Rareness starts early for disturbance-dependent grassland plant species

Ecological communities always contain a few common species and an abundance of uncommon species. Given that most plant mortality occurs in seeds and seedlings, recruitment success often predicts plant community assemblage and patterning, but observational patterns do not reveal whether plant populations are seed or habitat limited. Grassland plant species make up a sizable portion of the overall native flora in northeastern North America (N.A.), but approximately 30 % of the area’s threatened and endangered flora are grassland species, possibly leftovers from the post-glacial landscape. Yet, close relatives of many rare grassland species thrive in the same range. We investigated whether seed or habitat limitation explained rarity and commonness in remnant grassland species. We used seed addition experiments coupled with microhabitat manipulations (burning and herbivore exclusion) in three different habitat types to evaluate recruitment (germination and seedling survival) limitation for three rare and three common grassland species. Rare grassland species successfully recruited where burning reduced initial competitor density, but seedling survival suggested they were severely limited by interspecific competition. Both the rare and common plant species survived equally well in forest habitats where herbaceous density was low whereas neither survived in the edge habitats. Only the common plants thrived in the high-competition meadow habitat, further suggesting that the rare grassland species are poor competitors. Commonness and rarity are temporal designations that can change as disturbance alters the landscape. Glacial retreat and low precipitation in northeastern N.A. created a landscape suitable for poor competitors that tolerated poor conditions. Our results suggest that rare remnant grassland plants, unlike their close relatives, display more biotic than abiotic limitation as they do not compete well with other plants. These results suggest that suitable habitat is not a spatial location but a temporally transient assemblage of species requirements.