target plant. The three high- and three low-elevation planting siteswere located within the University of British Columbia MalcolmKnapp Research Forest near Haney, B.C. All the sites had been domi-nated by coniferous forests and harvested. A bulldozer was used toclear all vegetation from the sites and remove the organic soil layer inearly July 1992. Plots were laid out using a 2-m spacing. Planting tookplace from July 21 to 23, 1992, with treatments randomly assigned tothe plots. Plants were approximately 4 cm tall at the time of planting.During the experiment the sites were weeded on up to a weekly basis.Height and diameter measurements were made on a monthly basisduring the growing seasons of 1993 and 1994 on all plants. The rootsand shoots of the plants were harvested in August 1994 and totalbiomass was determined from the plant mass dried at 65C. Themonthly census data were converted to total dry mass estimates foreach plant using a linear regression of height diameter2 versus massof harvested plants (r2 = 0.964). Relative growth rates (RGR) werecalculated on a monthly basis in 1993 and 1994. RGRs were deter-mined for each plant by taking the change in the log-transformedestimate of plant mass between census periods and dividing by thenumber of days in the period. Methods are more fully described else-where (Markham 1996).
The seedlings started showing signs of herbivore damage in earlyMay 1993, with damage peaking in late August, some trees beingtotally defoliated. On all but the most severely attacked trees, herbi-vore damage was confined to leaf tissue in the lower part of the crown.Where herbivore damage was heavy, the leaves in the top of the crownand soft stem tissue near the ends of branches were also eaten. Treesthat were defoliated produced new leaves that were generally free ofherbivore damage.
On August 17, 1993, and July 29, 1994, we quantified the herbi-vore damage on each tree in the experiment into the following sixclasses: (0) no damage, (1) 125% of leaves damaged, (2) 2650% ofleaves damaged, (3) 5175% of leaves damaged, (4) 76100% ofleaves damaged, and (5) complete removal of all leaf tissue. A pre-liminary sampling of wild red alder showed that these classes couldconsistently be distinguished on experimental trees. The herbivoredamage classes were used as a dependent variable to compare thedegree of herbivore damage between sites and as an independentvariable when examining the relationship between herbivore damageand plant growth.
The effect of herbivore damage on the growth of plants was com-pared by examining both the mass and growth of plants experiencingdifferent levels of herbivore damage. The effect of herbivore damageon plant mass and growth was analyzed using single-factor ANOVAswith log mass or RGR as the dependent variable and herbivore dam-age class as the independent variable for each date that mass andgrowth were estimated. Differences between herbivore damageclasses were determined using a Ryans Q test (Day and Quinn 1989).Mortality of plants (24%, with 50% of the total mortality occurring onone high-elevation site) and the differences in the numbers of plants
receiving different levels of herbivore damage meant that these analy-ses were performed using unequal sample sizes.
Feeding preferencesA feeding preference experiment was set up to determine if changesin herbivore damage between years were related to changes in thepalatability of previously defoliated trees. The experimental designand statistical analysis follow Peterson and Renauds (1989) recom-mendations of using replicated controls for changes in plant mass inthe absence of herbivores. On July 22, 1994, leaves and sawflies werecollected. Undamaged leaves from previously defoliated and undam-aged plants were collected from sites G and G40, the only sites withdefoliated trees in 1993. Although 10 trees on these sites had beendefoliated in 1993, only seven of these could provide undamagedleaves at the time of sampling. Sawflies were collected from thesesites but were not collected from the trees providing the leaf samples.
In the laboratory, leaves from a single defoliated tree were ran-domly matched with leaves from a single undamaged tree. Threefeeding preference dishes were made from each leaf pair by placingleaf sections of approximately the same fresh mass (0.335 0.115 g,mean 1SD) into petri dishes. Two of the dishes were used as repli-cate feeding preference trials with five sawflies placed in each dish.The remaining dish received no sawflies and was used as a control forchanges in leaf weight independent of consumption by sawflies. Thesawflies were allowed to feed for 16 h after which all leaf sectionswere weighed. The difference in the change in weight of the two leafsections was calculated as the percentage of the initial leaf weight.The mean difference of the replicate dishes was compared with thedifferences in weight change of leaves in the absence of sawflies usinga paired t-test.
The year of the sawfly outbreak, herbivore damage class var-ied significantly both between planting elevations (p = 0.013)and among sites within elevations (p < 0.000 for both high- andlow-elevation sites) according to log-likelihood ratio analysis(Table 1). The mean of the mean herbivore damage class foreach site was 3.15 0.64 ( 1SD) on the low-elevation sitescompared with 0.867 0.69 on the high-elevation sites. Thehighest levels of damage occurred on sites G and G40, withplants having an average of greater than 50% of their leavesdamaged. One site (H110) had no evidence of damage on anyplant. Herbivore damage the following year (1994) was lessthan 25% of leaves per tree damaged on all but one site (G40).Mean plant mass at the end of the growing seasons variedsignificantly between sites, with plants on the low-elevationsite attaining a greater mass.
G (65) G40 (85) K (250) H30 (530) H90 (650) H110 (550)
ln mass 2.81.2a 1.71.0b 1.20.9c 1.00.8c 1.20.9c 0.20.3dDamage class 3.3 3.7 2.4 0.9 1.4 0.0
ln mass 5.11.3a 4.11.6b 3.11.4c 2.81.4c 3.41.5bc 1.61.7cDamage class 0.8 1.5 0.8 0.6 0.6 1.0
Note: Mass is given as ln-transformed values 1SD, with sites that are not significantly different followed by acommon letter. Elevation (m) of each site is given in parentheses.
Table 1.Mean plant mass at the end of the growing season and mean herbivore damage class in 1993 and1994 on each site.
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On high-elevation planting sites, where plants experiencedsignificantly lower levels of herbivory, there was no relation-ship between the level of herbivore damage and plant growthor size of plants. On low-elevation planting sites, plants withhigh levels of herbivore damage were significantly larger thanplants with low levels of herbivore damage at the end of thegrowing season (Fig. 1). By the end of the growing season,plants with no herbivore damage were significantly smallerthan plants with less than 50% of their leaves damaged. Thelargest plants were those with greater than 50% of their leavesdamaged. These differences were detected in May 1993 beforeany sawflies could be seen on the plants. The RGRs variedover the growing season for plants with different levels ofherbivore damage. From April until June 1993, there was apositive relationship between the degree of herbivore damageand RGR. By August, however, the relationship was negative,plants with all of their leaf tissue removed having significantlylower RGRs than all other plants. Plants with 50100% leafdamage had significantly lower RGRs than plants with lessthan 50% leaf damage. Plants with a high degree of herbivoredamage in 1993 remained significantly taller throughout the
1994 growing season. Herbivore damage in 1993 had no effecton RGRs in 1994.
Most trees in the study had a lower herbivore damage classin 1994 than in 1993 (Table 2). However, there was a positiverelationship between damage in 1993 and 1994 for plants ex-periencing high levels of herbivory in 1993. Eight of the nineplants that had greater than 50% damaged leaves in 1994 alsohad greater than 50% damaged leaves in 1993, even though
Fig. 1. Mean plant sizes and relative growth rates (RGR) for plants on low-elevation sites receiving different levels of herbivore damage in1993. +, no damage; , 125% leaves damaged; h, 2650% leaves damaged; e, 5175% leaves damaged; , 76100% leaves damaged; Y,complete leaf tissue removal. Vertical bars join plant damage classes that are not significantly different at p = 0.05 according to a Ryans Q test.
Class (1993) 0 1 2 3 4 5
0 14 19 3 1 0 0
1 8 9 3 0 0 0
2 9 6 0 0 0 0
3 14 23 2 0 1 0
4 0 24 1 4 1 0
5 1 3 3 1 1 0
Table 2.Frequency of plants in each herbivore damage class in1993 and 1994.
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