Office National des Forets ONF

Saint-Martin-de-Ré, France

Office National des Forets ONF

Saint-Martin-de-Ré, France
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Goulamoussene Y.,University Of Guyane | Bedeau C.,Office National des Forets ONF | Descroix L.,Office National des Forets ONF | Linguet L.,University Of Guyane | Herault B.,University Antilles
Biogeosciences | Year: 2017

Natural disturbances are the dominant form of forest regeneration and dynamics in unmanaged tropical forests. Monitoring the size distribution of treefall gaps is important to better understand and predict the carbon budget in response to land use and other global changes. In this study, we model the size frequency distribution of natural canopy gaps with a discrete power law distribution. We use a Bayesian framework to introduce and test, using Monte Carlo Markov chain and Kuo-Mallick algorithms, the effect of local physical environment on gap size distribution. We apply our methodological framework to an original light detecting and ranging dataset in which natural forest gaps were delineated over 30 000 ha of unmanaged forest. We highlight strong links between gap size distribution and environment, primarily hydrological conditions and topography, with large gaps being more frequent on floodplains and in wind-exposed areas. In the future, we plan to apply our methodological framework on a larger scale using satellite data. Additionally, although gap size distribution variation is clearly under environmental control, variation in gap size distribution in time should be tested against climate variability. © Author(s) 2017.


Goutal N.,Office National des Forets ONF | Goutal N.,French National Institute for Agricultural Research | Boivin P.,University of Geneva | Ranger J.,French National Institute for Agricultural Research
Soil Science Society of America Journal | Year: 2012

Soil compaction is a major contributor to forest soil degradation. To make a decision on soil remediation, both the soil compactness after heavy traffic of forest machinery and the natural soil recovery rate must be estimated. We estimated the impact of heavy traffic on soil specific volume (V) and its recovery rate on two forest sites by yearly collecting steel cylinders close to field capacity, as currently recommended, at different depths (D) during 3 to 4 yr. Though collected at water contents (w) as homogeneous as possible, the comparison of sample volumes led to inconsistent results. Using w as covariate was necessary to quantify the initial compaction and the V recovery with time and D. Moreover, compared to the soil V and w determined with shrinkage analysis, some field values were very large, suggesting an artifact due to hammering the cylinder at large w. The surface layer (0-10 cm) of the less compacted site showed no residual compactness 3 yr after heavy traffic and the 10- to 20-cm layer compactness decreased significantly with time. The compactness of the second site decreased significantly only in the 0- to 10-cm layer, and the recovery was still ongoing after the third year of monitoring. This site had less swelling clays and larger clay content. The recovery of the soil volume was attributed to shrink-swell processes. Longer monitoring is required to validate these trends, and further research should evaluate the need for more accurate monitoring based on shrinkage analysis and the use of soil organic carbon (SOC) and clay content as covariates. © Soil Science Society of America.


Bontemps J.-D.,French National Institute for Agricultural Research | Herve J.-C.,French National Institute for Agricultural Research | Duplat P.,Office National des Forets ONF | Dhote J.-F.,Office National des Forets ONF | Dhote J.-F.,French National Institute for Agricultural Research
Oikos | Year: 2012

Height growth is a trait that contributes to tree species fitness. How height growth responds to environmental changes may therefore provide indications on species ability to compete and maintain, and on changes in tree community composition. Common beech Fagus sylvatica and sessile oak Quercus petraea are the predominant late-successional broadleaved species in Europe, and they differ in their shade-tolerance. On common beech (a shade tolerant species), recent observations across Europe have shown a growth decline during recent climate warming. Because sessile oak is a warmth- and light-demanding species, we therefore hypothesised that it may gain in competitiveness relative to common beech. We conducted analyses of historical height growth in several regions spanning the distributional range of the two species across a temperate-continental gradient in France. Common beech and sessile oak were sampled in two and four regions, respectively, and were compared in two neighbouring regions. We documented the climatic and nutritional conditions of regional samples. Height growth of 408 trees of various ages was reconstituted from stem analyses. We estimated 20th-century regional chronologies of height growth using a statistical modelling approach that filtered out the effects of ontogeny and site fertility. In regions where both species were sampled, modelled height trajectories were compared at different periods over the 20th century. Growth chronologies revealed 1) long-term growth rate increases of a magnitude of 50-100% over 100 years in both species, more acute in the continental domain, 2) recurrent historical inversions in growth fluctuations between species, 3) a recent divergence, with growth decline in common beech versus a dramatic growth increase in sessile oak, more acute in colder regions. The analysis of height trajectories indicated a recent reduction in common beech competitiveness relative to sessile oak. In the face of future climate warming, we conclude that increased prevalence of beech-oak mixtures may arise. © 2011 The Authors. Oikos © 2011 Nordic Society Oikos.


Bontemps J.-D.,Agro ParisTech | Bontemps J.-D.,French National Institute for Agricultural Research | Duplat P.,Office National des Forets ONF
Forestry | Year: 2012

Since the height horizon remains undetected in the vast majority of height series sampled in forest stands, even of notable ages, the realism of the traditional asymptotic-size modelling assumption is questioned. The aims of the study were to present an original non-asymptotic growth model and to test its accuracy against asymptotic-size equations. The equation proposed is a first-order four-parameter autonomous differential equation. The related sigmoid size curve has a parabolic branch of time. It was tested on 349 old growth series of top height (1047 stem analyses) selected to explore the maximum observed ranges of age and site conditions in seven temperate tree species growing in pure and even-aged stands. The fitting accuracy of this equation and three classical asymptotic-size growth equations (Richards, Hossfeld IV and Korf equations) were compared, with parameterizations of increasing flexibility. For the different parameterizations, the proposed growth equation showed higher performances than asymptotic growth equations, attributed to its non-asymptotic property and to the mathematical independence between parameters related to the inflection point and late growth. Top height growth was therefore accurately modelled by a sigmoid curve not based on the asymptotic-size assumption. This equation may be of general relevance to tree growth modelling.


Guitet S.,Office National des Forets ONF | Guitet S.,French National Institute for Agricultural Research | Guitet S.,IRD Montpellier | Herault B.,Center de Cooperation Internationale de la Recherche Agronomique Pour le Developpement | And 3 more authors.
PLoS ONE | Year: 2015

Precise mapping of above-ground biomass (AGB) is a major challenge for the success of REDD+ processes in tropical rainforest. The usual mapping methods are based on two hypotheses: a large and long-ranged spatial autocorrelation and a strong environment influence at the regional scale. However, there are no studies of the spatial structure of AGB at the landscapes scale to support these assumptions.We studied spatial variation in AGB at various scales using two large forest inventories conducted in French Guiana. The dataset comprised 2507 plots (0.4 to 0.5 ha) of undisturbed rainforest distributed over the whole region. After checking the uncertainties of estimates obtained from these data, we used half of the dataset to develop explicit predictive models including spatial and environmental effects and tested the accuracy of the resulting maps according to their resolution using the rest of the data. Forest inventories provided accurate AGB estimates at the plot scale, for a mean of 325 Mg.ha-1. They revealed high local variability combined with a weak autocorrelation up to distances of no more than10 km. Environmental variables accounted for a minor part of spatial variation. Accuracy of the best model including spatial effects was 90 Mg.ha-1 at plot scale but coarse graining up to 2-km resolution allowed mapping AGB with accuracy lower than 50 Mg.ha-1. Whatever the resolution, no agreement was found with available pan-tropical reference maps at all resolutions.We concluded that the combined weak autocorrelation and weak environmental effect limit AGB maps accuracy in rainforest, and that a trade-off has to be found between spatial resolution and effective accuracy until adequate "wall-to-wall" remote sensing signals provide reliable AGB predictions. Waiting for this, using large forest inventories with low sampling rate (<0.5%) may be an efficient way to increase the global coverage of AGB maps with acceptable accuracy at kilometric resolution. Copyright: © 2015 Guitet et al.


Bottinelli N.,French National Institute for Agricultural Research | Hallaire V.,French National Institute for Agricultural Research | Hallaire V.,Agrocampus Ouest | Goutal N.,Office National des Forets ONF | And 2 more authors.
Geoderma | Year: 2014

Processes and rate of macroporosity changes following heavy traffic in forest ecosystems are seldom studied. The aim of this study was to determine the ability of forest soils to regenerate their macroporosity naturally. The study was performed on 2 silty temperate-forest soils classified as sensitive to compaction located in north-eastern France. Macroporosity was measured in control and trafficked plots at 3 depths (0-7, 15-30 and 30-45cm) over 2-3years. Soil macroporosity characteristics (shape, size and orientation) were assessed on polished sections through 2D-image analysis and micromorphic observations. Immediately after heavy traffic, macroporosity decreased by 96 to 49% from 0 to 45cm in depth. Natural regeneration of macroporosity occurred in the upper 7cm of soil, while the soil below remained compacted. Small and medium macropores (0.05-0.8mm2) dominated by rounded and irregular pores regenerated completely. Large macropores (>0.8mm2) originally dominated by vughs, mammilated vughs and channels rarely regenerated and were gradually replaced by horizontally oriented planar pores. Our results suggest that initial stages of natural macroporosity recovery are likely due to plant-root penetration and physical processes (shrink-swell, freeze-thaw), whereas recovery due to fauna activities appears later.© 2013 Elsevier B.V.


Bontemps J.-D.,French National Institute for Agricultural Research | Herve J.-C.,Directorate General of Armaments | Leban J.-M.,French National Institute for Agricultural Research | Dhote J.-F.,Office National des Forets ONF
Trees - Structure and Function | Year: 2011

Environmental drivers of forest productivity increases have been much debated. Evidence for the suggested role of increasing nitrogen supply is lacking over long-term time scales. Tracking the footprint of environmental factors by using long-term growth records may thus prove decisive. We analysed growth chronologies of common beech in two areas of contrasting nutritional status in France. Dominant height growth was used as a proxy for productivity. Growth was compared between old and young paired stands sampled at the same sites to factor out effects of ageing and site. Growth chronologies were estimated with a statistical modelling procedure. The environmental causality of growth changes was addressed by combining (1) a comparison of growth changes between regions, (2) a regional comparison of growth chronologies with chronologies of environmental factors and (3) growth-environment relationships established from climate/soil data. Historical growth increases followed very similar courses in the two areas. Remarkably, the magnitude of change was 50% lower in the area that had reduced nutritional status and nitrogen deposition. Historical variations in environmental factors and growth were congruent with the roles of nitrogen availability and deposition, and of atmospheric CO2 increase. Low-frequency variations in climate and growth were not coincident. However, our analysis demonstrated the role of climatic anomalies in short-term growth variations. Growth-environment relationships further indicated a nitrogen constraint. These observations corroborate the enhancing role of increased nitrogen availability on forest biomass accumulation previously reported in ecosystem experiments and process-based modelling explorations. © 2010 Springer-Verlag.


Bontemps J.-D.,French National Institute for Agricultural Research | Herve J.-C.,Directorate General of Armaments | Dhote J.-F.,Office National des Forets ONF
Forest Ecology and Management | Year: 2010

While forest productivity is usually inferred from height growth indices, retrospective analyses of tree rings have been intensively used to assess long-term trends in forest productivity. However, radial growth is sensitive to the degree of competition between trees and influenced by management practices or local disturbances. Whether radial growth is accurate for diagnosing and quantifying productivity changes remains a debated question. In a previous study (Bontemps et al., 2009), we assessed historical variations in dominant height growth of even-aged stands of common beech in north-eastern France as a proxy for their productivity changes. The analysis was based on a sampling design including 14 pairs of young/old (75/150 yr) neighbour stands growing under the same site conditions. Dominant height was reconstructed from stem analyses and was compared between generations using a statistical modelling procedure. In this analysis, we tested whether radial and height growth of dominant trees may provide compatible indications on long-term trends. We therefore measured and analysed the radial growth of dominant trees at breast height for the same sampled trees. The effects of site, developmental stage, and calendar date were separated by applying a similar modelling approach. Consideration of the developmental stage effect led to the formulation of an original growth equation. Analysis of radial growth revealed: (i) a long-term positive increase; (ii) a magnitude of +50% over the last century; and (iii) growth declines in the 1940s and 1990s. These features were remarkably similar to those reported on dominant height, and indicated that radial growth of dominant trees delivered a sound picture of productivity changes. The radial growth chronology also differed by showing a more acute acceleration phase in the early century, and a recent but significant difference between stand generations. © 2010 Elsevier B.V. All rights reserved.


PubMed | IRD Montpellier, Office National des Forets ONF, CIRAD - Agricultural Research for Development, French National Institute for Agricultural Research and Center de cooperation Internationale de la Recherche Agronomique pour le Developpement
Type: Journal Article | Journal: PloS one | Year: 2015

Precise mapping of above-ground biomass (AGB) is a major challenge for the success of REDD+ processes in tropical rainforest. The usual mapping methods are based on two hypotheses: a large and long-ranged spatial autocorrelation and a strong environment influence at the regional scale. However, there are no studies of the spatial structure of AGB at the landscapes scale to support these assumptions. We studied spatial variation in AGB at various scales using two large forest inventories conducted in French Guiana. The dataset comprised 2507 plots (0.4 to 0.5 ha) of undisturbed rainforest distributed over the whole region. After checking the uncertainties of estimates obtained from these data, we used half of the dataset to develop explicit predictive models including spatial and environmental effects and tested the accuracy of the resulting maps according to their resolution using the rest of the data. Forest inventories provided accurate AGB estimates at the plot scale, for a mean of 325 Mg.ha-1. They revealed high local variability combined with a weak autocorrelation up to distances of no more than10 km. Environmental variables accounted for a minor part of spatial variation. Accuracy of the best model including spatial effects was 90 Mg.ha-1 at plot scale but coarse graining up to 2-km resolution allowed mapping AGB with accuracy lower than 50 Mg.ha-1. Whatever the resolution, no agreement was found with available pan-tropical reference maps at all resolutions. We concluded that the combined weak autocorrelation and weak environmental effect limit AGB maps accuracy in rainforest, and that a trade-off has to be found between spatial resolution and effective accuracy until adequate wall-to-wall remote sensing signals provide reliable AGB predictions. Waiting for this, using large forest inventories with low sampling rate (<0.5%) may be an efficient way to increase the global coverage of AGB maps with acceptable accuracy at kilometric resolution.


Rioux J.-A.,Montpellier University | Carron S.,65 rue Paul Rimbaud | Dereure J.,Montpellier University | Perieres J.,Montpellier University | And 5 more authors.
Parasite | Year: 2013

This study was conducted around Céret (Pyrénées- Orientales, mean elevation 200 m) to test the statistical reliability of 12 stations devoted to sampling the Leishmania infantum vectors Phlebotomus ariasi and P. perniciosus in the South of France. Each station included a retaining wall and the surrounding phytoecological environment (total area: 2,000 m 2). The wall had rectangular drainage cavities (weep holes) in which flight interception traps (sticky paper) were inserted and stretched every 10 days from May to October. For both vector species, the statistical analysis of 10-day and annual frequencies led to the following conclusions: (1) P. ariasi densities were significantly higher than P. perniciosus densities, (2) densities per species were significantly different at the 12 stations: none of them could be considered as representative of local vector densities, which depend on the wall structure (exposure, shade, vertebrate hosts), (3) the 10-day variation trends were not significantly different between stations, indicating that these variations are not determined by the station structure but rather by a common external factor (likely meteorological) and (4) the phytoecological features at the stations were not correlated with the sandfly densities. Most of the observations obtained with P. ariasi and P. perniciosus are also relevant for the non-vectorial species S. minuta. In conclusion, future research on the dynamics of leishmaniasis outbreaks relative to climate change and agricultural-silvicultural modifications should be very cautiously carried out, while focusing especially on the vector sampling quality and the use of phytoecological maps as vector density indicators. © J.-A. Rioux et al., published by EDP Sciences, 2013.

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