Résumé
Cronbach's alpha is an estimate of the reliability of a test score if the items are essentially tau-equivalent. Several authors have derived results that provide al-ternative interpretations of alpha. These interpretations are also valid if essential tau-equivalency does not hold. For example, alpha is the mean of all split-half reliabilities if the test is split into two halves that are equal in size. This note presents several con-nections between Cronbach's alpha and the Spearman-Brown formula. The results provide new interpretations of Cronbach's alpha, the stepped down alpha, and stan-dardized alpha, that are also valid in the case that essential tau-equivalency or parallel equivalency do not hold. The main result is that the stepped down alpha is a weighted average of the alphas of all subtests of a specific size, where the weights are the de-nominators of the subtest alphas. Thus, the stepped down alpha can be interpreted as an average subtest alpha. Furthermore, we may calculate the stepped down alpha without using the Spearman-Brown formula.
Auteurs, date et publication :
Auteurs Renato Cordeiro de Amorim
Publication : Journal of Classification
Date : 2025
Volume : 32
Issue : March
Pages : 46–62
Catégorie(s)
#CIRAD #FORET ParacouRésumé
While attention on logging in the tropics has been increasing, studies on the long‐term effects of silviculture on forest dynamics and ecology remain scare and spatially limited. Indeed, most of our knowledge on tropical forests arises from studies carried out in undisturbed tropical forests. This bias is problematic given that logged and disturbed tropical forests are now covering a larger area than the so‐called primary forests. A new network of permanent sample plots in logged forests, the Tropical managed Forests Observatory (TmFO), aims to fill this gap by providing unprecedented opportunities to examine long‐term data on the resilience of logged tropical forests at regional and global scales. TmFO currently includes 24 experimental sites distributed across three tropical regions, with a total of 490 permanent plots and 921 ha of forest inventories.
Auteurs, date et publication :
Auteurs Plinio Sist , Ervan Rutishauser , Marielos Peña-Claros , Alexander Shenkin , Bruno Hérault , Lilian Blanc , Christopher Baraloto , Fidèle Baya , Fabrice Benedet , Katia Emidio da Silva , Laurent Descroix , Joice Nunes Ferreira , Sylvie Gourlet-Fleury , Marcelino Carneiro Guedes , Ismail Bin Harun , Riina Jalonen , Milton Kanashiro , Haruni Krisnawati , Mrigesh Kshatriya , Philippa Lincoln
Publication : Applied Vegetation Science
Date : 2015
Volume : 18
Issue : 1
Pages : 171–174
Catégorie(s)
#CIRAD #FORET ParacouRésumé
In this letter, the temporal coherence of tropical forest scattering at P-band is addressed by means of a ground-based experiment. The study is based on the TropiScat campaign in French Guiana, designed to support the Biomass mission, which will be the ESA 7th Earth Explorer mission. For Biomass, temporal coherence is a crucial parameter for coherent processing of polarimetric synthetic aperture radar (SAR) interferometry and SAR tomography in repeat-pass acquisitions. During the experiment, data were continuously collected for six months during both the rainy and dry seasons. For the rain-free days in both seasons, the coherence exhibits a daily cycle showing a high decorrelation during daytime, which is likely due to motion in the canopy. Up to a 20-day baseline, the coherence is much higher in the dry season than in the rainy season (textgreater 0.8). From 20 to 40 days, it presents the same order of magnitude in both seasons [0.6, 0.7]. For larger temporal baselines, it becomes lower in the dry season. The results can be used to assess the long-term coherence of repeat-pass observations over a tropical forest. However, an extension of this study to several years and over other forest spots would be necessary to draw more general conclusions.
Auteurs, date et publication :
Auteurs Alia Hamadi , Pierre Borderies , Clement Albinet , Thierry Koleck , Ludovic Villard , Dinh Ho Tong Minh , Thuy Le Toan , Benoit Burban
Publication : IEEE Geoscience and Remote Sensing Letters
Date : 2015
Volume : 12
Issue : 3
Pages : 557–561
Catégorie(s)
#CIRAD #FORET ParacouRésumé
The objective of this letter is to provide a better understanding of the impact of temporal decorrelation on the tomographic phase of the P-band synthetic aperture radar (SAR) mission BIOMASS, selected as the Seventh Earth Explorer by the European Space Agency. In the context of Phase A BIOMASS activities, the tropical forest site of Paracou, French Guiana, was illuminated at P-band during the airborne campaign TropiSAR 2009 and the ground-based campaign TropiScat 2011. P-band data from TropiSAR were used to generate a high-resolution 3-D reconstruction of the Paracou forest, whereas TropiScat data provided information about temporal correlation considering different time lags and different heights within the vegetation layer. The ensemble of the two datasets were used to generate a synthetic SAR data stack that emulates BIOMASS acquisitions over the Paracou forest site, accounting for BIOMASS geometry and resolution, as well as for the forest temporal decorrelation. Different data stacks were produced by varying the revisit time between two consecutive passes from 1 to 17 days. The resulting vertical structure reconstruction and forest height retrieval were observed to yield valuable results as long as the revisit time is 4 days or less.
Auteurs, date et publication :
Auteurs Dinh Ho Tong Minh , Stefano Tebaldini , Fabio Rocca , Thuy Le Toan
Publication : IEEE Geoscience and Remote Sensing Letters
Date : 2015
Volume : 12
Issue : 6
Pages : 1297–1301
Catégorie(s)
#CIRAD #FORET ParacouRésumé
The fire ant Solenopsis saevissima is a major pest frequent in human-disturbed areas of its native range where it forms ‘supercolonies'. We determined that its natural habitat in French Guiana is likely the sporadically flooded riparian forest and aimed to evaluate this ant's impact on the abundance and diversity of other ants by comparing different habitats at two sites. We noted a significant decrease in ant species richness between the rainforest and human-disturbed habitats (but not between the former and the naturally disturbed riparian forest), and between extreme habitats and all others. The number of ant nests per surface unit (i.e., quadrats of equal surface area), a proxy of ant abundance, globally followed this pattern. S. saevissima was absent from pristine rainforest (as expected) and from extreme habitats, showing the limits of its adaptability, whereas some other native ants can develop in these habitats. Ant species richness was significantly lower in the presence of S. saevissima in the riparian forest, forest edges and meadows, illustrating that this ant species has a negative impact on the ant communities in addition to the impact of natural- and man-made disturbances. Only some ant species can develop in its presence, and certain of these can even thrive. Because it has been recorded in Africa, Guadeloupe and the Galápagos Islands, we concluded that, due to the increasing volume of global trade and forest destruction, S. saevissima could become a pantropical invasive species.
Auteurs, date et publication :
Auteurs Alain Dejean , Régis Céréghino , Maurice Leponce , Vivien Rossi , Olivier Roux , Arthur Compin , Jacques H.C. Delabie , Bruno Corbara
Publication : Biological Conservation
Date : 2015
Volume : 187
Pages : 145–153
Catégorie(s)
#CIRAD #CNRS #FORET Nouragues #FORET ParacouRésumé
Studies of the periodicity of wood formation provide essential data on tree age and on factors that control tree growth. The aim of this work was to investigate cambial phenology and its relation with leaf phenology and climatic seasonality in two briefly deciduous tropical rainforest species belonging to the genus Parkia. Wood microcores were collected every 15 days from April 2009 to February 2012 from five trees of each species. The microcores were stained with cresyl violet acetate to facilitate counting the number of cells in the cambial zone, in the radial enlargement zone and wall-thickening zone. At the same time, we observed leaf shedding pattern in the crown of the same trees. In both species, cambial activity was significantly reduced during the leafless period. In P. nitida, these two concomitant events were observed during the dry season whereas in P. velutina they can occur anytime in the year with no apparent link with seasonality. In conclusion, the period of reduced cambial activity in some tropical rainforest trees may be independent of rainfall seasonality and not necessarily follow an annual cycle. It appears that leaf phenology is a good proxy to estimate cambial activity.
Auteurs, date et publication :
Auteurs Hélène Morel , Thomas Mangenet , Jacques Beauchêne , Julien Ruelle , Eric Nicolini , Patrick Heuret , Bernard Thibaut
Publication : Trees
Date : 2015
Volume : 29
Issue : 4
Pages : 973–984
Catégorie(s)
#CIRAD #FORET ParacouRésumé
Motivated by the analysis of the impact of ecological processes on spatial distribution of tree species, we introduce in this paper a novel approach to detect spatial cluster of points. Our procedure is based on an iterative transformation of the distance between points into a measure of closeness. Our measure has the advantage of being independent of an arbitrary cluster shape and allowing adjustment for covariates. The comparison of the observed measure of closeness to a reference point process leads to a hierarchical clustering of spatial points. The selection of the optimal number of clusters is performed using the Gap statistic. Our procedure is illustrated on a spatial distribution of the Dicorynia guianensis species in the French Guiana terra firme rainforest.
Auteurs, date et publication :
Auteurs Avner Bar-Hen , Mathieu Emily , Nicolas Picard
Publication : Spatial Statistics
Date : 2015
Volume : 14
Pages : 400–411
Catégorie(s)
#CIRAD #FORET ParacouRésumé
Nitrogen (N) and phosphorus (P) availability both control microbial decomposers and litter decomposition. However, these two key nutrients show distinct release patterns from decomposing litter and are unlikely available at the same time in most ecosystems. Little is known about how temporal differences in N and P availability affect decomposers and litter decomposition, which may be particularly critical for tropical rainforests growing on old and nutrient-impoverished soils. Here we used three chemically contrasted leaf litter substrates and cellulose paper as a widely accessible substrate containing no nutrients to test the effects of temporal differences in N and P availability in a microcosm experiment under fully controlled conditions. We measured substrate mass loss, microbial activity (by substrate induced respiration, SIR) as well as microbial community structure (using phospholipid fatty acids, PLFAs) in the litter and the underlying soil throughout the initial stages of decomposition. We generally found a stronger stimulation of substrate mass loss and microbial respiration, especially for cellulose, with simultaneous NP addition compared to a temporally separated N and P addition. However, litter types with a relatively high N to P availability responded more to initial P than N addition and vice versa. A third litter species showed no response to fertilization regardless of the sequence of addition, likely due to strong C limitation. Microbial community structure in the litter was strongly influenced by the fertilization sequence. In particular, the fungi to bacteria ratio increased following N addition alone, a shift that was reversed with complementary P addition. Opposite to the litter layer microorganisms, the soil microbial community structure was more strongly influenced by the identity of the decomposing substrate than by fertilization treatments, reinforcing the idea that C availability can strongly constrain decomposer communities. Collectively, our data support the idea that temporal differences in N and P availability are critical for the activity and the structure of microbial decomposer communities. The interplay of N, P, and substrate-specific C availability will strongly determine how nutrient pulses in the environment will affect microbial heterotrophs and the processes they drive.
Auteurs, date et publication :
Auteurs Nicolas Fanin , Stephan Hättenschwiler , Paola F. Chavez Soria , Nathalie Fromin
Publication : Frontiers in Microbiology
Date : 2016
Volume : 6
Pages : 1507
Catégorie(s)
#CIRAD #FORET ParacouRésumé
The preparation of tropical wood surface sections for time‐of‐flight secondary ion mass spectrometry imaging is described, and the use of delayed extraction of secondary ions and its interest for the analysis of vegetal surface are shown. The method has been applied to the study by time‐of‐flight secondary ion mass spectrometry imaging with a resolution of less than one micron of a tropical wood species, Dicorynia guianensis, which is one of the most exploited wood in French Guiana for its durable heartwood. The heartwood of this species exhibits an economical importance, but its production is not controlled in forestry. Results show an increase of tryptamine from the transition zone and a concomitant decrease of inorganic ions and starch fragment ions. These experiments lead to a better understanding of the heartwood formation and the origin of the natural durability of D. guianensis.
Auteurs, date et publication :
Auteurs Quentin P. Vanbellingen , Tingting Fu , Claudia Bich , Nadine Amusant , Didier Stien , Serge Della-Negra , David Touboul , Alain Brunelle
Publication : Journal of Mass Spectrometry
Date : 2016
Volume : 51
Issue : 6
Pages : 412–423
Catégorie(s)
#CIRAD #FORET ParacouRésumé
By harvesting scattered large trees, selective logging increases light availability and thereby stimulates growth and crown expansion at early‐life stage among remnant trees. We assessed the effects of logging on total and merchantable bole (i.e., lowest branch at crown base) heights on 952 tropical canopy trees in French Guiana. We observed reductions in both total (mean, −2.3 m) and bole (mean, −2.0 m) heights more than a decade after selective logging. Depending on local logging intensity, height reductions resulted in 2–13 percent decreases in aboveground tree biomass and 3–17 percent decreases in bole volume. These results highlight the adverse effects of logging at both tree and stand levels. This decrease in height is a further threat to future provision of key environmental services, such as timber production and carbon sequestration.
Auteurs, date et publication :
Auteurs Ervan Rutishauser , Bruno Hérault , Pascal Petronelli , Plinio Sist
Publication : Biotropica
Date : 2016
Volume : 48
Issue : 3
Pages : 285–289