Extract the Chave et al. 2014 environmental index thanks to the coordinates of the data. The function is time-consuming at its first use as it downloads a raster in the working directory. However, as soon as the raster is downloaded once, the function then runs fast (if the working directory is not changed or if the raster is copied in the new working directory).
Coordinates of the site(s), a matrix/dataframe with two columns (e.g. cbind(longitude, latitude)) (see examples).
The Chave's environmental index,
E, has been shown to be an important covariable of the diameter-height relationship of trees. It is calculated as:
E = 1.e-3 * (0.178 * TS - 0.938 * CWD - 6.61 * PS)
where TS is temperature seasonality as defined in the Worldclim dataset (bioclimatic variable 4), CWD is the climatic water deficit (in mm/yr, see Chave et al. 2014) and PS is the precipitation seasonality as defined in the Worldclim dataset (bioclimatic variable 15).
The E index is extracted from a raster file (2.5 arc-second resolution, or ca. 5 km) downloadable at http://chave.ups-tlse.fr/pantropical_allometry.htm
The function returns
E, the environmental index computed thanks to the Chave et al 2014 formula.
Jerome CHAVE, Maxime REJOU-MECHAIN, Ariane TANGUY
Chave et al. (2014) Improved allometric models to estimate the aboveground biomass of tropical trees, Global Change Biology, 20 (10), 3177-3190
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