Stable carbon (C) isotopes, in particular employed in labeling experiments, are an ideal tool to broaden our understanding of C dynamics in trees and forest ecosystems. Here, we present a free-air exposure system, named isoFACE, designed for long-term stable C isotope labeling in the canopy of 25 m tall forest trees. Labeling of canopy air was achieved by continuous release of CO 2 with a d 13 C of -46.9%. To this end, micro-porous tubes were suspended at c. 1 m distance vertically through the canopy, minimizing CO 2 gradients from the exterior to the interior and allowing for C labeling exposure during periods of low wind speed. Target for CO 2 concentration ([CO 2 ]) increase was ambient ?100 lmol mol -1 . Canopy [CO 2 ] stayed within 10% of the target during more than 57% of the time and resulted in a drop of d 13 C in canopy air by 7.8%. After 19 labeling days about 50% of C in phloem sugars and stem CO 2 efflux were turned over and 20-30% in coarse root CO 2 efflux and soil CO 2 . The iso-FACE system successfully altered d 13 C of canopy air for studying turn-over of C pools in forest trees and soils, highlighting their slow turn-over rates.Keywords Canopy CO 2 concentrations Á European beech (Fagus sylvatica L.) Á Free-air carbon isotope labeling infrastructure (isoFACE) Á Soil respired CO 2 Á Stable carbon isotope (d 13 C) Á Stem CO 2 efflux (respiration)
Impacts of elevated ground-level ozone (O(3)) on nitrogen (N) uptake and allocation were studied on mature European beech (Fagus sylvatica L.) and Norway spruce (Picea abies [L.] Karst.) in a forest stand, hypothesizing that: (i) chronically elevated O(3) limits nutrient uptake, and (ii) beech responds more sensitively to elevated O(3) than spruce, as previously found for juvenile trees. Tree canopies were exposed to twice-ambient O(3) concentrations (2 × O(3)) by a free-air fumigation system, with trees under ambient O(3) serving as control. After 5 years of O(3) fumigation, (15)NH(4)(15)NO(3) was applied to soil, and concentrations of newly acquired N (N(labelled)) and total N (N(total)) in plant compartments and soil measured. Under 2 × O(3), N(labelled) and N(total) were increased in the bulk soil and tended to be lower in fine and coarse roots of both species across the soil horizons, supporting hypothesis (i). N(labelled) was reduced in beech foliage by up to 60%, and by up to 50% in buds under 2 × O(3). Similarly, N(labelled) in stem bark and phloem was reduced. No such reduction was observed in spruce, reflecting a stronger effect on N acquisition in beech in accordance with hypothesis (ii). In spruce, 2 × O(3) tended to favour allocation of new N to foliage. N(labelled) in beech foliage correlated with cumulative seasonal transpiration, indicating impaired N acquisition was probably caused by reduced stomatal conductance and, hence, water transport under elevated O(3). Stimulated fine root growth under 2 × O(3) with a possible increase of below-ground N sink strength may also have accounted for lowered N allocation to above-ground organs. Reduced N uptake and altered allocation may enhance the use of stored N for growth, possibly affecting long-term stand nutrition.
Abstract. The present study compares the dynamics in carbon (C) allocation of adult deciduous beech (Fagus sylvatica) and evergreen spruce (Picea abies) during summer and in response to seven-year-long exposure with twice-ambient ozone (O 3 ) concentrations (2 × O 3 ). Focus was on the respiratory turn-over and translocation of recent photosynthates at various positions along the stems, coarse roots and soils. The hypotheses tested were that (1) 2 × O 3 decreases the allocation of recent photosynthates to CO 2 efflux of stems and coarse roots of adult trees, and that (2) according to their different O 3 sensitivities this effect is stronger in beech than in spruce.Labeling of whole tree canopies was applied by releasing 13 C depleted CO 2 (δ 13 C of −46.9 ‰) using a free-air stable carbon isotope approach. Canopy air δ 13 C was reduced for about 2.5 weeks by ca. 8 ‰ in beech and 6 ‰ in spruce while the increase in CO 2 concentration was limited to about 110 µl l −1 and 80 µl l −1 , respectively. At the end of the labeling period, δ 13 C of stem CO 2 efflux and phloem sugars was reduced to a similar extend by ca. 3-4 ‰ (beech) and ca. 2-3 ‰ (spruce). The fraction of labeled C (f E,new ) in stem CO 2 efflux amounted to 0.3 to 0.4, indicating slow C turnover of the respiratory supply system in both species.Elevated O 3 slightly stimulated the allocation of recently fixed photosynthates to stem and coarse root respiration in spruce (rejection of hypothesis I for spruce), but resulted in a significant reduction in C flux in beech (acceptance of hypotheses I and II). The distinct decrease in C allocation to Correspondence to: T. E. E. Grams (grams@tum.de) beech stems indicates the potential of chronic O 3 stress to substantially mitigate the C sink strength of trees on the longterm scale.
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