How do higher con-centrations of carbon
dioxide in the air and larger depositions of nitrogen affect forest trees? An experiment carried out by WSL and other research Inst~
tutions supplied some answers that were, In part, surprising.
Fxperimenting with model mini-forests
If people start to consume significantly more sugar and fat, their weight will normally increase. Does something similar happen to forest trees when they receive more carbon dioxide (CO2) and nitrogen? This question has preoccupied scientists for years. After all, carbon dioxide levels in the air are continually rising and depositions of nitrogen in the forest are increasing.
Forests in open-top chambers
Numerous experiments have been per-formed with plants and young trees cultivated in pots, but it is unclear to what extent these results apply to whole forests. A forest is made up not just of groups of individual trees, but consists of a multitude of living organisms which mutually influence each other. This is why scientists talk about the forest as an ecosystem. In 1994 WSL, in collabor-ation with numerous other research institutes cooperating under the
pro-height in cm
gramme "European Cooperation in the Field of Scientific and Technical Re-search" (COST) began a comprehensive experiment. For the first time in Europe it was planned to investigate, under the most natural conditions possible, the effects of increased atmospheric carbon dioxide concentrations and larger nitro-gen depositions on model forest eco-systems. In these model ecosystems, the trees compete for water, nutrition, space and light just as they would do in their natural environment. Researchers, therefore, planted 16 open-top chambers with model mini-forests in the grounds at WSL. Each chamber was divided into two halves: the first contained acidic soil low in nutrients, while the other half contained calcareous soil rich in nutrients. The model mini-forests on each half were both made up of eight spruce and eight beech trees, the most common tree species in Swiss forests, plus some typical herbaceous plants for the ground vegetation.
The mini-forests were treated in four different ways: The control forests grew in the normal air of the surroundings without increased nitrogen depositions.
For the second treatment, the co11ce11/ratw11s a11d 111creased 111troge11 depo s1tio11s. These t1e:es wae 11111<h larger tlw11 t/Josc 111 th£' co11trol group (far left), a11d (I/so bigger 1/Ja11 those trees that had ra,•wed ,111
-/y cme tre(l/111e11t (e/euated CO2 senmd gro11fi (mm the left. or add1tw11t1/ 11itmge11, third fro111 the left). U11/ike the sprnce, t/Je beech-trees (110/ ,/mum) res/nmdcd 011/y :,;lightly to t/Je CO2 1111d 11itroge11 tre(l/111e11ts.
treatment, additional nitrogen was in-troduced through irrigation, while in the fourth both the carbon dioxide and the nitrogen were increased.
Not a CO2 sink
The model ecosystems were studied continuously for four years. Finally, in
The experiment was carried out within the framework of the European research programme "The influence of elevated CO2 levels, climate change and air pollution on tree physiology (ICAT) ".
Participants included, besides WSL, the Universities of Berne, Basie, Neuchatel and Geneva, the ETH Zurich and the the autumn of 1998, the time was ripe. Paul Scherrer Institute, as well as various The researchers came equipped with institutes abroad. The Board of the Swiss secateurs and saws to harvest needles, Federal Institutes of Technology and the leaves, twigs, stems, and roots and to
take soil samples. The analyses, expert evaluations and discussions of the findings will still take some time.
Nevertheless, it is already possible to draw some conclusions:
- Doubling the CO2 concentration led to only a slight increase in growth in the model mini-forest. This implies that the additional carbon dioxide was not absorbed to a great extent. Thus the hope that forests, by acting as a so-called CO2 sink, could "remove" the excess carbon dioxide produced by humans is misguided.
- The beech and spruce trees reacted differently to the treatments. Doubling CO2 and raising the level of nitrogen did indeed stimulate the growth of the spruce. The beech trees, on the other hand, scarcely reacted. On the acidic soil that was low in nutrients, beech trees actually grew less well under doubled CO2 concentrations than under normal conditions. Spruce, therefore, was the winner of this "biodiversity compe-tition" under conditions with elevated CO2 and increased nitrogen deposition. This finding surprised scientists who had, if anything, expected the opposite.
Federal Office for Education and Science provided financial support. For the first time in Europe social scientists took part in this kind of natural science study. They observed the collaboration between the different disciplines and investigated how people reacted to such experiments and findings, as well as the problems involved in drafting appropriate legis-lation. Evaluation of their findings is still in progress.
Contact:
Dr. Ji.irg Bucher,
Forest and Environment Protection, Birmensdorf, Tel. 01-739 22 73, E-mail: bucher@wsl.ch
The model 1111111-(orests were planted 111
these 16 open-top c/Jambers.
State of one model m1111-(orest (om years after pla11t111g (last year of the expenme11t). The beech-trees are growmg m11c/J better 011 the calcareous so,/ (ng/Jt /Jal() than 011 t/Je ac1d1c soil (left /Jal(). The spruce trees, 011 the nt/Jer hand, are hardly respo11d111g tu the d1((ere11t sod compos1t1011s.
A111111al Report WISL 1998 25