Structure and diversity of a natural temperate sessile oak (Quercus petraea L.) – European Beech (Fagus sylvatica L.) forest

The paper details an investigation of the stand structure and patterns of diversity observed in the Runcu-Grosi Natural Reserve (western Romania), one of the best preserved natural mixed sessile oak forests in Europe. The effect of the proportion of sessile oak in the stand composition on specific patterns was also studied.Comparable with other natural forests, the stands of Runcu-Grosi are characterised by a high volume (mean living tree volume of 577 m3 ha−1 in the sessile oak dominated plots and 675 in the pure beech plots), a highly differentiated diameter distribution and a large quantity of dead wood, especially in the mixed plots (approx. 135 m3 ha−1, compared to 75 in the pure beech plots). The higher volume of coarse woody debris in the mixed plots is due to the higher proportion of dead sessile oak in the reserve, which accounted for about 70% of the total dead wood in both the sessile oak and the beech dominated plots. The over proportional representation of dead sessile oak, and a very low number of oak saplings taller than 1.3 m, indicated a declining tendency of sessile oak in the stand in future. The sessile oak dominated stands exhibited a higher level of tree species diversity than those dominated by beech. Where beech was the dominant species, a high percentage of the sessile oak was present in pure groups (36%), whereas in the oak dominated stands, most of the sessile oaks were surrounded either by shade tolerant beech or by other species (ca. 75%), and were classed as either dominant or codominant trees.These species-specific patterns correspond with the silviculture of mixed oak-beech forests: without interventions to favour oak, the dominance of beech will increase.

► The studied sessile oak-beech forest exibits the characteristics of a virgin forest.
► The sessile oak dominated stands have a high diversity level.
► The coarse woody debris volume consists mostly of sessile oak trees (70%).
► Without a major disturbance to the canopy a decline of sessile oak will occur.