Root growth and respiration in elevated CO2
(700 μmol mol−1) was studied in three tree species,
Fraxinus excelsior L., Quercus petraea. L. and Pinus
sylvestris L. grown in open-top chambers (OTCs) during a long-term
exposure
(20 months), during which root systems were allowed to develop without
restriction imposed by pots. Root growth,
measured as root length using root in-growth bags was increased significantly
in trees exposed to elevated CO2,
although the magnitude of the response differed considerably between
species and with time of sampling, the
greatest effect observed after 6 months in ash (ratio of elevated: ambient,
e[ratio ]a; 3·40) and the smallest effect observed
in oak (e[ratio ]a; 1·95). This was accompanied by changes in specific
root length, with a significant decrease in all species
after 6 months, suggesting that root diameter or root density were
increased in elevated CO2. Increases in root
length might have resulted from an acceleration in root cell expansion,
since epidermal cell size was significantly
increased in the zone of elongation in ash root tips (P<0·05).
Contrasting effects of elevated CO2 were observed for root
carbohydrates, with significant increases in soluble
sugars for all species (P<0·05), but both increases
and
decreases in starch content were observed, depending on
species, and producing a significant interaction between species and
CO2 (P<0·001). Exposure to elevated CO2
increased the total root d. wt for whole trees of all three species after
8 months of exposure, although the magnitude
of this effect, in contrast to the root in-growth study, was greatest in
Scots pine and smallest in ash. No significant
effect of elevated CO2 was observed on the root[ratio ]shoot
ratio.
Further detailed analysis of whole root systems after
20 months confirmed that species differences in root responses to elevated
CO2 were apparent, with increased
coarse and fine root production in elevated CO2 for Scots pine
and ash respectively. Lateral root number was
increased in elevated CO2 for all species, as was mean root
diameter. Root respiration rates were significantly
reduced in elevated CO2 for all three species. These results
provide firm evidence that exposure of trees to future
CO2 concentrations will have large effects on root system
development, growth, carbohydrate status and
respiration. The magnitude and direction of such effects will differ,
depending on species. The consequences of
such responses for the three species studied are discussed.