OOS 63-1 - Saving for a rainy day: What can seed banks tell us about vegetation change under drought and disturbance?

Background: Climate change is driving shifts in precipitation patterns, including increases in multi-year drought in rangelands across North America. Although grazing is a key use of these systems, the combination of drought and grazing related disturbance can transform ecosystems through plant community turnover. The seedbank may act as a reserve pool of species and influence the ability of a community to track changing climate patterns or return to an initial community composition following a perturbation. Understanding how seed banks change alongside vegetation, particularly in response to future climate patterns and management actions, is a critical aspect to anticipate both change and recovery.

Methods: Here we leveraged a four-year experiment in a semi-arid Colorado grassland which manipulated rainfall (three levels: -%66 reduction, ambient, +%66 addition) and grazing management (three levels: growing season, dormant season, and no grazing). We assessed the composition of the seed bank each fall though greenhouse grow outs. We ask two questions: 1) How do seed banks change in response to grazing and drought? 2) Does change in seedbank communities track changes in aboveground composition?

Results: The combination of dormant-season grazing and drought led to increases in the overall abundance of seeds in the seedbank after three years of treatments. This corresponded to an increase in Shannon diversity driven, in part, by increases in the abundance of annual and biennial species. While there was substantial interannual variation across the ungrazed and dormant-season grazing treatments, grazing during the spring reduced variation due to rainfall treatment and background variation. Using Principal Coordinate Analysis, we found that seed banks exposed to a combination of drought and grazing (dormant or growing season) tended to become more similar to each other (decreased beta-dispersion within treatments) and diverged from the composition of the aboveground community. The aboveground community composition shifted more slowly than the seed bank, also towards greater annual-biennial dominance under drought and grazing.

Conclusions: Drought and grazing can alter the size, diversity, and composition of seed bank communities, with the most rapid changes due to increases in annual-biennial species. These changes in seed bank composition are more rapid than changes in aboveground community composition. However, small but significant increases in annual-biennial plants aboveground, combined with the increasing differentiation of the seed bank towards annual dominance, may offer a warning sign, suggesting that drought and grazing might be pushing composition closer to a threshold of increased annual and reduced perennial grass dominance.