In response to declines in biodiversity, a large body of research has documented the positive effects of plant species diversity - and, more recently, genetic diversity within plant species - on ecosystem functioning. In some cases, genetic diversity may have an effect as large or larger than the effect of species diversity. However, it is unknown whether the relative importance of species and genetic diversity shifts over time. Temporal shifts may be particularly important in dynamic ecosystems such as sand dunes. Sand dunes along Lake Michigan are dominated by the foundational species Ammophila breviligulata. This grass promotes sand stabilization, allowing plants not adapted to sand burial to colonize during primary succession. Current restoration practices include planting genetic monocultures of Ammophila, but it is unclear whether Ammophila’s role as a foundational species changes over time or in different diversity treatments. We established a long-term field experiment manipulating genetic diversity within Ammophila (1, 3, or 6 genotypes) and plant species diversity (1, 3, or 6 species) to test how diversity influences ecosystem functioning and whether the relative importance of species and genetic diversity shifts over time. Here, we present results from eight years of data showing how plant diversity influences sand accumulation.
As expected, monocultures of the foundational species, Ammophila, accumulated the most sand over the eight year census (46.5 cm). However, Ammophila genetic diversity did not influence sand accumulation. Generally, plots without Ammophila lost sand (-49.6 cm). However, preliminary results indicate that increased species diversity led to a reduction in sand loss, and this effect is strengthening over time. For plots without Ammophila, sand loss with high species diversity was 2.7 times lower than sand loss with low and medium diversity. Inclusion of Ammophila in low and medium species diversity treatments increased average sand accumulation up to 4.1 times, but only 2.9 times in high species diversity treatments. The difference between sand accumulation in high species diversity plots with and without Ammophila has been shrinking every year. This indicates that while Ammophila may promote early, short-term sand accumulation during restoration, species diversity may be a stronger driver of sand accumulation in the long-term. We expect that high species diversity will eventually promote sand accumulation as much as the presence of the foundational species. These results suggest that dune restoration should include both high species diversity and high genetic diversity in dominant species to maintain ecosystem functioning over time.