In Phase I and II, studies on the competitiveness of mature European beech and Norway spruce trees ( Fagus sylvatica, Picea abies ) under biotic and abiotic stress focused on above-ground and below-ground space occupation as well as on carbon and water economy. In Phase III, the approach focused on the effects of elevated ground-level O₃ concentrations on space exploitation of nutrient resources, i.e. nitrogen. The acquisition and allocation of nitrogen was studied in the mature beech and spruce trees at the free-air O₃ fumigation facility “Kranzberger Forst” (SE-Germany).

Hypothesis

- Chronically elevated ground-level ozone (O₃ ) concentrations may change nutrient supply by reduced below-ground carbon allocation.

- Beech responds more sensitive in its competitiveness to elevated ozone than spruce, due to a higher resource turnover as compared to spruce.

(see B4 poster).

At the free-air O₃ fumigation facility “Kranzberger Forst” (SE-Germany) canopies of 60-70 yr old European beech and Norway spruce trees ( Fagus sylvatica, Picea abies ) were exposed to double-ambient O₃ concentrations (2xO₃) over seven years.

In the 6 ^th year of the ozone treatment, a ¹⁵N tracer was applied to 1 m² soil surface per tree, and effects of elevated ozone on nitrogen (N) acquisition in trees, mycorrhizal root tips and soil was studied over 16 months. Roots of the ¹⁵N labelled plots were harvested at the end of the experiment. The uptake and partitioning of the newly acquired N (= ¹⁵N) was calculated as based on the modelled tree biomasses of the study trees (project B1). The nitrogen balance of the stand was established including additionally measured N leaching and N mineralisation.

In foliage of beech, particularly in the sun crown, ¹⁵N concentration was lower under 2xO₃ as compared to 1xO₃ , and correlated to transpiration. In contrast, ¹⁵ N concentration in spruce foliage was similar under both ozone regimes, whereas total N was reduced in needles and twigs. Relative to the low total N concentrations in spruce foliage, the incorporation of ¹⁵N, however, was increased under 2xO₃ . In roots, ¹⁵ N concentrations did not differ under both O₃ regimes. However, total N was reduced in mycorrhizal root tips of beech under 2xO₃ as compared to 1xO₃ . Higher ¹⁵ N concentrations in mycorrhizal root tips of beech compared to spruce indicated different strategies in N-acquisition.

On whole-tree level, ¹⁵N uptake per kg biomass tended to be reduced in both species under 2xO₃ . However, relative to the total N pool of the trees, ¹⁵N uptake was reduced in beech but increased in spruce. The tree-internal partitioning of new N also differed between the ozone treatments: In beech, relatively more of the new N remained in roots, particularly in fine roots and mycorrhiza under 2xO₃ as compared to 1xO₃ , whereas in spruce, allocation of new N to roots was decreased under 2xO₃ , particularly in medium-sized roots.

The N balance showed a relatively high N availability on stand level, and a moderate influence of the O₃ effects.

•  In both species, N acquisition seems to be impaired under elevated ground-level ozone.

•  In beech, reduced stomatal conductance and hence transpiration rates may be responsible for lowered N uptake and allocation to foliage.

•  In spruce, N demand is increased under 2xO₃ , particularly in foliage, possibly due to defence and repair mechanisms, resulting in an increased allocation of new N to above-ground organs.

•  For both species, the changes in N acquisition and allocation in response to elevated ozone may lead to a higher consumption of storage pools and therefore weaken the nutrient equilibrium and availability in the long-term.

•  In beech, these effects may be more severe compared to spruce, as the assimilation organs seem to be affected more directly by the reduced N supply. The findings match well with the reduced growth of beech under 2xO₃ , as found in mature (Project B1) as well as in young beech trees (Project B5).

•  The relatively moderate O₃ effects on tree level may in long-term alter the N cycle on stand level, e.g. through higher N availability in the soil.