M. Crookshanks et al., ELEVATED CO2 AND TREE ROOT-GROWTH - CONTRASTING RESPONSES IN FRAXINUS-EXCELSIOR, QUERCUS-PETRAEA AND PINUS-SYLVESTRIS, New phytologist, 138(2), 1998, pp. 241-250
Root growth and respiration in elevated CO2 (700 mu mol mol(-1)) was s
tudied 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 respon
se differed considerably between species and with time of sampling, th
e greatest effect observed after 6 months in ash (ratio of elevated:am
bient, e:a; 3.40) and the smallest effect observed in oak (e:a; 1.95).
This was accompanied by changes in specific root length, with a signi
ficant decrease in all species after 6 months, suggesting that root di
ameter or root density were increased in elevated CO2. Increases in ro
ot length might have resulted from an acceleration in root cell expans
ion, since epidermal cell size was significantly increased in the zone
of elongation in ash root tips (P < 0.05). Contrasting effects of ele
vated CO2 were observed for root carbohydrates, with significant incre
ases in soluble sugars for all species (P < 0.05), but both increases
and decreases in starch content were observed, depending on species, a
nd producing a significant interaction between species and CO2 (P < 0.
001). Exposure to elevated CO2 increased the total root d. wt for whol
e 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:shoot ratio. Further detailed an
alysis of whole root systems after 20 months confirmed that species di
fferences in root responses to elevated CO2 were apparent, with increa
sed coarse and fine root production in elevated CO2 for Scots pine and
ash respectively. Lateral root number was increased in elevated CO2 f
or all species, as was mean root diameter. Root respiration rates were
significantly reduced in elevated CO2 for all three species. These re
sults provide firm evidence that exposure of trees to future CO2 conce
ntrations will have large effects on root system development, growth,
carbohydrate status and respiration. The magnitude and direction of su
ch effects will differ, depending on species. The consequences of such
responses for the three species studied are discussed.