NORTHERN COLORADO PLANT COMMUNITIES 225 



shrubs, while grassland is made up of herbaceous plants among which 

 there need be no true grasses at all. Coniferous forests of pines are 

 drought-resisting or xerophytic, while alders, willows and aspens which 

 require a moderate amount of water may be described as mesophytic. 

 Engelmann spruces live in rather moist soil but it is cold and often " sour," 

 hence the habitat is physiologically dry because these conditions retard 

 absorption of water by roots. Spruce forests may on this account be 

 called xerophytic, although various authors — the present writer in- 

 cluded — have at times spoken of spruce forests as mesophytic. 



List of Plant Communities. — The following list will serve for 

 reference in the later discussion. It is not intended as complete but to 

 be suggestive. 



Boulder Park and Vicinity 



1. Lodgepole Pine Forest — Pinus murrayana "Oreg. Com." 



2. Limber Pine Forest — Pinus flexilis James. 



3. Engelmann Spruce Forest — Picea engelmanni (Parry) Engelm. 



4. Aspen Forest — Populus tremuloides Michx. 



5. Willow and Alder Thicket — Salix chlorophylla Anders., Salix brachycarpa Nutt., 

 Alnus tenuifolia Nutt. 



6. Rock Pine Forest— Pinus scopulorum (Engelm.) Lemmon. 



7. Willow, Birch and Honeysuckle Scrub —Salix spp., Betula glandulosa Michx., Lonicera 

 involucrata Banks. 



8. Potentilla Scrub — Potentilla fruticosa Linn. (Dasiphora fruticosa Rydb.). 



9. Sage-Brush Scrub —Artemisia tridentata Nutt. (perhaps Artemisia arbuscula Nutt.). 



10. Swamp or Moor —Car ex spp. 



11. Meadow (moderately moist grassland). 



12. Dry Grassland. 



13. Gravel Slide. 



14. Sand Bank and Sand Bar. 



15. Aquatic Vegetation. 



16. Vegetation 0} Roadsides and Waste Places (ruderal vegetation). 



17. Forest Under growths. 



At Rollins Pass (Corona, Colo.) 



1. Pond Vegetation — Potamogeton, various algae, etc. 



2. Scrub —Salix spp. 



3. Swamp or Moor— Car ex spp., Caltha, Trollius, Elephantella. 



4. Meadow — Rydbergia, Campanula, Trifolium, Mertensia, Sieversia, Poa spp. 



5. Rock Desert Vegetation ("Dry Tundra")— Eritrichium, Silene, Tetraneuris, Sela- 

 ginella, Antennaria, Artemisia. 



