Approximation of Fire-Return Intervals with Point Samples in the Southern Range of the Coast Redwood Forest, California, USA

Abstract

A legacy of past fires is evident in the form of blackened basal hollows found throughout the southern range of the coast redwood (Sequoia sempervirens [D. Don] Endl.) forest. A deeper look reveals cambial scars dating back centuries, telling a story of low-to moderate-intensity fires that burned periodically across California's Central Coast bioregion. While attempts have been made to reconstruct the fire history of this forest type, estimates of the fire-return interval vary widely, and the relationship of the fire-return interval to varying cultural influences is not fully understood. We analyzed 373 fire scars from 70 cross-sections removed from stumps, downed logs, and live trees in the coastal Santa Cruz Mountains of California, USA, in order to estimate fire-return intervals (FRI) for individual trees, mean FRI across samples, and seasonality of historical fires. The mean FRI, averaged across point samples, was 60.6 yr with a median of 40.1 yr. Fire scars were most prevalent in the dormant and latewood portions of annual growth rings. A sub-sample of 19 cross-sections, for which we were able to deter- RESUMENEl legado de fuegos pasados es evidente en forma de huecos ennegrecidos en la base de troncos encontrados en los bosques de sequoia roja (Sequoia sempervirens [D. Don] Endl.) de la región costera sur. Una mirada más profunda revela cicatrices en el cambium que datan de centurias pasadas, contándonos una historia de fuegos de baja a moderada intensidad que quemaron periódicamente a través de la Bioregión de la Costa Central de California. Mientras que varios intentos han sido realizados para reconstruir la historia del fuego en este tipo de bosque, estimaciones del intervalo de retorno varían ampliamente, y la relación entre el intervalo de retorno del fuego con influencias culturales variables no está completamente entendida. Nosotros analizamos 373 cicatrices de un total de 70 cortes transversales obtenidas de tocones, troncos caídos, y árboles vivos en la costa de las montañas de Santa Cruz en California, EEUU, para estimar los intervalos de retorno del fuego (FRI por sus siglas en inglés) para ár-boles individuales, el FRI promedio entre muestras, y la estacionalidad de fuegos históri-cos. El FRI medio, promediado de muestras puntuales, fue de 60,6 años con una mediana de 40,1 años. Las cicatrices de fuego fueron más prevalentes en las porciones latentes y de leño tardío de los anillos anuales de crecimiento. Una sub-muestra de 19 secciones transversales, Fire Ecology Volume 11, Issue 3, 2015 doi: 10.4996/fireecology.1103080 Jones and Russell: Fire-Return

Fuel load, stand structure, and understory species composition following prescribed fire in an old-growth coast redwood (Sequoia sempervirens) forest

“…With human settlement in the region, the majority of fires became anthropogenic in origin (McBride 1983). Indigenous burning generally resulted in low-intensity surface fires, with higher frequencies occurring in close proximity to grasslands, oak (Quercus L.) woodlands, and indigenous settlements (Greenlee and Langenheim 1990;Stephens and Fry 2005;Lorimer et al 2009;Jones and Russell 2015). Over time, the frequency and seasonality of fire has changed with cultural land use practices (Stuart 1987;Greenlee and Langenheim 1990).…”

Tree-Ring Indicators of Fire in Two Old-Growth Coast Redwood Forests

“…Warm and dry summers constrain inter-annual redwood growth with a general trend of increasing sensitivity to drought and maximum temperature from north to south (Carroll et al 2014). Most fires in the redwood region occur during dry late summer to early fall, as shown by twentieth century records (Gripp 1976) and sub-annual position of fire scars (Brown and Swetnam 1994, Brown and Baxter 2003, Jones and Russell 2015. In the North Coast Ranges of California, USA, fire often occurs during dry years (Skinner et al 2009), and across the western USA, climate contributes to regionally synchronous fire years while terrain, fuels, and aspect influence landscape-level variability in fire severity (Swetnam 1993, Heyerdahl et al 2001, Bigio et al 2016.…”

“…Most redwood fire-return intervals derive from ring counts of undated tree-ring records (Jacobs et al 1985, Stephens and Fry 2005, Jones and Russell 2015, and we know of only one published fire history based on crossdated samples from pre-Euro-American-settlement trees (Brown and Swetnam 1994). Redwood exhibits frequent discontinuous or missing rings Averill 1924, Carroll et al 2014), declining radial growth with age that can result in ring widths <0.1 mm (Sillett et al 2010, Carroll et al 2014, and anomalous growth near basal buttressing (Sillett et al 2015).…”

“…Redwood, giant sequoia, and other members of the Cupressaceae respond to injury by producing inducible traumatic resin ducts (TRD) in the xylem (Hudgins et al 2004, Krokene et al 2008, and fire-associated TRD occur up to 6 m above the ground on redwood trunks (Brown and Swetnam 1994). TRD provide evidence of fire in redwoods (Jones and Russell 2015), but the mechanics and variation of fire-induced TRD are poorly understood. Other anatomical reactions to heat exposure in conifers include resin soaking, discolored wood, and callus tissue formation (Smith et al 2016).…”

“…Other anatomical reactions to heat exposure in conifers include resin soaking, discolored wood, and callus tissue formation (Smith et al 2016). Intra-annual density fluctuations (IADFs) such as false rings and double latewood correspond with fire in both redwood and giant sequoia (Brown and Swetnam 1994, Jones and Russell 2015. IADFs result from disruption of normal xylogenesis during the growing season and reflect variations in environmental conditions often caused by climate (Vieira et al 2010, De Micco et al 2016.…”