Effects of simultaneous ozone and nitrogen exposure on two tree species : Fagus sylvatica (L.) and Picea abies (L.) Karst.

Tropospheric ozone (O3) and nitrogen deposition (N) are considered as two of the most
important anthropogenic pollution factors for temperate forests today. Even though many
studies demonstrate the impact of either one of the two pollutants on forest trees, little is still
known on combinatory effects. However, as both, ozone concentrations and nitrogen
depositions exceed the critical levels and loads for forests in Switzerland it is essential to
investigate the impacts of a simultaneous exposure of trees to these two pollutants. Both, the
scarcity and the need for further information in that respect motivated this thesis.
To investigate the combinatory effects on forest trees, the two main tree species of
Switzerland, Fagus sylvatica and Picea abies, were chosen as study species – seedlings as
well as mature trees.
From literature we know that similar parameters are affected by either N or O3 exposure, e.g.
shoot elongation, biomass accumulation, or root carbohydrate concentrations. Regarding these
parameters trees might react to concurrent exposure in a simple additive way, i.e. positive
and/or negative trends sum up to a total effect. In this case individual effects do not interact.
Adversely, O3 and N effects interact, which may lead to either aggravation or alleviation of
the sum of individual effects.
I chose two different approaches to reveal possible interactions but also to elucidate the
individual effects of trees being exposed to the two pollutants N and O3:
a) A three-year controlled fumigation (O3) and fertilisation (N) experiment with tree seedlings
was conducted to investigate allometrical, physiological, and phenological parameters.
b) Mature trees along an O3 and N gradient were tested for changes in root carbohydrate
concentrations.
For spruce seedlings a significant interaction of the two pollutants was found for the root
starch concentrations where the negative effect of O3 counteracted a positive effect of N in an
antagonistic way. Other combinatory effects were mainly additive, without any interaction
effect detectable: Monosaccharide concentrations of fine roots were raised by both Nfertilisation
(+ 10 %) and O3 fumigation (+ 10 %). Also the formation of needle discolorations
(chlorotic mottling) was increased by both pollutants. Conversely, shoot elongation and starch
concentrations of stems were enhanced by N but counteracted by O3, without interaction of
the two pollutants. Trehalose concentrations also showed an additive effect, but here, nitrogen
fertilisation showed a decreasing and ozone fumigation an increasing trend. For the nutrient
concentrations and nutrient ratios an additive effect was found for potassium (both negative)
and the N : K ratio (both positive).
In other parameters measured, only one of the two treatments was responsible for the
observed changes: Increased N levels resulted in higher above- and belowground biomass
accumulation, without a shift of the shoot : root ratio. On the other hand, this ratio was
influenced by O3 (- 10 %) although neither above- nor belowground biomass was
significantly affected by ozone fumigation. Also soluble carbohydrates in spruce fine roots
(except monosaccharides; see above) were mainly influenced by N alone: Sugar alcohol
concentrations, including mannitol, decreased (- 17 %) while di- and trisaccharides
concentrations were elevated (+ 10 %) with increasing N. Furthermore, the nutrient
concentrations and ratios in the needles showed mostly N effects: N concentration increased,
whereas P, Mg, and Mn concentrations decreased, leading to enhanced N : P, N : K, and N :
Mg ratios.
Beech seedlings showed similar patterns in their reaction to N and O3 treatment compared
with spruce seedlings, however, beech tended to be more sensitive to higher N loads. In
general, I found supportive fertilisation effects with increasing N up to 40 kg N ha-1 yr-1 which
were reversed for the highest level of 80 kg N ha-1 yr-1 compared to controls. This reaction of
beech seedlings makes description of results somewhat more complicated.
Despite this complexity I found statistically comprehensible interaction effects of N and O3,
e.g. in shoot elongation, leaf area, or sugar alcohols. In most cases, these significant
interactions were characterised by an amplification of the N effects by O3 - in positive as well
as in negative ways. To exemplary illustrate this statement I describe the results for the shoot
elongation in some detail: at 20 kg N ha-1 yr-1 elongation was significantly increased (F: + 4
%). This nitrogen-induced change was amplified in plants fumigated with ozone (O3: + 19%),
as was shown by a positive interaction term. Nitrogen fertilisation of 40 kg N ha-1 yr-1
increased shoot growth in all plants (F: + 17 and O3: + 13 %), independent of the fumigation.
For the highest fertilisation level, however, nitrogen reduced shoot elongation below that of
controls (F: -8 %), which was again amplified in plants fumigated with ozone (O3: - 28 %).
In various parameters I detected simple additive effects of the two pollutants: e.g. biomass
accumulation belowground, leaf water content, specific leaf area, starch amounts in shoots
and roots, starch concentrations in stems, monosaccharide concentrations in fine roots,
nutrient concentrations (P, K, Mg), and nutrient ratios N : P, N : K, N : Mg.
Finally, also in beech seedlings I found parameters that were only influenced by one of the
two pollutants, namely nitrogen: e.g. leaf necroses, aphid infestation, biomass accumulation
aboveground, shoot : root ratio, and di- and trisaccharide concentrations.
When comparing the results of the two tree species, beech seedlings seemed to be more
susceptible towards N than spruce trees. This was mainly indicated by the reversion of the
“positive” N effects at the highest fertilisation level of 80 kg N ha-1 yr-1 for beech. Although
the sensitivity of beech and spruce seedlings towards O3 did not seem to differ considerably
the interaction effects did. In spruce, the only interaction effect detected, was an antagonistic
one for starch concentrations in coarse roots. In beech however, ozone amplified the nitrogen
effects in positive, as well as in negative ways (e.g. shoot elongation).
In the gradient study on mature trees, similar effects of O3 and N exposure on root starch
concentrations of beech and spruce were detected. In both tree species root starch
concentrations declined with increasing nitrogen and ozone concentrations. An antagonistic
interaction term for the two pollutants was found, which indicated that the two pollutants
alleviated the effect of each other at high concentrations, in beech as well as in spruce trees.
Comparing seedlings and mature trees in their reaction towards the combination of the two
pollutants the studies indicate a higher sensitivity of mature trees. Significant decreases in the
root starch concentrations under elevated O3 and N, as well as a significant interaction were
found. In the beech seedlings root starch concentrations did not decline significantly under
ozone fumigation. However, ozone concentrations in the fumigation experiment were
relatively low compared to the sites used in the gradient study (ozone fumigation: AOT40dl
7.2 – 22.1 ppm h; gradient study: AOT40dl 5.6 – 36.0 ppm h). Mature spruce showed the same
reaction pattern like mature beech: root starch concentrations decreased with increasing
pollution concentrations, showing a significant antagonistic interaction. In the spruce
seedlings, however, root starch concentrations increased with nitrogen fertilisation, which
contrasts the observations with mature trees. But here again, the ozone concentrations in the
fumigation experiment were lower, compared to the sites in the gradient study (ozone
fumigation: AOT40dl 7.2 – 22.1 ppm h; gradient study: 6.4 to 39.5 ppm h). Overall the
findings suggest a higher sensitivity of mature trees of both species, compared with seedlings.
Main statements and conclusions
- The interaction effects of the two pollutants are far from being uniform. The combination
of ozone and nitrogen results in amplification (e.g. shoot elongation in beech seedlings) or
alleviation (e.g. root starch concentrations in mature spruce and beech) of the effects,
whereas some parameters are subject to additive “interactions” (e.g. shoot elongation in
spruce seedlings), or are only influenced by one of the pollutants (e.g. biomass in spruce
seedlings (N)).
- As an overall effect of nitrogen fertilisation, shoot elongation, aboveground biomass
accumulation (at least up to 40 kg N ha-1 yr-1), and monosaccharide concentrations are
enhanced, whereas phosphorus concentrations in needles and leaves decrease. Ozone
fumigation has general decreasing effects on shoot elongation and also on starch
concentrations.
- Beech and spruce seedlings react similar towards ozone and nitrogen exposure, but beech
seems to be more sensitive, especially towards nitrogen fertilisation.
- Mature trees react stronger in their response towards ozone and nitrogen exposure,
concerning root starch concentrations. This supports the notion that mature trees might be
more sensitive towards the two pollutants than seedlings.
- Ozone exposure and nitrogen fertilisation induce changes in the fungus-specific sugar
alcohols mannitol and trehalose, which suggest impacts on the mycorrhization of the
seedlings.
- The observed changes in the nutrient concentrations and nutrient ratios are concerning,
especially as they are likely to proceed further under ongoing and probably more severe
exposure. Particularly phosphorus deficiencies might become crucial for tree health.
- Due to the detected changes in the investigated parameters and the observed needle
discolorations, leaf necroses, and aphid infestations it is very likely that tree health in
general will decrease under simultaneous ozone and nitrogen exposure.