“…Soil types and humus forms do not vary at the same scale of time (Crocker and Major, 1955;Switzer et al, 1979;Turk et al, 2008), making their causal relationships highly variable in space and time, more especially in forest environments (Kuuluvainen et al, 1993;Ponge et al, 1998Ponge et al, , 1999. It has been shown that the thickness of forest floor and the structure of organo-mineral horizons, which are under the paramount influence of ecosystem engineers such as earthworms (Bernier, 1998;Hoogerkamp et al, 1983;Wironen and Moore, 2006; but see Burghouts et al, 1998), can vary according to the age of trees (Aubert et al, 2004;Chauvat et al, 2007;Godefroid et al, 2005), plant successional processes (Emmer and Sevink, 1994;Leuschner et al, 1993;Scheu and Schulz, 1996) and undergo cycles at the scale of centuries in naturally regenerating late-successional forests (Bernier and Ponge, 1994;Sagot et al, 1999;Salmon et al, 2008).…”