Ten fully stocked pinyon-juniper stands contained a total of 73 species in the understory, but the number of understory species in any one stand was moderately low (x̄ = 20). On each stand, species of at least five different plant groups were present in the understory (shrub, perennial grass, perennial forb, annual grass, or annual forb). A perennial grass, Sandberg bluegrass (Poa sandbergii), and a group of annual forbs with relatively high cover and constancy among stands appeared best adapted to coexist with the pinyon-juniper overstory. The proportion of total plant cover was greater on tree-associated microsites (duff and transition) than in the interspace between trees because of the greater surface area of the former in most stands. The transition microsite was the most favorable for understory species and provided understory cover in disproportionately greater amounts than the area it occupied.
Everett, Richard L. and Koniak, Susan
"Understory vegetation in fully stocked pinyon-juniper stands,"
Great Basin Naturalist: Vol. 41
, Article 15.
Available at: http://scholarsarchive.byu.edu/gbn/vol41/iss4/15