French Mediterranean Area Research Articles

Spatio-temporal characteristics of heavy precipitation events observed over the last decade on the eastern French Mediterranean coastal area

Study regioneastern part of the French Mediterranean coastal area. Study focusThis work focuses on the heavy precipitation events that have affected the eastern part of the French Mediterranean coastal area over the last decade and the river discharges associated with these events. The aim is to make a throughout analysis of the spatio-temporal characteristics of heavy precipitation events, and to evaluate whether a high flow event is associated with them when looking at river discharges on affected catchments. New hydrological insights for the regionBased on radar-based precipitation grids, 158 heavy precipitation events (HPEs) affected the study area over the period 2007–2020. In addition, 65 % of HPEs were associated with a high flow event (HFE) on one or more gauged catchments. Results show that HPEs were more frequent during the beginning of summer and autumn. A large portion of autumn and winter HPEs were associated with a HFE. Looking at the spatio-temporal characteristics of the HPEs, their mean duration was 20 hours, with 42 % of the events lasting less than 6 hours. Some common characteristics among the HPEs that caused HFEs were identified: all HPEs lasting more than 30 hours and all HPEs with catchment precipitation accumulation above 150 mm were associated with a HFE.

Does a convection-permitting regional climate model bring new perspectives on the projection of Mediterranean floods?

Abstract. Floods are the primary natural hazard in the French Mediterranean area, causing damages and fatalities every year. These floods are triggered by heavy precipitation events (HPEs) characterized by limited temporal and spatial extents. A new generation of regional climate models at the kilometer scale have been developed, allowing an explicit representation of deep convection and improved simulations of local-scale phenomena such as HPEs. Convection-permitting regional climate models (CPMs) have been scarcely used in hydrological impact studies, and future projections of Mediterranean floods remain uncertain with regional climate models (RCMs). In this paper, we use the CNRM-AROME CPM (2.5 km) and its driving CNRM-ALADIN RCM (12 km) at the hourly timescale to simulate floods over the Gardon d'Anduze catchment located in the French Mediterranean region. Climate simulations are bias-corrected with the CDF-t method. Two hydrological models, a lumped and conceptual model (GR5H) and a process-based distributed model (CREST), forced with historical and future climate simulations from the CPM and from the RCM, have been used. The CPM model confirms its ability to better reproduce extreme hourly rainfall compared to the RCM. This added value is propagated on flood simulation with a better reproduction of flood peaks. Future projections are consistent between the hydrological models but differ between the two climate models. Using the CNRM-ALADIN RCM, the magnitude of all floods is projected to increase. With the CNRM-AROME CPM, a threshold effect is found: the magnitude of the largest floods is expected to intensify, while the magnitude of the less severe floods is expected to decrease. In addition, different flood event characteristics indicate that floods are expected to become flashier in a warmer climate, with shorter lag time between rainfall and runoff peak and a smaller contribution of base flow, regardless of the model. This study is a first step for impact studies driven by CPMs over the Mediterranean.

Assessing the ability of a new seamless short-range ensemble rainfall product to anticipate flash floods in the French Mediterranean area

Abstract. Flash floods have dramatic economic and social consequences, and efficient adaptation policies are required to reduce their impacts, especially in the context of global change. Developing more efficient flash flood forecasting systems can largely contribute to these adaptation requirements. The aim of this study was to assess the ability of a new seamless short-range ensemble quantitative precipitation forecast (QPF) product, called PIAF-EPS (Prévision Immédiate Agrégée Fusionnée ensemble prediction system) and recently developed by Météo-France, to predict flash floods when used as input to an operational hydrological forecasting chain. For this purpose, eight flash flood events that occurred in the French Mediterranean region between 2019 and 2021 were reanalysed, using a hydrological-modelling chain similar to the one implemented in the French Vigicrues Flash operational flash flood monitoring system. The hydrological forecasts obtained from PIAF-EPS were compared to the forecasts obtained with different deterministic QPFs from which PIAF-EPS is directly derived. The verification method applied in this work uses scores calculated on contingency tables and combines the forecasts issued on each 1 km2 pixel of the territory. This offers a detailed view of the forecast performances, covering the whole river network and including the small ungauged rivers. The results confirm the added value of the ensemble PIAF-EPS approach for flash flood forecasting, in comparison to the different deterministic scenarios considered.

Examination of the parameters controlling the triple oxygen isotope composition of grass leaf water and phytoliths at a Mediterranean site: a model–data approach

Abstract. Triple oxygen isotopes (17O-excess) of water are useful to trace evaporation at the soil–plant–atmosphere interface. The 17O-excess of plant silica, i.e., phytoliths, inherited from leaf water, was previously calibrated in growth chambers as a proxy of atmospheric relative humidity (RH). Here, using a model–data approach, we examine the parameters that control the triple oxygen isotope composition of bulk grass leaf water and phytoliths in natura, at the O3HP experimental platform located in the French Mediterranean area. A grass plot was equipped to measure for 1 year, all environmental and plant physiological parameters relevant for modeling the isotope composition of the grass leaf water. In particular, the triple oxygen and hydrogen isotope composition of atmospheric water vapor above the grass was measured continuously using a cavity ring-down spectrometer, and the grass leaf temperature was monitored at plot scale using an infrared (IR) radiometer. Grass leaves were collected in different seasons of the year and over a 24 h period in June. Grass leaf water was extracted by cryogenic vacuum distillation and analyzed by isotope ratio mass spectrometry (IRMS). Phytoliths were analyzed by IR–laser fluorination–IRMS after chemical extraction. We showed that the traditional Craig–Gordon steady-state model modified for grass leaves reliably predicts the triple oxygen isotope composition of leaf water during daytime but is sensitive to uncertainties on the leaf-to-air temperature difference. Deviations from isotope steady state at night are well represented in the triple oxygen isotope system and predictable by a non-steady-state model. The 17O-excess of phytoliths confirms the applicability of the 17O-excessphyto vs. RH equation established in previous growth chamber experiments. Further, it recorded average daytime RH over the growth period rather than daily RH, related to low transpiration and silicification during the night. This model–data approach highlights the utility of the triple oxygen isotope system to improve the understanding of water exchange at the soil–plant–atmosphere interface. The in natura experiment underlines the applicability of 17O-excess of phytoliths as a RH proxy.

Role of land-cover and WUI types on spatio-temporal dynamics of fires in the French Mediterranean area.

This work aims at assessing, in the French Mediterranean area, the spatio-temporal trends of fires, including their causes, at fine scale (communities), comparing different periods between 1993 and 2017. These trends were compared to those of land-cover and wildland-urban interface (WUI) which were coupled with a spatial analysis of the ignitions in order to highlight the main drivers and preferential areas. Fire density was highly variable among communities, hotspots being located mostly close to big cities but spatially varying in time in contrast to fire occurrence and burned area. A decrease in the unknown cause proportionand a variation of the cause frequency were highlighted among periods, criminal fires being the most frequent and deleterious, especially before 2009, as well as those due to negligence during private activities, mostly after 2009. Land cover classes significantly varied among periods, artificialized and natural areas presenting a reversed trend compared with agricultural areas. Natural areas were the most affected by ignitions (60%), regardless of the period; this trend is slowly decreasing. WUI represented ∼30% of the study area, the different types varying spatially (denser clustered types mostly located in the South-East) and showed an increase over time, especially for both clustered types but with high variability among communities. Half of the ignitions occurred in WUI, with "very dense clustered" and "scattered" types being the most affected, especially in 2009. Better understanding the spatio-temporal evolution of fires and of their causes should allow refining the fire policies in terms of awareness raising, firefighting means, and land management.

Evaluation of convection-permitting extreme precipitation simulations for the south of France

Abstract. In the autumn, the French Mediterranean area is frequently exposed to heavy precipitation events whose daily accumulation can exceed 300 mm. One of the key processes contributing to these precipitation amounts is deep convection, which can be explicitly resolved by state-of-the-art convection-permitting models to reproduce heavy rainfall events that are comparable to observations. This approach has been tested and performed at climate scale in several studies in recent decades for different areas. In this research, we investigate the added value of using an ensemble of three climate simulations at convection-permitting resolution (approx. 3 km) to replicate extreme precipitation events at both daily and shorter timescales over the south of France. These three convection-permitting simulations are performed with the Weather Research and Forecasting (WRF) Model. They are forced by three EURO-CORDEX simulations, which are also run with WRF at the resolution of 0.11∘ (approx. 12 km). We found that a convection-permitting approach provides a more realistic representation of extreme daily and 3-hourly rainfall in comparison with EURO-CORDEX simulations. Their similarity to observations allows use for climate change studies and its impacts.

Winners and Losers in Energy Transition: Study Case of Wood Biomass Power-Plants Implementation in France

International policies promote renewable forms of energy to mitigate climate change. In Europe, the production of electricity using wood biomass represents one of the most popular energy alternatives. In 2012, France initiated a large-scale strategy to develop wood biomass energy. The biggest wood biomass power-plant project has been developed in the French Mediterranean area and its huge size raises several issues for the short- and long-term sustainability of local forests and associated economic sectors. The French Mediterranean forests provide four types of economic goods (private, club, common, and public goods) and multiple ecosystem services, which makes them complex to manage under an energy transition policy. In this paper, we applied three qualitative methods, namely interviews, participative workshops, and observant participation, and three conceptual models, namely (i) Ostrom’s (2010) self-organization key conditions, (ii) the types of economic goods classified according to their excludability and rivalry properties, and (iii) the ecosystem service categorization system of the Millennium Ecosystem Assessment (2005). With our methods, we show that the renewable strategy chosen in France replicates the current centralized production model based on fossil and nuclear fuels. Thus, we demonstrate that European, national, and local authorities fail to consider the multiple ecosystem services that forest management strategies should include to face the energy transition, climate change, and the other ecological challenges of the 21st century.

Correction to: Does recent fire activity impact fire-related traits of Pinus halepensis Mill. and Pinus sylvestris L. in the French Mediterranean area?

The original article was published with a small but significant mistake.

Does recent fire activity impact fire-related traits of Pinus halepensis Mill. and Pinus sylvestris L. in the French Mediterranean area?

Key message Climate change will induce a change in fire frequency in Mediterranean region and that could impact fire-adapted plant species. We showed that fire-related traits of some pine species are strongly related to other factors than fire but the recent fire history has nonetheless an impact on the variation of key traits for different fire adaptive strategies. ContextIn fire-prone Mediterranean areas, climate change is expected to exacerbate the fire pressure on ecosystems, altering the current fire regime and threatening species if they cannot acclimate.AimsStudying intraspecific variations of some fire-related traits in relation to variation in recent fire activity is thus an important step to better understand if this acclimation is possible.MethodsWe measured structural (bark thickness, shoot bulk density, self-pruning, leaf surface to volume ratio) and functional (serotiny level for Pinus halepensis only) traits in two pines species (Pinus halepensis and Pinus sylvestris) commonly found in southeastern France and that present different fire-adaptive strategies (resilience vs resistance, respectively). Populations were sampled according to different fire frequency modalities (0 vs 1 to 2 fires) along a geographical gradient, measuring numerous environmental and plant characteristics to be used cofactors in the analyses.ResultsAs expected, trait variation was strongly linked to environmental and tree characteristics as well as to ontogeny overriding the effect of fire modalities, even though using integrative models with random effect. However, fire modalities had an impact on the variance of some key fire-related traits of Pinus halepensis.ConclusionThis study will help to anticipate the future response of these Mediterranean pine species and further underlines the importance of investigating chemical traits, flammability, and genetic variation of highly heritable traits, such as serotiny.

Isoprene contribution to ozone production under climate change conditions in the French Mediterranean area

26 Tropospheric ozone is a strong oxidant which affects human health, agricultural yields, and 27 ecosystems functioning. Thus, it is very important to determine ozone formation in order to 28 control air pollution. It is well known that isoprene participates in ozone formation. In this 29 study, we assess the potential impact of climate change in the Mediterranean region on ozone 30 concentration, through drought-related increase or decrease in isoprene emissions after 1 (Short 31 Drought scenario-1 year of 35% annual rain restriction) and 3 (Long Drought scenario-3 32 repeated years of 35% annual restriction) years of drought stress. 33

Distribution and evolution of the major viruses infecting cucurbitaceous and solanaceous crops in the French Mediterranean area

Plant viral diseases represent a significant burden to plant health, and their highest impact in Mediterranean agriculture is on vegetables grown under intensive horticultural practices. In order to understand better virus evolution and emergence, the most prevalent viruses were mapped in the main cucurbitaceous (melon, squashes) and solanaceous (tomato, pepper) crops and in some wild hosts in the French Mediterranean area, and virus diversity, evolution and population structure were studied through molecular epidemiology approaches. Surveys were performed in summer 2016 and 2017, representing a total of 1530 crop samples and 280 weed samples. The plant samples were analysed using serological and molecular approaches, including high-throughput sequencing (HTS). The viral species and their frequency in crops were quite similar to those of surveys conducted ten years before in the same areas. Contrary to other Mediterranean countries, aphid-transmitted viruses remain the most prevalent in France whereas whitefly-transmitted ones have not yet emerged. However, HTS analysis of viral evolution revealed the appearance of undescribed viral variants, especially for watermelon mosaic virus (WMV) in cucurbits, or variants not present in France before, as for cucumber mosaic virus (CMV) in solanaceous crops. Deep sequencing also revealed complex virus populations within individual plants with frequent recombination or reassortment. The spatial genetic structure of cucurbit aphid-borne yellows virus (CABYV) was related to the landscape structure, whereas in the case of WMV, the recurrence of introduction events and probable human exchanges of plant material resulted in complex spatial pattern of genetic variation.

A framework for estimating species-specific contributions to community indicators

Community Weighted Means (CWM) are valuable tools describing community composition with respect to one given trait. They have been widely used as indicators in global change studies to measure biodiversity responses to environmental perturbations. However, how individual species contribute to such community indicators has hardly been investigated. One of the reasons lies in the absence of a methodological framework relating changes in community dynamics to species-specific population variations. Here, we present a comprehensive framework allowing a finer interpretation of changes in CWM, and we propose a way to compute species contributions to these indicators.We present an analytical framework allowing the quantification of species-specific contributions to changes in the mean (CWM) and the variance (Community Weighted Variance, CWV) of trait distributions in species assemblages monitored through time and space. We apply this approach to a case study investigating the impact of climate change on common bird assemblages in the French Mediterranean area between 2001 and 2012.This approach allows us to identify that a small proportion of the species drive the changes observed at the community level indicator, and allows the identification of those species. Moreover, we show that the species-specific contributions are not homogeneous between taxonomic groups and that migratory species tend to have a higher impact.This novel decomposition and interpretation of Community Weighted Means and Variances (for which specific software package is provided along with this article) sheds new light on the drivers of community modifications in response to environmental changes across time and space. Moreover, it represents a relevant and simple way to assess particular aspects of species-specific responses to environmental changes and it is straightforward to use for widely used ecological data on any species group.

Combining habitat suitability models and spatial graphs for more effective landscape conservation planning: An applied methodological framework and a species case study

In response to the negative effects of habitat fragmentation on biodiversity, habitat network conservation and restoration has become a central objective in conservation planning. Evaluating landscape functional connectivity and mapping habitat networks are key steps for implementing effective actions, but both remain challenging. Habitat suitability models (HSM) and spatial graphs provide conservation managers with useful information for landscape planning. To tackle their respective drawbacks, we proposed to combine HSM and spatial graphs in a four-step methodological framework: (i) collect and prepare input data; (ii) model habitat suitability using MaxEnt to map the species’ habitat suitability index (HSI); (iii) transform the HSI map into spatial graph inputs using GIS; and, (iv) perform spatial graph connectivity analysis using Graphab. The outputs of this species-specific and quantitative approach were maps of the species’ habitat network. Habitat patches and dispersal linkages were ranked according to their importance for overall habitat availability and landscape connectivity. This prioritization identified locations where conservation biologists and landscape planners should focus conservation and restoration efforts. This framework is illustrated here with a case study on the woodlark (Lullula arborea) - a bird species - in a French Mediterranean area, and the method’s limitations and alternatives are discussed. The quantitative-based graphical outputs of the framework can valuably support decision-making for landscape planning, complement local expert opinion, and encourage appropriate stakeholders to cooperate at regional scale.

An enteric methane emission calculator (DREEM) built to consider feed diversity: Case study of pastoral and sedentary farming systems

Ruminant livestock systems are significant sources of greenhouse gases (GHG). Livestock farming in regions with extreme climatic events have to face both scarcity and variability in feed resources. Herd mobility is a known major adaptation strategy to address seasonal availability of forage resources: it allows an increase in herd size, thereby improving labor productivity. The present study quantifies enteric methane (CH4) emissions from French Mediterranean sheep farming systems, focusing on the use of diversified pastoral feed resources, and developing a calculator (Diversity of feed REsources and Enteric Methane emissions, DREEM). The DREEM calculator was developed to estimate at the animal level enteric CH4 emissions (g/day) from empirical equations and be subsequently integrated, as a sub-table, into an economic and GHG (kg/year) balance model (Outil de Simulation du TRoupeau ovin ALaitant, OSTRAL) at the whole farm level. Several equations were taken from the literature to estimate enteric CH4 emissions in DREEM calculator. Nature of forage and feed, animal feeding levels and performance were referenced according to the animal feeding system and tables in France and taking into account the French Mediterranean area studied. DREEM was used to estimate enteric CH4 emissions from four sheep farming systems covering the main contrasting mobility and situations, from sedentary to highly mobile pastoral systems, in the French Mediterranean area. At the individual level, enteric CH4 emissions (g/day) of ewes in the sedentary system were slightly higher than those of ewes in other systems. These differences were due mainly to differences in animal feeding level (intake / body weight) and feed resources characteristics. Overall, enteric CH4 emissions of ewes and rams were slightly lower than French national inventory estimates. When enteric CH4 emissions of lambs were expressed in g/kg of carcass, were lower in the less pastoral farming systems than in the other systems, because lambs’ average daily gains were higher. In double transhuming farming systems, lambs late slaughtering age led to lamb’s CH4 contribution of 15% vs 2–5% in the other systems. Flock management, which depends on land use and ownership, greatly contributed to these results.

Identifying Consensus on Coastal Lagoons Ecosystem Services and Conservation Priorities for an Effective Decision Making: A Q Approach

Coastal lagoons ecosystems, while representing benefits for the local populations, have been subjected to high anthropogenic pressures for decades. Hence, conservation measures of these ecosystems are urgently needed and should be combined with their sustainable uses. To address these issues, new research avenues for decision support systems have emphasized the role of the assessment of ecosystem services for establishing conservation priorities by avoiding monetarization approaches. These approaches, because they flatten the various values of nature by projecting them on the single monetary dimension, are often rejected by the stakeholders. We undertake a Q analysis to identify levels of consensus and divergence among stakeholders on the prioritization of ecosystem services provided by two French Mediterranean coastal lagoons areas. The results highlighted that there is a strong consensus among categories of stakeholders in the study sites about the paramount importance of regulation and maintenance services. Three groups of stakeholders, each sharing the same points of view regarding ecosystem services conservation, were identified for each study site. As a non-monetary valuation, Q methodology is very instrumental for the new pluralistic approach of decision support by capturing the values expressed by the stakeholders, without triggering a rejection reflex due to the monetarization.

Correction to: Live fuel moisture content (LFMC) time series for multiple sites and species in the French Mediterranean area since 1996

The original version of this article has been corrected to reflect Open Access. Additionally, this correction stands to correct what was a broken hyperlink in the key message section of the article as well as an error related to the replacement of Supplementary File 13595_2018_729_MOESM2_ESM.xlsx.

Land use legacies on forest understory vegetation and soils in the Mediterranean region: Should we use historical maps or in situ land use remnants?

How to move forward if we cannot understand our present from our past? The same applies for the study of ecosystems. Evidence of ecological legacies in temperate post-agricultural forests has been provided on soil physico-chemical properties and understory vegetation richness and composition, which led to distinguish ancient and recent forests. However, no land use legacies dating back more than 50 years have been reported in Mediterranean forests. The definition of ancient and recent forests usually relies on a threshold date, most often provided by historical maps. In France, the Etat-Major map is the most precise, with a relatively high resolution and drawn over the whole country with a standardised method. However, the use of historical maps only for studying land use legacies in forest is questionable in a Mediterranean context characterised by a traditional agro-sylvo-pastoral system. Using a historical ecology approach, our purpose was to examine the legacies of various former land uses on soil and understory vegetation in Mediterranean forests and to compare the relevance of two historical sources: 1860 land use map (map-based PLU) and remnants recorded in the field (in situ PLU remnants). In the Regional Natural Park of Luberon (French Mediterranean area), we inventoried plant communities and analysed soil physico-chemical properties on 100 plots with various past land uses according the Etat-Major map (arable, pasture or forest) and in situ PLU remnants (terrace, stone removal or no remnants). Compared to ancient forest, former arable land hosted more species and had deeper and nutrient-richer soils while former pasture had shallower and nutrient-poorer soils. Similarly, and compared with no remnants, terraces hosted more species and had deeper and nutrient-richer soils whereas stone removal had shallower and nutrient-poorer soils. Understory species composition significantly varied according to map-based PLU and in situ PLU remnants. However, in situ PLU remnants were globally a much better indicator of land use legacies on soil and understory plant composition compared to map-based PLU. We thus confirmed the legacies of past land use on Mediterranean forest soil and understory vegetation. We also stressed that historical ecology should rely on both field evidence and historical maps, particularly in Mediterranean landscapes with a complex history of land uses.

Live fuel moisture content (LFMC) time series for multiple sites and species in the French Mediterranean area since 1996

Live fuel moisture content (LFMC) time series for multiple sites and species in the French Mediterranean area since 1996

Observed increase in extreme daily rainfall in the French Mediterranean

We examine long-term trends in the historical record of extreme precipitation events occurring over the French Mediterranean area. Extreme events are considered in terms of their intensity, frequency, extent and precipitated volume. Changes in intensity are analysed via an original statistical approach where the annual maximum rainfall amounts observed at each measurement station are aggregated into a univariate time-series according to their dependence. The mean intensity increase is significant and estimated at + 22% (+ 7 to + 39% at the 90% confidence level) over the 1961–2015 period. Given the observed warming over the considered area, this increase is consistent with a rate of about one to three times that implied by the Clausius–Clapeyron relationship. Changes in frequency and other spatial features are investigated through a Generalised Linear Model. Changes in frequency for events exceeding high thresholds (about 200 mm in 1 day) are found to be significant, typically near a doubling of the frequency, but with large uncertainties in this change ratio. The area affected by severe events and the water volume precipitated during those events also exhibit significant trends, with an increase by a factor of about 4 for a 200 mm threshold, again with large uncertainties. All diagnoses consistently point toward an intensification of the most extreme events over the last decades. We argue that it is difficult to explain the diagnosed trends without invoking the human influence on climate.

Autecology and growth of Aleppo pine ( Pinus halepensis Mill.): A comprehensive study in France

Abstract Aleppo pine (Pinus halepensis Mill.) is one of the dominant conifer species in the Mediterranean area. Many studies have investigated the role of climate variables (precipitations, temperature) in its occurrence and growth, but few have taken the effect of local site conditions into account, nor their interaction with climate. This study aimed at thoroughly assessing forest sites potential for Aleppo pine in France, and designing a practical tool to help forest managers implementing these results. The study is based on a network of 512 plots laid out in the French Mediterranean area, where 2158 trees were measured. A site index was computed using a height growth model calibrated on these plots. Using neural networks and partial least square regression models (PLS), we disentangled the effects of climatic factors and local site conditions in Aleppo pine presence and growth. Local water balance (assessed through topography, soil, geology and legacies of past human activities) was the main factor driving Aleppo pine productivity in the study area. Climate played a significant role as a whole but more important at the species distribution margins, frost and snow limiting Aleppo pine range northwards and in elevation. Based on these results, a practical tool was designed for two different types of mother rocks to help assessing forest site potential for Aleppo pine. As the span of climate conditions in south-eastern France includes most of those found in Aleppo pine distribution area, except the driest and hottest ones found in its southernmost locations, this study could easily be used or adapted in most other concerned countries. Because of the ongoing climate change and increasing drought in the Mediterranean area, the balance between local and climate factors may change in time and space. Our results show that without an accurate knowledge of local conditions, the impact of climate change cannot be predicted.