Effects of polyacrylamide gel and straw mulch on growth and survival of direct-seeded blue and interior live oaks in California rangeland

Interior live oak (Quercus wislizeni A. DC.) and blue oak (Quercus douglasii Hook. & Arn.) are native trees of California that face anthropogenic pressures such as woodcutting, rangeland clearing, livestock grazing, and exotic plant competition that impede maintenance of both mature and juvenile populations. These pressures are coupled with habitat loss due to urban development and agricultural conversion throughout their respective ranges. Investigators have proposed that continued low levels of seedling recruitment of blue oak and some other oak species will result in their long-term decline. The evidence supporting this hypothesis has been criticized because it is primarily based on short-term studies focusing on acorn and seedling life stages and inferred from current mature oak. However, government agencies and conservation groups continue to conserve remnant oak stands and replant sites historically dominated by oak species when feasible. In seeking to augment the planting strategies available to practitioners, I explored how straw mulch and cross-linked polyacrylamide gel (PAM) gel affect the growth and establishment of blue oak and interior live oak seedlings planted in restoration settings. I assessed the germination, mortality of seedlings and saplings, mid-day stem xylem potential, and performance for both species, over two years. Acorn germination was 90% and greater for both species over all treatments. In 2006, interior live oak mortality was 18 and 19 percent lower in the mulch and PAM gel+mulch treatments compared with the control. In 2007, the mulch and PAM+mulch treatments had no mortality and the PAM gel treatment was not significantly different from the control. None of the treatments significantly affected blue oak mortality relative to the control in either year. In 2006, both species were taller in the mulch and PAM gel+mulch treatments; in 2007, only the interior live oak height was significantly greater in the mulch and PAM gel+mulch treatments. There were no significant treatment effects on mid-day stem water potential in either year. Mulch improves the performance of both interior live and blue oak and should be considered for use in a restoration context, whereas, PAM gel is not recommended for planting blue or interior live oak acorns.