Adaptation to a broad soil moisture gradient in a dominant prairie grass: applications for restoration

Background/Question/Methods

Plant populations often show patterns of adaptation, and perform better when transplanted into their home environments than other suitable habitats. Selective agents driving adaptation, however, are rarely identified. In restoring plant communities, matching source populations with restoration sites along environmental gradients may be important. Intraspecific genetic variation in restored populations may buffer against this uncertainty by increasing the chance that a pre-adapted genotype is introduced. I tested for the effect of adaptation to soil moisture and genotypic diversity in the prairie grass Andropogon gerardii.

I collected seed from sites representing three soil moisture regimes (wet, mesic, and dry). Three natural populations were sampled from each level. Additionally, seed from three separate production fields was obtained from a commercial source, for a total of 12 populations. The fields were established from the same mixed collections from several populations. In fall 2011, I transplanted established plants from all 12 populations into three field sites, represent the same range of soil moisture as in the source populations. At these sites, plots of three plants represented either one population or all three populations for a given soil moisture. In fall 2012, I sampled plant height and survivorship.

Results/Conclusions

Soil moisture of origin and planting site both significantly affected plant height and survivorship, but I found no direct evidence that genotypic diversity improved performance. Interestingly, the populations from production fields outperformed the other populations overall. This could be due to a maternal effect from lower interspecific competition in production fields. Also, because these fields were established from several genotypes, increased genetic variation in these populations may be conferring greater performance. Finally, performance of 9 of the 12 populations was highest at the dry field site. This may be due to reduced competition, as this site had the lowest overall plant cover.

Adaptation to soil moisture would be indicated by a significant interaction between soil moisture of origin and planting site, and these were only marginally significant. Furthermore, populations did not consistently perform best in their home soil moisture regime. While the biological mechanisms behind these results are unclear, the evidence collectively may point to a clearer recommendation for restoring this species. Commercially produced seeds appear to perform better across a broad soil moisture gradient, so may be used successfully a range of sites and may in general be preferable to seed sources from natural populations.