In situ effects of elevated CO2 on plants and insects

10.1 Mature trees in a CO2 enriched world
Mature (app. 100 years old) temperate forest trees were exposed to an increased
atmospheric CO2 concentration for four growing seasons at the Swiss Canopy Crane site
(SCC) near Basel (Switzerland) using web-FACE (Free air CO2 enrichment) technology.
For 11 trees (5 different species) in elevated CO2 and 26 control trees growing at ambient
CO2 I documented seasonal phenology, measured basal area (BA) increment, main and
lateral shoot growth, leaf area/shoot length ratio and calculated a branching index. At the
end of 2001, one year after the onset of the experiment, I found a significant BA growth
stimulation in Fagus growing in elevated CO2. Fagus exhibited a second significant BA
stimulation in 2003, when Europe experienced a centennial drought (no response in 2002
and 2004). None of the other species showed a stem growth response to CO2 in any of the 4
years. Also when tested across all species, BA increment did not show a significant
response to elevated CO2 (neither when tested per year nor cumulative for four years). The
inclusion of Prunus and Tilia did not change the picture. Fagus showed a significant higher
lateral branching in elevated CO2 in 2002 when shoots developed from buds that were
formed during the first season of CO2 enrichment (2001). The effect disappeared in 2003.
In Quercus, there was a steady increase in leading shoot length in high CO2 trees resulting
in longer leading shoots after 4 years of CO2 enrichement. Phenology (bud break, leaf fall,
leaf duration) was highly species-specific and did not reveal a consistent effect of elevated
CO2. Our four-year data set reflects a highly dynamic and species-specific response of tree
growth to a step change in CO2 supply. In spite of some transient growth stimulation our
results do not support the notion that mature forest trees will grow better in a CO2 enriched
world and thus act as carbon sinks.
10.2 Plant-insect interaction in a CO2 enriched world
I studied two insect species feeding on in situ CO2 enriched plants at treeline. Miramella
alpina fed on two Vaccinium species and Zeiraphera diniana on Larix decidua shoots. In
both plant species elevated CO2 reduced nutritional quality, which affected the
performance of the insects feeding on them. Relative growth rates of Miramella were lower
in animals feeding on V. myrtillus compared to those feeding on V. uliginosum and
grasshoppers growth was affected by elevated CO2 depending on plant species and
nymphal development stage. Relative growth rate of Miramella correlated with CO2
induced changes in leaf water, nitrogen, and starch concentrations, depending on
grasshoppers' instar. Elevated CO2 resulted in reduced female adult weight; irrespective of
plant species elevated CO2 prolonged development time in animals feeding on V.
uliginosum only, but did not cause a significant difference in nymphal mortality. Newly
molted adults of Miramella produced lighter eggs and less secretion, (serving as egg
protection) under elevated CO2. When grasshoppers had a choice among four different
plant species grown either under ambient or elevated CO2, the consumption of V. myrtillus
and V. uliginosum leaves increased under elevated CO2 in females while it decreased in
males compared to ambient CO2-grown leaves. These findings suggest that rising
atmospheric CO2 distinctly affects leaf chemistry in two important dwarf shrub species at
the alpine treeline, leading to changes in feeding behaviour, growth, and reproduction of
the most important insect herbivore in this system.
The study of Zeiraphera (larch bud moth) revealed that larvae grew slower under elevated
CO2 compared to ambient CO2 when trees had remained undefoliated in the previous year.
If, however, trees had been defoliated, this response was reversed, with a faster growth of
larch bud moth on high CO2-exposed trees than on control trees. Pupal weight was not
affected by either CO2 or defoliation. Thus elevated CO2 and defoliation had only minor
effects of larch bud moth performance. Needle maturation over the course of the season
incurres significant compositional changes. N concentration was on average 38% lower and
lignin concentration 55% higher in early July than in mid June 2003. In conclusion, my
results suggest that elevated CO2 and defoliation induced changes in larch needle quality
have only little impact on larch bud moth performance at the alpine treeline, and, in
particular, indicate no reinforced negative effect of these two factors. However, the
pronounced changes of needle quality during needle maturation might suggest that any
shifts in tree phenology due to global change may be of greater importance for alpine larch
bud moth populations than the direct impact of CO2 on needle quality.
An increasing number of studies suggest that switching of host plants due to rising
atmospheric CO2 concentrations is probable (Goverde and Erhardt 2003, Hättenschwiller
and Schafellner 2004, Agrell et al. 2005). However all the experimental work on effects of
elevated CO2 on insect herbivores compares insects in ambient CO2 conditions and insects
feeding on plants, which have been abruptly subjected to the chosen elevated CO2 levels.
One fundamental point, which enables insects to switch host plants, is the degree of insect
mobility. A priori it would be presumed that nymphs are less mobile than adults, but this
was never experimentally tested. I showed that for example caterpillar or grasshopper
nymphs do not switch between tree species or even trees during their larval stages. The
poorer food quality becomes with rising CO2 concentrations the higher might be the need
to switch feeding plants in the future. However, the change in leaf chemistry as well as
related changes in insect behaviour (e.g. switching host plants for feeding or egg
deposition) will occur gradually. It is not known, whether adaptions of insect-plant
relations will occur over the next decades (Whittaker 1999, 2003).
10.3 Conclusion
This work revealed insight into a number of biotic influences of elevated CO2 under most
realistic experimental conditions. Across all tests it became clear that atmospheric CO2-
enrichment exerts rather subtle influences all related to biodiversity.
The major conclusion of my growth analysis in a mature deciduous forest is that after four
years these tall trees do not respond to elevated CO2 with more stem growth. If there is a
sustained response it would be small and much longer observation periods would be needed
to identify such a response, and it is almost certain that such responses will be species
specific as they were during the starting phase of this experiment. From what I see at the
Swiss forest FACE site, a first approximation would be that there is no gain in carbon
stocking in a CO2 rich future in such trees. A smaller CO2 responsiveness of larger, more
complex test systems was already obvious as CO2 research switched from seedlings (pots)
to saplings (open top chambers), a trend now finding strong support by data from the tallest
and oldest forest studied so far under free air CO2 enrichment. My plant-animal interaction
studies with plant material grown in situ under elevated CO2 are in line with other studies,
but do not permit a simple generalizing conclusion. I showed that food plant species matter
and even the sex of the feeding animals played a role. Performance of both insect species is
likely to become impaired by rising CO2 concentration. However, the overall picture is
highly complex.
Scientist are now beginning to study the effects of elevated CO2 on plants and insects in
"natural" (e.g. FACE experiments) systems although they have to deal with a lot more
variability compared to earlier works in controlled conditions, which makes the analysis
more difficult. So far, too little attention was put on species specifity although evidence is
emerging that generalisation of effects found in plants, as well as in insects is not possible
because they show distinct species specific reactions to elevated CO2, as I have shown in
my thesis.