Grass seedling demography and sage steppe restoration

Authors: James, Jeremy; Rinella, Matthew - Matt; Svejcar, Anthony

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2011
Publication Date: 7/1/2012
Citation: James, J.J., Rinella, M.J., Svejcar, A.J. 2012. Grass seedling demography and sage steppe restoration. Rangeland Ecology and Management. 65(4):409-417.

Interpretive Summary: Seeding is a key management tool for semi-arid and arid rangeland but sown seed often fails to establish. Over three years we quantified demographic transition spanning seed sowing to adult plant establishment for three widely used restoration grasses. We found that germination was rapid and high, often exceeding 50% two weeks after sowing. We also found that emergence of germinated seed was slow with up to 90% of the germinated seed failing to emerge. Discovery of this major bottleneck in seedling establishment can help identify simple management strategies that can drastically improve seeding success rates on rangeland.

Technical Abstract: Seeding is a key management tool for semi-arid and arid rangeland. In these systems, however, seeded species often fail to establish. The objectives of this study were to identify demographic transitions limiting seedling recruitment in a disturbed, sage steppe system and determine the degree to which these demographic limitations varied across years. To do this we seeded disturbed plots in 2007, 2008 and 2009 using monocultures of the native perennial bunchgrasses Pseudoroegenaria spicata and Elymus elymoides and the introduced bunchgrass Agropyron desertorum and we tracked the fate of seed and plants through key demographic transitions spanning germination to adult plant survival. Across the three study years and species we found that germination was high and rapid, with species obtaining 50% germination by December. For both native species, however, the percentage of germinated seed that emerged was low, averaging 16%. For the introduced A. desertorum, on average, 50% of the seeds that germinated emerged. Relative to the number of seeds sown, over 60% of the native grass individuals died after seeds germinated but before seedlings emerged. In contrast, only about 8% of the native grasses died following emergence and transitioning to an adult plant. Early emergence of seedlings did not increase survival probability or decrease midday plant water potential. Contrary to widely held notions, these demography data suggest factors like summer drought and competition from weeds may not be causes of many restoration failures. Instead these data suggest factors that operate in winter or early spring after seeds germinate but before seedlings emerge, such as soil freeze-thaw events, seed pathogens, or formation of physical crusts, may be key drivers of seeding failures. Management tools and seed mixes that overcome this emergence bottleneck may greatly improve restoration outcomes in these systems.