Effects of drought on the competitive interference of an early successional species (Rubus fruticosus) on Fagus sylvatica L. seedlings:: 15N uptake and partitioning, responses of amino acids and other N compounds

We assessed the role of water availability as a factor regulating the ability of beech seedlings to cope with competitive interference for nitrogen resources by an early successional species (Rubus fruticosus). A glasshouse experiment was performed with two levels of interference (beech with and without R. fruticosus) and three levels of irrigation (high, intermediate, none). N-15 uptake and partitioning of both species, and composition of N pools in leaves, roots and phloem of beech, were determined. Under all irrigation regimes, N-15 uptake by beech seedlings decreased when grown together with R. fruticosus. R. fruticosus had higher N-15 uptake rates than beech, under all water supply levels. When irrigation was reduced, a substantial decrease in N-15 uptake of beech seedlings and a concurrent increase in N-15 uptake by R. fruticosus were observed. Interference by R. fruticosus and low irrigation also affected the N-15 partitioning in beech seedlings and resulted in reduced allocation of N-15 to the roots. The combination of competitive interference and lack of irrigation led to an increase in soluble non-protein N in roots and leaves of beech, due to protein degradation. This response was attributed to an increase in levels of amino acids serving as osmoprotectants under these conditions. The concentration of proline in leaves of beech was negatively correlated to shoot water potential. A competition-induced reduction of total N in leaves of beech under high and intermediate irrigation was found. These results illustrate (1) the advantage of R. fruticasus in terms of N uptake when compared to young beech, particularly under inadequate water supply, and (2) the changes in IN composition of beech seedlings in order to cope with reduced soil water and interference by R. fruticosus.