In the fall of 1999, a four-year study on improved success of bottomland regeneration was conducted by Daniel C. Dey, Wayne Lovelace, John M. Kabrick, and Michael A. Gold. Specifically, methods were evaluated for regenerating pin oak and swamp white oak on former agricultural crop fields in the Missouri River floodplain at Smoky Waters and Plowboy Bend Conservation Areas. The study fields had been in crop production for years before this study.
Soils at the Plowboy Bend site were mapped as Sarpy Fine Sand. These soils are formed in sandy alluvium and consequently are excessively drained. Soils at the Smoky Waters site were mapped as Haynie Silt Loam, Leta Silty Clay, and Waldron Silty Clay Loam. These soils are formed in silty or clayey alluvium and range from somewhat poorly drained (Leta and Waldron) to moderately well drained (Haynie). The Plowboy Bend site is also protected by a levee, while the Smoky Waters is not protected by levees.
Both 1+0 bareroot and 11- and 19-liter RPM (Root Production Method®) seedlings were planted to evaluate the effect of seedling size and nursery stock type on the survival and growth of pin oak and swamp white oak seedlings.
Seedlings were planted in soil mounds created with a rice plow or in unmounded soil. They were planted with either a cover crop of a New Century redtop grass or with natural vegetation that normally colonizes abandoned bottomland crop fields.
Weather and wildlife played a role in the study. In two of the four years (’01, ’02), the study site at Smoky Waters was flooded for up to 3 weeks in June. Additionally, every winter after the first year, cottontail rabbits girdled and shoot clipped oak seedlings and oak sprouts, which significantly impacted oak survival and growth. The severity of the damage to planted oaks varied greatly between the cover crop treatments (redtop grass vs. natural vegetation fields).
RPM oak seedling survival rates were dramatically higher than traditional bareroot seedlings.
RPM seedlings remained high (better than 94%) during the first three years, while survival of bareroot seedlings showed a significant decline for both swamp white oak and pin oak.
After three years, bareroot swamp white oak seedlings had a survival rate averaging 76%, with pin oak bareroot seedlings averaging 54%. The difference in survival rate in RPM oaks was statistically insignificant between 11-and 19-liter seedlings, and also between pin oak and swamp white oak RPM seedlings. Notably, the survival of oak seedlings was not significantly affected by soil mounding or cover crop treatments.
However, an assessment of the survival of individual trees not damaged by rabbits showed redtop grass cover crop significantly increased survival over trees grown with natural vegetation. The assessment was based on logistic regression analysis conducted by Dey and others (2003). There were no significant differences in survival among trees on mounded and unmounded soils.
Diameter and Height Growth
RPM seedlings demonstrated faster growth and larger diameter than bareroot stock. Basal diameter increment after three years was significantly greater for RPM® seedlings than bareroot stock. This result was consistent regardless of species, based on an analysis of variance. There was no significant difference (P = 0.34) in basal diameter increment between the 11- and the 19-liter RPM® seedlings. The average basal diameter of all RPM oak seedlings increased by 0.8 cm in the first three years, whereas bareroot seedlings increased only 0.3 cm.
The basal diameter of pin oak 19-liter RPM® seedlings increased the most during the first three years, averaging 1.0 cm of new growth. The above analysis includes rabbit-damaged and rabbit-undamaged trees. Excluding rabbit-damaged trees, the average basal diameter increment was 1.6 cm for RPM seedlings and 0.2 cm for bareroot trees.
Average height increment after three years was negative for most species and nursery stock types due to wildlife damage. Cottontail rabbits caused extensive damage by girdling the stems of RPM® seedlings or by clipping the shoots of bareroot seedlings at ground-level, which caused shoot dieback and loss of height.
Three year height increment was significantly less for RPM® seedlings than bareroot, based on an analysis of variance. For bareroot seedlings that had been shootclipped by rabbits, annual sprout growth came close to, or slightly exceeded the initial height, resulting in small negative or positive increments in height. In contrast, net height increment was much lower in RPM® trees because rabbit girdling, which occurred in the lower 0.30 m of the stem, caused shoot dieback to near ground-level, and annual sprout growth was not enough to recover the original height.
Height growth of undamaged RPM® seedlings may be low because these trees were planted in an area with natural vegetation competition. There may be less light competition in the redtop grass fields during the growing season than in the natural vegetation fields.
Acorn Production swamp white oak RPM® seedlings planted at 18 to 24 months age produced acorns in each of the first four years following outplanting. Production occurred in a small proportion (3.5%) of the 2,522 swamp white oak RPM® seedlings in their first year in the field. Most of the production (60%) occurred in oaks from 19-liter containers, but larger 11-liter container RPM® trees also produced acorns.
During the first four years in the field, the average RPM® acorn production increased from 4.3 to 12.5 acorns per tree. Individual trees were able to produce as many as 125 acorns. A single pin oak RPM® seedling produced acorns for the first time in year 4.
Consistent, early production of acorns was surprising considering that open-grown oaks do not begin producing seed until they are 20 to 30 years old. In contrast, no bareroot oak seedlings have produced acorns after four growing seasons.
Large container RPM® seedlings had significantly greater survival and basal diameter growth than bareroot seedlings after three years. Three-year height increment for RPM® seedlings was negative and significantly less than that of bareroot seedlings largely due to the loss of initial height from rabbit herbivory on oak seedlings. There was no difference in growth or survival to-date between the 11- and 19-liter RPM® trees.
After three years, and two June floods at Smoky Waters Conservation Area, soil mounding did not improve height and diameter growth, or survival of oak seedlings at either site.
One- to two-year-old swamp white oak RPM® seedlings produced acorns in the first year after planting in Missouri River floodplain crop fields. The number of acorn bearing trees and production per tree increased during the first four years.
Under controlled conditions in the nursery, the RPM® process produces seedlings with very large root systems that have numerous large diameter lateral roots (or multiple taproots), and a high density of fine roots. These are desirable root characteristics that many have recognized as being essential for successful regeneration of species such as the oaks. Early results from this study indicate that planting RPM (Root Production Method®) seedlings of large size in a redtop grass ground cover appears to be a successful formula for regenerating oaks and restoring acorn production in agricultural floodplains.
PRODUCTION AND EARLY FIELD PERFORMANCE OF RPM (Root Production Method®) SEEDLINGS IN MISSOURI FLOODPLAINS
Daniel C. Dey, Wayne Lovelace, John M. Kabrick, and Michael A. Gold
Read the study in its entirety here.
A Scientific Approach to Conservation
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