Root biomass and rooting profile of naturally regenerated beech in mid-elevation Scots pine woodlands

The natural establishment of shade-tolerant forest species such as beech (Fagus sylvatica L.) occurs in naturally regenerated Scots pine (Pinus sylvestris L.) woodlands that develop on former pastures and cultivated lands. To examine possible effects of underground competition in beech establishment, we studied the root biomass and the rooting profile of 53 mixed Scots pine-natural beech woodlands in French mid-elevation volcanic areas. Stands were arranged along a maturation gradient. Roots were sampled using the root-auger technique (0 to 75 cm depth every 15 cm). In addition, 23 young beeches were uprooted to study the entire root system. Total beech fine-root biomass was closely correlated with most beech aerial characteristics, (e.g., height, diameter and girth), and correlated moderately with tree age. However, it correlated poorly with basic competition indices such as stand density and basal area. Conversely, competition indices including vertical dimensions of competing trees were correlated with the underground biomass, probably as a result of redundancy with beech height. The rooting profile (fine roots, < 5 mm) of beech and pine were quite similar, and did not change significantly along the stand maturation gradient. Beech has a heart-shaped root system while pine is more plate-like and dimorphic. Beech fine-root biomass progressively surpassed pine biomass throughout the soil layers, thus confirming that it is dynamic and competitive in mature mixed stands. The coexistence of the root systems of beech and pine in the same soil layers presumably results in strong underground competition.