I think a high PH (above 7) Would contain more calcium and magnesium in the soil which will increase the avaiability of nitrogen which in turn will speed up the decomposition of the leaf litter and convert it into humus which the plants can readily use.
A strongly acid soil, ie 4.5 may hinder the breakdown of organic matter, nitrogen in particular and nutrients can become "locked up" in an undecomposed state and will not be available to use by the plants.
As Skylight suggests try googling the key words for a more detailed answer Good Luck Tbird+