Usually, species composition differences between plots of vegetation submitted to contrasting conditions are followed by a shift in CWM of traits. But I have a case where species composition variation was not significant and CWM was. Does anyone ever get such trend? Can you suggest any literature containing similar results?
The discrepancy might have occurred because of your calculation of species composition? Did you use frequency? It is better to use density, biomass, or cover (as %). Cover is best because it allows for "relative" comparison of individual species across sites or times that vary significantly, reflects the relative contribution of a species to a community, and reflects dominance of a specific species on a site.
See:
Oecologia
April 2014, Volume 174, Issue 4, pp 1359–1366
Community-weighted mean functional effect traits determine larval amphibian responses to litter mixtures.
And:
Solid Earth, 8, 137–147, 2017, doi:10.5194/se-8-137-2017, Community-weighted mean traits but not functional diversity determine the changes in soil properties during wetland drying on the Tibetan Plateau.
I hope this helps :)
Thank you for your answer, Andrew.
I use the same calculation for species composition (aerial cover) and for CWM. So, I believe this is not a methodological problem.
I interpret this result as a functional turnover occurring with different species (that contain similar trait values).
I just wanted to see if anyone else ever found something similar and where to find it on literature.
:)
Can you tell me which traits were measured in the calculation of CWM please?
Dear Felicia,
the CWM of a community is the summed product of the abundance vector and the trait vector of species. Therefore, CWM cannot change without changing either the abundance or the trait values or both.
Trait values can change if for some reason the same species possess different values in different sites. It can occur only if you measured the trait for each species in each site separately to account for intraspecific variability. Did you do it so?
If you used a constant trait value for a species across all sites, that is the common case, your abundance/compositional data must have changed, but your statistical test may have not detected it as significant. It may have happened that you used a more sensitive test for CWM-s than for species composition, and a certain amount of change passed the pre-set limit of significance for the first but not for the second. Could you let me know how you tested difference in CWM and species composition, including the assessment of significance?