Site and tree factors in Oregon white oak acorn production in Western Washington and Oregon

Acorn production in Oregon white oak (Quercus garryana) is variable and not well understood. To determine tree, stand, and environmental characteristics influencing acorn production, a total of 284 trees in September and October 1999 were surveyed. Surveyed trees extended from Whidbey Island, Washington, to Roseburg, Oregon, and ranged in age from 11 to over 300 years. Acorn production was ranked on a 1-4 scale (nonproducing to heavily producing), and tree size, condition, site, stand, and vegetation characteristics were recorded. All acorns were counted on 18 trees. Acorn production started around 20 years and plateaued around 80 years. Trees with inverted vase-shaped crowns typical of dense forest stands produced fewer acorns than those with columnar- or mushroom-shaped crowns typical of more open areas. Open-grown trees growing in well-drained, loamy soils had the highest productivity within natural settings. Trees in urban parks with fertilization or irrigation also produced well. Trees underburned 6-10 years before the survey produced better than trees not underburned or trees underburned more recently. Trees growing in slough-sedge wetlands, steep south-facing rocky balds, or with unhealthy crowns produced the least. Understanding the factors that influence production of Oregon white oak acorns could help managers increase acorn crop size as many of the factors can be altered by management activities.