Variation in outcomes of ecological restoration projects is commonplace, and at times these outcomes do not match practitioners’ goals. This mismatch suggests a gap in our ecological knowledge of how communities assemble: if we cannot successfully rebuild a plant community, we have yet to learn something about its assembly. A better understanding of how restored ecological communities assemble will thus inform both ecology and restoration. It is suspected that year effects— environmental conditions at the outset of a restoration project—may contribute to variation among restoration efforts. However, a lack of experimental tests limits scientific understanding of the mechanisms driving year effects and their long term impact. We conducted an experiment to evaluate year effects on assembling plant communities in tallgrass prairie restorations. We restored prairie at the same site and followed identical methods in three different years and, within each planting year, manipulated precipitation with rain-out shelters. Our past work shows that seedling establishment varied across years, simulated plating-year precipitation treatments, and years within precipitation treatments. We now ask how plant communities vary across planting years and precipitation treatments both in (1) the planting year and (2) in the second year after planting.
Results/Conclusions
Plant communities in the planting year were significantly different across watering treatments (R2=0.08, p<0.001) and planting years (R2=0.20, p<0.001). However, in the second year following restoration, as the prairies developed, there was no longer a significant difference between communities that had been planted in different years (R2=0.15, p=0.13) or under different watering treatments (R2=0.06, p=0.07). Total sown forb abundance, a metric of restoration success, also differed across planting years initially but converged in the second year. The year after good planting conditions (i.e. in plots with high forb establishment), the number of sown forbs decreased 25%; and the year after poor planting conditions (i.e. plots with low forb establishment, e.g. after drought treatment), the number of sown forbs increased 57% but did not recover to non-drought levels. We conclude that inter-annual variation in both precipitation and some non-precipitation factor(s) are important drivers of year effects; however, these effects already begin to dampen by the second year. This suggests that the common assumption that planting year conditions may lead to variable restoration success is not necessarily true; however, further research is needed to address how planting year conditions alter the success of restoration projects over longer time scales and across various site conditions (e.g. soil moisture).