94 Lambert et al.: Effect of Container Size at Time of Planting on Tree Growth Rates al. 2008a; Struve 2009). Additionally, the scale of plantings for restoration projects would likely make any of these prac- tices cost prohibitive even if they were found to be beneficial. Determining the most successful and cost-effective plant- ing size under less than ideal conditions is critical to maxi- mizing restoration success and meeting permit success cri- teria. The purpose of this study was to determine the relative cost effectiveness of three planting sizes for three commonly used species and to assess whether they achieve common suc- cess criteria for height, percent survival and percent cover. METHODS Site Descriptions Possum Branch Preserve This primary study site is located in northern Pinellas County, Florida, U.S. (S 1/2 S-16/T-28/R-16). Within this 10.2-ha site, a constructed 2.2-ha forested seep slope was planted with various tree species, including two commonly used wetland restoration species, red maple (Acer rubrum) and baldcypress (Taxodium dis- tichum). Soils consisted of marine clays deposited from the his- toric dredging of the adjacent Lake Tarpon Outfall Canal. Initial colonizing vegetation included halophytic species such as seaside heliotrope (Heliotropium curassavicum) and little hogweed (Por- tulaca oleracea), suggesting saline soils; however, these species did not persist after the first rainy season and initial saturation of the site with freshwater. For this reason, and the known toler- ance of baldcypress to low salinities (Wilhite and Toliver 1990), soil testing was not performed for salinity or other parameters. Brooker Creek Preserve The second site is an area within Brooker Creek Preserve in the far northeast corner of Pinellas County (N 1/2 of S-11/T-27/R-16) known as the Upland Enhancement Area of the Bi-County Thruway Mitigation site. Historic use of the area was a cattle pasture cleared from pine flatwoods; no grade alteration or clearing had occurred for many years prior to this study. As an upland restoration project, the primary tree species planted was longleaf pine (Pinus palustris). STUDY DESIGN At the Possum Branch Preserve site, trees were planted in July and August 2002, with the initial set of measurements taken from September through December 2002. There were 600 trees at this location: 100 trees from each of three container sizes (#1, #3, and #7) for both red maple and baldcypress. The Brooker Creek Preserve site was not part of the initial study design, but the scheduled project planting in November 2002 offered the opportunity to represent longleaf pine as an additional species. There were 100 trees at this secondary location: 50 trees of both #1 and #3 container sizes. Initial measurements for longleaf pine were taken from January through February 2003. A similar an- nual schedule was followed for the remaining four years of the study. The later planting date at Brooker Creek Preserve would have provided cooler conditions and thus less heat stress at the time of planting, but would have reduced the overall growing season for the longleaf pines. Tree installation was accomplished by two different biological consulting firms, one at each study ©2010 International Society of Arboriculture site, in a manner typical of a mitigation/restoration planting ef- fort; no special instructions were given. Trees arrived on site in good condition and conformed to the standards for wetland plants as defined by Florida Grades and Standards for Nursery Plants (Florida Dept. of Agric. and Consumer Serv. 1998). Planting tech- nique commonly applied to large scale restoration plantings, how- ever, emphasizes speed and low unit cost. This means most trees were handled by the trunk, little attention was given to the proper size hole, nursery stakes were neither removed nor were field sup- port stakes added, and soil was generally added on top of the root ball for stability. Although no watering or tamping was done, this should have been minimally important due to the saturated soils at both planting locations. Tree spacing was variable within the designated planting zones, but was approximately 3 m on center. Measured parameters included tree height, canopy spread, and survival for each of the planted trees. Additional estimates of growth, specifically caliper and diameter at breast height (dbh) were taken but are not reported here, as they were found to be highly correlated with height. Reproductive status and anecdotal observations were also recorded concerning cone production and gall formation on the baldcypress. Height was measured using a telescoping measuring rod. The tallest point on the tree was used (tips of leaves or needles), as location of terminal buds or branch tips could not be determined consistently. For leaning trees, trees were held vertical for the height measurement. When this be- came impractical, height was measured at right angles to the lean (Bonham 1989). Canopy Spread was measured using a folding engineer’s rule or the telescoping measuring rod as the canopy became larger. If the canopy was not symmetrical, the largest di- ameter was used (Denton 1990). Survival was noted annually for each tree. A tree was recorded as dead if no leaves were present and no green tissue was evident upon scraping the trunk near the base. At the initial measurement, container size was recorded and a unique consecutively numbered tag was attached to each tree. Site maintenance at Possum Branch Preserve was performed monthly by a contracted firm and consisted primarily of back- pack herbicide application to control nuisance species and periodic trimming with a bladed weed trimmer. No monthly maintenance occurred in the Brooker Creek Preserve site as the upland planting area was relatively free of nuisance spe- cies. Vines that jeopardized sample trees were removed annu- ally prior to measuring canopy; these vines were particularly prevalent in the red maple planting zones. Trees were pruned as needed after measurements were taken; this was done to establish a dominant leader which is particularly important for the red maples (Gilman and Grabosky 2006), and to re- move lower branches and thereby reduce vine entanglement. Both study areas were subjected to the effects of tropi- cal storm Frances on September 5, 2004, and tropical storm Jeanne on September 26, 2004. This resulted in a prolonged period of heavy rain associated with sustained winds of 72 kph and gusts over 97 kph. Planted red maples and longleaf pines suffered from wind throw during these storms, with tropical storm Jeanne causing the most damage. The largest, fullest trees appeared to have been most susceptible to the ef- fects of the wind. All planted baldcypress remained erect. Differences among container sizes for height and canopy spread were determined using Oneway ANOVA. Pairwise dif- ferences among container sizes within each species for height and canopy spread were determined using Tukey-Kramer
March 2010
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