Growth and photosynthetic responses of encroaching tree seedlings to CO2 and stress interactions

1. Woody encroachment in southern African savanna has been partly
attributed to rising atmospheric CO2 fertilising the growth of C3 trees
but less so that of competing C4 grasses. However, growth conditions
(resource availability, competition, rooting space, and herbivory) must be
suitable for the effects of elevated CO2 (eCO2) to be realised. 2. This
research investigated the interactions between the positive effect of eCO2
on tree seedling growth and limitations imposed by drought, disturbance,
and competition with C4 grasses. Seedlings of the prolific encroacher C3
tree Vachellia karroo were grown at ambient (400 ppm) or eCO2 (800 ppm) in
Open-Top Chambers and exposed to a variety of stresses and disturbances
typical of savanna systems. Photosynthetic, growth and allocation
responses to eCO2 and other treatments were determined. 3. Unsurprisingly,
we show strong growth and water-saving responses of V. karroo seedlings to
eCO2 when in the absence of competition and herbivory. However, the
addition of either competition or simulated herbivory in the first season
of growth moderated this, while neither drought nor shading diminished the
eCO2 effect relative to similarly treated plants grown at ambient [CO2].
4. Synthesis. We demonstrate that eCO2-induced C3 stimulation in
encroaching savanna species such as V. karroo will be inconsistent across
time and space. This research does not detract from the suggestion that
increasing atmospheric CO2 is implicated in woody encroachment, but rather
that eCO2 benefits to C3 tree seedlings are only realised when growth
conditions are suitable. Inconsistencies in eCO2 response will translate
into spatial and temporal variation in seedling responses to eCO2 and
CO2-driven woody encroachment, explaining some of the variability observed
in woody encroachment across geographic regions and disturbance gradients.