Barriers to oak recruitment

Blue oak in oak woodland at Sierra Foothill Research and Extension Center, CA. Photo Credit: W. Schlegel

Why are so few oaks able to recruit in oak woodlands? Is a demographic bottleneck to blame? How do seasonal grazing practices and annual grasses influence this bottleneck?

Oak woodland and savanna habitats have suffered significant losses over the last century, primarily due to urban expansion and agricultural development. In the remaining 3 million ha of oak habitat, there is a concern that natural oak recruitment is not sufficient to maintain current populations. In particular, the inability of oak seedlings to transition to sapling and larger stages may often constrain recruitment. At the Sierra Foothill Research and Extension Center, we are investigating the landscape-scale relationship between the timing of grazing and blue oak (Quercus douglasii) recruitment, particularly with regard to the transition of seedlings into the sapling stage.

The influences of livestock and non-native annual grasses are generally implicated as the major causes of this failure to recruit. Alternative livestock grazing regimes that reduce the duration and intensity of herbivory pressure on woody vegetation relative to herbaceous vegetation may enhance the success of oak saplings while maintaining an important source of forage production in California. 

We will test several pathways by which the timing of grazing on the landscape could influence oak recruitment. Direct herbivory effects may be more severe when other green forage options are reduced.

In addition, the timing and duration of grazing could influence residual dry matter present in fall when acorns are dispersed and germinate, and nutrient availability and the vigor of herbaceous neighbors in spring when oak seedlings begin growth. Differences in herbaceous species composition resulting from grazing practices may influence the success of seedling survival due to changes in competitive intensity and nutrient and water use. Residual dry matter and herbaceous species composition could also influence damage from other herbivores such as deer, small mammals or insects.

We hope this work will inform management regarding the timing and duration of grazing, suggesting ways to optimize forage production, while simultaneously improving the native representation of the herbaceous community and oak recruitment.

People involved: Dr. Susana Rodriguez-Buritica, University of Arizona-Tucson, Dr. Stan Harpole, Iowa State University, Dr. Mitch McClaran, University of Arizona-Tucson