We tested the extent to which growth responses to elevated carbon
dioxide (CO2) are temperature-dependent and
change through early seedling ontogeny among boreal tree species of contrasting
relative growth rates (rgr).
Populus tremuloides Michx, Betula papyrifera Marsh, Larix
laricina (Du Roi) K. Koch, Pinus banksiana Lamb.,
and Picea mariana (Mill.) B.S.P. were grown from seeds for 3 months
in controlled-environment chambers at two
CO2 concentrations (370 and 580 μmol mol−1)
and five temperature regimes of 18/12, 21/15, 24/18, 27/21
and
30/24°C (light/dark). Growth increases in response to CO2
enrichment were minimal at the lowest temperature
and maximal at 21/15°C for the three conifers and at 24/18°C
or higher for the two broadleaved species,
corresponding with differences in optimal temperatures for growth. In both
CO2 treatments, rgr among species
and temperatures correlated positively with leaf area ratio (lar)
(r[ges ]0·90, P<0·0001). However, at
a given lar,
rgr was higher in elevated CO2, owing to enhanced whole-plant
net assimilation rate. On average in all species
and temperatures at a common plant mass, CO2 enrichment increased
rgr (9%) through higher whole-plant net
assimilation rate (22%), despite declines in lar in high CO2
(11%). Reductions in lar are thus an important
feedback mechanism reducing positive plant growth responses to CO2.
Proportional allocation of dry mass to roots
did not vary between CO2 treatments. Early in the experiment,
proportional increases in plant dry mass in elevated
CO2 were larger in faster-growing Populus tremuloides
and B. papyrifera than in the slower-growing conifers.
However, growth increases in response to CO2 enrichment fell
with time for broadleaved species and increased
for the conifers. With increasing plant size over time, compensatory adjustments
to CO2 enrichment in the factors
that determine rgr, such as lar, were much larger in
broadleaves than in conifers. Thus, the hypothesis that
faster-growing species are more responsive to elevated CO2 was
not supported, given contrasting patterns of
growth response to CO2 with increasing plant size and age.