What Shapes Phenology and Fecundity in a Masting Tree Species? Location, Location, Location

Andy Lentz, UC Santa Barbara Sedgwick Reserve

Frank Davis, University of California, Santa Barbara, Bren School of Environmental Science and Management; Victoria L. Sork, University of California Los Angeles, Department of Ecology and Evolution and Institute of the Environment and Sustainability.

Flower production and acorn production are critical components of viability of oak populations. Moreover, acorn production is a critical food resource for many insects and wildlife. For the last 14 years, we have been documenting the timing and quantity of male and female flowers and the corresponding acorn production in 100 valley oak adults located in an 80-ha oak savanna in the University of California Sedgwick Natural Reserve, Los Olivos, California. Given concerns about climate change affecting reproduction, we assessed the extent to which temperature has influenced the timing of the emergence of flowering, population synchrony in the timing of flowering, and the impact of these two processes on acorn production. Each year from 2007-2020 around February through April, we visited 100 (minus 1 that died) adult valley oaks twice per week to record the developmental status of buds, emergence of male catkins, and prevalence of female flowers. Each fall, we censused acorn abundance by counting acorns for 15 seconds twice per tree. Daily temperature and precipitation data was available from the weather station at Sedgwick reserve and, starting in 2011, temperature data recorded every 4 hours was from temperature sensors placed on 50 trees scattered throughout the study region. In addition, flower season temperatures were modeled for all 100 trees using digital elevation data from high-resolution LIDAR imagery . We found that the timing of leaf emergence and flowering among individual trees was highly correlated across years, so we focused on the extent to which spatial location and temperature affected acorn production. We found that the location of a tree was a major factor determining flower phenology and which trees produced large and small acorn crops. Trees at warmer microsites during the flowering season flowered earlier than most other trees. Trees that flowered when many other trees flowered and that were closer to the water table produced more acorns. As climates get warmer and water table drops, acorn production may be affected. Thus, rapid climate change could have a disruptive effect on future acorn production.