| | 1. Rhizomes or stolons well developed, clearly present; perennial herb |
| |
| |
| | 3. Glume back finely scabrous throughout; spikelet falling as 1 unit; inflorescence axes stiff |
| 2 A. semiverticillata |
| | 3' Glume back +/– glabrous, finely scabrous only on keel; floret axis breaking above persistent glumes; inflorescence axes thread-like, flexible |
| |
| | 4. Ligule 0.5–2 mm, generally wider than long; stolons < 5 cm; inflorescence widely ovate in outline, 1° branches spreading, spikelets not crowded |
| 3 A. capillaris |
| | 4' Ligule 2–5 mm, longer than wide; stolons 5–100 cm; inflorescence elliptic to lanceolate in outline, 1° branches generally all ascending, spikelets overlapping, crowded |
| 2 A. stolonifera |
| |
| | 5. Floret callus hairs 1.5–2 mm, generally > 1/2 lemma; ligule 4–7 mm |
| 3 A. hallii |
| | 5' Floret callus hairs generally minute, sparse, or 0; ligule generally < 3 mm (except A. gigantea) |
| |
| | 6. Palea 0 or minute and << lemma |
| |
| | 7. Leaf blade 1–6 mm wide; lemma tip minutely toothed; rhizome < 10 cm; anthers generally > 1 mm |
| 3 A. pallens |
| | 7' Leaf blade < 1 mm wide; lemma tip +/– acute; rhizome < 5 cm; anthers < 1 mm |
| 3 A. variabilis |
| | 6' Palea well developed, +/– 1/2 to slightly < lemma |
| |
| | 8. Inflorescence +/– oblong in outline, somewhat dense, branches generally all ascending to erect; montane to alpine, generally > 1500 m |
| 2 A. thurberiana |
| | 8' Inflorescence generally ovate in outline, open, most branches spreading; open, generally disturbed places, generally < 1000 m |
| |
| | 9. Rhizomes < 5 cm, slender, not clearly scaly; ligule wider than long; inflorescence branches with spikelets on distal half |
| 3 A. capillaris |
| | 9' Rhizomes < 25 cm, +/– thick, scaly; ligule longer than wide; inflorescence branches with spikelets +/– throughout |
| 2 A. gigantea |
| | 1' Rhizomes or stolons 0; annual or perennial herb |
| |
| | 10. Lemma awned from back or near tip |
| |
| | 11. Inflorescence open, generally oblong to ovate in outline; spikelets not crowded, inflorescence axes clearly visible |
| |
| | 12. Inflorescence generally oblong to lanceolate in outline, 1° branches generally ascending |
| |
| | 13. Lemma awned at or above middle; anthers < 0.6 mm; lower leaf sheaths generally glabrous |
| 3 A. exarata |
| | 13' Lemma awned below middle; anthers 1–1.5 mm; lower leaf sheaths finely tomentose |
| A. hooveri |
| | 12' Inflorescence generally ovate in outline, lowest 1° branches spreading, upper branches generally ascending |
| |
| | 14. Lemma back puberulent below middle; floret axis prolonged beyond lemma +/– 1 mm, short-hairy-tufted; palea > 1 mm |
| A. avenacea |
| | 14' Lemma back glabrous or fine-scabrous; floret axis not prolonged beyond lemma; palea 0 or minute |
| |
| | 15. Lemma awned near tip, awn 3–10 mm, wavy; callus hairs < 1/2 lemma, dense; anther 1, persistent in fruit; annual |
| A. elliottiana |
| | 15' Lemma awned below middle, awn < 2 mm, +/– straight; callus hairs << lemma, sparse; anthers 3, deciduous; perennial herb |
| 2 A. scabra |
| | 11' Inflorescence dense, generally cylindric; spikelets crowded, overlapping, inflorescence axes not clearly visible |
| |
| | 16. Primary inflorescence branches generally > 0.5 cm, often evident at base |
| |
| | 17. Leaf blade generally < 1 mm wide, +/– inrolled; floret callus glabrous; inflorescence base often partly enclosed by upper leaf; anthers 1–2 mm |
| 2 A. blasdalei |
| | 17' Leaf blade generally 2–10 mm wide, flat; floret callus minutely hairy; inflorescence clearly stalked; anthers +/– 0.5 mm |
| |
| | 18. Back of glume fine-scabrous throughout; palea 0.5–0.7 mm; ligule 1.5–2 mm |
| 3 A. densiflora |
| | 18' Back of glume +/– glabrous (keel fine-scabrous); palea +/– 0.3 mm; ligule 2.5–4 mm |
| 3 A. exarata |
| | 16' Primary inflorescence branches generally < 0.5 cm, obscured by densely clustered spikelets |
| |
| | 19. Lemma teeth 4, unequal (two < 1 mm, other two 1–1.5 mm); lemma awned below middle; callus densely short-hairy |
| A. tandilensis |
| | 19' Lemma teeth 0 or 2, equal; lemma awned at or above middle; callus generally sparsely hairy, hairs minute |
| |
| | 20. Lemma awned above middle, awn < 3.5 mm, straight; glume tips acute; perennial herb |
| 3 A. densiflora |
| | 20' Lemma awned +/– at middle, awn 3.5–10 mm, generally bent; glume tips narrowly acuminate to awn-like; annual |
| |
| | 21. Lemma body 2–4 mm, awn 8–10 mm |
| A. hendersonii |
| | 21' Lemma body 1.5–2 mm, awn 4–8 mm |
| A. microphylla |
| |
| | 22. Spikelets crowded and often overlapping on same branch, spikelet stalks and 2° axes not clearly visible |
| |
| | 23. Palea slightly < lemma (or +/– 1/3 × lemma in A. densiflora) |
| |
| | 24. Spikelet breaking below glumes, falling as 1 unit; stems often decumbent to long-trailing |
| 2 A. semiverticillata |
| | 24' Floret breaking above persistent glumes; stems generally erect |
| |
| | 25. Lower inflorescence branches < 1.5 cm; ligule 1.5–2 mm; anthers ± 0.5 mm |
| 3 A. densiflora |
| | 25' Lower inflorescence branches > 2 cm; ligule 2–5 mm; anthers > 1 mm |
| 2 A. stolonifera |
| | 23' Palea 0 or < 1/3 lemma |
| |
| | 26. Leaf blade < 1 mm wide, generally inrolled or folded |
| |
| | 27. Anthers > 1 mm; inflorescence branches generally < 0.5 cm; coastal habitats < 100 m |
| 2 A. blasdalei |
| | 27' Anthers < 1 mm; inflorescence branches 0.5–1.5 cm; inland mtns, 1600–4000 m |
| 3 A. variabilis |
| | 26' Leaf blade generally 2–7 mm wide, generally flat |
| |
| | 28. Floret callus with hairs 1.5–2 mm, generally slightly > 1/2 lemma |
| 3 A. hallii |
| | 28' Floret callus with hairs 0 or < 0.5 mm, < 1/2 lemma |
| |
| | 29. Lower inflorescence branches 1–2 cm; anthers 0.3–0.6 mm |
| 3 A. exarata |
| | 29' Lower inflorescence branches 2–5 cm; anthers 0.7–1.8 mm |
| 3 A. pallens |
| | 22' Spikelets not crowded, generally well spaced on same branch, axes clearly visible |
| |
| | 30. Primary inflorescence axes branched 1–2 × above middle, spikelets 0 on lower 1/2 |
| |
| | 31. Leaf blades 2–4 mm wide; glumes 2–3 mm; palea 1/4–1/5 lemma; anthers 0.6–0.8 mm; moist areas |
| A. oregonensis |
| | 31' Leaf blades 0.7–2 mm wide; glumes 1.5–2.7 mm; palea 0 or minute, << lemma; anthers < 0.7 mm; well drained areas |
| |
| | 32. Leaves basal and cauline; inflorescence +/– 2 × longer than wide; 1° axes +/– stiff |
| A. idahoensis |
| | 32' Leaves mostly basal; inflorescence +/– long as wide; 1° axes flexible, lower +/– arched |
| 2 A. scabra |
| | 30' Primary inflorescence axes branched 1–2 × from base upwards, spikelets distributed throughout |
| |
| | 33. Floret callus with hairs 1.5–2 mm, generally slightly > 1/2 lemma; anthers > 1.5 mm |
| 3 A. hallii |
| | 33' Floret callus glabrous or with minute hairs << lemma; anthers generally < 1.5 mm (except some A. pallens) |
| |
| | 34. Palea 0 or minute, << lemma |
| |
| | 35. Leaf blades 1–6 mm wide; plants generally 10–70 cm; anthers 0.7–1.8 mm |
| 3 A. pallens |
| | 35' Leaf blades generally < 1 mm wide; plants 4–30 cm; anthers 0.5–0.7 mm |
| 3 A. variabilis |
| |
| | 36. Inflorescence branches spreading to ascending; anthers 1–1.5 mm; wet or disturbed areas, generally < 1500 m |
| |
| | 37. Ligule 0.5–2 mm, generally wider than long; inflorescence branches with spikelets on distal half |
| 3 A. capillaris |
| | 37' Ligule 2–6 mm, longer than wide; inflorescence branches with spikelets +/– throughout |
| 2 A. gigantea |
| | 36' Inflorescence branches ascending to erect; anthers < 1 mm; moist to +/– dry areas, generally > 1500 m |
| |
| | 38. Inflorescence 2–5 cm, slightly flattened; plants generally < 15 cm; alpine |
| A. humilis |
| | 38' Inflorescence generally 4–11 cm, not clearly flattened; plants 8–50 cm; < subalpine |
| 2 A. thurberiana
|