ASSOCIATION BETWEEN FIRE RETURN INTERVAL AND POPULATION DYNAMICS IN FOUR CALIFORNIA POPULATIONS OF TECATE CYPRESS (CUPRESSUS FORBESII)

The Tecate cypress (Cupressus forbesii) is a tree species associated with chaparral ecosystems in southern California and northern Baja California, Mexico. It is fire-adapted, its regeneration triggered by the opening of serotinous cones when adult trees are burned. Surveys made in the 1980s by others suggested that some Tecate cypress populations were declining, and some authors suggested that increased fire frequency in southern California was a major factor for this decline. We asked whether current population trends were still negative for Tecate cypress 20 years later, and whether population growth was associated with fire return interval length. Based on demographic, survival, and growth-rate data, we calculated the rate of population increase (λ) for the 4 known California populations of Tecate cypress (Coal Canyon-Sierra Peak in Orange County and Guatay Mountain, Otay Mountain, and Tecate Peak in San Diego County). The rate of population increase was positively associated with fire return interval length, which varied among populations. A 40-year or longer fire return interval corresponded to λ > 1, suggesting the population trend is positive, while a fire return interval shorter than 40 years corresponded to λ < 1, suggesting that populations burning more often than once every 40 years are declining.