1 Variation in the competitive ability of plant species may determine their
persistence and abundance in communities. We quantified the competitive ef
fects of grasses and woody plants in native mixed-grass prairie on the perf
ormance of transplant species and on resources.
2 We separated the effects of grasses, shrubs and intact vegetation contain
ing both grasses and shrubs by manipulating the natural vegetation using se
lective herbicides to create four neighbourhood treatments: no neighbours (
NN), no shrubs (NS), no grasses (NG) and all neighbours (AN). Treatments we
re applied to 2 x 2 m experimental plots located in either grass- or shrub-
dominated habitats. The effects of grasses and shrubs on resource availabil
ity (light, soil moisture, soil available nitrogen) and on the growth of tr
ansplants of Bouteloua gracilis, a perennial tussock grass, and Elaeagnus c
ommutata, a common shrub, were measured over two growing seasons.
3 Resource availability was two- to fivefold higher in no neighbour (NN) pl
ots than in vegetated plots (NS, NG, AN) with grasses and shrubs having sim
ilar effects. Light penetration declined linearly with increasing grass or
shrub biomass, to a minimum of about 30% incident light at 500 g m(-2) shoo
t mass. Soil resources did not decline with increasing neighbour shoot or r
oot mass for either grasses or shrubs, suggesting that the presence of neig
hbours was more important than their abundance.
4 Transplant growth was significantly suppressed by the presence of neighbo
urs, but not by increasing neighbour shoot or root biomass, except for a li
near decline in Bouteloua growth with increasing neighbour shoot mass in pl
ots containing only shrubs. Competition intensity, calculated as the reduct
ion in transplant growth by neighbours, was similar in both grass- and shru
b-dominated habitats for transplants of Bouteloua, but was less intense in
shrub-dominated habitats for the shrub Elaeagnus. Variation in the persiste
nce and abundance of plants in communities may therefore be more strongly c
ontrolled by variation in the competitive effects exerted by neighbours tha
n by differences in competitive response ability.