The impact of seeding method on diversity and fine-scale plant distribution in restored grasslands

Background/Question/Methods

Communities are typically compared based on species diversity and composition.  However, communities may also differ in fine-scale plant distribution which may affect species turnover rates and resource use.  To determine to what extent initial seed distributions may affect diversity and fine-scale plant distributions, we tested for quadrat-scale differences in diversity, composition, and plant distribution in established tallgrass prairie plantings in Iowa, U.S.A.  A diverse grassland seed mix was either drill seeded, which places seeds in equally spaced rows, or broadcast seeded, which places seeds uniformly across the ground surface, into 24 plots in each of three sites in 2005.  We predicted that closer neighbors in drilled plantings would result in lower species diversity and larger conspecific patch sizes due to greater competitive exclusion during establishment.  Species abundance was measured through point-intercept sampling in a 1 m² quadrat in each plot.  Common species distributions were mapped by dividing each quadrat into 64- 12.5 x 12.5 cm cells and recording the most abundant species in each cell.  Plant distributions were then quantified from quadrat maps using patch-based measures of landscape composition.

Results/Conclusions

There were no differences in quadrat-scale diversity (species richness or evenness), planted forb or exotic species abundance, number of patches, or mean patch size between plantings.  This suggests that planting type did not differentially affect quadrat-scale establishment or local invasion and that closer neighbors in drilled plantings did not result in greater competitive exclusion.  However, native warm season (C₄) grasses, which are similar in their resource requirements, were more abundant and occurred in patches with a larger mean-squared radius in drilled quadrats.  Greater dispersion of C₄ grasses in drilled quadrats was likely due to more open space available for grasses to spread vegetatively among rows.  In addition, quadrat-scale light capture was greater in drilled plantings, suggesting that fewer microsites are available for colonization in drilled over broadcast plantings.  Although these communities are similar in diversity at present, they may diverge in time due to differences in plant distribution and resource use.  The extent to which spatial relationships among seedlings influences plant distributions in establishing communities needs to be further investigated as a potentially important determinant of diversity maintenance and ecosystem function in grasslands.