Land-use history trumps planting diversity effects in prairie restoration

Background/Question/Methods

Experimental work in grasslands has illustrated how greater plant species diversity can support greater function, such as primary productivity and invasion resistance; however, the extent to which these findings translate to ecological restoration efforts is relatively unknown. Restored systems support substantial environmental variability, which may lead to break down of intended diversity-function relationships. In part, this variability may result from legacies of previous land-use, which can result in the rapid establishment of exotic species following cessation of agricultural uses. In order to assess the importance of seed mix richness to restoring function, we surveyed 30 prairie restorations in southwest Michigan in former agricultural land. The restorations were between three and eight years old and had been sown with between 8 and 71 species using a standard native seed drill. We sampled percent cover of all plant species and aboveground biomass in 1 m² quadrats along transects.

Results/Conclusions

If greater diversity is to improve restorations, restorations must first be able to establish a diverse target community. We found that although both the richness and relative abundance of sown species was a linear increasing function of seed mix richness, less than half of the sown species established.  Instead, the species richness of native and exotic volunteers was more than twice that of sown species and we observed no relationship between seed mix richness and the diversity of the entire plant community. Increased seed mix richness also did not translate into greater function. Neither aboveground biomass nor invasion resistance was correlated with species richness. Rather, these functions were related to historical factors: land-use history and restoration age.  Exotic species cover was greater in prairies restored from former hay fields, relative to restorations with row crop or old field histories, whereas aboveground biomass was negatively correlated with restoration age.

Experimentally demonstrated relationships, such as between species diversity and ecosystem function, can be less important in restoration. Historical legacies inhibit the successful establishment of the target community, which results in the breakdown of these predicted effects. To improve the success of restorations researchers must improve understanding of the mechanisms behind historical factors.