Effect of pure and multi-species beech (Fagus sylvatica) stands on soil characteristics and earthworms in two northern German forests

The effect of mixed beech forests versus pure beech forests on soil parameters and earthworms was investigated in two forests (Hevenbruch and Schattin) of northern Germany differing in their nutrient supply. Within a total of 45 stands, including 23 pure and 22 mixed beech stands, the following parameters were determined: soil pH and organic matter in two soil layers, litter thickness, earthworm density, earthworm ash-free biomass, and species richness. Mixed beech stands had higher pH values, lower litter thickness, and higher earthworm biomass in both forests. With increasing beech dominance soil pH is decreasing and litter thickness is increasing. Earthworm biomass was positively correlated with soil pH and negatively with increasing dominance of beech. In particular, endogeic earthworm density was negatively correlated with increasing dominance of beech. Earthworm biomass and density of endogeic earthworms were negatively correlated with litter thickness. We interpret the results as an effect of higher acidification and lesser digestibility of litter in pure beech stands in comparison with mixed beech stands. Pure beech stands decrease the alkalinity of soils and decelerate the litter decomposition in forests via the decline of the endogeic earthworm population. This process mainly occurs in a pH range between 3.0 and 3.8.

► Mixed tree forest increase pH of upper soil layer.
► Mixed tree forest enhance earthworm activity.
► Mixed tree forest accelerate litter decomposition.