Individual study: The effect of an eastern cottonwood Populus deltoids nurse crop on establishment and growth of Nuttall oak Quercus nuttallii in a bottomland hardwood restoration in the Lower Mississippi Alluvial Valley, Mississippi, USA
Gardiner E.S., Stanturf J.A. & Schweitzer C.J. (2004) An afforestation system for restoring bottomland hardwood forests: biomass accumulation of Nuttall oak seedlings interplanted beneath Eastern cottonwood. Restoration Ecology, 12, 525-532
Much bottomland hardwood forest in southeastern USA has been since European settlement. Now afforestation effort, particularly in the Lower Mississippi Alluvial Valley, has generated a demand for alternative afforestation systems. The potential of an afforestation system involving initial establishment of the rapidly growing native eastern cottonwood Populus deltoids, followed by planting the understory with Nuttall oak Quercus nuttallii is being investigated. In this study, the growth and biomass accumulation by Nuttall oak seedlings was assessed to determine whether this species can established and develop beneath a cottonwood overstory.
Study site: The study was established in the Lower Mississippi Alluvial Valley on a former agricultural land on Sharkey Clay soils in Sharkey County, Mississippi, USA. The site is situated in the humid, subtropical region of the temperate zone with average annual precipitation of 1,318 mm, and air temperature averaging 27.8 °C in July to 7.5°C in February.
Establishment of experimental stands: Agricultural production ended on the study site with the harvest of a soybean Glycine max crop in autumn1994. In March 1995 three 8.1 ha eastern cottonwood Populus deltoids stands were established. Cottonwood cuttings were hand-planted on a 3.7 × 3.7 m grid, and mechanically cultivated to control weeds over the first two growing seasons. After the second growing season (February 1997), bare-root Nuttall oak Quercus nuttallii seedlings were interplanted between every other cottonwood row. Additionally, a Nuttall oak stand was established adjacent to the three interplanted cottonwood stands to provide open-grown seedlings as experimental controls. Fifty oak seedlings were selected in each stand and protected from small mammal herbivory with 0.6 m tall x 0.6 m diameter wire mesh shelters. It should be noted that growth of eastern cottonwood is very site specific, excessive flooding, for example, is not appropriate for establishment of eastern cottonwood.
Stand conditions: Sampling of oak seedlings began after three growing seasons. The eastern cottonwood nurse crop had developed rapidly, reducing light availability to interplanted oak seedlings to approximately 30% of light available in the open. The amount of herbaceous competition in the cottonwood understory was less than 35% of that in the open.
Stem growth: After three years Nuttall oak height was not influenced by the eastern cottonwood canopy, seedlings averaging about 126 cm tall both under the cottonwood and in the open. This height represented growth of more than twice the height at planting. Seedling root-collar diameter was influenced by the afforestation treatment. Nuttall oak seedlings under the cottonwood stands were 20% smaller in root-collar diameter (21.4 ± 1.3 mm) compared to those in the open (26.7 ± 2.2 mm).
Biomass accumulation: The amount of biomass accumulated by oak seedlings differed according to treatments. Seedlings under cottonwood accumulated less biomass than open-grown seedlings, accumulated 54% less leaf mass, 59% less stem mass, and 50% less root mass relative to seedlings in the open. Nuttall oak seedlings in the open developed a total mass more than twice that of seedlings under the cottonwood.
Biomass distribution: The comparatively greater amount of leaf biomass in open-grown seedlings was partially attributed to a larger number of leaves (250% more) relative to interplanted seedlings. Leaves of open-grown seedlings, however, were generally smaller in area (52%), but there was no detected treatment effect on total leaf area.
Irrespective of the large differences observed for biomass accumulation, the proportional distribution of biomass within seedlings was not altered by stand environment as interplanted oak seedlings accumulated leaf, stem and root biomass in equal proportions to those in the open.
Conclusions: These results suggest that an afforestation system involving rapid establishment of eastern cottonwood cover, followed by understory planting with Nuttall oak and perhaps other native tree species, may provide an alternative afforestation method on former bottomland hardwood sites.
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