Fire disturbance, forest structure, and stand dynamics in montane forests of the southern Cascades, Thousand Lakes Wilderness, California, USA

Abstract

We examined tree diameter, age structure, and successional trends in 100 montane forest plots to identify the effects of variation in the return interval, severity, and extent of fires on forest structure and dynamics in the southern Cascade Range, California. We classified 100 forest plots into 8 groups based on stand structural characteristics. Median point fire return intervals were shortest in lower montane mixed conifer and Jeffrey pine-white fir stands (13-25 y) and upper montane red fir-white fir stands (14.5-19.5 y), intermediate in lodgepole pine stands (50-76.5 y), and longest in high-elevation red fir-mountain hemlock stands (100 y). Fire severity was mainly moderate to high in all forest structural groups except red fir-mountain hemlock. In the late 19 th century, large, mostly high-severity fires burned through all forests. Fire extent varied among structural groups, burning from 13% to 85% of plots in a group on average. Stands differed in composition, but size and age structures were similar across structural groups, with few trees > 100 y old and peaks of establishment between 1895 and 1955 for all groups except red fir-mountain hemlock. Distinct pulses of tree recruitment followed the most recent (1883, 1885, 1889, 1918) large and mainly high-severity fires. Suppression of fire since 1905 has increased understory density of shade-tolerant, fire-intolerant species and caused forest compositional shifts, particularly in lower-elevation Jeffrey pinewhite fir and mixed conifer stands, and lodgepole pine stands on well-drained sites. Structural or compositional change is less pronounced in upper montane red fir-white fir and red fir-mountain hemlock forests. The combination of gently sloping terrain with few fire breaks, extensive, moderate-to high-severity fires in all forest types and gradient positions and fire suppression has promoted homogenization of forest structure that may lead to large and severe fires in the future.

Efecto De La Severidad Del Fuego en Las Características De La Estructura Forestal en Rodales De Coníferas

“…With respect to the species distribution, Figure 4 shows that jóvenes como adultos, promoviendo el desarrollo de masas forestales de distintas edades y dimensiones (Bekker & Taylor, 2010). Sin embargo, otros mecanismos también influyen en la distribución espacial de los árboles; por ejemplo, la heterogeneidad microambiental es un conductor potencialthe correlation function of mark indicates that in the area with low severity, the species mix almost in all distances analyzed; however , in medium and high severities, the correlation was positive (K mm [r] > 1).…”

“…De acuerdo con las estadísticas nacionales, la cantidad de incendios va en aumento; tan sólo hasta el mes de septiembre de 2013, cerca de 415,000 ha se han visto afectadas registrándose más de 10,000 incendios distribuidos en todo el país (Comisión Nacional Forestal [CONAFOR], 2013). Sin embargo, en ecosistemas como los bosques de coníferas, la presencia y la severidad con que un incendio se desarrolla contribuyen a mantener o eliminar la dinámica de crecimiento de las especies, modificar su composición, diversificar la estructura de dimensiones y crear un mosaico heterogéneo de edades (Collins, Everett, & Stephens, 2011;Omi, 2005;Phillips & Waldrop, 2008;Pomerenning, 2006) no sólo a escala de rodal sino también de paisaje (Alanís et al, 2010;Bekker & Taylor, 2010;Gill, Woinarski, & York, 1999;González, Szejner, Muñoz, & Silva, 2011). De esta forma, después de un incendio, los bosques experimentan cambios inmediatos en los patrones de distribución del arbolado (Beaty & Taylor, 2008;Lecomte, Simard & Bergeron, 2006;Li-Juan & Jian-Ping, 2003;Stephens & Fry, 2005;Taylor, 2010) y en la variación de características de la estructura forestal (Ávila et al, 2012;Yu, Wiegand, & Yang, 2009).…”

“…In medium and high severities, the pattern of dimensional distribution showed negatively correlation, indicating that trees are distributed in a spatial mixture of different dimensions. It has been reported that severities from moderate to high intensity, affect the spatial structure of the forest because they eliminate many trees both young and adults, promoting the development of forest stands of different ages and sizes (Bekker & Taylor, 2010). However, other mechanisms also influence the spatial distribution of trees; for example, the micro-environmental heterogeneity is a potentially important conductive of spatial patterns of trees in some locations (North, Innes, & Zald, 2007).…”

“…According to Mexican statistics, the number of fires is increasing, only until September 2013, about 415,000 ha have been affected, recording more than 10,000 fires distributed across the country (Comisión Nacional Forestal [CONAFOR], 2013). However, in ecosystems such as coniferous forests, the presence and the severity with which a fire develops help to maintain or eliminate the growth dynamics of the species, change its composition, diversify the dimensional structure and create a patchwork of age (Collins, Everett, & Stephens, 2011;Omi, 2005;Phillips & Waldrop, 2008;Pomerenning, 2006) not only at the stand level but also landscape level (Alanís et al, 2010;Bekker & Taylor, 2010;Gill, Woinarski, & York, 1999;González, Szejner, Muñoz, & Silva, 2011). Thus, after a fire, forests experience immediate changes in tree distribution patterns (Beaty & Taylor, 2008;Lecomte, Simard & Bergeron, 2006;Li-Juan & Jian-Ping, 2003;Stephens & Fry, 2005;Taylor, 2010) and the variation of characteristics of forest structure (Ávila et al, 2012;Yu, Wiegand, & Yang, 2009).…”

Changing tree composition by life history strategy in a grassland‐forest landscape

“…Whether climate has warmed to historical levels and potentially, precipitation has decreased (or the length between precipitation events has increased), or counter-intuitively, growing season temperatures have cooled and growing season precipitation has increased (McEwan et al 2011), tree species that have increased in current forests show a range of shade and drought tolerance, from early-successional eastern redcedar to mesic, near-riparian tree species (Hanberry et al 2012a). Further patterns that conflict with climate change as a driving factor of forest change include (1) historically dominant oaks and pine are decreasing and contracting in range, not shifting in range to track climate, (2) although climate outlines species distributions, most species do not realize their potential distribution, as evidenced by naturalization outside of ranges, (3) tree establishment and growth do not always match with weather oscillations (e.g., Bekker and Taylor 2010, Kaye et al 2010, Schoennagel et al 2011. However, changes that (1) began at least a century ago, (2) occur across a continent, (3) operate on a temporal scale of tree generations, (4) and have little documentation make experimental and even correlational studies difficult.…”