Response of Fagus sylvatica seedlings to root trenching of overstorey Picea abies

In a trenching experiment the impact of root competition by overstorey Norway spruce on growth and allocation of above-ground woody biomass of advanced planted beech was investigated in two stands over two growing seasons after trenching. Measurements of soil water potential were taken inside and out side trenched areas by tensiometers at 16 measuring points in one stand from May to September 1999. Height and diameter of the seedlings, as well as branch length and diameter, were measured of beech growing in trenched and untrenched conditions in September before trenching (1998) and afterwards (1999 and 2000). Tensiometers inside the trenches reached the expiring value of -0.085 Mpa 30-40 days later, or never, compared with those in untrenched areas. Whereas relative growth rate (RGR) in height was not affected by trenching, RGR bot h in diameter and in estimated dry weight of the main stem were substantially enhanced in the first growing season after trenching (1999). A different result was obtained in 2000, when RGR in seedling diameter and estimated dry weight of the main stem was slightly reduced by trenching. The only variable that showed increased RGR of the trenched seedlings in bot h years was the estimated dry weight of the branches, indicating different above-ground biomass allocation pat terns of seedlings with and without root competition.