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
Understanding how plants are constructed—i.e., how key size dimensions and the amount of mass invested in different tissues varies among individuals—is essential for modeling plant growth, carbon stocks, and energy fluxes in the terrestrial biosphere. Allocation patterns can differ through ontogeny, but also among coexisting species and among species adapted to different environments. While a variety of models dealing with biomass allocation exist, we lack a synthetic understanding of the underlying processes. This is partly due to the lack of suitable data sets for validating and parameterizing models. To that end, we present the Biomass And Allometry Database (BAAD) for woody plants. The
BAAD contains 259 634 measurements collected in 176 different studies, from 21 084 individuals across 678 species. Most of these data come from existing publications. However, raw data were rarely made public at the time of publication. Thus, the BAAD contains data from different studies, transformed into standard units and variable names. The transformations were achieved using a common workflow for all raw data files. Other features that distinguish the BAAD are: (i) measurements were for individual plants rather than stand averages; (ii) individuals spanning a range of sizes were measured; (iii) plants from 0.01– 100 m in height were included; and (iv) biomass was estimated directly, i.e., not indirectly via allometric equations (except in very large trees where biomass was estimated from detailed sub-sampling). We included both wild and artificially grown plants. The data set contains the following size metrics: total leaf area; area of stem cross-section including sapwood, heartwood, and bark; height of plant and crown base, crown area, and surface area; and the dry mass of leaf, stem, branches, sapwood, heartwood, bark, coarse roots, and fine root tissues. We also report other properties of individuals (age, leaf size, leaf mass per area, wood density, nitrogen content of leaves and wood), as well as information about the growing environment (location, light, experimental treatment, vegetation type) where available. It is our hope that making these data available will improve our ability to understand plant growth, ecosystem dynamics, and carbon cycling in the world’s vegetation
Auteurs, date et publication :
Auteurs D.-S. Falster , R.-A. DUURSMA , MASAE I-I , D.-R. BARNECHE , R.-G. FITZJOHN , A. VA ˚ RHAMMAR , M. AIBA , M. ANDO , N.-I. ANTEN , M.-J. ASPINWALL , J.-L. BALTZER , C. BARALOTO , M. BATTAGLIA , J.-J. BATTLES , B. BOND-LAMBERTY , M. VAN BREUGEL , J. CAMAC , Y. CLAVEAU , L. COLL , M. DANNOURA
Publication : Ecology
Date : 2025
Volume : 96
Issue : 5
Pages : 1445
Catégorie(s)
#CIRAD #FORET Itatinga #INRAERésumé
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
There is a fundamental trade-off between leaf traits associated with either resource acquisition or resource conservation. This gradient of trait variation, called the economics spectrum, also applies to fine roots, but whether it is consistent for coarse roots or at the plant community level remains untested. We measured a set of morphological and chemical root traits at a community level (functional parameters; FP) in 20 plant communities located along land-use intensity gradients and across three climatic zones (tropical, mediterranean and montane). We hypothesized (i) the existence of a root economics spectrum in plant communities consistent within root types (fine, < 2mm; coarse, 2-5mm), (ii) that variations in root FP occur with soil depths (top 20cm of soil and 100-150cm deep) and (iii) along land-use gradients. Root FP covaried, in line with the resource acquisition-conservation trade-off, from communities with root FP associated with resource acquisition (e.g. high specific root length, SRL; thin diameters and low root dry matter contents, RDMC) to root FP associated with resource conservation (e.g. low SRL, thick diameters and high RDMC). This pattern was consistent for both fine and coarse roots indicating a strong consistency of a trade-off between resource acquisition and conservation for plant roots. Roots had different suites of traits at different depths, suggesting a disparity in root function and exploitation capacities. Shallow, fine roots were thinner, richer in nitrogen and with lower lignin concentrations associated with greater exploitation capacities compared to deep, fine roots. Shallow, coarse roots were richer in nitrogen, carbon and soluble concentrations than deep, coarse roots. Fine root parameters of highly disturbed, herbaceous-dominated plant communities in poorer soils were associated with foraging strategies, that is greater SRL and lower RDMC and lignin concentration than those from less disturbed communities. Coarse roots, however, were less sensitive to the land-use gradient.Synthesis. This study demonstrates the existence of a general trade-off in root construction at a community level, which operates within all root types, suggesting that all plant tissues are controlled by the trade-off between resource acquisition and conservation.
Auteurs, date et publication :
Auteurs Ivan Prieto , Catherine Roumet , Remi Cardinael , Christian Dupraz , Christophe Jourdan , John H. Kim , Jean Luc Maeght , Zhun Mao , Alain Pierret , Noelia Portillo , Olivier Roupsard , Chantanousone Thammahacksa , Alexia Stokes
Publication : Journal of Ecology
Date : 2015
Volume : 103
Issue : 2
Pages : 361-373
Catégorie(s)
#CIRAD #FORET CoffeeFluxRésumé
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
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
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
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 ParacouAuteurs, date et publication :
Auteurs M. Christina , Nouvellon Y , Laclau J-P , Stape J- L , Campoe O-C , G. Le Maire
Publication : Canadian Journal of Forest Research
Date : 2025
Volume : 46
Issue : 3
Pages : 297-309