Northern red oak in the western Lake States area of the USA exists
on the most xeric edge of its distribution range.
Future climate-change scenarios for this area predict decreased water availability
along with increased atmospheric
CO2. We examined recent photosynthate distribution and growth
in seedlings as a function of CO2 mole fraction
(400, 530 and 700 μmol mol−1 CO2), water
regime (well watered and water-stressed), and ontogenic stage. Water
stress effects on growth were largely offset by elevated CO2.
Water stress increased root mass ratio without concurrently increasing
allocation of recent photosynthate to the
roots. However, apparent sink strength of water-stressed seedlings at the
completion of the third growth stage
tended to be greater than that of well watered seedlings, as shown by continued
high export, which may contribute
carbon reserves to support preferential root growth under water-stressed
Elevated CO2 decreased apparent shoot sink strength associated
with the rapid expansion of the third flush.
Carbon resources for the observed enhanced growth under elevated CO2
could be provided by enhanced
photosynthetic rate over an increased leaf area (Anderson & Tomlinson,
1998, this volume).
Increased sink strength of LG seedlings under water-stressed conditions,
together with decreased apparent
shoot sink strength associated with growth in elevated CO2 provide
mechanisms for offsetting water stress effects
by growth in elevated CO2.
Careful control of ontogeny was necessary to discern these changes and
provides further evidence of the need
for such careful control in mechanistic studies.