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A standardized neighbor removal experiment was conducted in 12 plant communities located on three continents to test the null hypothesis that competition intensity (CI) was independent of the amount of plant biomass present. Six plots were chosen in each community to cover the range oflocal variation in plant biomass. In each plot the relative growth rate (RGR) of transplanted Poapratensis (Poaceae) seedlings was compared in the presence and absence of neighbors. Neighbors were removed experimentally using R. J. READER ET AL. Ecology, Vol. 75, No. 6 herbicide. Removing neighbors increased RGR of transplants significantly in most plots. CI increased with an increase in the amount of neighbor biomass present in one community where the range of neighbor biomass was greater than in any other community. In contrast, CI did not change significantly with an increase in neighbor biomass in other communities where the range of neighbor biomass was smaller. For the communities combined, CI was not related to neighbor biomass in a consistent fashion. These results indicate that competition may reduce growth over a wide range of habitat productivity, but the relationship between CI and neighbor biomass may differ among communities.

Large areas of North American prairie are dominated by Eurasian plant species introduced either for range improvement or accidentally as weeds. We examined the impact of introduced plants on both native vegetation and bird communities in a mosaic of North American mixed‐grass prairie and Eurasian vegetation. We established ten transects five in areas of native prairie and five in areas dominated by introduced plant species. Each transect comprised five sampling stations separated by 100 m. Vegetation was sampled in four 0.5m2 quadrats at each station. The cooers of eight of the ten most common plant species varied significantly (p < 0.05) between native and introduced vegetation. One common native plans Andropogon scoparius, was absent in introduced vegetation Singing birds were identified to species at each station on three occasions during the breeding season All bird species found were native to prairie. The total number of birds did not vary between vegetation types Two out of eight bird species, upland sandpiper and Sprague's pipit were signifcantly more abundant in native prairie than in introduced vegetation No bird species were significantly more common in introduced vegetation. A correlation matrix calculated for all bird species and the ten most abundant plant species divided the bird community into two groups. The first group (western meadow lark, upland sandpiper, Sprague's pipit, Baird's sparrow and savannah sparrow) was positively correlated with native plant species and negatively with introduced plants, while the second (vesper sparrow, clay‐colored sparrow, and grasshopper sparrow) was negatively correlated with native species and positively correlated with introduced Discriminant analysis separated transects from native and Eurasian vegetation on the basis of their respective bird communities. The results illustrate that the introduction of Eurasian plant species to North American prairie not only replaces the native plant community, but also produces significant changes in the species composition of a higher trophic level.

The relationship between leafy spur8e (Eqhorbio esda L.) and the species composition of mixed-8r8ss pr8irie was eumlned on both a lar8e scale, within a 200-km2 erea, and on a local scale, wfthin a single infestation. On the lar8e scale, cover vahres of 8 of the 10 most common species v8rkd siylificantly (p

Alvars are areas with a distinctive dry grassland vegetation growing in thin soil over level limestone, and they are documented in Scandinavia, the eastern United States, and central Canada. Ordination and classification analysis techniques were used to describe alvar vegetation in Canada at two scales: within one alvar and among four alvar sites. Within one alvar, changes in species composition corresponded to changes in soil depth and biomass. There were two main vegetation types: (i) alvar meadows with complete vegetation cover and (ii) rock flats with incomplete vegetation cover over limestone rock. Among alvars, species composition was related primarily to geographic location. The southern site was distinct from the eastern and northern sites. Relationships between soil depth, plant biomass, and vegetation could also be detected. At within and among alvar scales, tall perennial graminoids dominated sites with deep soil while small annuals and stress-tolerant perennials dominated shallow soil sites. Average biomass levels were strongly positively correlated with soil depth across vegetation types. Average species richness was curvilinearly related to biomass. Our results describe Canadian alvar vegetation and illustrate important differences among alvar sites, showing that a number of these sites need protection to conserve alvar vegetation. Key words: grassland, drought, soil depth, species richness, biomass, conservation.

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