Loblolly pine (Pinus taeda) and coppiced sweetgum (Liquidambar styraciflua)
seedlings were grown in competition with a native weed community using soil and
seed bank collected near Appomattox, Virginia. Seedlings and weeds were exposed
to CO2 (ambient and elevated) and water (water stressed and well watered)
treatments for approximately one growing season in closed top chambers.
Weed growth had an effect on tree growth, but the amount of variation in tree
biomass explained by weed biomass was very low. It appears that the tree
seedlings benefited more from available resources than the herbaceous weeds. The
influence of competition with loblolly pine and elevated CO2 did not have an
influence on total weed biomass; however, it did favor C3 weed community
development regardless of water availability. This suggests that weed community
composition may shift toward C3 plants in a future elevated CO2 atmosphere.
Loblolly pine height, diameter, needle, shoot and total biomass were
significantly greater in the well watered treatment than the water stressed
treatment. Pine root, needle, shoot and total biomass were significantly greater in
the elevated treatment than the ambient treatment. While not significant, root
biomass of water stressed pine seedlings was 63% greater in the elevated CO2
treatment than the ambient treatment. There was a significant water and CO2
interaction for pine root:shoot ratio. Under elevated CO2, root:shoot ratio was
significantly greater in the water stressed treatment than the well watered
treatment. In contrast, root:shoot ratio in the ambient treatment was nearly
identical under both water treatments. These results indicate that loblolly pine will
respond favorably in an elevated CO2 atmosphere, even under dry conditions.
The coppiced sweetgum seedlings responded favorably to well watered conditions
with significant increases in leaf area, specific leaf area, leaf, shoot and total
biomass compared to water stressed conditions. Leaf, root, shoot+stump and total
biomass of sweetgum significantly increased and specific leaf area decreased under
elevated CO2 compared to ambient CO2, but differences were smaller than previous
findings. This indicates that coppicing may dampen the growth response to
elevated CO2, at least in the initial growth stage after coppicing.
