Survival and 14-Year Growth of Black, White, and Swamp White Oaks Established as Bareroot and RPM®-Containerized Planting Stock

Bareroot Seedling Production In November of 1995, acorns were sown at 20/ft to a depth of 1 in. and covered with 2 in. of mulch. One thousand lb/acre of 28-14-14 NPK had been previously incorporated into the tilled silty loam soil before mounding to create 4-ft-wide raised beds. An additional 1,000–1,500 lb/acre of the 28-14-14 NPK were applied in increments of 300–400 lb/acre throughout the remainder of the growing season. Seedlings were lifted in early March of 1997, graded for size (15% culled) and stored at 34° F until planted. At the time of planting, another 25% of the seedlings were culled, resulting in a mean seedling height of 0.81 ft and a caliper (1 in. above root collar) of 0.15 in. Seedling Establishment RPM -containerized seedlings were field-planted during early October 1996. Bareroot seedlings were field-planted in March 1997. The planting site was located at the Horticulture and Agroforestry Research Center, New Franklin, Missouri (39° 02 N and 92° 46 W). Soils are Menfro silt loam with a 2% slope. The longterm mean annual precipitation for the study area is 38 in. Prior to planting, 5-ft strips (20 ft center-to-center) were treated with a combination of glyphosate and simazine for weed control. Seedlings (RPM and bareroot) were planted in the strips at 10-ft intervals. Four pairs of RPM and four pairs of bareroot planting stock were randomly planted within single species rows for each of the three oak species. Each species was randomly assigned to 4 of 12 strips. Data Collection Survival of RPM and bareroot planting stock was monitored annually through age 5 before an initial thinning. During a second dormant season thinning in February 2010 (13 years after outplanting), an additional 10, 15, and 5 trees from RPM planting stock and 10, 7, and 6 trees from bareroot stock of swamp white, black, and white oaks, respectively, were harvested. Selection was made randomly in the office to remove one tree of each surviving pair. The trees to be removed were dictated by the need to create strategically positioned canopy gaps. Trees were cut at groundline and measured for dbh, total height, aboveground fresh weight, and stem-only fresh weight (tree weight minus branch weight). A 1,000-lb load cell connected to a SGCN Dillon electronic meter was hung from the bucket of a skid loader to determine tree and stem-fresh weight. Trees were attached to the load cell by a short cable and lifted until each tree cleared the ground. Data Analysis Survival data (first 5 years for all 32 RPM and bareroot planting stock) were analyzed as a split plot in space. Trees on four of six treatment combinations had 100% survival and, since a logit value cannot be calculated on 100 or 0%, a dead tree (0) was added to each treatment within each of the four replications (rows) for each species. The main plot contained the effect of species and the subplot contained the effect of treatment and species treatment interaction. Replication within species was used as the denominator to test main plot effects. Since each replication had multiple trees for each treatment (eight trees, four pairs for each treatment), the replication within species treatment interaction was used as the denominator to test the subplot effects to avoid pseudoreplication. Procedure GLIMMIX in SAS, with logit link and a binomial distribution, was used for the analysis. Differences between logit means were tested using Fisher’s least significant difference (LSD). These differences were expressed as odds ratio (antilog of the difference between two average logits). Growth data from harvested trees were analyzed as a split plot in space in which the main plot contained the effects of species and row within species and the subplot contained the effect of planting stock and the interaction of species with planting stock. Treatment means for each replication were analyzed using PROC MIXED in SAS with Fisher’s LSD used to determine mean differences (SAS Institute 2002). Results and Discussion Early 5-Year Survival of Bareroot and RPM Oak Seedlings For black and white oaks, RPM survival was greater than bareroot. All RPM black and white oak survived, compared to 63 and 75% survival, respectively, of bareroot seedlings. At a 95% confidence interval, survival differences between RPM and bareroot planting stock were significantly better for both black and white oak RPM s (P 0.019 and 0.048, respectively; Table 1). Higher mortality of black and white oak trees of bareroot origin compared to containerized stock is consistent with what others have reported (Dixon et al. 1981, Parker et al. 1986). In floodplain plantings, Dey et al. (2004) reported RPM oak seedlings consistently had higher survival ( 94%) than the bareroot 1–0 planting stock (76%). Harris and Bassuk (1993) noted that nursery-dug trees may lose more than 90% of their tap root and lateral roots during lifting and transplanting severely impacting survival and growth response. Kormanik et al. (1995) found that 1–0 bareroot seedlings of northern red oak with more than 12 first-order lateral roots (FOLRs) survived and performed better in clearcuts than smaller seedlings with fewer than seven FOLRs. While specific measurements were not taken on the root systems of either the RPM or bareroot planting stock used in our study, Shaw et al. (2003) found that RPM planting stock of pin and swamp white oak had 3–7 times the dry mass and 4–9 times the volume of 1–0 bareroot seedlings (Figure 1). It is apparent from the findings of our research, and those of others, that the use of planting stock with intact large root systems, such as those produced using the RPM technology, has great potential for significantly improving the survival of some oak species Table 1. Comparison of 5-year survival for three oak species using RPM and bareroot (BR) stock planted at the Horticulture and Agroforestry Research Farm, New Franklin, Missouri. Number planted % Surv Odds ratio P-value Black oak RPM 32 100 21.9 0.019 Black oak BR 32 63 White oak RPM 32 100 12.1 0.048 White oak BR 32 75 Swamp white oak RPM 32 100 1.0 1.000 Swamp white oak BR 32 100 RPM (all species) 96 100 6.4 0.026 BR (all species) 96 79 a Percent survival calculated from actual data, prior to logit analysis. b Odds ratios were calculated by taking the antilog of the difference between average logit of RPM minus average logit of bareroot. 44 NORTH. J. APPL. FOR. 30(1) 2013 and could be a valuable tool in the regeneration of oak and other hardwood species. Fourteen-Year Growth Responses Following 14 growing seasons, diameters were consistently greater for the RPM planting stock of black and white oak, ranging in advantage from 35% in black oak to 33% in white oak, as compared to bareroot stock. A 6% increase in diameter of swamp white oak was not significant. RPM black and white oak also exhibited significant height growth advantages of 11 and 26%, respectively, over their bareroot counterparts (Table 2). With the emphasis placed on biomass yield in today’s markets for carbon sequestration and feedstock for energy, the fresh weight of stems and aboveground tree (stem plus branches) is of great importance. Total aboveground tree weight of RPM white and black oak was significantly greater (2.16 and 1.74 times greater, respectively) than that of bareroot planting stock. RPM swamp white oak had 8% greater fresh weight than its bareroot counterpart. The patterns for differences in stem weights (stem minus branches) were similar, with RPM white oak planting stock producing 2 times greater weight, followed by 76 and 14% increases for RPM black and swamp white oak, respectively, as compared to bareroot trees. Many studies have demonstrated the importance of seedling quality in the successful regeneration of oak species (Johnson 1993, Spetich et al. 2002, Dey et al. 2010). Our results suggest a strong correlation between the size and quality of the planting stock and its success 13 years after outplanting. Survival and overall growth of RPM -produced white and black oak seedlings were significantly greater than survival and growth of their 1–0 bareroot counterparts. While RPM planting stock of swamp white oak outgrew bareroot seedlings, the differences in survival and growth were not significant. These results have important implications in the establishment and growth of oak plantations whether for conventional timber, biomass, or other forest values. Our data demonstrate significant survival and growth advantages of using RPM over 1–0 bareroot planting stock for white and black oak when intensively managed as a plantation on high-quality sites. The cost of RPM planting stock is much greater than for bareroot seedlings. Although a cost-benefit analysis was not conducted in this study, it is likely that RPM planting stock is not suited to all regeneration settings. However, numerous forest values exist that might justify the use of RPM planting stock for tourism and recreation, watershed protection, carbon sequestration, creation of wildlife habitat, and ecological restoration (Pearce 2001). With the difficulties experienced in successfully regenerating many oak species, and the emphasis being placed today on maximizing growth responses, the use of superior oak seedlings with large root systems appears to have merit. Figure 1. Comparison of root volume between 1-year-old RPM (a) and bareroot (b) planting stock of white oak (A) and swamp white oak (B). Table 2. Comparison of 14-year diameter at breast height (dbh), height (ht), tree, and stem-fresh weight for three oak species using RPM or bareroot (BR) planting stock. N dbh (in.) P-value ht (ft) P-value Tree weight (lb) P-value Stem weight (lb) P-value Black oak (RPM ) 15 8.40 0.0002 38.2 0.0088 622 0.0003 405 0.0002 Black oak (BR) 7 6.23 34.4 357 229 White oak (RPM ) 5 7.76 0.0084 31.7 0.0010 499 0.0043 326 0.0050 White oak (BR) 6 5.85 25.2 231 161 Swamp white oak (RPM ) 10 7.41 0.4088 33.6 0.1931 409 0.6481 274 0.4217 Swamp whi