Contribution of previous year's leaf N and soil N uptake to current year's leaf growth in sessile oak

The origin of N which contributes to the synthesis of N reserves of in situ forest trees in autumn and to the growth of new organs the following spring is currently poorly documented. To characterize the metabolism of various possible N sources (plant N and soil N), six distinct 20-year-old sessile oaks were ¹⁵N labelled by spraying ¹⁵NH₄¹⁵NO₃: (i) on leaves in May, to label the N pool remobilized in the autumn for synthesis of reserves, (ii) on soil in the autumn, to label the N pool taken up from soil and (iii) on soil at the beginning of the following spring, to label the N pool taken up from soil in the spring. The partitioning of ¹⁵N in leaves, twigs, phloem, xylem, fine roots, rhizospheric soil and microbial biomass was followed during two growing seasons. Results showed a significant incorporation of ¹⁵N into the soil–tree system; more than 30 % of the administered ¹⁵N was recovered. Analysis of the partitioning clearly revealed that in autumn, roots' N reserves were formed from foliage ¹⁵N (73 %) and to a lesser extent from soil ¹⁵N (27 %). The following spring, ¹⁵N used for the synthesis of new leaves came first from ¹⁵N stored during the previous autumn, mainly from ¹⁵N reserves formed from foliage (95 %). Thereafter, when leaves were fully expanded, ¹⁵N uptake from the soil during the previous autumn and before budburst contributed to the formation of new leaves (60 %)

Comparison of leaf water use efficiency of oak and sycamore in the canopy over two growing seasons

The seasonal trends in water use efficiency of sun and shade leaves of mature oak (Quercus robur) and sycamore (Acer pseudoplatanus) trees were assessed in the upper canopy of an English woodland. Intrinsic water use efficiency (net CO2 assimilation rate/leaf conductance, A/g) was measured by gas exchange and inferred from C isotope discrimination (δ13C) methods. Shade leaves had consistently lower δ13C than sun leaves (by 1–2‰), the difference being larger in sycamore. Buds had distinct sun and shade isotopic signatures before bud break and received an influx of 13C-rich C before becoming net autotrophs. After leaf full expansion, δ13C declined by 1–2‰ gradually through the season, emphasising the importance of imported carbon in the interpretation of leaf δ13C values in perennial species. There was no significant difference between the two species in the value of intrinsic water use efficiency for either sun or shade leaves. For sun leaves, season-long A/g calculated from δ13C (72–78 μmol CO2 [mol H2O]−1) was 10–16% higher than that obtained from gas exchange and in situ estimates of leaf boundary layer conductance. For shade leaves, the gas exchange–derived values were low, only 10–18% of the δ13C-derived values. This is ascribed to difficulties in obtaining a comprehensive sample of gas exchange measurements in the rapidly changing light environment