Root respiration plays an essential role in forest ecosystem carbon cycling and plant carbon budgets. However, knowledge on the factors influencing root respiration is rare. In this thesis, root respiration measurements were carried out to test for impacts of two abiotic factors and root inherent morphology. The first experiment, on Acer platanoides in the lab, determined root respirations reaction to five nitrogen forms ( nitrate, ammonium, glycine, diglycine and triglycine), and four concentrations (0 mM to 45mM) of nitrate and ammonium. Additionally, data was analysed to examine the relationship between root respiration and morphology in two root order classes, terminal orders 1+2 and the more coarse root order 4. A second experiment was conducted on roots of mature Fagus sylvatica trees in situ. The aim was to estimate how root respiration changes under different drought stress regimes, varying in drought period. Root morphology was highly correlated with respiration. Root order 1+2 exerted higher respiration rates per dry weight (RRDWT) compared to order 4. RRDWT tended to decline with increasing root diameter and tissue densities (RTD and RDMC), thus being positively related with root morphological traits (SRA and SRL). No immediate significant influence of N-forms and N concentration was found. Fagus sylvatica root biomass was not significantly influenced by drought regimes. A significant reduction in RRDWT was detected in drought stressed treatments after rewetting. Root respiration was higher in organic than in mineral soil horizons. In conclusion, the two woody species possessed a rather low plasticity in root respiration under varying abiotic factors while root morphology inherently determined respiration in a more apparent way.