Effects of flooding regime and seedling treatment on early survival and growth of Nuttall oak

Summary

Since the mid-1800s, approximately 7 million hectares of bottomland hardwood forests in the Lower Mississippi River Valley, U.S.A., have been cleared, drained and converted to farmland. However, many of the cleared floodplain soils were found to be marginal agricultural lands with a history of crop failure. Since the inception of the Wetlands Reserve Program in 1995, attempts to reestablish forest cover on such marginal lands have increased. Reforestation success has however, been mixed, with seedling mortality and poor growth often attributed to drought or flooding. Poor planting practices and herbivory have limited the success of seedling establishment at many sites.

Due to its wildlife value, Nuttall oak Quercus texana (as well as other large seeded tress) is a popular choice of speices for reforestation projects. It had been suggested that oak seedlings grown in containers may have a higher survival probability as their roots are protected by the soil that they were grown in at the nursery. After planting, this soil reduces shock associated with transplanting and helps seedling roots to remain moist for longer. Container seedlings, however, are more costly to produce and plant than bareroot seedlings, and information was therefore needed to help managers evaluate the practicalities and cost-effectiveness of using each. This experiment was designed to determine if there is a difference in the effects of seasonal flooding, seedling stock type (bareroot and container-grown, with and without mycorrhizal inoculation) on Nuttall oak seedling survival and growth in Sharkey soil.

Nuttall oak Quercus texana seedlings were cultivated using common nursery methods. Three seedling cultivation treatments were applied: acorns grown in plastic seedling cones; acorns grown in plastic seedling containers and inoculated with fungal mycelium; and acorns grown directly into a nursery bed (bareroot seedlings):

Container seedlings: In late May 1994, acorns (collected in late April and kept in cold storage) were individually sown in 164-cm³ (3.8 × 21 cm) plastic seedling cones (Ray Leach "Cone-tainer" Nursery, Canby, OR, U.S.A.) in a mix of peat moss and vermiculite. They were then cultivated in a greenhouse, watering as needed. Seedlings were fertilized and liquid iron added, biweekly. In late June, cones with fully developed seedlings were placed on trays (49/tray) and moved to outdoor tables covered with 50% shade cloth. In early July, every other tray of seedlings was inoculated with Pisolithus tinctorius fungal mycelium. Theses were inoculated again in August and December 1994. Seedlings were transported to the planting site in a covered van in early January 1995.

Bareroot seedlings: Bareroot seedlings, 46 cm or larger, were obtained from the same seed source and seed lot as the container-grown seed. They were lifted from the nursery bed in early January and in sealed shipping bags during transportation to the planting site.

Characteristics prior to planting: The inoculated container seedlings had significantly more primary lateral roots (>0.5 mm) than the non-inoculated container seedlings, and had twice as many lateral roots (>5 mm) than the bareroot seedlings. However, the root volume of the bareroot seedling (mostly taproot) was roughly four times that of either container type. Root volume, stem diameter at the root collar, stem dry weight, and height were about the same for the two container seedling types.

Planting: Seedlings were planted in January 1995 at Yazoo National Wildlife Refuge. The field selected had been planted with soybeans for several years, but repeated failure due to inundation resulted in it being scheduled for wetland restoration by the U.S. Fish and Wildlife Service in 1994, when it became part of the Wildlife Refuge. The soil type comprised Sharkey clay i.e. poorly drained, very slowly permeable clay soils. When dry, it commonly develops cracks several centimetres deep and wide. Two elevation contours (representing different flooding regimes) were chosen, 28.1 m amsl level ('low') and 29.4 m amsl ('high'). The seedlings were planted with four replications (30 seedlings/replication) at each elevation on the floodplain, for each of the three seedling types (i.e. 720 seedlings in total).

Weather: During March–April 1995, rainfall in west-central Mississippi was considered normal to above normal and during this period the lower planting elevation was inundated for 21 days (flooding twice for a total of 21 days between 8 March and 1 May). The higher elevation did not flood during the 5-year study period. Low rainfall during the latter half of the first growing season (June–September 1995) produced unusually dry conditions. Average monthly precipitation was again lower than average during the first half of the second growing season (1996). Rainfall was normal to above normal during the last half of the second growing season and through 1997, but in 1998 and 1999, drought conditions persisted from March to December.

Plant competition: Plant competition at each planting elevation during the first growing season was compared by recording percentage vegetation cover, height and aboveground biomass in a 1-m² quadrat located at the centre of the spaces between each plot.

Nuttall oak seedling survival: Survival was significantly greater at the low, intermittently flooded elevation for all treatment types). In the first growing season in early June 1995, the average survival of both container-grown types exceeded 96%, whereas bareroots averaged 45%. By the end of 1995, 97% of the seedlings had been clipped slightly above the root collar by small rodents. Resprouting is common in rodent-damaged hardwood seedlings and by April 1996, many of the oak seedling had initiated new shoot growth. Some herbivory and resprouting was also observed in subsequent years (being abundant also in 1996). Survival was again assessed in the third and fifth years after planting.

Of the rodent damaged seedling, about half resprouted. For bareroots, there was approximately 40–70% recovery by sprouting in the third year, with higher survival in the lower elevation. For container seedlings there was a marked difference between the high (0–10% survival) and low (37–73% survival) planting elevations. The inoculated treatment had the best recovery (up to 90%). The fifth-year survival data were very similar to third-year data.

For both third- and fifth-year data, overall survival in the container-only (without fungal inoculation) treatment was significantly lower than that in other two treatments. Survival of bareroot seedlings did not differ significantly from the inoculated container seedlings. Survival at the higher planting elevation was generally poor. At the high elevation, by the end of the fifth growing season, overall survival of bareroot seedlings (32%) was much higher than both inoculated container seedlings (17%) and container seedlings (7%).

Seedling height: Between the third and fifth years, all seedlings more than doubled in height (see Table 1, attached). At the end of the third and fifth growing seasons, the height of bareroot seedlings was greater than container-grown seedlings. For bareroots, average height at the high elevation (190 cm) was significantly greater than that at the low elevation (159 cm) by the fifth year. In both types of container-grown seedlings, the average height in the low elevation exceeded the high elevation, although not significantly so.

Plant competition: The commonest species at the low elevation was annual marsh elder Iva annua, with 75% average cover. No other species had more than 5% cover in the canopy of the competing vegetation. At the higher planting elevation, plots had a dense growth of Johnson grass Sorghum halepense, with powderpuff Mimosa strigillosa slightly more prevalent than at the lower elevation.

Conclusions: Survival of all seedling types was greatest at the lower wetter, elevation, indicating that drought and related effects on plant competition were more limiting to seedling survival than flooding stress. At the end of the fifth growing season, survival of the bareroot seedlings was greater than that of the noninoculated container seedlings at the higher planting elevation. Survival of the bareroot seedlings was not different from that of the inoculated container seedlings at either elevation. Bareroot seedlings grew taller than the non-inoculated and the inoculated container seedlings during the 5-year study. As seedlings grown in the plastic seedlings cones did not survive better than the bareroot seedlings at either planting elevation, the bareroot stock (cheaper to grow and easier to plant) appear to be the economically superior choice for regeneration in Sharkey soil.


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