I have a dataset of 15 environmental variables (soil physical and chemical properties) and about 25 "species" variables for about 40 sampling sites. I want to do a CCA to analyse the effects of the environmental data on the species pattern. However, these variables are measured on quite different scales and units (eg. pH, clay content and salt concentration in %, total N in mg/g etc.). The species variables are all measured in the same unit.
For PCA, I would definitely standardize these environmental data to zero mean and unit variance, otherwise the ones with higher absolute values would shift the results inappropriately. But I couldn't find any information about if standardization is necessary or even appropriate for CCA.