Contrasting patterns and combined effects of moose and insect herbivory on striped maple (Acer pensylvanicum)

Basic and Applied Ecology 12 (2011) 64–71

Contrasting patterns and combined effects of moose and insect herbivory on striped maple (Acer pensylvanicum) W. Scott Schwenk∗ , Allan M. Strong Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington, VT 05405, USA Received 28 May 2010; accepted 5 October 2010

Abstract Although both mammalian and insect herbivores can strongly affect plant growth and biomass, interactions between these taxa feeding upon shared plant resources seldom have been studied experimentally in northern forests. In this study, we manipulated density of leaf-chewing insects and took advantage of a natural gradient of moose (Alces alces) foraging activity to examine their separate and combined effects on sapling striped maple (Acer pensylvanicum). Winter moose browse was patchy over space and time, but when present it resulted in a median bud loss of 72%. Saplings responded to moose browse by increasing leading shoot growth during the following summer as well as by developing leaves from epicormic buds. However, they did not fully compensate for the loss of biomass, and produced fewer leaves than in the prior year. Leaf-chewing insect herbivory occurred during the summer and was ubiquitous and more uniform than moose browse, but of a lesser magnitude (average 7% leaf area removed). Insect herbivory suppressed shoot growth and leaf production. Leaf damage was greater on browsed than unbrowsed trees, suggesting that browsing may improve leaf quality for insect herbivores, further constraining sapling growth. Although striped maple mortality was low, the average number of leaves produced per sapling declined during the four years of the study, indicating that combined mammalian and insect herbivory coupled with resource limitation will prevent most saplings from reaching maturity.

Zusammenfassung Während pflanzenfressende Säuger und Insekten Wachstum und Biomasse von Pflanzen stark beeinflussen können, wurden die Interaktionen zwischen diesen Taxa, wenn sie gemeinsame Pflanzenressourcen nutzen, in nördlichen Wäldern nur selten experimentell untersucht. In der vorliegenden Studie manipulierten wir die Abundanz der blattfressenden Insekten und nutzten einen natürlichen Gradienten der Frassaktivität von Elchen (Alces alces), um ihre getrennten und kombinierten Effekte auf Schösslinge des Amerikanischen Streifenahorns (Acer pensylvanicum) zu untersuchen. Die winterliche Beweidung durch Elche war ungleichmäßig in Raum und Zeit, aber wo sie stattfand, betrug der Median des Knospenverlusts 72%. Die Schösslinge beantworteten die Beweidung mit einer Steigerung des Wachstums des Hauptsprosses im folgenden Sommer sowie mit dem Austreiben von Blättern aus epikormen Knospen. Indessen kompensierten sie den Biomasseverlust nicht vollständig und produzierten weniger Blätter als im Vorjahr. Blattfrass durch Insekten trat im Sommer überall auf und war gleichmäßiger

∗ Corresponding

author. Tel.: +1 802 656 2233. E-mail addresses: [email protected], [email protected] (W.S. Schwenk).

1439-1791/$ – see front matter © 2010 Gesellschaft für Ökologie. Published by Elsevier GmbH. All rights reserved. doi:10.1016/j.baae.2010.10.002

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verteilt als die Elchbeweidung, war aber quantitativ geringer als diese (im Mittel wurden 7% der Blattfläche entfernt). Insektenfrass unterdrückte das Sprosswachstum und die Blattproduktion. Die Blattschäden waren an beweideten Bäumen größer als an unbeweideten. Dies legt nahe, dass die Beweidung durch Elche die Nahrungsqualität für blattfressende Insekten anhebt, wodurch das Wachstum der Schösslinge weiter eingeschränkt wird. Während die Mortalität des Streifenahorns gering war, ging die Anzahl der je Schössling produzierten Blätter im Lauf der vier Untersuchungsjahre zurück, was andeutet, dass kombinierter Elch- und Insektenfrass zusammen mit begrenzten Ressourcen die meisten Schösslinge daran hindern wird, das Reifealter zu erreichen. © 2010 Gesellschaft für Ökologie. Published by Elsevier GmbH. All rights reserved. Keywords: Acer pensylvanicum; Alces alces; Herbivore interactions; Tree growth

Introduction To reach maturity, woody plants must survive a gauntlet of challenges. While competing for light, moisture and nutrients, they must also overcome the impacts of herbivores that consume their photosynthetic tissues and structural support. Mammalian herbivores can strongly affect plant performance in forested systems, particularly by browsing on saplings and other low plants. For example, moose (Alces alces) and deer can induce changes in plant morphology, reduce tree growth and survival, retard or accelerate forest succession, and shift competitive balances in favour of less palatable species (McInnes, Naiman, Pastor, & Cohen 1992; Thompson, Curran, Hancock, & Butler 1992; Danell, Bergström, & Edenius 1994; Horsley, Stout, & deCalesta 2003). Although the impacts of insect herbivores typically are not as dramatic as those of mammals, under certain circumstances insects can also profoundly affect forest plant growth and survival (Davidson, Gottschalk, & Johnson 1999; Ayres & Lombardero 2000). While the effects of individual herbivore taxa on woody plants have been commonly studied, less is known about how mammalian and insect herbivores combine and interact in feeding upon shared plant resources, especially within forests. The combined effects of mammals and insects are important to understand because they frequently co-occur and can affect plants and each other in a variety of direct and indirect ways. The timing of herbivory has been found to affect plant responses (Danell et al. 1994; Guillet and Bergström 2006; den Herder, Bergström, Niemelä, Danell, & Lindgren 2009) as has the form of herbivory. For example, by browsing on terminal buds, mammalian herbivory often breaks apical dominance, which can cause substantial structural changes via production of new lateral shoots (Marquis 1996; Haukioja & Koricheva 2000). This feeding action occurs only in select insect taxa (e.g., Curculionidae). A number of studies have also documented that herbivory by one species can change plant quality and subsequent herbivory by another species (Martinsen, Driebe, & Whitham 1998; Ohgushi 2005; den Herder et al. 2009). Indirect negative effects of one herbivore on another constitute a form of exploitative competition, whereas positive effects can be considered indirect mutualism (Wootton 1994). These two kinds of interactions are likely to have very different implications for the host plant.

To better understand these interactions, we investigated how herbivory by moose and leaf-chewing insects individually and collectively affected biomass and growth of a shared food plant, striped maple (Acer pensylvanicum). In this study, we conducted two different experimental manipulations of insect herbivory, overlain upon natural levels of moose herbivory in a northern forest ecosystem. We then evaluated plant responses, including how effects differed among multiple years.

Materials and methods Field sites Our study was conducted at three elevations within Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA: a 300 m a.s.l. site with minimal evidence of moose browse, a 500 m a.s.l. site with limited evidence of browse, and a 750 m a.s.l. site with extensive signs of recurring browse. Commonly browsed species included striped maple, sugar maple (Acer saccharum), hobblebush (Viburnum lantanoides), and balsam fir (Abies balsamea). Canopy dominants consisted of yellow birch (Betula alleghaniensis), sugar maple, and American beech (Fagus grandifolia), with red spruce (Picea rubens), balsam fir, and paper birch (Betula papyrifera) increasing in abundance at higher elevations. Striped maple is most common at higher elevations but is present across a range of elevations at HBEF (Bormann, Siccama, Likens, & Whittaker 1970). Striped maple supports a suite of generalist, leaf-chewing insects, especially free-living Lepidoptera larvae (Marquis & Passoa 1989; Schwenk, Strong, & Sillett 2010). Caterpillars have long been considered the dominant herbivores at HBEF (Gosz, Holmes, Likens, & Bormann 1978). In recent decades, however, moose populations have been increasing in northern New England and nearby states and Canadian provinces (Timmermann 2003) and signs of their presence are commonly observed at HBEF. Moose feed upon a number of woody plant species, including striped maple (Routledge and Roese 2004). We observed that browse by moose on striped maple was more common at higher elevations, where the understory was denser, the canopy was more open, and conifers are more common than at lower elevations. Brows-

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ing activity at HBEF mainly occurred during the winter, but occasionally leaf stripping occurred during the summer at our study sites.

Manipulation of insect herbivory on saplings with natural levels of moose browse To observe the extent and effects of moose and insect herbivory on striped maple, we observed natural levels of moose browse while manipulating insect herbivory to broaden the range of insect herbivory experienced by trees. In 2004, we selected 45 saplings that were approximately 1.5–3.5 m in height at each of the three elevations; we subsequently added three saplings to replace or supplement observations for study trees that died. At each elevation, we randomly assigned onethird of trees to an insect herbivore removal treatment that consisted of regular spraying with an insecticide, spinosad (Conserve® SC, Dow AgroSciences), and hand-removal of leaf-chewing insects. We randomly assigned another onethird of trees to an exclosure treatment designed to increase insect herbivory by preventing access to trees by insectivorous birds. The bird exclosure consisted of translucent netting attached to an aluminum and PVC frame constructed around each tree. Herbivores were not manipulated on the remaining one-third of trees (controls). Elsewhere, we have reported that these treatments did not have a significant effect on striped maple growth over the entire study period (Schwenk et al. 2010). However, the removal treatment significantly decreased leaf damage by insect herbivores; the bird exclosure treatment was associated with a small, non-significant increase in leaf damage. We monitored the extent of moose and insect herbivory from 2004 through 2006 and collected plant growth and biomass data for the years 2004 through 2007. We calculated the extent of moose herbivory by recording the number of buds lost to moose browse between each summer (2004–2006) and the following spring (2005–2007). We estimated the amount of annual herbivory by leaf-chewing insects based on the proportion of leaf area missing from a sample of 10 leaves collected from each tree in September 2004–2006. We used a transparent grid (5 mm gridpoint spacing) to estimate percent leaf-area missing from each sample. We measured tree growth by recording the length of the shoot with the greatest length extension (the leader) each summer after the conclusion of annual shoot growth. As an index of total biomass, we recorded the maximum number of leaves on each tree for 2004–2007. We also estimated light availability for each striped maple using LI-COR quantum sensors to measure photosynthetic photon flux density (PPFD) at four locations around each tree 1 m above the ground (Schwenk et al. 2010) because light levels have a large effect on striped maple growth (Wilson and Fischer 1977). We evaluated the individual and combined effects of prior moose browse and insect herbivory on log tree growth (2005–2007) using general linear mixed models for repeated

measures (PROC MIXED, SAS Institute 2006) that also included light (log(% light transmitted)) and year as predictor variables and repeated measures on individual trees (random effects). The initial model included all possible interactions among browse, herbivory, and year; higher order interactions were sequentially removed if they were non-significant. The same mixed model approach was applied to net change in leaves (2005–2007) due to moose and insect herbivory, with the addition of the number of leaves in the prior year as a covariate (because a larger magnitude of change might be expected for trees with more leaves).

Simulated insect herbivory experiment with natural levels of moose browse We also evaluated the effects of moose and insect herbivory by experimentally simulating summer insect herbivory in one year on trees subject to natural levels of winter moose browse. In 2005, 96 striped maples (750 m elevation) were randomly assigned to one of four treatments: 0% (control), 12.5%, 25%, or 75% defoliation. Trees were defoliated by manually clipping the appropriate proportion of whole leaves in July, after most leaves had emerged and attained full size. The 12.5% level was chosen to be comparable to relatively common ambient levels of herbivory. The 75% level represented a high intensity of herbivory, such as might occur during an insect outbreak, which we expected would result in a strong effect on trees. Growth of the longest shoot and the total number of leaves were recorded for each tree immediately before simulated defoliation and one year later, in July 2006. (One tree died and was excluded from analysis.) We also recorded whether trees were browsed by moose during the winter of 2005–2006. We used PROC GLM of SAS for an analysis of covariance to assess the effect of treatment on shoot growth, with percent defoliation treatment as a continuous predictor variable, the presence of moose browse a categorical factor, and pre-treatment shoot growth a covariate. We performed a similar analysis for the change in the number of leaves between 2005 and 2006, using pre-treatment number of leaves as a covariate.

Response of leaf-chewing insects to moose browse We assessed the response of leaf-chewing insects to moose browse in 2005 and 2006, the two years for which we had data on both current insect herbivory and prior year moose browse. Data were analyzed using SAS PROC MIXED, with moose browse (winters 2004–2005 and 2005–2006) and year as predictor variables, individual trees (control group only) as subjects with repeated measures, and extent of leaf damage as the response variable. We did not include low elevation trees in this analysis because virtually no moose browse occurred there. In addition to the 15 control trees from each elevation previously described, in 2005 we added three mid-elevation

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Table 1. Extent of herbivory by moose (percent buds removed) and insects (percent leaf area removed) on striped maple saplings, 2004–2006. Insect herbivory was experimentally manipulated using herbivore removal and bird exclosure treatments; results are presented for control and all three treatments combined. Sample size = 135 trees per year, one-third allocated to each treatment, resulting in 405 observations (includes partial results for two trees that died and two replacement trees; insect herbivory data are missing for 16 occasions). Percent herbivory

0 >0–5 >5–10 >10–20 >20–30 >30–40 >40–60 >60–80 >80

Moose browse: # of tree observations

Insect herbivory: # of tree observations

Control only

All trees

Control only

All trees

93 0 0 3 3 4 7 10 15

296 0 0 6 8 7 19 29 40

0 58 39 28 6 0 0 0 0

1 223 95 54 15 1 0 0 0

and five high elevation control trees to expand the pool of trees that were not heavily browsed during winter 2004.

increase in shoot growth in 2005, a 49% increase in 2006, and a 13% increase in 2007. Light availability was also positively related to shoot growth (F1,139 = 73.1, P < 0.0001).

Results

Effects of moose browse and insect herbivory on leaf production

Patterns of moose browse and insect herbivory The patterns of mammalian herbivory were distinct from insect herbivory, reflecting the differences in size and abundance of these herbivores (Table 1). Browse by moose was patchy (27% of trees browsed in 2004, 34% in 2005, and 20% in 2006), but usually extensive when it occurred (median 72% bud removal). Of the 137 trees, 66 were not browsed at all during the study period (45 of 47 low elevation, 20 of 45 midelevation, and 1 of 45 high elevation). Ten trees were browsed in all three winters. Virtually no striped maples escaped herbivory by leaf-chewing insects, but the extent of damage was relatively low. The extent of leaf area removed from control trees was approximately 11.0% in 2004, 4.0% in 2005, and 6.3% in 2006. Unlike moose herbivory, there was not a consistent elevational pattern in the magnitude of insect herbivory during the study.

Effects of moose browse and insect herbivory on shoot growth Striped maple shoot growth (Fig. 1) responded positively to moose browse in the preceding winter (F1,228 = 58.9, P < 0.0001) but negatively to the extent of leaf damage by insects in the prior year (F1,199 = 7.3, P = 0.007). The effect of moose browse was less strong in 2007 than in 2005 and 2006 (browse × year F1,187 = 4.6, P = 0.011; year F1,149 = 9.9, P < 0.0001). Both types of herbivory had the potential for large magnitudes of effects. At the maximum extent of insect herbivory recorded, 33%, the modeled reduction in shoot growth was 42%. At an equivalent degree of moose browse (33%), the expected effect would be a 33%

We observed three growth patterns in striped maple that appeared to be responses to loss of potential leaves due to winter browsing by moose on shoots and buds. First, saplings produced long lateral shoots from buds released from apical dominance. These shoots produced up to four or more pairs of leaves in a single season, compared to the typical (unbrowsed) single pair per shoot (Hibbs and Fischer 1979, personal observation). Second, heavily browsed saplings produced leaves from epicormic buds that emerged from the trunk or remaining browsed branches during the spring. These leaves frequently were deformed, small, and completed growth later in the season than leaves from intact buds. Finally, the leaves produced from intact buds on browsed trees often were considerably larger than the leaves found on unbrowsed trees. These growth patterns permitted striped maple saplings to partially compensate for the loss of buds due to browsing, but nevertheless moose browse was associated with a net loss of leaf production (Fig. 2). The magnitude of the effect was generally similar across years; significant interaction terms primarily indicated that the effect was weakest for trees in 2006 associated with greater insect damage (browse × damage × year F2,210 = 5.7, P = 0.004, browse × damage F1,294 = 7.7, P = 0.006, browse F1,351 = 187.3, P < 0.0001). Expressed on a percentage basis, the model results indicate that in 2005 the number of leaves increased 10% for unbrowsed trees compared to, for example, a 25% loss of leaves for trees that lost 50% of their buds to moose browse (and incurred median insect damage). In 2006, leaves decreased 1% for unbrowsed trees vs. a 33% decrease with 50% bud loss, and in 2007 leaves increased 14% for unbrowsed trees vs. a 22% decrease with 50% bud

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Shoot growth (mm)

200

150

2005 response to moose

100

2006 response to moose 2007 response to moose 2005 response to insects

50

2006 response to insects 2007 response to insects

0 0%

20%

40%

60%

80%

100%

Percent herbivory: moose bud removal or insect removal of leaf tissue

Fig. 1. Shoot growth (leader) of striped maple saplings (n = 135 per year) as a function of herbivory (percent removal of leaf tissue by insects and buds removed by moose). Vertical line represents the maximum insect herbivory observation during the study (33%). Moose browse curves are based on median insect herbivory (3.8%), insect herbivory curves are based on median moose browse (0%), and all curves are based on median light availability.

loss. Considering the same results (Fig. 2) from the perspective of insect herbivory, a greater degree of insect damage was consistently associated with a reduction in leaf production at a given level of moose browse (main effect of damage F1,249 = 12.5, P = 0.0005), except in 2006. In that year, and only on trees with substantial moose browse, trees with greater insect herbivory had higher leaf production than trees with less insect herbivory (interaction terms cited previously). Light (F1,158 = 45.8, P = < 0.0001; positive effect)

and year (F2,190 = 3.4, P = 0.037; greatest in 2007) were also significantly related to leaf production.

Tree responses to simulated insect herbivory experiment with natural moose browse Results of the simulated summer leaf herbivory experiment were similar to those previously described. Shoot growth was negatively related to simulated herbivory (F1,91 = 11.5,

Net gain or loss of leaves from prior year

20

2005 - low previous insect herbivory 2005 - high previous insect herbivory

10

2006 - low previous insect herbivory 2006 - high previous insect herbivory

0

2007 - low previous insect herbivory 2007 - high previous insect herbivory

-10 -20 -30 -40 -50 -60 0%

20%

40%

60%

80%

100%

Percent buds browsed by moose prior to growing season

Fig. 2. Net annual change in maximum number of leaves attained on striped maple saplings (n = 135 per year) as a function of moose browse, at low and high degrees of insect herbivory. Low herbivory is represented by the 20th percentile of leaf damage across all observations (1.3%) and high herbivory is represented by the 80th percentile (9.6%).

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Percent of leaf tissue consumed by insects

Browsed

Shoot growth (mm)

Unbrowsed

600

400

200

0

30%

25%

2005 2006

20%

15%

10%

5%

0% 0%

0%

25%

50%

75%

Experimental removal of leaf tissue

Fig. 3. Effect of simulated July herbivory on next year’s shoot growth (leader) of 95 striped maple saplings. Lines represent ANCOVA results with natural moose browse as a categorical factor (solid line = browsed, dashed line = unbrowsed). Simulated herbivory significantly reduced shoot growth (P = 0.001); at comparable levels of simulated herbivory, shoot growth on moose browsed trees was significantly greater than on unbrowsed trees (P < 0.0001). Treatments are offset slightly for clarity.

P = 0.001) during the following year (Fig. 3). A trend of reduced leaf production due to simulated herbivory was not statistically significant, however (F1,91 = 3.3, P = 0.072). Moose browsed 65 of 95 trees, with browse intensity ranging from 11% to 100% of buds removed (median = 77%). Browse strongly stimulated shoot growth (F1,91 = 18.6, P < 0.0001; Fig. 3) while causing a net loss of leaves compared to the previous year (F1,91 = 35.9, P < 0.0001).

Leaf-chewing insect responses to moose browse The intensity of browse by moose in one growing season was positively related to extent of leaf tissue removed by insects during the subsequent growing season (F1,46 = 10.4, P = 0.002; Fig. 4). Insect herbivory was more extensive in 2006 than 2005 (F1,30 = 6.6, P = 0.015) but there was no evidence for a significant browse × year interaction.

Discussion Moose and insects affected saplings in different ways due to divergent intensity, distribution, and timing of herbivory. Most moose browse occurred between September and May, and browsed trees experienced an often drastic loss of buds. They responded by producing a much longer leader than unbrowsed trees. While we did not quantify the growth of other shoots, we observed that on browsed trees the increased growth frequently occurred on multiple shoots. Increased shoot growth in winter-browsed trees appears to be a common phenomenon, based on simulated moose

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25%

50%

75%

100%

Percent buds browsed by moose before growing season

Fig. 4. Insect leaf damage on 38 striped maple saplings as a function of the intensity of moose browse prior to the growing season (solid line = 2005, dashed line = 2006). Insect damage was positively related to prior moose browse (P = 0.002).

browse on birch (Betula pendula and B. pubescens) and Scots pine (Pinus sylvestris) (Edenius, Danell, & Bergström 1993; Persson, Danell, & Bergström 2005; den Herder et al. 2009) and natural moose browse on poplar (Populus balsamifera) and willow (Salix nova-angliae and S. planifolia) (Molvar, Bowyer, & Van Ballenberghe 1993; Roininen, Price, & Bryant 1997). Despite increased shoot growth, saplings did not attain pre-browse biomass or photosynthetic capacity, as measured by number of leaves, during the growing season following browse. Measuring photosynthetic capacity as total leaf area rather than number of leaves confirmed this effect of browse, based on two years of data (Schwenk, unpublished data). This finding also corroborates other studies of natural and simulated moose browse that found woody plants did not fully compensate for biomass lost to browse (Danell et al. 1994; McLaren 1996; Persson et al. 2005), although some authors have reported full compensation and even “overcompensation” following browse (Hjältén 1999). Striped maple saplings were affected differently by leafchewing herbivory, which occurred from May to September. Leaf-chewing insects and simulated herbivory decreased rather than increased shoot growth, suggesting that insect damage may reduce accumulation of energy stores available for growth in the following year. Insect herbivory was also negatively related to leaf production. Because insect herbivory was much more uniform than browse and usually of a lesser magnitude, growth discrepancies among trees due to leaf-chewing insects were not as dramatic as moose browse. Over the past twenty years, however, annual abundance of caterpillars on northern hardwood trees has fluctuated by 20fold in our region (Reynolds, Ayres, Siccama, & Holmes 2007), suggesting that in years with elevated caterpillar abundance, effects on sapling growth and biomass may be substantial.

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Environmental conditions, including resource availability, affected how trees responded to herbivory. Because light positively affected growth, browsed saplings in high light environments may have a greater opportunity to escape moose browse through elevated shoot growth than saplings growing in low light environments. McLaren (1996) also reported that light availability influenced the response of trees to moose browse in a study involving simulated browse of balsam fir. However, as reported by Mathisen et al. (2010), the elevated light availability in part may be due to recurrent moose browse, and escape from browsing pressure may be difficult at such sites. Additionally, significant differences among years indicated that prevailing climatic conditions not only affected shoot growth and leaf production but also modified the effects of herbivory on plants. For example, under the apparently more favourable growth conditions of 2007, shoot growth was not as strongly affected by winter browse as in 2005 and 2006. How did moose browse and insect herbivory combine to affect plant growth and biomass? Within years, leaf-chewing insects appeared to place relatively uniform, moderate constraints on shoot growth and leaf production; the magnitude of effects varied depending on annual fluctuations in their populations as well as climatic factors affecting plant growth. Moose browse further constrained leaf production. Although winter moose browse counteracted the negative effect of insect herbivory on shoot growth, if saplings are drawing upon a fixed pool of stored reserves to respond to moose browse, repeated browsing eventually may lead to reduced growth responses (Brookshire, Kauffman, Lytjen, & Otting 2002). Furthermore, the apparent indirect mutualism we observed, with greater damage by leaf-chewing insects on browsed trees, implies a synergistic effect of increased herbivore stress on browsed trees. We did not test why leafchewing herbivores may prefer browsed tree foliage, but other studies suggest possible explanations. Price (1991) argued that many herbivores prefer to feed on vigorously growing plants or plant modules, which corresponds to the rapidly growing shoots on browsed trees in our study. More specifically, Danell et al. (1994) and Danell, Haukioja, and Huss-Danell (1997) reported that winter browsing of birches increased foliar concentrations of N, K, and Ca and decreased concentrations of secondary compounds such as tannins, and that leaf-eating insects were more abundant on these trees. The altered leaf morphology we observed suggests that structural changes also could render them more edible to insects. Overall (not including two trees that died), the number of leaves per control tree declined over the course of the study by an average of 5.9% at the low elevation (least light, limited browse), 23.6% at the mid-elevation (intermediate light and browse), and 44.9% at the high elevation (greatest light, heavy browse). Notwithstanding the high survival and remarkable of resilience of saplings to moose browse we documented, it appears that few saplings will reach maturity if current conditions persist. Continued heavy browsing by moose at high elevations could alter competitive interactions among

plants, with less palatable species, such as spruce, increasing in abundance at the expense of striped maple (McInnes et al. 1992; Thompson et al. 1992).

Acknowledgements We are grateful to Joanna Hatt, Nicholas Kovacs, Kiryn Lanning, Briita Orwick, and Sarah Wilkins for their work in the field. We also benefited from the advice and assistance of Tray Biasiolli, Roxanne Bogart, Alison Brody, Gabe Colbeck, Richard Holmes, Therese Donovan, Alan Howard, Karen Klinger, Nicholas Rodenhouse, Paul Schaberg, Scott Sillett, and Ray Webster. Financial support was provided by grants from the US Forest Service McIntire-Stennis Cooperative Forest Research Program, the Northeastern States Research Cooperative, and the U.S. National Science Foundation (DEB 0108488, DEB 0423259, DEB 0640823).

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