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Impact of simulated mechanical defoliation caused by Zeiraphera canadensis (spruce bud moth) on food… Then, May Anne 2015-04

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UNIVERSITY OF BRITISH COLUMBIADEPARTMENT OF FOREST AND CONSERVATION SCIENCESFRST 498 B.Sc. Thesis in ForestryImpact of Simulated Mechanical Defoliation caused by Zeiraphera canadensis (Spruce Bud Moth) on Food Resource Availability on Picea glauca (White Spruce) in Subsequent YearsMAY ANNE, THENSUPERVISORSDR. ALLAN CARROLLDR. YOUSRY EL KASSABYAPRIL 2015
AbstractZeiraphera canadensis (spruce bud moth) larvae feed on newly burst buds of Picea glauca (white spruce).  It has been observed that Z. canadensis herbivory on apical shoots leads to loss of apical dominance and the release of dormant buds in subsequent years due to shoot damage. It has also been observed that there is greater success of larval colonization in years following herbivory. This paper explores whether Z. canadensis herbivory increases the amount of food resource available for subsequent generations by simulating mechanical defoliation in a controlled experiment. The results rejected our original hypothesis and total buds produced was found to decrease with increasing herbivory. We did not observe  positive resource regulation feedback in the spruce bud moth and white spruce system. Key words: Zeiraphera canadensis, Spruce Bud Moth, growth compensation, herbivory, food resource feedback, mechanical defoliation, Picea glauca, White Spruce 
May Anne ThenApril 20151. IntroductionNative insect folivores are important actors in complex forest ecosystems, with important roles in water and nutrient cycles, as agents affecting successional changes, as plant growth stimulants and other ecosystem processes (Schowalter and Lowman 1999, Trumble et al. 1993). It is known that folivore populations can not only cause small scale changes (at the tree level) as well as larger effects, such as reducing host plant density and productivity at the landscape level during population eruptions (Schowalter 2006). Host-plant interactions of herbivorous insects can also pose serious implications for forest management, especially when populations reach damaging levels as ‘pests’ due to their significant impacts on tree growth and wood quality (Schowalter 2006). The spruce bud moth (SBM) and white spruce complex is a well studied system, with many studies since the 1990s focussing on the temporal and spatial patterns of the insect-host relationship and on SBM survivorship. The spruce bud moth (SBM) larvae,  Zeiraphera canadensis Mut. & Free. (Lepidopptera: Tortricidae) is a native phyllophagous webworm that feeds on foliar buds. Its preferred host is the white spruce, Picea glauca (Moench) Voss, though it also feeds on black spruce trees (Picea marinara). Its native distribution matches that of white spruce and black spruce in Canada and populations have been associated with eruptive spruce budworm levels in the Eastern most provinces (Natural Resources Canada 2011). An individual insect’s fitness is determined by its response to environmental conditions (Schowalter 2006). In return, feeding can alter future habitat structure and resource distribution spatially and temporally, either by causing damage to plants after excessive feeding and biomass loss or by inducing growth (Romoser and Stoffolano 1998). Previous studies have shown that the   of   1 24May Anne ThenApril 2015susceptibility of white spruce to SBM may not related to nutritional quality but related to tree physiology (Quiring 1992). Due to the small time window that buds remain suitable for spruce bud moth colonization, the amount of resource available following herbivory can be important for future generations (Quiring 1993). Studies have shown that SBM fitness and survivorship is related to food resource availability; spatially and temporally. Larvae first emerge in May and feed on newly flushed foliar buds until June-July, eventually spinning a characteristic silk bud cap to protect itself from predators (Natural Resources Canada 2011).  Over the past two decades, research has demonstrated that there is a close relationship between bud burst phenology and SBM emergence (Quiring and McKinnon 1999; Quiring 1994), with developing buds remaining suitable for first-instar larvae for only a few days after bud burst, after which survivorship decreased significantly (Quiring 1992). Spatial and temporal variation in trees may be an important mechanism for reducing herbivory (Quiring 1993). However, there evidence to show that an insect which has been exposed to constitutive or induced tree defence mechanisms repeatedly over time can develop adaptations against them (e.g. Karban and Niiho 1995;  Carroll & Hoffmann 1980).Plant architecture is often altered following heavy levels of herbivory, which has impacts on future plant growth and susceptibility to herbivores (Schowalter 2006). Bud-feeders like the SBM have the potential to kill developing shoots and induce growth of lateral shoots (e.g. Clark and Clark 1985).  Carroll and Quiring (1993) also observed that a history of herbivory lead to shorter shoots and greater scarring, with multiple leaders created in following year after herbivory. These impacts were greater on terminal branches than on proximal branches, had greater effect on shoot length reduction and defoliation (Carroll and Quiring 1993). Herbivory was also seen to cause large reductions in vertical growth (Schowalter 2006). Volume increment   of   2 24May Anne ThenApril 2015was found to be influenced, though this was only significant after a few consecutive years of heavy damage (Carroll et al. 1993; Piene 2003). Significant height growth reductions coupled with continued large radial growth increments by heavily damaged trees during the first few years of attack by Z. canadensis caused distinctive stem growth patterns evident in oblique sequence (Carroll et al. 1993b). This is confirmed by anecdotal observations which identify the morphological difference between non-infested trees which are generally conical in shape as compared to SBM infested trees which have a more shrub-like structure, due to ramified branching patterns as multiple leaders fight for dominance (Natural Resources Canada 2011). Heavily damaged trees also reported with greater growth losses between high and low damage categories (Carroll et al. 1993b). However, white spruce appears to be very tolerant to tissue loss in forested stands, where canopy closure causes rapid decline of SBM populations (Carroll et al. 1993b).Timing of bud burst influenced by previous herbivory (Carroll 2003). Though there are studies suggesting a decrease in fitness and reduction in incremental growth post herbivory, not much is known about the quantitative food resource post herbivory. There are also seemingly competing hypotheses with regard to plant response. Tree Growth Response to HerbivoryFurther studies have revealed a more complex web of relationships between herbivory and plant productivity, which traditionally has been viewed as a process that reduced primary plant production (Schowalter 2006). The health of the plant, its developmental stage and intensity of herbivory can affect survival, productivity, and growth form differently.   of   3 24May Anne ThenApril 2015There are a few theories regarding the way trees with determinate growth, such as white spruce, could respond to the loss of plant tissue due to herbivory. Defence mechanisms could reduce tree damage by producing less food resources following herbivory to reduce herbivore population or alternatively tolerate herbivory by compensating with growth or reproduction (while employing other mechanisms) to  maintain plant vigour which can have stabilizing effects no herbivory (Strauss and Agrawal 1999).  If a plant employs tolerance against herbivory as a strategy, a positive feedback cycle of increased herbivory and increased growth may commence until the plant no longer has the resources needed for compensatory growth. At this point, a negative feedback loop may begin instead, which would have negative effects on insect population (Paul 2010). It is well documented that Z. Canadensis herbivory can lead to loss of apical dominance and release of dormant buds. Most apical buds damaged from Z. Canadensis feeding on elongating shoots, lead to the release of dormant buds, mainly proximal ones (Carroll 2003). Damaged shoots had more active buds near previous year’s bud scales and there was more successful colonization of basal bud that were on damaged shoots (Carroll 2003). Buds on damaged shoots flushed earlier than on intact shoots and earlier bud burst best synchronized with egg hatch (Carroll 2003).One theory of plant response to herbivory is the overcompensation theory or herbivore optimization hypothesis whereby that primary production is maximized at low to moderate levels of herbivory (Bast and Reader 2013; Pedigo et al. 1986) where greater regrowth may be observed post herbivory. The plant regrowth due to injury may exceed that of normal growth of a non-injured plant (Bast and Reader 2013). This theory was formed under the assumption that more dormant meristems are released to replace the injured dominant meristems given that there   of   4 24May Anne ThenApril 2015is sufficient nutritional to sustain the development and  growth of these meristems (Bast and Reader 2013). This study showed that the absence of overcompensation in Black Spruce was mostly due to a relatively small supply of dormant meristems on the trees that were tested (Bast and Reader 2013).The resource regulation hypothesis suggests that the generalized plant response to herbivory is to produce more of the herbivore’s preferred resources (Paul 2010). The selection pressure on host-plants of philopatric herbivores,  could have lead to resource regulation favouring food that benefit offspring (Paul 2010). In this way, resource regulation can have a stabilizing effect on insect population dynamics by maintaining a supply of high-quality plant resources over successive years of feeding (Paul 2010). Additionally, resource regulation may increase intra-tree heterogeneity of resources for herbivores by altering the physiological age structure of resources (Paul 2010). In contrast, the impact of resource regulation on the evolution of plant strategies and population dynamics may be relatively low (Craig et al. 1988). The juvenilization cycle (Craig et al. 1986) involves the activation of dormant buds, which is energetically costly for the plant. This takes advantage of the host plant’s need to compensate for growth loss following different forms of disturbances (Belsky et al. 1993; Rosenthal and Kotanen 1994). This theory assumes that herbivore is kept below a threshold and has not been tested in many systems.The carbon nutrient balance hypothesis predicts that plants that are growing in nutrient rich environments will respond to damage with vigorous regrowth and reduced investment in the production of defence compounds (Coley et al. 1985). A study conducted by McKinnon and Quiring (1998) found that fertilization treatments increased growth rate and were correlated with   of   5 24May Anne ThenApril 2015increases in foliar Nitrogen and water, and decrease in carbon based secondary compounds (defence compounds, such as tannins). One would expect that plants which overcompensate for growth following herbivory would have evolved induced defences to protect new growth, but the literature suggests this does not always happen (Coley et al. 1985). This could be due to the energetic cost of induced defence versus compensatory growth (Paul 2010). As a philopatric herbivore, a constant supply of quality and quantity food source from a host tree is important for future SBM generations. Z. Canadensis herbivory is known to increase bud colonization success in following years, increase food quality for future generations and improve larval survival rate (Schowalter 2006). Additionally, SBM damage to apical shoots leading to the activation of dormant buds may provide a larger quantity of food available for future generations. This, along with the observation that a continuous infestation SPB on white spruce can lead to highly branched shrub-like tree structures, this paper will examine whether co-evolutionary processes has lead to the induction of increased bud burst post SBM herbivory, therefore increasing quantity of food resources for future generations.Hypotheses & Research questionsIt has been noted in previous studies that natural defoliation can complicate the estimation of damage caused (Carroll and Quiring 1993). In this study, we test the effect of SBM herbivory by simulating feeding using two levels of mechanical defoliation in a controlled experiment to measure tree growth response the following years. Statistical analyses were conducted to determine whether treatment had significant impact on quantity of buds produced in the years following herbivory. Specifically, this paper will test the hypothesis that the total   of   6 24May Anne ThenApril 2015number of buds produced in subsequent years increases following herbivory.  Since the loss of apical dominance (due to clipped terminal branches) increases the potential for dormant bud development and competition for new leader, I additionally hypothesize that an increase in shoot length in medial lateral and proximal shoots will occur following herbivory.  of   7 24May Anne ThenApril 20152. Materials & MethodsThe experiment was conducted from 1990 to 1991 by Dr. Allan Carroll from Nackawic, New Brunswick, Canada. Thirty white spruce trees  were selected and measured for experimentation, all of which were from an Ottawa Valley white spruce half-sib family planted in 1982. Three treatment levels were used to simulate different intensities of SBM mechanical defoliation and were randomly assigned to the thirty trees. The three treatment levels were: (1) Heavy defoliation (H), where the terminal and distal lateral branches were clipped at midpoint (2) Light defoliation (L), where only the terminal branches were clipped at midpoint and (3) Control (C), where no treatment was applied. The treatment was applied to 1st and 2nd order branches of the top three whorls of each tree on the 31st May 1990. After treatment, the trees were left for a year before one terminal and one distal lateral branch from each tree was collected for measurements in the laboratory in 1991. Figure 1. Schematic diagram illustrating terminal (T) and distal lateral (DL) branches of 1990 branch, 1991 shoots and terminal or distal lateral (T/DL), medial lateral (ML) and proximal lateral (PL) positions of 1992 buds (adapted from Carroll and Quiring 1980).  of   8 24DLDLT1990 Branch1991 shootT/DLMLPLMay Anne ThenApril 2015MeasurementsIn the laboratory, shoot length of the original 1990 shoots were recorded, as well as the shoot lengths of the new 1991 shoots produced. Additionally, the number of buds produced on the respective 1991 shoots were recorded, by type, hereafter referred to as 1992 buds. From the raw data, total number of 1991 shoots produced and total number of 1992 buds produced was counted. Data AnalysisStata v13.1 was used to conduct regression analyses to determine the effect of (1) tree treatment on the total number of 1991 shoots produced per tree (2) tree treatment on the total number of 1992 shoots per tree (3) tree treatment on 1991 shoots by 1990 branch type (4) tree treatment on 1992 buds by 1990 branch type (5) treatment on 1991 shoot length means by shoot type.The tree group data was calculated by summing the number of shoots produced by the 1990 terminal branch and the 1990 distal lateral branch of the same tree.. Due to the distinct spatial pattern of SBM foraging behaviour, the position of the buds is important and was considered when calculating and comparing the mean shoot lengths.  of   9 24May Anne ThenApril 20153. ResultsThe regression analysis of tree treatment level and type of 1990 branch as factors revealed that they were both significant factors (table 1). Table 1. Summary of coefficients from simple linear regression showing significance of  tree treatment level and type of 1990 branch as independent factors. Table 2. Numerical means and standard error of 1991 shoots and 1992 buds produced across treatment levels, by type of branch in 1990 (distal lateral, terminal) and total per tree (total). Red means were not statistically significant (p>0.05) from control.Number of 1991 shoots per treeThe descriptive statistics associated with the total number of shoots produced in 1991 (1991 shoots) across the three treatment levels are summarized in table 2. The control  group (0) had the smallest numerical mean (M=34.1), followed by low treatment (M=17.7) and high treatment (M=14). Based on a simple regression analysis of total 1991 shoots produced by trees across treatment, treatment contributed significantly to the difference between means (p=0.000), yielding a coefficient of -10.05 (t=-7.77,  p< 0.0005). Compared to the control treatment, the light treatment accounted for -16.4 (t=-7.37, p<0.0005) of the 91 total shoots and the heavy Factors Coefficient SE t PTreatment -7.4 0.79 -9.32 0.0001990 Shoot Type-6.2 1.59 -3.90 0.0001991 Shoots 1992 BudsDistal Lateral Terminal Total Distal Lateral Terminal TotalControl 9 (±1) 24.9 (±1.59) 34.1(±1.696) 195.1 (±11.14) 156.7 (±7.58)195.1 (±11.14)Light 9 (±1) 8.9 (±1.44) 17.87(±1.71) 117.2 (±15.03) 44(±5.54) 117.2 (±15.03)High 4 (±1) 10.1(±1.13) 14.00 (±1.28) 119 (±12.74) 85.7 (±12.78)119 (±12.74)  of   10 24May Anne ThenApril 2015treatment accounted for -38.05 (t=-9.03, p<0.0005) of the total shoots produced. This suggests that tree treatment has a negative effect on total shoots produced in 1991.Figure 2. Mean ± 2SE of number of 1991 shoots (orange) and 1992 buds (blue) produced, by tree, over control, light and heavy treatments. Number of 1992 buds per treeThe descriptive statistics associated with the total number of buds produced in 1992 (1992 buds) across the three treatment levels are summarized in table 2. As seen in figure 2, the group with the numerically smallest mean was the light treatment (M=117.2), followed by the high treatment (M=119) and then the control group (M=195.1). Based on a simple regression analysis of total 1992 buds produced by trees across treatment, treatment contributed significantly to the difference between means (p=0.000), yielding a coefficient of -38.05.   of   11 24May Anne ThenApril 2015Compared to the control treatment, the light treatment accounted for -77.9 (p=0.000) of the 92 total buds and the heavy treatment accounted for -76.1 (p=0.000) of the total buds produced. This suggests that mechanical defoliation has a negative effect on total 1992 buds produced, though the difference between light and heavy treatment is not as pronounced. Total 1991 shoots produced, by branch typesFigure 3. Mean ± 2SE of total number of 1991 shoots produced on the respective 1990 branch types (terminal and distal lateral), across treatments.Since branch type was sub-sampled for every tree, a regression was also performed to compare means of total 1991 shoots and total 1992 buds, by 1990 branch type (distal lateral; terminal), across the three treatment levels. The summary of results are presented in table 1. Overall, the observed trend is that more 1991 shoots as well as 1992 buds were produced on the   of   12 24May Anne ThenApril 2015terminal branch than on the distal lateral branch, across all treatments (figure 3). The difference between terminal and distal lateral branches in the light treatment group (where only terminal branches were clipped) however, was not significantly different. This suggests the loss of dominance of the terminal branch. The highest numerical mean of 1991 shoots on the distal lateral branch belonged to the control group (M=9.2), followed by light treatment (M=8.8) and high treatment (M=3.9). A regression analysis showed that the coefficient (-0.4 ± 1.047) for treatment effect for light treatment was not significantly different from control (p=0.706).  This suggests that light treatment may not have an effect on growth of 1991 shoots on distal lateral branches. This result is logical because the distal lateral was not clipped in the light treatment. On the terminal branches, the control group had the highest numerical mean (M=24.9), followed by the high treatment group (M=10.1) then the light treatment group (M=8.9). The regression analysis showed that coefficients that confirmed this -16 (± 1.980) in light treatment and -14.8 (±1.990) in high treatment .Overall, the terminal branch produced more shoots in 1991 than the distal lateral branch in the control and high treatments. In the light treatment, there were no significant difference between distal lateral and terminal branches (figure 3). This is consistent with less shoots being produced on cut branches. This effect of treatment was more pronounced on the terminal branches than distal lateral (figure 3).   of   13 24May Anne ThenApril 2015Total 1992 buds produced, by branch typesFigure 4. Mean ± 2SE of total number of 1992 buds produced on the respective 1990 branch types (terminal and distal lateral), across treatments.In the distal lateral branch type, the light treatment group had the highest numerical mean (M=73) followed by the control group (M=38) and then high treatment (M=33). On the terminal branch type, the observed pattern follows the 1991 shoots produced on the terminal branch, where more buds (according to numerical means) are produced at heavy treatment (M=85.7) than at light treatment (M=44). Both light and heavy treatment are less than the control group (M=156.7). In the control and heavy treatment groups, more buds were produced on the terminal than the distal lateral branch, though the pattern was more pronounced on the control group. The light   of   14 24May Anne ThenApril 2015treatment group had the opposite pattern. With treatment, the difference in mean1992 buds produced by branch type seems to decrease significantly (figure 4). In the heavy treatment group, where both terminal and distal lateral branches were clipped, more buds were still produced on the terminal than the distal lateral, however, the number of buds produced on the terminal branch was significantly lower than the control group (figure 4). In the light treatment group, the clipped terminal branches produced much less buds than the control terminal branches, however the the distal lateral compensated with significantly more buds than control (figure 4). Average 1991 shoot length, by branch typesFigure 5. Mean ± 2SE of 1991 proximal lateral (pl), medial lateral (ml), distal lateral (dl) and terminal (t) shoots on 1990 distal lateral branches and terminal branches across treatments; with ♦ denoting treated (clipped) branches.  of   15 24♦ ♦ ♦May Anne ThenApril 2015The means and standard errors of the average 1991 shoot length, by type is summarized in figure 5. Overall, the means of average lengths are significantly different across treatment groups (figure 5).  Distinct growth patterns of 1991 shoot length is observed within the 1990 distal lateral control group and the 1990 terminal control group. The relative position of the shoot was directly correlated to the mean length (the 1991 proximal laterals being the shortest and the 1991 terminals being the longest). In the 1990 distal lateral however, there were not enough observations of proximal lateral shoots to calculate a mean and standard error. Comparing between control groups of the 1990 branch type, all 1991 shoot types on the  terminal branch were longer than on the  distal lateral branch. This pattern is also observed in the heavy treatment group where both terminal and distal lateral branches were clipped. However, greater variance in means of 1991 shoot types was observed in the heavy treatment than in the control treatment. The variance in 1991 shoot length seems to increase with treatment level. This suggests an increase in growth in some branches when both terminal and distal lateral branches were clipped. In the light treatment group, the terminal 1991 shoot means were lower on the terminal than in the distal lateral, suggesting a loss of apical dominance on the former and compensatory incremental growth on the latter.When both branches are clipped (high treatment), the length of shoots produced the following year follow the same pattern as the control group, with higher variance than the control group. When only the terminal branches are clipped (light treatment), the shoots produced on the clipped 1990 terminals are shorter than the control and the shoots produced on the unclipped 1990 distal laterals are longer than the control.   of   16 24May Anne ThenApril 2015Summary of Important Results1. Tree treatment has an overall negative effect on total shoots produced in 1991 and a more pronounced negative effect on production of 1992 buds. 2. There are significant differences between pattern of buds produced between terminal and distal lateral branches in the light and heavy treatments, suggesting a loss of dominance. 3.  The average length of 1991 shoots produced reflects loss of apical dominance as terminals were clipped with treatment. 4. DiscussionBased on the result that total number of 1991 shoots and total number of 1992 buds per tree decreased with treatment level, we can conclude that mechanical defoliation treatments simulating the effects of SBM herbivory has an overall negative impact on the quantity of food resource available the following year. This rejects our original hypothesis, which was based on the overcompensation theory (Bast and Reader, 2013) and resource regulation hypothesis (Paul 2010). This is also contrary to the findings of Carroll and Quiring (1993) and Piene (2003), which found that shoot breakage had a positive correlation to shoots produced in the year following herbivory, as multiple leaders are formed. In Piene’s study, all defoliated trees managed to recover foliar mass after 3 years.These findings could be due to the other factors that influence growth overcompensation, such as environmental conditions and plant adaptation to herbivory (Loreau 1995,  Williamson et al. 1989).  The literature suggests that this hypothesis is best supported in sap feeding insects, that can cause source-sink changes in host plant resources (Schowalter 2006). The tree’s   of   17 24May Anne ThenApril 2015environmental conditions is also a significant factor in determining its capacity to compensate for herbivory. Low or moderate levels of herbivory often increase photosynthesis and stimulate plant productivity (Carroll and Hoffman 1980), whereas severe herbivory usually results in mortality or decreased fitness (Williamson et al. 1989).  Since environmental factors such as plant nutrient and moisture can affect resource regulation in plants, it may be useful to record these parameters in future experiments, to provide further insight. There are aspects of the resource regulation hypothesis that still supports our results, namely, that even positive feedback cycles of herbivory induced plant growth will end when plant damage will eventually lead to the exhaustion of plant resources (Paul 2010). This is due to the loss of photosynthetic potential from shoot damage, depleting the resources needed for compensatory growth which is more important in trees with determinate growth patterns, since buds are set over a shorter windows (Schowalter 2006). A theoretical model by De Mazancourt et al. (1998) and Loreau (1995) concluded that herbivores which enhance or introduce nutrients may be more likely to induce a compensatory growth response in host plant. This is may also be related to growth rate decline observed with higher browsing in previous studies (Sharma and Tarkington 2009).However, when looking at the growth pattern at the branch level, there is a more complex relationship between number of buds produced on terminal and distal lateral branch types. In the first year growth post herbivory (1991 shoots), treated branches produced significantly less buds than the control, with the uncut branches remaining relatively similar to the control. In the following year however (1992 buds), we see that the means of total buds were closer to the control, and the untreated distal lateral branch in the light treatment had produced more buds than the clipped terminal, perhaps compensating for the loss of growth and apical dominance of   of   18 24May Anne ThenApril 2015the terminal branch. If bud growth was measured for a few more years, we may be able to infer whether there is a lag effect of compensatory growth that cumulates over time. Further analysis on the differences in number of buds produced over a longer period of time could provide insights into whether there is a lag effect of compensatory growth (Hunter 1988). Plants have a hierarchy of meristems, which are important in determining resource allocation. Compensatory growth can be activated as a result of loss of dominant meristems (Benner 1988; Thomas and Watson 1988). The specific tissues utilized by herbivores will determine whether a resource regulation cycle will be initiated (Paul 2010). It is worth noting that direct comparisons of shoot growth between damaged and undamaged shoots may still present a source for inaccuracy in our results (Sower and Shorb 1984).Plant loss and regrowth may alternate between positive and negative feedback cycles, where plant regrowth overcompensates for loss due to herbivory in the former and where plant growth reduces to recover from depletion of nutrient reserves in the latter (Paul 2010). In order to test whether these feedbacks are found in the SBM and white spruce system, a longer time frame is needed, to observe patterns in growth following ongoing herbivory.Another effect of positive feedback cycles that could be further investigated is the potentials increase in heterogeneity of buds. Resource regulation may manifest in different ways, such as altering the position of buds produced (Hunter 1992; Hunter and Price 1992). While some studies have looked into this already, studies combining this factor with quantitative analysis of number of buds produced may provide more detail (Quiring and McKinnon 1999). Our results may also suggest that the tree may have lost significant photosynthetic resources due to the mechanical defoliation and did not have enough resources to sustain growth. We can see that perhaps the tree is beginning to recover in the following year, in its ability to   of   19 24May Anne ThenApril 2015begin compensating in 1992 bud development. It would be interesting to test whether there is a trade-off between defence mechanisms, i.e. protection against herbivory (resistance) or tolerance, i.e. regrowth to compensate for plant loss. Future studies could look at whether there are compositional changes in foliar content to suggest a change in metabolism following herbivory. Paul (2010) also suggests that resource regulation cycles can be terminated by other abiotic and biotic factors, for example  an increase in mortality due to naturalenemies. Another reason could be that the observed reduction in shoot and bud production following herbivory is a strategy to minimize herbivory in following years as a defence mechanism to keep food resources for the SBM low. Resource regulation in the form of increased growth following herbivory may not be indicative of improved larval performance in following years, since this is dependent on many other factors. Our results do demonstrate however, that despite observed larval success in the literature following years of herbivory (Caroll and Quiring 1993),  the total amount of food source available in the form of buds is in fact lower after mechanical defoliation. Kaitaniemi et al. (1997) observed that induced defence compounds in new foliage growth lead to reduced fitness in folivore. These induced defence mechanisms evolve as a result of specific herbivore interactions, since its effectiveness is dependent on expending valuable resources as a response to precise instead of a general response to damage (Bryant 1981). Small populations of sparsely distributed folivores will not likely co-evolve to induce defences despite heavy damage on individual plants.Overall, our average 1991 shoot length results are consistent with the findings of Carroll and Quiring (1993), which found that high intensities of herbivory lead to greater shoot length reduction than lower intensities. Studies also show that greater reductions in shoot length is   of   20 24May Anne ThenApril 2015observed when feeding was near the apical shoot meristems (clipped terminal branches), as compared to feeding in more proximal positions (clipped distal lateral branches) (Carroll and Quiring 1993). The observed loss of apical dominance and the release of medial lateral and proximal laterals was also an interesting pattern. This increasing dominance of medial lateral and proximal laterals, and their increasing abundance could be impact food source availability in subsequent years as tree morphology changes over time. The pattern observed of 1991 shoot length by 1991 shoot type could support the findings of McKinnon and Quiring (1999), whereby the production of early bursting basal buds (proximal laterals) compensated for the delay in terminal budburst. This study argues that the susceptibility of white spruce to SBM is related to the within tree heterogeneity in bud burst phenology and to the nutritional quality of subsequent buds after herbivory. These unmeasured factors in our experiment could explain the overall observations that SBM populations can be maintained, and even increase, on the same tree after years of continuous herbivory. The effects of herbivory can be more pronounced on nutritional quality and the manipulation of bud types produced (hence timing of budburst) than on total buds produced (McKinnon and Quiring 19999; Quiring 1994). I am interested to find out more about the relationship of shoot length reduction with the number of buds produced following herbivory, as this relationship may shed light on which factor is most correlated with degree of defoliation.The response of plant growth to herbivory also has larger ecosystem ramifications, namely with other herbivores that are associated with SBM, such as the spruce bud worm (Chonistoneura spp.) Due to their short life cycles, insect populations can respond quickly to changes in their environment. These changes in insect populations can in turn, affect ecosystem functions and structure making the both intrinsically linked (Schowalter 2011). Feedback   of   21 24May Anne ThenApril 2015processes, such as resource supply and demand are interlinked regulatory mechanisms that have knock on effects across the food web (Schowalter 2011). Predation and herbivory levels respond to resource availability and responses at different trophic levels brings about negative feedbacks that keep resources in balance (Schowalter 2011). These negative feedbacks are seen as a  primary mechanism to stabilize population size, species interactions, and process rates in ecosystems. However, some insect interactions also provide potentially destabilizing positive feedbacks, which increases the likelihood of future herbivory. Reduction in variation and structure in ecosystems suggests that ecosystem self-regulation is present, potentially through insect interactions as they are relatively more sensitive to environmental changes (Odum 1969; Patten and Odum 1981). In order to collect more robust data,  more observations could have been sampled. Several models were insignificant due to lack of observations in some cases. Another way to simulate different intensity of herbivory could be to vary the length clipped from terminal shoots, instead of the number of branches clipped (Rodgers et al. 1995). While this experiment simulated the mechanical impacts of defoliation, other factors present in the feeding process may be important in inducing different tree responses. For example, chemical compounds released from saliva may induce different defence mechanisms as compared to mechanical breakage, especially in a co-evolved system like this. Additionally, the nutrient content from faeces during insect feeding may be an important resource for plant growth which is absent in this experiment. The tree growth response may not include defensive mechanisms against herbivory due to the purely mechanical nature of the experiment conducted.Placing this study in the context of food resource availability for future SBM populations, it is also important to note the importance of considering the variance in bud break phenology in   of   22 24May Anne ThenApril 2015different host tree provenances, among other spatial and temporal factors (Rossi and Bousquet 2014). Evidence suggests that white spruce phenology may also be adapted to local climatic conditions despite retaining sizeable genetic diversity within populations (Rossi and Bousquet 2014). Quiring et al. (1991) also noted the genetic differences among different susceptibility of spruce trees which could have affected our results. Other factors affecting SBM vigour include nutritional quality of foliage; as found in Roland and Myer’s study (1984), where bud phenology as well as nutritional quality of foliage lead to better performance of populations after herbivory. To examine this, pupal weights could be measured to compare vigour of SBM feeding on trees producing foliage post herbivory compared to a control.   of   23 24May Anne ThenApril 20155. ConclusionOverall, we have rejected our hypothesis that total food resources in the form of buds available for spruce bud moth in subsequent years following initial defoliation would increase. Mechanical defoliation then, had an overall negative effect on total shoots produced in subsequent years, decreasing total food resource available to future generations of SBM. Despite an increase in bud production from 1991 to 1992, this did not compensate for the loss caused by insect herbivory. In order to determine whether there are any lag effects of resource regulation, data of shoots and buds produced needs to be available over a longer period of time. The average length of shoots Additionally, there are many other factors, such as environmental conditions, that could be of interest in determining the growth response of white spruce that could be tested in the future. The average length of shoots produced following herbivory reflects the loss of apical dominance as terminals were clipped with treatment. AcknowledgementsI would like to acknowledge Dr. Allan Carroll for the original experimental design, data collection and overall guidance throughout the project. I also wish to thank UBC M.Sc. candidates Asif Raza and Sarah Huber for support regarding statistical tools and software use. I am also grateful to UBC B.Sc. candidates Klaudia Wegschaider and Nidhi Joseph for the critical discussions that lead to valuable insights during data analysis. statistical tools and softwares available. 
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