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The area burned by Southern California wildfires has increased in recent decades, with implications for human health, infrastructure, and ecosystem management. Meteorology and fuel structure are universally recognized controllers of wildfire, but their relative importance, and hence the efficacy of abatement and suppression efforts, remains controversial. Southern California's wildfires can be partitioned by meteorology: fires typically occur either during Santa Ana winds (SA fires) in October through April, or warm and dry periods in June through September (non-SA fires). Previous work has not quantitatively distinguished between these fire regimes when assessing economic impacts or climate change influence. Here we separate five decades of fire perimeters into those coinciding with and without SA winds. The two fire types contributed almost equally to burned area, yet SA fires were responsible for 80% of cumulative 1990-2009 economic losses ($3.1 Billion). The damage disparity was driven by fire characteristics: SA fires spread three times faster, occurred closer to urban areas, and burned into areas with greater housing values. Non-SA fires were comparatively more sensitive to age-dependent fuels, often occurred in higher elevation forests, lasted for extended periods, and accounted for 70% of total suppression costs. An improved distinction of fire type has implications for future projections and management. The area burned in non-SA fires is projected to increase 77% (±43%) by the mid-21st century with warmer and drier summers, and the SA area burned is projected to increase 64% (±76%), underscoring the need to evaluate the allocation and effectiveness of suppression investments.

Wildland fires in Southern California can be divided into two categories: fall fires, which are typically driven by strong offshore Santa Ana winds, and summer fires, which occur with comparatively weak onshore winds and hot and dry weather. Both types of fire contribute significantly to annual burned area and economic loss. An improved understanding of the relationship between Southern California's meteorology and fire is needed to improve predictions of how fire will change in the future and to anticipate management needs. We used output from a regional climate model constrained by reanalysis observations to identify Santa Ana events and partition fires into those occurring during periods with and without Santa Ana conditions during 1959–2009. We then developed separate empirical regression models for Santa Ana and non-Santa Ana fires to quantify the effects of meteorology on fire number and size. These models explained approximately 58% of the seasonal and interannual variation in the number of Santa Ana fires and 36% of the variation in non-Santa Ana fires. The number of Santa Ana fires increased during years when relative humidity during Santa Ana events and fall precipitation were below average, indicating that fuel moisture is a key controller of ignition. Relative humidity strongly affected Santa Ana fire size. Cumulative precipitation during the previous three winters was significantly correlated with the number of non-Santa Ana fires, presumably through increased fine fuel density and connectivity between infrastructure and nearby vegetation. Both relative humidity and the preceding wet season precipitation influenced non-Santa Ana fire size. Regression models driven by meteorology explained 57% of the temporal variation in Santa Ana burned area and 22% of the variation in non-Santa Ana burned area. The area burned by non-Santa Ana fires has increased steadily by 1.7% year−1 since 1959 (p

Wildfire ignition requires a combination of an open spark, and suitable weather and fuel conditions. Models of fire occurrence and burned area provide a good understanding of the physical and climatic factors that constrain and promote fire spread and recurrence, but information on how humans influence ignition patterns is still lacking at a scale compatible with integrated fire management. We investigated the relative importance of the physical, climatic and human factors regulating ignition probability across Southern California's National Forests. A 30-year exploratory analysis of one-way relationships indicated that distance to a road, distance to housing and topographic slope were the major determinants of ignition frequency. We used logistic and Poisson regression analyses to model ignition occurrence and frequency as a function of the dominant covariates. The resulting models explained ∼70% of the spatial variability in ignition likelihood and 45% of the variability in ignition frequency. In turn, predicted ignition probability contributed to some of the spatial variability in burned area, particularly for summer fires. These models may enable estimates of fire ignition risk for the broader domain of Southern California and how this risk may change with future population and housing development. Our spatially explicit predictions may also be useful for strategic fire management in the region.

Ecosystem-based management advocates that forestry disturbances should aim to emulate natural disturbances to mitigate the landscape-level impact of forest management. This study compares the impact of clear-cuts followed by a prescribed burn (CCPB) with clear-cuts alone (CC) and current careful logging practices (CLAAG: 'careful logging around advanced growth') on understory composition within black spruce (Picea mariana Miller (BSP)) paludified forest stands at the plot, site, and treatment levels using a functional-type approach. Vascular and nonvascular taxa showed significant differences in composition at the plot level among treatments. We found that pioneer taxa occurred mainly in CCPB sites, while late-successional taxa characterized CC sites. CLAAG sites had higher taxa richness than CCPB and CC sites, and we found that CCPB treatments were most likely to promote vascular taxa compositions that are more similar to those observed after natural disturbances. Additionally, the relative abundance of Sphagnum spp., responsible for paludification, was significantly reduced in sites treated by prescribed burning. This study therefore presents results suggesting that prescribed burning might represent a sustainable alternative to current harvesting techniques in the Clay Belt of eastern Canada that could help in preserving biodiversity (in terms of understory species assemblage) while maintaining or even enhancing forest productivity. Key words: boreal forest ecosystem, black spruce forest stands, prescribed fire, clear-cut, biodiversity, understory composition, conservation management, paludification. Les strategies d'amenagement forestier ecosystemique preconisent d'imiter les effets des perturbations naturelles pour attenuer l'impact de traitements sylvicoles a l'echelle du paysage. Cette etude compare les effets de coupe totale suivie de brulage dirige (CCPB) avec ceux de coupe totale (CC) et de pratique sylvicole appliquee limitant la perturbation du sol (CLAAG : << careful logging around advanced growth >>) sur la composition des especes de sous-bois, au sein de peuplements d'epinette noire (Picea mariana Miller (BSP)) touches par la paludification. Les analyses, effectuees a l'echelle de la placette, du site et du traitement ont permis via l'analyse des taxons et des types fonctionnels d'examiner les effets respectifs de chaque traitement. Des differences de composition significatives parmi les especes vasculaires et non-vasculaires ont ete observees a l'echelle de la placette selon le type de traitement considere. Nous avons constate que les especes pionnieres etaient associees aux sites CCPB tandis que les especes de fin de succession etaient caracteristiques des sites CC. Une richesse specifique plus elevee a ete observee parmi les sites CLAAG que dans les sites CCPB et CC. Nous avons par ailleurs trouve que les traitements CCPB etaient davantage enclin a promouvoir des patrons de composition d'especes vasculaires similaires a ceux observes apres des perturbations naturelles comme les feux de foret. Nous avons egalement constate une abondance relative plus faible des especes de sphaigne, responsible du phenomene de paludification, au sein des sites traites par brulage dirige. Les resultats de cette etude suggerent ainsi que le brulage dirige represente une alternative durable aux pratiques sylvicoles actuelles en permettant de conserver la biodiversite (en termes d'assemblages d'especes) et de maintenir voire d'augmenter la productivite des peuplements exploites. Mots-cles: ecosysteme forestier boreal, peuplements d'epinette noire, brulage dirige, coupe totale, biodiversite, composition des especes de sous-bois, gestion de la conservation, paludification., Introduction In boreal forests, stand-replacing wildfires are considered to be the primary disturbances that drive natural forest dynamics (Bergeron et al. 2001; Harvey et al. 2002, Bouchard et al. 2008; [...]

Abstract: As phytohormones lie at the very core of molecular mechanisms controlling the plant physiology and development, they have long been hypothesized to be involved in insect‐induced plant manipulations. Cytokinins (CKs) are phytohormones now widely recognized to be utilized by leaf‐mining and gall‐inducing insects in the control of the physiology of their host plant. In some leaf‐mining moth species, larvae can supply the hormones themselves, bacterial symbionts contributing to the production of CKs. Our objective was to investigate whether closely related leaf‐miner species sharing the same ecological niche but differing in their Wolbachia infection status develop similar strategies to manipulate their host plant. An extensive identification and quantification of CKs has been used to elucidate physiological patterns relevant for the plant–insect interactions. Our results show that modulation of plant CK levels is impaired in the Wolbachia‐free leaf‐mining moth Phyllonorycter mespilella (Hübner) (Lepidoptera: Gracillariidae) contrasting with results previously observed in the closely related moth species Phyllonorycter blancardella (Fabricius) that produce and deliver CKs to the plant through an intricate interaction with Wolbachia. Our study suggests that mechanisms underlying colonization of the host plant and adaptation to fluctuating environmental conditions are different between the two leaf‐miner species and that P. mespilella larvae most likely do not produce CKs. This species rather only buffers the degradation of CKs naturally occurring during the senescence of leaves leading to few active CKs being maintained at a sufficiently high level to induce and maintain a ‘green island’ phenotype (photosynthetically active green patches in otherwise senescing leaves). This study further provides converging experimental evidence pointing toward the influence of bacterial symbionts in the ability of leaf‐mining moths to control the physiology of their host plant with consequences for their ecology and evolutionary diversification. [ABSTRACT FROM AUTHOR]