Abstract
Many species of oak across the world exhibit low rates of recruitment and regeneration. In California, Quercus douglasii (blue oak) has experienced little to no recruitment over the past 100 to 150 years. A variety of factors, including competition from non-native plants, livestock grazing, fire suppression, and habitat fragmentation, have individually and collectively been postulated as drivers for low recruitment of blue oak. Despite the prevalence of livestock grazing and attendant soil compaction throughout the range of blue oak, soil compaction has not been studied as a possible factor affecting blue oak recruitment. Therefore, the objective of this study is to examine whether soil compaction affects blue oak seedling emergence and growth. I hypothesized that increasing soil compaction would increase the number of days to emergence and decrease the growth of blue oak seedlings compared to emergence and growth in uncompacted soil. To test this hypothesis blue oak acorns were grown in field collected soil hand compacted to different bulk density levels. Blue oak acorns and soil were collected at the Sierra Foothill Research and Extension Center (SFREC) located in Yuba County, California. Two pre-germinated acorns were planted in 72 PVC pots filled with soil compacted to contrasting levels of bulk density. Bulk density levels mimicked the range of those found at the SFREC in grazed and ungrazed areas. After germination time to shoot emergence was collected and pots thinned to one seedling each. Seedlings were grown at a nursery in Sacramento for 7 months, after which they were removed from the PVC pots and separated from the soil. Following seedling harvest soils were analyzed for final bulk density and basal stem diameter, shoot length, and taproot length of seedlings were measured. Seedling fine and coarse root lengths were measured using WinRHIZO software. Biomass of above and belowground parts of each plant was determined. To test the effect of soil bulk density on seedling growth analysis of variance was conducted. Time to shoot emergence differed significantly across treatment levels, with seedlings grown at the control bulk density level emerging earlier compared to those grown at low and high bulk densities. In addition, taproot length was significantly greater at low and high soil bulk densities compared to the control. Root to shoot mass ratio was significantly higher in low compared to high soil bulk density, but neither differed from the control. No significant differences in seedling growth were observed in the nine other seedling growth variables with respect to compaction treatment. The results indicate that soil bulk density levels used in this study affected some seedling growth variables but did not substantially inhibit growth of blue oak seedlings. This suggests that soil compaction caused by grazing cattle at moderate stocking rates may affect blue oak seedling emergence but may not affect later stages of seedling growth and thus recruitment in blue oak populations.