Seed Viability and Fire-Related Temperature Treatments in Serotinous California Native Hesperocyparis Species

Fire-prone serotinous California Hesperocyparis L. (cypress) have been experiencing low seedling recruitment, underscoring our need to better understand these species’ responses to fire. We investigated the specific heating conditions required to break cone serotiny and to promote seed dispersal by focusing on five Hesperocyparis species of interior California: Hesperocyparis nevadensis (Abrams) Bartel, Paiute cypress; H. bakeri (Jeps.) Bartel, Baker cypress (also known as Modoc cypress); H. forbesii Jeps. Bartel, tecate cypress; H. macnabiana (A. Murray bis) Bartel, McNab’s cypress; and H. sargentii (Jeps.) Bartel (Sargent’s cypress). A muffle furnace was used to conduct eight temperature treatments of 250 °C to 700 °C, ranging in duration from 30 seconds to 5 minutes of exposure. The heat-released seeds were tested for viability using a tetrazolium red stain. Logistic regression analysis of seed viability indicated that duration of heating alone was highly significant (P < 0.005) for all species, regardless of temperature, with durations of 1 min or less resulting in the greatest viability. Hesperocyparis forbesii and H. nevadensis were capable of tolerating temperatures as high as 700 °C. Models predicting seed viability reflected interspecific differences, with H. macnabiana and H. sargentii having higher seed viability than H. nevadensis and H. forbesii, which had higher seed viability than H. bakeri. Lab results coupled with field observations following fire suggest that fire can trigger a massive release of seeds, overwhelming the inherently low viability and allowing for greater potential for adequate seedling establishment.