Fire history, stand origins, and the persistence of McNab cypress, northern California, USA

Abstract

Despite their substantial contribution to the uniqueness and diversity of the California Floristic Province's flora and vegetation, surprisingly little is known about the region's native cypress species (genus Hesperocyparis Bartel & R.A. Price). Current ideas about the fire regimes that maintained these species in the past are based largely or solely on assumptions about the expression and function of life-history characteristics. Empirical studies of fire history are generally lacking. I used a combination of dendrochronological methods, field observations, and contemporary fire records to investigate the history of fire and its role in stand development in McNab cypress (Hesperocyparis macnabiana [A. Murray bis] Bartel), an uncommon serotinous cypress endemic to northern California. Although even-aged stands were present within all 20 populations surveyed, 4 populations also contained stands exhibiting uneven age structure. Fire records and field observations confirmed that even-aged stands originated following stand-replacing fires, but no link between fire and recruitment was found for uneven-aged stands. Rather, trees within uneven-aged stands were generally older, larger, and exhibited higher rates of senescence than those found in even-aged stands, suggesting that inter-fire tree mortality and inter-fire establishment may be linked. When aggregated, the dates of stand-replacing fires (derived from stand recruitment dates and fire records) revealed that relatively few stand-replacing fires occurred in McNab cypress populations during the 50 yr period between 1940 and 1989 (1 fire per decade) compared to either the previous 100 yr period (at least 3 fires per decade) or the subsequent 20 yr period (3.5 fires per decade). This suggests that short fire return intervals do not appear to be a significant threat to McNab cypress persistence at the present time, and there is little ecological justification for fire exclusion.