Could Tanoak Mortality Affect Insect Biodiversity? Evidence For Insect Pollination in Tanoaks

Abstract

Tanoaks, Notholithocarpus densiflorus (Hook. & Arn.) Manos, Cannon & S. H. Oh, are being killed by sudden oak death, caused by the pathogen Phytophthora ramorum Werres, de Cock & Man in't Veld. However, very little is known about the basic ecology of the species. Here we investigate the pollination ecology of tanoaks using insect-visitor observations along with a pollinator-exclusion study. Insect-visitor observations were conducted by citizen-scientist volunteers at three different sites in the Midpeninsula Regional Open Space District lands in the Coast Range of California in 2009. Pollinator exclusions were conducted over two years (2009, 2010), using veil bags to prevent insects from reaching female flowers at the Blodgett Forest Research Station in the Sierra Nevada foothills. Microsatellite markers were used to infer selfing or outcrossing for the developing acorns. The citizen scientists observed 148 insect visitors to tanoak flowers over 11.5 hours of observation (in 65 observation periods). Pollinator exclusion resulted in lower fruit set and higher rates of selfing. The data suggest that tanoak is primarily an insect-pollinated species, though some level of wind pollination is likely. There is a diverse community of insects visiting tanoak flowers. In order to understand the importance of tanoaks to the native insect community, future research needs to focus on identifying the composition of the insect community, and the extent to which they rely on tanoak pollen and nectar as a food source.