Deep rooting revisited: comparing the rooting patterns of European beech, Sessile oak, Scots pine, and Douglas fir in sandy soil to 3.8 m depth

Deep rooting is an important element in the strategy of trees to cope with drought, but the depth distribution of the absorbing fine root system is not well known for most of the common tree species. With the aim to compare the deep rooting of the economically important species Fagus sylvatica, Quercus petraea, Pinus sylvestris and Pseudotsuga menziesii on deep sandy soil in a quantitative way, we excavated the root system of mature trees to 380 cm depth and analyzed the depth dependence of fine (≤ 2 mm) and larger roots and related the patterns to gradients in soil physical and chemical conditions. All four species rooted to 380 cm depth; the total mass of fine roots and large roots in the profile decreased in the sequence Fagus sylvatica ≈ Quercus petraea > Pseudotsuga menziesii > Pinus sylvestris, whereas the depth to which 99 % of fine root mass was found decreased in the sequence Quercus petraea > Fagus sylvatica > Pinus sylvestris > Pseudotsuga menziesii (218, 208, 164 and 114 cm, respectively), indicating deepest rooting for oak and shallowest for Douglas fir. Vertical soil moisture gradients, the downward increase in soil bulk density and the decrease in N are discussed as putative drivers of the exponential decrease in fine root mass with soil depth. We conclude that assessing the belowground drought vulnerability of trees should include both records of observed maximum rooting depth and the quantitative analysis of fine root distribution in the top- and subsoil.