280 Chorbadjian et al.: Defensive Chemistry and Herbivore Resistance of Austrian Pine and Paper Birch ml n-pentane for 48 hours (Raffa and Steffeck 1988; Codella and Raffa 1995). Terpene concentrations in extracts were measured using a Shimadzu GC-17A gas chromatograph equipped with an Rtx-5 fused silica column (15 m × 0.25 mm inside diameter × 0.25 µm film thickness). Nitrogen was used as a carrier gas at a linear velocity of 29.6 cm/sec. The temperature program was 60°C for five minutes, increased by 5°C per minute to 110°C, then increased by 10°C per minute to 200°C, and then by 25°C per minute to 250°C, and finally held at 250°C for five minutes. Total analysis time was 31 minutes. The injector and detec- tor temperatures were 220°C and 230°C, respectively. Quan- tification was based on pure standards, except germacrene-D, which was identified based on EI spectra because no standard was available. Its concentration was estimated according to the response factor for another sesquiterpene, trans-caryophyllene. Results are presented as the sum of individual mono- and ses- quiterpenes in milligrams per gram of fresh foliage weight. Tree Resistance to Insects Treatment effects on birch resistance to defoliating insects were measured in bioassays initiated for forest tent caterpillar on May 9, 2003, gypsy moth on May 9, 2003 and May 25, 2004, and white- marked tussock moth on August 25, 2004. Resistance of Austrian pine to European pine sawfly was quantified in bioassays initiated on April 21, 2003; May 10, 2004; and May 11, 2005. These dates correspond to the natural feeding phenology of these insects. Growth of larvae feeding on foliage from experimental trees was measured in laboratory bioassays that lasted for 10 days. For birch-feeding caterpillars, a group of five neonate larvae was weighed collectively and placed in a polystyrene Petri dish (15 × 3 cm) containing a base of plaster mixed with activated charcoal with foliage that had just been harvested from one of the experimental plants. To control for ontogenetic variation in leaf quality, all insects received leaves of the same age (youngest fully expanded leaf). The plaster base of each dish was saturated with distilled water to maintain turgor of detached leaves over the course of the bioassay, and foliage was replaced with fresh leaves every 48 hours. Bioassays were conducted in a growth chamber at 25ºC with a 16:8 (L:D) photoperiod. Larval survival was recorded, and living larvae were weighed as a group at the end of the 10-day bioassay, and mean final weight was calcu- lated by dividing total weight by number of larvae. Larval growth (mg) was calculated as the difference between mean final and mean initial mass. Bioassays with European pine sawfly larvae were conducted as previously described, except that 10 larvae were confined in each petri dish and fed one-year-old needles. Data Analyses Effects of block, paclobutrazol, fertilization, and their interac- tions on tree growth and physiological measurements, leaf traits, and chemistry, as well as insect growth and survival were as- sessed by analysis of variance (PROC GLM, Type III sums of squares; SAS Institute, Inc. 2003). Following significant F-tests (α = 0.05), the PDIFF option following the LSMEANS state- ment was used to make pairwise comparisons (α = 0.05). Fer- tilization and paclobutrazol treatments were considered fixed factors. Initial larval weight was included as a covariate in the model when analyzing larval growth. Vapor pressure deficit was used as a covariate when analyzing photosynthesis and stoma- ©2011 International Society of Arboriculture tal conductance. Initial measures of trunk diameter were used as covariates for analysis of tree growth measurements. All vari- ables met assumptions of homoscedasticity and normal distri- bution of residuals. Data are presented as least squares means ± one standard error. Pearson’s correlation coefficients (PROC CORR SAS 9.3) were used to quantify relationships among de- pendent variables (e.g., between leaf chemistry and tree growth). Prior to correlation analysis, bivariate plots were inspected to ensure that none of the analyzed relationships were curvilinear. RESULTS Tree Growth Paclobutrazol significantly decreased the height and radial trunk growth of paper birch in 2003 and, to a lesser extent, in 2004 (Figure 1). The growth-inhibiting response to paclobutrazol was observed in fertilized as well as non-fertilized paper birch trees (significant main effect of paclobutrazol, no significant paclobutrazol * fertilization interaction) (Table 1). Similarly, paclobutrazol substantially decreased the height and trunk diam- eter growth of Austrian pine in all three years (Figure 1). The growth-inhibiting response to paclobutrazol was also observed in fertilized as well as non-fertilized Austrian pine trees (no signifi- cant paclobutrazol * fertilization interaction). Fertilization had no effect on height or diameter growth of either species (Table 1). Photosynthesis Neither paclobutrazol, fertilization, nor their interaction had an effect on net photosynthesis rate of paper birch or Austrian pine on any date measured, with the exception of August 14, 2003, when paclobutrazol decreased photosynthesis of paper birch by 17% relative to untreated trees (Figure 2; Table 2). Leaf Morphology Paclobutrazol decreased the area of individual paper birch leaves in 2003 and 2004 (Table 1; Figure 3). Paper birch leaves from paclobutrazol-treated trees also exhibited noticeable curl- ing in 2003 and were a darker shade of green. Paclobutrazol increased specific leaf mass (leaf mass per unit area) of paper birch in 2003 but had no effect in 2004 (Table 1; Figure 3). Paclobutrazol had no effect on leaf area of one-year- old Austrian pine needles in 2004 or 2005, or specific leaf mass in 2004 (Table 1; Figure 3). However, when mea- sured in 2005, paclobutrazol increased specific mass of pre- vious-year needles (formed in 2004) (Table 1; Figure 3). Fertilization decreased specific leaf mass of paper birch in 2004, 70.7 ± 2.1 g/m2 versus 76.7 ± 1.7 g/m2 , for fertilized and non-fertilized trees, respectively, but otherwise had no ef- fect on birch leaf morphology (Table 1). Fertilization had no effect on needle morphology of Austrian pine (Table 1). Phytochemistry and Foliar Nitrogen Paclobutrazol had no effect on the nitrogen concentration of paper birch or Austrian pine, except for a slight increase in paper birch foliar nitrogen in August 2004 (Table 3). However, fertilization did increase foliar nitrogen of paper birch on June 24, 2003 (3.15 ± 0.11% versus 2.83 ± 0.11%, for fertilized and non-fertilized
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