42 research outputs found
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 <sup>15</sup>N labelled by spraying
<sup>15</sup>NH<sub>4</sub><sup>15</sup>NO<sub>3</sub>: (i)Â on leaves in May, to label the N pool
remobilized in the autumn for synthesis of reserves, (ii)Â on soil in the
autumn, to label the N pool taken up from soil and (iii)Â on soil at the
beginning of the following spring, to label the N pool taken up from soil in
the spring. The partitioning of <sup>15</sup>N in leaves, twigs, phloem, xylem,
fine roots, rhizospheric soil and microbial biomass was followed during two
growing seasons. Results showed a significant incorporation of <sup>15</sup>N into
the soil–tree system; more than 30 % of the administered <sup>15</sup>N was
recovered. Analysis of the partitioning clearly revealed that in autumn,
roots' N reserves were formed from foliage <sup>15</sup>N (73 %) and to a
lesser extent from soil <sup>15</sup>N (27 %). The following spring, <sup>15</sup>N
used for the synthesis of new leaves came first from <sup>15</sup>N stored during
the previous autumn, mainly from <sup>15</sup>N reserves formed from foliage
(95 %). Thereafter, when leaves were fully expanded, <sup>15</sup>N uptake from
the soil during the previous autumn and before budburst contributed to the
formation of new leaves (60 %)
Storage and transpiration have negligible effects on  13C of stem CO2 efflux in large conifer trees
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