Effects of prescribed burning on leaves and flowering of Quercus garryana

Many woodland understories are managed with prescribed fire. While prescribed burns intended to manipulate understory vegetation and fuels usually do not cause excessive tree mortality, sublethal canopy damage may occur and can affect tree vigor and reproductive output. We monitored Quercus garryana trees in western Washington, USA with multiple canopy thermocouples during three prescribed burns. Peak temperatures recorded in tree canopies ranged from 36 to 649C. We assessed leaf damage immediately after burning, and flower, leaf and acorn production in the following year in the vicinity of each thermocouple. Leaf scorch first occurred with peak thermocouple temperatures around 45C, was variable up to 75C, but above 75C all leaves were killed. Buds, including their reproductive and leaf organs were more resistant to heat damage than leaves, but leaf scorch had predictive value in forecasting bud organ damage. Staminate and pistillate inflorescences and acorn production per bud decreased and bud mortality increased with maximum thermocouple temperature. In two burns where the highest peak temperatures reached 137C, there was no difference in leaf production between burned and control plots in the spring following burning. However, no staminate or pistillate inflorescences were produced when thermocouple peak temperatures went above 55 or 68C, respectively. While heat damage to bud organs was detected, production of reproductive organs was also curtailed at temperatures lower than could reasonably be attributed to heat damage. Thus, it is probable that some other fire-related factor, possibly smoke, was also involved.