The Influence of Elevated Carbon Dioxide and Water Availability on Herbaceous Weed Development and Planted Loblolly Pine (Pinus taeda) and Coppice Sweetgum (Liquidambar styraciflua) Growth

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Virginia Tech

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.

herbaceous weed community, carbon dioxide, sweetgum, Loblolly pine, biomass, competition