Depth-related fine root dynamics of Fagus sylvatica during exceptional drought

Fine roots are the most important plant organs for water and nutrient acquisition. Water and ion uptake is most sensitively altered by the formation of new roots in favourable sites. Fine root turnover, i.e. shedding and replacing fine roots, is therefore important in maximizing root system efficiency. Some evidence exists that accelerating fine root turnover is appropriate to counterbalance negative effects caused by drought albeit formation of new roots in drying soil is a high-risk investment.In the present study, fine root (≤2 mm diameter) dynamics in a stand of mature Fagus sylvatica (European beech) was observed during summer 2003 that was extraordinary dry in Central Europe. Depth-related fine root formation and mortality (between 3.4 and 41.6 cm soil depth) were observed and related to soil water content and soil temperature. The environmental parameters measured appeared as significant but surprisingly weak determinants for fine root formation and none of them influenced fine root mortality significantly. Fine root mortality was not counterbalanced by root formation but pronounced fine root shedding was found for non-suberized and superficial fine roots. Despite severe soil drying, fine roots were primarily invested in upper and thus unfavourable soil strata.The fine root system of F. sylvatica underlay an almost continuous replacement but there was no evidence for altered fine root turnover as an important response to soil drying. Rather the relative conservative fine root investment pattern and the large losses of fine roots indicate high construction costs at low benefits on the whole plant level and thus may limit the vigour of F. sylvatica if comparable drought periods occur frequently.