Alternative community states maintained by fire in the Klamath Mountains, USA

Abstract

Summary 1.The earliest examples of alternative community states in the literature appear to be descriptions of natural vegetation said to both depend on and promote fire. Nonetheless, alternative community states determined by fire have rarely been documented at landscape scales and in natural vegetation. This is because spatial autocorrelation may confound analyses, experimental manipulations are difficult and a long-term perspective is needed to demonstrate that alternative community states can persist for multiple generations. 2. We hypothesized that alternative community states occur in a largely forested landscape in the Klamath Mountains, north-western California, USA, where shrub-dominated sclerophyllous vegetation establishes after fire that is lethal to forests. Forests redevelop if succession is not arrested by fire. Our hypothesis would require that sclerophyll and forest vegetation states each be maintained by different self-reinforcing relationships with fire. 3. To test this hypothesis, we examined pyrogenicity of forest and sclerophyll vegetation as a function of time since the previous fire, accounting for spatial autocorrelation. Fire exclusion served as a de facto experimental treatment. Areas where fire had proceeded to occur served as controls. 4. Our findings are consistent with the occurrence of alternative community states established and maintained by different self-reinforcing feedbacks with fire. Sclerophyll vegetation was more pyrogenic, especially where time-since-fire (TSF) was relatively short, a favourable relationship for this fire-dependent vegetation. Forests were much less pyrogenic, especially where TSF was long, favouring their maintenance. Fire exclusion therefore has led to afforestation and rapid retreat of fire-dependent vegetation. 5. Synthesis: We have documented how different self-reinforcing combustion properties of forest and sclerophyll vegetation can naturally produce alternative states coexisting side-by-side in the same environment. Such fire-mediated alternative states may be underappreciated, in part, because they are difficult to demonstrate definitively. In addition, the dynamics they exhibit contrast with common perceptions that fire hazard increases deterministically with TSF in forests and shrublands. Addressing the impacts of fire exclusion will probably require a management shift to better allow fire to perform its ecological role in shaping landscape diversity and maintaining firedependent biota.

Highly episodic fire and erosion regime over the past 2,000 y in the Siskiyou Mountains, Oregon

“…Thus, varying amplitudes of interannual-todecadal-scale climatic variability (e.g., El Niño Southern Oscillation) could have a major impact on the prevalence of fire climate in our study area (16). Episodic fire patterns also may be promoted by fire-vegetation feedbacks if vegetation switches among alternate stable states (shrublands versus forest) with different levels of pyrogenicity (3). Unfortunately, the open-shrub vegetation types (e.g., Quercus) are poorly represented in the USL pollen record and cannot help test this hypothesis.…”

“…historical fire | climate variability | ecological resilience | logging | sediment charcoal F ire plays a key role structuring ecosystems of the floristically diverse Siskiyou and Klamath Mountains of southwest Oregon and Northwest California (1,2), driving successional pathways (3) and affecting the erosion rate of soils (4,5). Fire-history data obtained from tree-ring records suggest that before Euro-American settlement forests supported a mixed-severity fire regime characterized by minor amounts of stand-replacing high-severity fire (6)(7)(8).…”

Tamm Review: Management of mixed-severity fire regime forests in Oregon, Washington, and Northern California

“…No two landscapes were alike. Variation in landscape patterns of physiognomic types too created unique fire regime interactions between types (Lauvaux et al, 2016;Odion et al, 2010). For example, in landscapes with mixed forest and grassland/shrubland conditions, grass-fire/ shrub-fire cycles were often influential to adjacent forest fire frequency and severity.…”

“…Varying patterns of physiognomic conditions (sparse woodland, pre-forest, forest, herbland, and shrubland) are also clearly apparent in most historical MSForests as a consequence of disturbance frequency and intensity, and climatic influences (Hessburg et al, 1999a(Hessburg et al, , 1999bLenihan et al, 2003;Maxwell et al, 2014;Millar et al, 2007;Neilson, 1986Neilson, , 1995. For example, historically it was common for some MSForest settings to remain for a time in alternate woodland, shrubland, or grassland states due to relatively high fire frequency, coupled with occasional high-severity fire (Beisner et al, 2003;Odion et al, 2010). But with a warming climate creating conditions for more high-severity fire (Westerling et al, 2003;Lenihan et al, 2006), and the occurrence of larger and more frequent high-severity burned areas (Miller et al, , 2012Harris and Taylor, 2015), there is now a greater potential for severely burned patches to be converted to these alternative stable states (Lauvaux et al, 2016;Long et al, 2014a;Harris and Taylor, 2015;Perry et al, 2011;Savage and Mast, 2005).…”

Landscape‐scale modeling of reference period forest conditions and fire behavior on heavily logged lands

“…Reference conditions are also used as a source of information for identifying restoration goals and the types of treatment that might be used in places where contemporary forest conditions are approaching a bounded range of variation (Moritz et al 2013) or have moved outside their historical range of variability (Morgan et al 1994, White and Walker 1997, Swetnam et al 1999, Taylor 2004, Larson and Churchill 2012, Wiens et al 2012. Presumably, moving highly altered forests towards conditions within the historical range of variability should increase their resilience to wildfire and reduce the risk of unexpected outcomes such as vegetation type conversions caused by unusually severe wildfire related to high fuel loads from logging and or fire exclusion (Weatherspoon and Skinner 1996, Landres et al 1999, Savage and Mast 2005, Odion et al 2010. Consequently, identifying reference conditions is a key step in the ecosystem managementrestoration planning process but it is a particularly challenging task in locations where most if not all of the pre-settlement forests have been removed by logging and other types of human activity (Landres et al 1999, Swetnam et al 1999, Scholl and Taylor 2010, Wiens et al 2012.…”