizomes up to 13 cm long and 1.5 mm in diameter. Native. Subspecies media occurs throughout the range; subspecies elatior (Sibth. & Sm.) Rohlena occurs in south-east Europe. Briza media is a widely distributed common species of dry and calcareous, but also moist and acidic grassland, of little herbage value. The distribution of B. media in the British Isles is shown in Fig. 1. The species is widely distributed throughout the British Isles on Chalk, Carboniferous, Magnesian and Devonian Limestone, and is also found on circumneutral soils and on some acid soils where these are well-drained. The species is only occasional in the north of Scotland, the Shetland Isles and Orkney Isles and on some of the Western Isles. It has been recorded from the Isle of Man, the Channel Isles and the Isle of Wight but not from the Isles of Scilly. In Wales the species is common on the Lias of South Dyfed and on the calcareous substrata in the south of Powys but occurs less frequently on calcareous substrata of Gwent, Gwynedd and Clwyd (Hyde & Wade 1957). It is widely distributed across central Ireland, but is less common in both the north and the south of Ireland. The distribution of Briza media in the British Islea. (○) Pre-1950; (•) 1950 onwards. Each dot represents at least one record in a 10-km square of the National Grid. Mapped by Henry Arnold, Biological Records Centre, Centre for Ecology and Hydrology, using Alan Morton's DMAP programme, mainly from records made by members of the Botanical Society of the British Isles. Briza media is recorded throughout western Europe but is absent from the Faeroe Islands and Iceland, and is noted only as a casual in the extreme north of Europe in Flora Europaea. It is absent from the Mediterranean islands of the Balearics, Crete, Sicily and Cyprus, and does not occur in the Azores. In northern Europe, B. media is found in the Baltic, Karelia (southern part), Ladoga-Ilmen, Dvina-Pechora (western and southern parts), Upper and Middle Dniepr, the Upper Volga, Volga-Kama, Volga-Done (northern part), Ural (central and southern), Carpathians and the Caucasus (Tsvelev 1984). Dahl (1998) refers B. media to a southern boreal sub-element, and Preston & Hill (1997) assign it to the European temperate floristic element. Outside Europe, B. media has been recorded from Syria and Turkey, and from Kashmir, Myanmar, Nepal and Tibet. The species is also recorded from South Africa, and from Australia and New Zealand (Vergl. Chor.) as an alien. In North America, the species is introduced and occurs in Connecticut, Massachusetts, Michigan and Vermont in the United States and in British Columbia and Ontario in Canada (Hitchcock 1971). Briza media is found to 65° N in Europe (Sweden), to c. 30° N in Asia and to c. 50° N in North America. In England, B. media has an altitudinal range from sea-level to 640 m (Cumbria), from sea-level to 515 m in Wales (Tal-y-Fan), from sea-level to 655 m in Scotland (Atholl) and from sea-level to 427 m in Ireland (Wicklow) (Alt. Range Br. Pl.). It is found to 1870 m in the Alps (Pfl. Exk.), to 3000 m in the Caucasus, to 1000 m in the Carpathians, to 1450 m in the Pyrenees, to 3500 m in Nepal and to 4500 m in the north-west Himalayas. Briza media is exposed to annual precipitation of around 1700 mm on the Isle of Mull and to conditions with less than 1 mm precipitation during July and August in southern Spain, where average monthly maximum temperatures are around 43 °C (Meteorological Office 1972). It experiences temperatures as low as −35 °C in January in Estonia; however, the effects of these low temperatures are moderated by an insulating blanket of up to 20 cm of snow, which normally persists from November to the beginning of April (Lippmaa 1931). No evidence in Britain of wind or insolation damage has been noted, and the species is found on steep dry slopes in the Jura Alps subject to strong insolation, and to Föhn-swept slopes in Germany (Braun-Blanquet 1939). Briza media is found both on flat ground and on slopes up to about 60°, but is most common on slopes between 20 and 40° (Grime et al. 1988). The species shows no aspect preference in the British Isles nor in western and central Europe. Briza media is a component of semi-natural calcareous pastures, scree slopes, quarry spoil and road verges, and has been recorded from old meadows and enclosed pastures (Grime et al. 1988). It is widespread on brown earths, shallow free-draining rendzinas and grey rendzinas overlying chalk and limestone in England, but is less frequent on these in Wales and northern Scotland. Grubb et al. (1969) recorded B. media from tall chalk heath at Lullington Heath, Sussex, and Ivimey-Cook & Proctor (1966) recorded it from limestone heath in the Burren, Co. Clare. The species has also been recorded from wet heathy roadsides and from water meadows on rich soils (Edees 1972), from limestone swamps (Horwood & Noel 1933), grazed fen flushes (Sinker et al. 1985) and from calcareous mires (Grime et al. 1988). It is also recorded from calcareous peats and from clays with impeded drainage (Sinker et al. 1985), from Keuper marls and Carboniferous shales (Newton 1971), from Coal Measures (Graham 1988), from Liassic clays and marls, from Bunter sandstone, from incipient podsols (Balme 1953), from neutral to acidic grassland and heaths (Grose 1957) and from the very acid sands of the Weald, with pH < 4.0 (C. Stace, personal communication). It is recorded from volcanic rocks and stabilized sand dunes in Wales (Benoit & Richards 1961), from coastal sands in Yorkshire (Crackles 1990), and from dry dune pasture at Braunton Burrows, north Devon (Willis et al. 1959). Briza media occurs on brown forest soils, on high base status humic gleys, peaty gleys and flushed peat in Scotland (Gauld & Robertson 1985) and also from raw soils on sandy river shingle (Birse 1980). Briza media is recorded as occurring in soils of between pH 4.0 and 8.0 but most frequently in soils of around pH 7.0 (Ecol. Atl.). Soil analyses (methods according to Chem. Anal.) for 15 British sites supporting B. media gave a range of exchangeable calcium and magnesium, extracted with M ammonium acetate (pH 9.0), from 1200 to 7500 mg kg−1 and from 30 to 500 mg kg−1, respectively, while potassium ranged from 30 to 540 mg kg−1. Phosphate phosphorus, extracted with 0.5 m sodium bicarbonate (pH 8.5), ranged from 5 to 39 mg kg−1 and total nitrogen from 0.1 to 1.9%. In Continental Europe, in addition to soils overlying calcareous rocks, B. media occurs on calcareous moraines (Sterner 1922), on dry shell deposits (Willems 1982), on slate (Andersson 1970) in southern Sweden, on porphyr and gypsum (Meusel 1940), on dry acid soils (Ellenberg 1988) in Germany, and from biotite and granite gneiss in Switzerland (Marschall 1947). While B. media may be considered as most typically a plant of fairly dry, well-drained calcareous soils, it has a very wide edaphic tolerance, growing successfully in distinctly acidic soils, wet and poorly drained sites and on soils of many different textures. Briza media is predominantly a species of calcareous grasslands and occurs with a high constancy in the following communities defined by the National Vegetation Classification (Rodwell 1991, 1992). CG1 Festuca ovina–Carlina vulgaris grassland is found on freely (often excessively) draining rendzina soils with a high base status, on steep and rocky, but stable, slopes on hard limestones, usually with a southerly to westerly aspect and so with a tendency to summer droughting. Often heavily grazed by sheep and rabbits, this community is confined to Devonian Limestone in Devon and to Carboniferous Limestone in Wales and the Mendips. Briza media has an overall constancy of III in CG1 but attains a constancy of IV in the subcommunities of Helianthemum canum and Trinia glauca and a constancy of V in the Koeleria macrantha subcommunity. Constant species, as defined by the NVC, for CG1 and the following communities are given in Table 1. CG2 Festuca ovina–Avenula pratensis grassland consists of a rich mixture of grasses and dicotyledons in a closed sward and is traditionally grazed by sheep and rabbits. It occurs most frequently in relatively dry and warm, lowland climates on free-draining calcareous soils derived from calcareous bedrock, often prone to summer droughting. Briza media has a constancy of IV in this community. CG3 Bromus erectus grassland achieves maximum extent on lightly grazed or ungrazed grasslands over Chalk in the south-eastern areas of Britain, and over the Oolite of the Cotswolds and Northhamptonshire. Briza media has an overall constancy of III in this community, but a constancy of IV in the subcommunities of Centaurea nigra and Knautia arvensis–Bellis perennis. CG4 Brachypodium pinnatum grassland is also mainly associated with lightly or ungrazed calcareous swards in south-eastern Britain, but is more generally found on the cooler and damper areas of the Chalk and Oolite than is the Bromus erectus grassland, which is more continental in character. Briza media has an overall constancy of III in this community. CG5 Bromus erectus–Brachypodium pinnatum grassland occurs where B. pinnatum is favoured by amelioration of extreme Continental conditions, but not so greatly as to exclude Bromus erectus. The community is most characteristic on the calcareous, base-rich soils of the Oolite on the north-western fringe of lowland limestones of the Cotswolds and Northhamptonshire/Lincolnshire scarps. Briza media has an overall constancy of IV and of V in the Hieracium spp. subcommunity. CG6 Avenula pubescens grasslands are found on a variety of gently sloping lowland limestones, mainly in the south of England. The soils are deep and moist, though mostly free-draining alluvial rendzinas, or calcareous brown earths, and in some cases more mesophytic soils occur on flat limestones. This community is lightly grazed by cattle; on sloping limestones it is grazed by rabbits. Briza media has an overall constancy of only II in this community, but achieves a constancy of III in the Dactylis glomerata–Briza media subcommunity. CG8 Sesleria albicans–Scabiosa columbaria grassland is found only on free-draining calcareous, steep slopes of Magnesian Limestone in Durham, principally on rendzinas rich in calcium and magnesium carbonates. The climate is cool and dry, and the community is a plagioclimax vegetation maintained by grazing of domestic animals and rabbits. Briza media has a constancy of IV in CG8. CG9 Sesleria albicans–Galium sterneri grassland is found in the northern Pennine submontane or montane climate, over shallow, freely draining but moist, calcareous lithomorphic soils on drift-free Carboniferous Limestone exposures. It forms an important part of upland farm hill-pasture and as such is frequently grazed, mainly by sheep. In CG9, Briza media reaches an overall constancy of IV and a constancy of V in the Carex capillaris–Kobresia simpliciuscula subcommunity. Briza media occurs with a low constancy (I) in the calcareous grassland community of Festuca ovina–Hieracium pilosella–Thymus praecox/pulegioides (CG7) and in the Festuca ovina–Agrostis capillaris–Thymus praecox grassland (CG10). Briza media is also a component of the following mesotrophic grasslands. MG3 Anthoxanthum odoratum–Geranium sylvaticum grassland. This is a northern submontane community almost entirely restricted to a few valleys in northern England where traditional hay-meadow management occurs, and comprises a dense community of grasses and herbs. Briza media attains an overall frequency of I but a frequency of III in the B. media subcommunity. The constant species for this and the following grassland community are given in Table 2. MG5 Cynosurus cristatus–Centaurea nigra grassland occurs throughout the British lowlands. Briza media has a constancy of III in the subcommunity of Galium verum, found mostly over calcareous bedrocks, and a constancy of III in the Danthonia decumbens subcommunity, mainly occurring on the upland margins of northern England and Wales. The species is also found at a low constancy in the mesotrophic grassland communities of Arrhenatherum elatius (MG1), Arrhenatherum elatius–Filipendula ulmaria (MG2), Alopecurus pratensis–Sanguisorba officinalis (MG4), Cynosurus cristatus–Caltha palustris (MG8), and Holcus lanatus–Deschampsia cespitosa grassland (MG9), and also in the calcifuge sward of Festuca ovina–Agrostis capillaris–Galium saxatile grassland (U4). Briza media is also found as a minor component of the Salix repens–Campylium stellatum dune-slack community (SD14). Briza media is a component of several mire communities; constant species for these are given in Table 3. M10 Carex dioica–Pinguicula vulgaris mire is typically a soligenous mire kept very wet by base-rich, calcareous and oligotrophic waters. This is predominantly a community of north-western Britain, where the cool, wet climate influences the structure and floristics of the vegetation. Most stands are grazed by large herbivores and trampling by these plays a large part in maintaining an open community. Briza media has a constancy of IV in the Briza media–Primula farinosa subcommunity and an overall constancy of II. M13 Schoenus nigricans–Juncus subnodulosus mire is confined to peat or mineral soils irrigated by base-rich, highly calcareous and oligotrophic waters. This community is often found below springs and seepage lines, or on flushed margins of valley mires, and is restricted to the warmer southern parts of Britain. Some stands have been affected by mowing, burning and peat-digging; grazing sometimes affects the floristics and structure of the vegetation. Briza media has a constancy of V in the Briza media–Pinguicula vulgaris subcommunity and an overall constancy of II. M22 Juncus subnodulosus–Cirsium palustre fen-meadow is predominantly a community of moderately mesotrophic and moist, base-rich peats and mineral soils found in lowland southern Britain. It occurs either around well-developed flushes, springs and mires or delineates the influence of more ill-defined areas of surface waters. The identity of the community is maintained by mowing and/or grazing. Briza media has a constancy of IV in the Briza media–Trifolium spp. subcommunity and an overall constancy of II. M26 Molinia caerulea–Crepis paludosa mire is a very local community of moderately base-rich, moist, calcareous peats and peaty mineral soils in the submontane grasslands of the northern Pennines. It can be found either as a stable component around open waters and mires, but also on flushed slopes in soligenous situations, often subject to grazing. Briza media has a constancy of V in the Festuca rubra subcommunity and an overall constancy of III. Briza media is also a minor component in the Molinia caerulea–Cirsium dissectum fen-meadow (M24) and the Cratoneuron commutatum–Carex nigra spring community (M38). Briza media is recorded from Scotland (Birse 1980) from the Polygono–Helictotrichetum pratensis association which is a woodland replacement community created by grazing on rendzinas and shallow brown, calcareous soils. The characteristic species are Avenula pratensis, Carex caryophyllea, Helianthemum nummularium, Koeleria macrantha, and Polygonum viviparum. Birse (1980) also describes B. media as a minor component of the Anthyllido–Rhacomitrietum canescentis association, a pioneer community of river flood plains. In Ireland, B. media is recorded from the mesotrophic grassland Cynosurus cristatus–Centaurea nigra (MG5), which is the typical grassland of well-drained pastures over limestone, usually grazed by cattle. O’Sullivan (1982) records B. media from the Junco–acutiflori–Molinietum association of wet meadows, together with Cynosurus cristatus, Lotus corniculatus, Phleum pratense, Trifolium pratense and Trifolium repens. Briza media is also recorded by Ní Lamhna (1982) from the Festuco–Galietum maritimi sand dune vegetation of Malahide Island, north of Dublin, which is found on middle dunes of grass-dominated, ungrazed or very lightly grazed, vegetation. Differential species are Anthoxanthum odoratum, Briza media, Dicranum scoparium, Euphrasia occidentalis, Koeleria macrantha, Potentilla sterilis, Pteridium aquilinum, Rosa pimpinellifolia, Thymus praecox and Trifolium repens. Ivimey-Cook & Proctor (1966) recorded B. media from the Galium verum–Asperula cynanchica nodum, a calcareous dune community of the Burren, Co. Clare, together with Agrostis stolonifera, Asperula cynanchica, Carex flacca, Ctenidium molluscum, Euphrasia salisburgensis, Festuca ovina, F. rubra, Galium verum, Lotus corniculatus, Plantago lanceolata, Prunella vulgaris, Thymus praecox and Viola riviniana, and also from the Hyperico–Dryadetum association. This is a shrub-dominated community found on leached organic soil over bare limestone. Characteristic species are Calluna vulgaris, Danthonia decumbens, Dryas octopetala, Empetrum nigrum, Hylocomium splendens, Hypericum pulchrum, Hypnum cupressiforme var. ericetorum, Neckera crispa, Polygala vulgaris and Rosa pimpinellifolia. Ivimey-Cook & Proctor (1966) also recorded B. media from a Burren calcareous spring community, together with Anagallis tenella, Cardamine pratensis, Carex lepidocarpa, C. panicea, Juncus articulatus, Molinia caerulea, Pinguicula vulgaris and Succisa pratensis. Briza media has been observed by the author in a lightly shaded railway cutting, and on roadsides in the shade of pines in Sutherland. It has been recorded from a stream-side in a pine plantation in Spain, in oak woods in the Pindos Mountains of Greece, in sparse oak-wood in southern Sweden, in forest-steppe in central Europe, in Juniper drifts in Estonia and in a deciduous coniferous forest in Nepal. In continental Europe, B. media occurs in a number of associations of the Brometalia erecti throughout western Europe; these occur on dry steep slopes throughout the Alpine range, in dry rocky places in France and Belgium, the deeper moister soils of the Danube and Rhine valleys and also on more humid slopes of the Alps. Briza media also occurs in some associations of the class Arrhenatheretalia. A range of communities containing B. media, with a frequency of III and above, is given in Tables 4 and 5 to illustrate its geographical and ecological range. The latter is, like the edaphic range, very wide, embracing dry and damp habitats, including lowland, montane and alpine sites, so involving a high number of vascular plant species associated with B. media. It is reported that B. media is a poor competitor which fails to persist in derelict grassland (Šikula 1978; Grime et al. 1988), while Watt (1974) described it as showing a reduced frequency in ungrazed chalk grassland enclosed for 34 years. That B. media is not a good competitor is supported by the work of McLellan et al. (1997), who noted in a removal experiment that B. media was more productive with increasing gap size between plants. They concluded that the species appears to be negatively affected by a high density of neighbours and that it is suppressed by diffuse competition at normally observed densities. Sinker et al. (1985) reported that B. media persisted in grassland mown annually, and Baker (1937) observed it was present in hay meadows which were grazed during the period of autumn to early spring and then mown in mid-July However, Mitchley & Willems (1995) noted that in Dutch grasslands, subjected to an autumn mowing regime, this was insufficient to prevent taller grasses, such as Brachypodium pinnatum, from suppressing most other growth, including that of B. media. Briza media appears able to withstand a reasonably heavy grazing regime: it is a component of high constancy in for example the calcareous grassland communities of Festuca ovina–Carlina vulgaris (CG1) and Festuca ovina–Avenula pratensis (CG2) which are reported as being often heavily grazed by sheep and rabbits, and B. media is also described from a woodland plagioclimax community in Central Perthshire, Scotland, which is maintained principally by the grazing of cattle, red deer, rabbits and hares (Gauld & Robertson 1985). Šikula (1978) noted that the species is readily grazed by cattle and Sinker et al. (1985) reported that it is tolerant of heavy grazing; it is also recorded as frequent on sites heavily grazed by rabbits in Hertfordshire (Dony 1967). However, although the species might be tolerant of heavy grazing, a simulated grazing experiment by the author, with clipping at three height regimes every two weeks for 10 weeks, resulted in the greatest production of material in the unclipped control plants. Sinker et al. (1985) reported that B. media is very tolerant of trampling. Plants on the edges of paths produce more horizontal rosette-type growth, but no inflorescences have been observed by the author on these prostrate, trampled plants. Lloyd (1968) noted that fire significantly stimulated the production of inflorescences in B. media in the summer following a spring burn, but by the following year no significant stimulus to flower production remained. Briza media is usually found as scattered plants in ungrazed grassland, with a few groups of tillers joined by short rhizomes. In grazed grassland and on screes it grows as small tussocks. Briza media never produces large tussocks; even after 2 years in cultivation the average tussock diameter was only about 12 cm. Table 6 illustrates that Briza media is more productive in damp to wet habitats in comparison to dry ones, with longer leaves, culms and panicles. Leaf width, however, is not significantly different. The number of spikelets is also significantly greater in damp, ungrazed grassland compared with dry, ungrazed grassland. Although reported as absent from woodland (Grime et al. 1988), the species has been found in lightly shaded sites by the author, where it has flowered and set seed. Leaf and culm measurements and spikelet number in a shaded railway cutting were not significantly different from those of ungrazed grassland plants (Table 6). However, B. media does not flower where the irradiance is less than c. 50% of full sun (see VI (E)). The species sets seed throughout its range. As B. media grows almost as far north as the Arctic Circle and to 4550 m in the Himalayan Range, it is presumably frost tolerant, and no evidence of frost damage has been observed in Britain. Davison (1964), carrying out field experiments, noted that seedlings of B. media suffered no mortality during the winter, and tolerated snow cover from December to March. When B. media is subjected to water deficits the inner leaves of a tiller become inrolled and the outer leaves quickly dry and become ‘ridged’ in appearance and droop. Many of the leaves also become twisted giving a spiralling effect. As the tips of the leaves necrose they often develop a purplish coloration before becoming straw-coloured and dying. The species appears to be principally a drought avoider, as most of its leaves are produced by the end of April before the onset of periods of drought. Briza media does not seem to develop any rooting strategies to combat drought as seen for example in Koeleria macrantha (Dixon 2000). Reader et al. (1992) observed that the rooting depth of B. media in unwatered soil was no different from that in watered soil, and they concluded that its roots are unable to ‘seek out’ water in unwatered soil. As Briza media occurs in damp to wet meadows and is a component of calcareous mire communities (Wheeler 1980), it appears moderately tolerant of waterlogging. In a pot experiment in which plants were subjected to normal watering, one-third, two-thirds and total waterlogging conditions for 1 year, the one-third waterlogged plants had the longest leaves and the greatest shoot and root dry weights: there was no significant difference in shoot weight between those watered normally and those subjected to two-thirds and total waterlogging. Leaf lengths for the normally watered and the two-thirds waterlogged plants were not significantly different, and that for the totally waterlogged plants was only just significantly less than for the one-third waterlogged. However, plants with two-thirds and total waterlogging had significantly lower root dry weights than those of the normally watered plants. Briza media has a dense fibrous rootstock with short thick rhizomes which terminate in leafy shoots. It appears to have little plasticity in its rooting depth and Reader et al. (1992) observed that species which regenerate both from seed and vegetatively, as does B. media, show less plasticity in seedling rooting depth than do species which regenerate from seed only. Mean rooting depth of B. media in well-watered loam in West Yorkshire was c. 40 cm, while Kotańska (1970) reported a mean rooting depth of 30 cm in a xerothermic grassland on a shallow brown rendzina. The leaf sheaths of B. media are brown, fibrous and moderately persistent, as are the leaves, decomposing slowly, and so adding to the litter layer. Solitary growth in Briza media results in small tussocks, which increase mainly by the production of short leafy tillers produced at the tips of the short rhizomes. In longer vegetation B. media grows as straggly groups of just a few tillers. Briza media invests heavily in roots, rather less in inflorescences and much less so in leaves. Twenty young plants (each of 3 tillers) cultivated in good loam in West Yorkshire for a year gave a mean dry weight (± SE) per plant of 12.7 (± 1.4) g inflorescences (culms and panicles), 28.0 (± 3.2) g roots and 8.3 (± 1.3) g shoots (living and dead). Thus the culms comprised 26%, the roots 57% and the tillers 17% of the total dry weight. This is the same pattern as that shown by Avenula pubescens, which invested 57% in roots, 30% in inflorescences and 12% in tillers (Dixon 1991). At the time of harvesting (mid July) the living shoots provided about three-quarters of the total shoot material. Stomatal counts from plants in grazed limestone grassland in North Yorkshire gave a mean of 61.4 (± 4.4) mm−2 for the adaxial leaf surface and 46.5 (± 3.2) mm−2 for the abaxial surface: the counts were made at three positions along the length of 20 leaves. The stomata occur in rows between the veins, with rather more stomata around the middle of the leaf and fewer towards the tip. Briza media is listed as being vesicular-arbuscular mycorrhizal by Harley & Harley (1987) and they reported a high colonization rate from July to September. Read & Haselwandter (1981) noted that B. media from the northern calcareous Alps had a typically 60% colonization (Glomus sp.), while Mejstrik (1972), describing a Molinietum caeruleae association from Bohemia, also reported that B. media was strongly colonized. Grime et al. (1987) observed that B. media produced its highest yield when ungrazed and with mycorrhizal infection, and its lowest yield also when ungrazed but with no mycorrhizal infection, compared with grazed, with and without mycorrhizal infection. They noted, however, that the effects of grazing were not significant; that is the effect of grazing was to nullify the influence of the mycorrhiza. Briza media is a hemicryptophyte. Its leaves die back gradually once flowering has commenced, but plants retain a number of green leaves throughout winter until early spring when new growth starts. Vegetative growth is slow and the species never produces large tussocks, but a clone can extend beyond the extent of the original tussock by rhizome growth of at least 6 cm year−1. It flowers in the first summer after germination and can produce a moderate number of inflorescences; in cultivation in a good loam these ranged from 7 to 70 (mean 29.4 ± 4.4) after one year's growth. McLellan et al. (1997) reported that B. media largely recruits vegetatively and they comment that thi
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