Limits to reproduction and seed size-number trade-offs that shape forest dominance and future recovery.

The relationships that control seed production in trees are fundamental to understanding the evolution of forest species and their capacity to recover from increasing losses to drought, fire, and harvest. A synthesis of fecundity data from 714 species worldwide allowed us to examine hypotheses that are central to quantifying reproduction, a foundation for assessing fitness in forest trees. Four major findings emerged. First, seed production is not constrained by a strict trade-off between seed size and numbers. Instead, seed numbers vary over ten orders of magnitude, with species that invest in large seeds producing more seeds than expected from the 1:1 trade-off. Second, gymnosperms have lower seed production than angiosperms, potentially due to their extra investments in protective woody cones. Third, nutrient-demanding species, indicated by high foliar phosphorus concentrations, have low seed production. Finally, sensitivity of individual species to soil fertility varies widely, limiting the response of community seed production to fertility gradients. In combination, these findings can inform models of forest response that need to incorporate reproductive potential.

Globally, tree fecundity exceeds productivity gradients.

Lack of tree fecundity data across climatic gradients precludes the analysis of how seed supply contributes to global variation in forest regeneration and biotic interactions responsible for biodiversity. A global synthesis of raw seedproduction data shows a 250-fold increase in seed abundance from cold-dry to warm-wet climates, driven primarily by a 100-fold increase in seed production for a given tree size. The modest (threefold) increase in forest productivity across the same climate gradient cannot explain the magnitudes of these trends. The increase in seeds per tree can arise from adaptive evolution driven by intense species interactions or from the direct effects of a warm, moist climate on tree fecundity. Either way, the massive differences in seed supply ramify through food webs potentially explaining a disproportionate role for species interactions in the wet tropics.

Assessing the environmental and social co-benefits and disbenefits of natural risk management measures.

Risk management measures (RMM) participate in the sustainability of cities and communities through the protection of these socio-eco-environmental systems against threatening events, and by ensuring system recovery. They include structural measures that are grey or green/blue solutions, or hybrid solutions combining the two former types. These measures can provide environmental and social co-benefits (e.g., improved biodiversity, recreational services) and disbenefits (e.g., the development of unwanted flora, concentrations of pollutants). The aim of this article is to provide an approach to assess and compare RMMs by considering these different dimensions. An application to three natural hazards - floods, coastal floods and wildfires - is proposed. The approach takes the form of a procedure to assess the co-benefits/disbenefits of the various RMMs and some technical specifications. It allows comparing the performances of one RMM against another and collectively discussing the choice of RMMs that takes into account a wide range of dimensions. The approach is based on the formulation of eight sustainability criteria and thirty-one indicators. The results were graphically displayed as several types of diagram: one radar chart per RMM, compiling all the indicators; one radar chart by type of risk studied (flood, wildfire and coastal flooding) based on averages of indicators per criterion; a table of the global score assigned to each RMM calculated with an arithmetic mean or a weighted mean. The approach relies on an interdisciplinary research team and involves end-users in a focus group for the validation step. This approach constitutes a transparent base for decision-making processes in the context of sustainable spatial planning against natural risks.

Increased likelihood of heat-induced large wildfires in the Mediterranean Basin.

Wildfire activity is expected to increase across the Mediterranean Basin because of climate change. However, the effects of future climate change on the combinations of atmospheric conditions that promote wildfire activity remain largely unknown. Using a fire-weather based classification of wildfires, we show that future climate scenarios point to an increase in the frequency of two heat-induced fire-weather types that have been related to the largest wildfires in recent years. Heat-induced fire-weather types are characterized by compound dry and warm conditions occurring during summer heatwaves, either under moderate (heatwave type) or intense (hot drought type) drought. The frequency of heat-induced fire-weather is projected to increase by 14% by the end of the century (2071-2100) under the RCP4.5 scenario, and by 30% under the RCP8.5, suggesting that the frequency and extent of large wildfires will increase throughout the Mediterranean Basin.

Modelling fire hazard in the southern Mediterranean fire rim (Bejaia region, northern Algeria).

The southern rim of the Mediterranean Basin (MB) has a long fire history but fire hazard is poorly investigated in comparison to the northern rim. We built a fire database using MODIS data (2001-2015) for an area typical of the northern coastal Algeria (Bejaia region) in order to decipher the role of environmental and anthropic controls on the fire frequency and the area burnt. We found a high role of bioclimate, which controls the fuel dryness, ignitability, and biomass. Maximal fire frequency and burnt areas were recorded in northern sub-humid areas with high amounts of forests and shrublands, and fire was limited in the southern sub-arid area. Humans set most fires, and preferentially burn forests, shrublands, pastures, groves, and agricultural lands. The maximal fire frequency and burnt area occurs in wildland urban interfaces characterized by forest-shrublands mosaics with disseminated habitats. Fire activity is low to medium in rural-urban interfaces characterized by agropastoral areas with high habitat density and large habitat patches. Small to large crown fires occur in forests and shrublands, while small surface fires predominate in agropastoral areas and groves. Large fires (> 100 ha) are rare (10%) but contribute for ca. 50% to the total area burnt. These fire features are typical of many rural countries of the southern rim of the MB, and contrast with those on the northern rim. Based on this, we propose to improve the prevention, the detection, and the management of forest fires in the long term and to protect forests that host high biodiversity in Algeria.

Wildfire Policy in Mediterranean France: How Far is it Efficient and Sustainable?

A new fire policy reinforcing aggressive fire suppression was established in Mediterranean France in response to the devastating wildfires of the 1990s, but to what extent this has changed fire activity yet remains poorly understood. For this purpose, we compared the number and location of ignitions and of burned areas between two 20-year periods (1975-1994 vs. 1995-2014), in parallel to the changes in fuel covering, human activity promoting ignitions, and fire weather. The number of fires decreased almost continuously since 1975, but sharply after 1994, suggesting an effect of better fire prevention due to the new policy. But the major change in fire activity is a considerable reduction in fire size and burned areas after 1994, especially during summer and in the most fire-prone places, in response to massive efforts put into fire suppression. These reductions have occurred while the covering by fuel biomass, the human pressure on ignition, and the fire weather index increased, thus making the study area more hazardous. Our results suggest that a strategy of aggressive fire suppression has great potential for counterbalancing the effects of climate changes and human activities and for controlling fire activity in the short term. However, we discuss whether such a suppression-oriented approach is sustainable in the context of global changes, which cast new fire challenges as demonstrated by the devastative fires of 2003 and 2016. We advocate for a more comprehensive fire policy to come.

[Effects of fire recurrence on fire behaviour in cork oak woodlands (Quercus suber L.) and Mediterranean shrublands over the last fifty years]. / Effets de la récurrence des incendies sur le comportement du feu dans des suberaies (Quercus suber L.) et maquis méditerranéens sur les cinquante dernières années.

Past fire recurrence impacts the vegetation structure, and it is consequently hypothesized to alter its future fire behaviour. We examined the fire behaviour in shrubland-forest mosaics of southeastern France, which were organized along a range of fire frequency (0 to 3-4 fires along the past 50 years) and had different time intervals between fires. The mosaic was dominated by Quercus suber L. and Erica-Cistus shrubland communities. We described the vegetation structure through measurements of tree height, base of tree crown or shrub layer, mean diameter, cover, plant water content and bulk density. We used the physical model Firetec to simulate the fire behaviour. Fire intensity, fire spread, plant water content and biomass loss varied significantly according to fire recurrence and vegetation structure, mainly linked to the time since the last fire, then the number of fires. These results confirm that past fire recurrence affects future fire behaviour, with multi-layered vegetation (particularly high shrublands) producing more intense fires, contrary to submature Quercus woodlands that have not burnt since 1959 and that are unlikely to reburn. Further simulations, with more vegetation scenes according to shrub and canopy covers, will complete this study in order to discuss the fire propagation risk in heterogeneous vegetation, particularly in the Mediterranean area, with a view to a local management of these ecosystems.

Wildfire risk for main vegetation units in a biodiversity hotspot: modeling approach in New Caledonia, South Pacific.

Wildfire has been recognized as one of the most ubiquitous disturbance agents to impact on natural environments. In this study, our main objective was to propose a modeling approach to investigate the potential impact of wildfire on biodiversity. The method is illustrated with an application example in New Caledonia where conservation and sustainable biodiversity management represent an important challenge. Firstly, a biodiversity loss index, including the diversity and the vulnerability indexes, was calculated for every vegetation unit in New Caledonia and mapped according to its distribution over the New Caledonian mainland. Then, based on spatially explicit fire behavior simulations (using the FLAMMAP software) and fire ignition probabilities, two original fire risk assessment approaches were proposed: a one-off event model and a multi-event burn probability model. The spatial distribution of fire risk across New Caledonia was similar for both indices with very small localized spots having high risk. The patterns relating to highest risk are all located around the remaining sclerophyll forest fragments and are representing 0.012% of the mainland surface. A small part of maquis and areas adjacent to dense humid forest on ultramafic substrates should also be monitored. Vegetation interfaces between secondary and primary units displayed high risk and should represent priority zones for fire effects mitigation. Low fire ignition probability in anthropogenic-free areas decreases drastically the risk. A one-off event associated risk allowed localizing of the most likely ignition areas with potential for extensive damage. Emergency actions could aim limiting specific fire spread known to have high impact or consist of on targeting high risk areas to limit one-off fire ignitions. Spatially explicit information on burning probability is necessary for setting strategic fire and fuel management planning. Both risk indices provide clues to preserve New Caledonia hot spot of biodiversity facing wildfires.

Bark flammability as a fire-response trait for subalpine trees.

Relationships between the flammability properties of a given plant and its chances of survival after a fire still remain unknown. We hypothesize that the bark flammability of a tree reduces the potential for tree survival following surface fires, and that if tree resistance to fire is provided by a thick insulating bark, the latter must be few flammable. We test, on subalpine tree species, the relationship between the flammability of bark and its insulating ability, identifies the biological traits that determine bark flammability, and assesses their relative susceptibility to surface fires from their bark properties. The experimental set of burning properties was analyzed by Principal Component Analysis to assess the bark flammability. Bark insulating ability was expressed by the critical time to cambium kill computed from bark thickness. Log-linear regressions indicated that bark flammability varies with the bark thickness and the density of wood under bark and that the most flammable barks have poor insulating ability. Susceptibility to surface fires increases from gymnosperm to angiosperm subalpine trees. The co-dominant subalpine species Larix decidua (Mill.) and Pinus cembra (L.) exhibit large differences in both flammability and insulating ability of the bark that should partly explain their contrasted responses to fires in the past.

Wildfire frequency varies with the size and shape of fuel types in southeastern France: implications for environmental management.

Characterizing time intervals between successive fires in the recent history is of main interest for fire hazard prevention and sustainable environmental management as it indicates what the typical fire return interval for each type of ecosystem is. We tested the extent to which fire return intervals (FRIs) depend on fuel type and age, and we compared FRI values between two fire-prone areas of south-eastern France (Provence). These areas had similar weather and roughly similar fuel types but fuels occurred in patches with different sizes and shapes in the landscape. We built a fire database (1960-2010) and we fitted Weibull distributions of FRI in order to compute the probability density function and the hazard of burning. Our results indicate maximal probability of burning again for shrublands (garrigues and maquis), and minimal values for mixed broadleaf-conifer forests and broadleaved forests. Most fuel types of Provence showed no effect of fuel age on the probability of burning again. Only the unmanaged maquis showed a linear increase of fire hazard in time due to a rapid postfire fuel build up. Rather long fire-free intervals and low age-dependency for most forest fuels of Provence suggest that reducing their biomass may not be sufficient to reduce fire risk. In contrast, the flammable shrublands have rather short fire return intervals and represent a high fire hazard for the whole study area. The two areas had statistically significant difference of fire return intervals for a same fuel type (e.g. 18-22 years for shrublands, 20-24 years for pine forests, and 24-27 years for oak forests). This suggested that size, shape and connectivity of fuels play a major role in the probability of burning again and should be taken into account for fire management. The present policy of fire prevention puts efforts into public information and prevention, and preferential management of fuels at risk in the vicinity of roads and wildland-urban interfaces where fires occur preferentially. However, fire suppression may also take advantage of favouring low-flammable fuels with low age-dependency on strategic places in the landscape.

Cork oak vulnerability to fire: the role of bark harvesting, tree characteristics and abiotic factors.

Forest ecosystems where periodical tree bark harvesting is a major economic activity may be particularly vulnerable to disturbances such as fire, since debarking usually reduces tree vigour and protection against external agents. In this paper we asked how cork oak Quercus suber trees respond after wildfires and, in particular, how bark harvesting affects post-fire tree survival and resprouting. We gathered data from 22 wildfires (4585 trees) that occurred in three southern European countries (Portugal, Spain and France), covering a wide range of conditions characteristic of Q. suber ecosystems. Post-fire tree responses (tree mortality, stem mortality and crown resprouting) were examined in relation to management and ecological factors using generalized linear mixed-effects models. Results showed that bark thickness and bark harvesting are major factors affecting resistance of Q. suber to fire. Fire vulnerability was higher for trees with thin bark (young or recently debarked individuals) and decreased with increasing bark thickness until cork was 3-4 cm thick. This bark thickness corresponds to the moment when exploited trees are debarked again, meaning that exploited trees are vulnerable to fire during a longer period. Exploited trees were also more likely to be top-killed than unexploited trees, even for the same bark thickness. Additionally, vulnerability to fire increased with burn severity and with tree diameter, and was higher in trees burned in early summer or located in drier south-facing aspects. We provided tree response models useful to help estimating the impact of fire and to support management decisions. The results suggested that an appropriate management of surface fuels and changes in the bark harvesting regime (e.g. debarking coexisting trees in different years or increasing the harvesting cycle) would decrease vulnerability to fire and contribute to the conservation of cork oak ecosystems.

Recurrent fires and environment shape the vegetation in Quercus suber L. woodlands and maquis.

The effects of fire recurrence on vegetation patterns in Quercus suber L. and Erica-Cistus communities in Mediterranean fire-prone ecosystems of south-eastern France were examined on stands belonging to 5 fire classes, corresponding to different numbers of fires (from 0 to 4) and time intervals between fires since 1959. A common pool of species was identified among the plots, which was typical of both open and closed maquis. Fire recurrence reduced the abundance of trees and herbs, whereas it increased the abundance of small shrubs. Richness differed significantly between the most contrasting classes of fire recurrence, with maximal values found in control plots and minimal values in plots that had burned recurrently and recently. Equitability indices did not vary significantly, in contrast to Shannon's diversity index which mostly correlated with richness. Forest ecosystems that have burnt once or twice in the last 50 years were resilient; that is to say they recovered a biomass and composition similar to that of the pre-fire state. However, after more than 3-4 fires, shrubland communities displayed lower species richness and diversity indices than unburned plots. The time since the last fire and the number of fires were the most explanatory fire variables, governing the structure of post-fire plant communities. However, environmental factors, such as slope or exposure, also made a significant contribution. Higher rates of fire recurrence can affect the persistence or expansion of shrublands in the future, as observed in other Mediterranean areas.

Landscape--wildfire interactions in southern Europe: implications for landscape management.

Every year approximately half a million hectares of land are burned by wildfires in southern Europe, causing large ecological and socio-economic impacts. Climate and land use changes in the last decades have increased fire risk and danger. In this paper we review the available scientific knowledge on the relationships between landscape and wildfires in the Mediterranean region, with a focus on its application for defining landscape management guidelines and policies that could be adopted in order to promote landscapes with lower fire hazard. The main findings are that (1) socio-economic drivers have favoured land cover changes contributing to increasing fire hazard in the last decades, (2) large wildfires are becoming more frequent, (3) increased fire frequency is promoting homogeneous landscapes covered by fire-prone shrublands; (4) landscape planning to reduce fuel loads may be successful only if fire weather conditions are not extreme. The challenges to address these problems and the policy and landscape management responses that should be adopted are discussed, along with major knowledge gaps.

Tempol, a superoxide dismutase mimetic, prevents cerebral vessel remodeling in hypertensive rats.

Increased reactive oxygen species (ROS) production is involved in the pathogenesis of hypertension and stroke. The effects of ROS on cerebral vessels from hypertensive rats have not been studied. We hypothesized that tempol, a superoxide dismutase mimetic, would prevent middle cerebral artery (MCA) remodeling in stroke-prone spontaneously hypertensive rats (SHRSP). Six-week-old male SHRSP were treated with tempol (1mM) for 6weeks. The MCA was then removed and mounted in a pressure myograph to study tone generation, vessel reactivity, and passive vessel structure. Data are shown as mean±SEM, tempol vs. control. Plasma thiobarbituric acid reactive substances (TBARS) were decreased by tempol treatment (14.15±1.46 vs. 20.55±1.25nM of malondialdehyde [MDA]/ml, p=0.008). Maximum serotonin-induced constriction was increased by tempol treatment, without changes in dilation to adenosine diphosphate or tone generation. At an intralumenal pressure of 80mmHg, tempol caused a dramatic increase in the MCA lumen diameter (246±5 vs. 207±3µm, p<0.001), outer diameter (281±5 vs. 241±3µm, p<0.001), lumen cross-sectional area, and vessel cross-sectional area. Collagen IV mRNA expressions were increased by 2.4-fold after tempol treatment. These results suggest that ROS are involved in the remodeling of the cerebral vasculature of SHRSP and that ROS scavenging can attenuate this process.

Six-year time course of light-use efficiency, carbon gain and growth of beech saplings (Fagus sylvatica) planted under a Scots pine (Pinus sylvestris) shelterwood.

Two-year-old Fagus sylvatica L. saplings were planted under the cover of a Pinus sylvestris L. stand in the French Massif Central. The stand was differentially thinned to obtain a gradient of transmitted photosynthetically active radiation (PAR(t); 0-0.35). Eighteen Fagus saplings were sampled in this gradient, and their growth (basal stem diameter increment) was recorded over six years. Over the same period, morphological parameters (leaf area, number and arrangement in space) were monitored by 3D-digitization. Photosynthetic parameters were estimated with a portable gas-exchange analyzer. Photosynthesis was mainly related to light availability, whereas sapling morphology was mainly driven by sapling size. Annual stem diameter increment was related to the amount of light-intercepting foliage (silhouette to total leaf area ratio (STAR) x total sapling leaf area (LA)) and light availability above the saplings (PAR(t)). However, light-use efficiency, i.e., the slope of the relationship between STAR x LA x PAR(t) and stem diameter increment, decreased over time as a result of a relative decrease in the proportion of photosynthetic tissues to total sapling biomass.