Factors affecting emergence and first-year survival of seedlings of Garry oaks ( Quercus garryana) in British Columbia, Canada

Abstract

Regeneration is essential to sustain populations of Garry oaks ( Quercus garryana). Processes that mediate oak regeneration can be influenced by where acorn-hoarding animals take acorns and how deep acorns are buried at hoarding sites. We planted acorns to determine the role of habitat type and burial depth on emergence and first-year survival of Garry oak seedlings. In all habitats that we examined, vertebrates removed >53% of acorns placed on the ground surface. However, in approximately one half of the habitats, vertebrates removed only between 7 and 48% of acorns buried in the ground cover or soil. Habitats with high predation varied in terms of vegetation structure, but included all those with sparse herb, dense shrub, and moderate to high tree cover. Predation on buried acorns was low in habitats with various structural characteristics, including those with dense herb cover and sparse shrub and tree cover. The mortality rate of buried acorns that were not removed by predators was low (<17%) in all but one habitat. Perhaps because of desiccation, the mortality rate of surface acorns was >40% in all but one habitat. Regardless of burial depth, once seedlings emerged, seedling survival was >65% in most habitats. Some habitats with high seedling emergence had relatively low seedling survival, and vice versa. In habitats on a south-facing slope, characterized by rocky outcrops and absence of conifers, many seedlings died from desiccation. Other seedling mortality may have been caused by shoot browsing, root browsing, or insect herbivory. Seedling survival was not related to percent cover of overstorey vegetation, indicating that first-year Garry oak seedlings were not protected by shade and can survive in habitats with low light.