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Compensation through rosette formation: the response of scarlet gilia (Ipomopsis aggregata: Polemoniaceae) to mammalian herbivory Katie M. Becklin and H. Elizabeth Kirkpatrick
Abstract: Plants could potentially compensate for floral herbivory by regrowing flowering stalks and by forming additional vegetative stems. Because scarlet gilia (Ipomopsis aggregata (Pursh) V. Grant) is described as monocarpic, its ability to regrow multiple flowering stalks following the removal of its primary inflorescence has been cited as the species’ primary means of compensating for herbivory. However, ancillary rosette formation could also contribute to compensation in subsequent years. To determine if herbivory induces ancillary rosette formation and whether energy diverted to vegetative regrowth reduces reproductive output, we analyzed the response of scarlet gilia to elk herbivory in the Wenatchee National Forest of Washington State. Control plants were protected from herbivory by wire enclosures; clipped plants were hand-cut to simulate herbivory; and grazed plants were left vulnerable to elk herbivory. Ninety percent of plants that lost inflorescences regrew multiple flowering stalks; these plants produced fewer fruits and seeds than protected plants, indicating that scarlet gilia undercompensated for herbivory despite greater aboveground biomass. The plants that regrew multiple flowering stalks were also more likely to form ancillary rosettes, which could increase compensation over multiple seasons. Although herbivory reduced initial fecundity, grazing generated morphological changes that could enable the plant to achieve a greater degree of compensation over time. Key words: compensation, fecundity, plant–herbivore interactions, regrowth, ancillary rosettes. Re´sume´ : Les plantes ont la capacite´ de compenser pour l’herbivorie florale en e´mettant de nouvelles tiges florales et en formant des tiges ve´ge´tatives supple´mentaires. Puisqu’on de´crit l’Ipomopsis aggregata (Pursh) V. Grant comme monocarpique, on a cite´ sa capacite´ a` produire de multiples tiges florales, suite a` l’ablation de son inflorescence primaire, comme me´canisme primaire de compensation de l’herbivorie chez cette espe`ce. Cependant, la formation de rosettes auxiliaires pourrait aussi contribuer a` la compensation au cours des anne´es subse´quentes. Afin de de´terminer si l’herbivorie induit la formation de rosettes auxiliaires et si l’e´nergie de´tourne´e vers la croissance ve´ge´tative re´duit la productivite´ florale, on a analyse´ la re´action de l’I. aggregata a` l’herbivorie par le wapiti, dans la Wenatchee National Forest, dans l’e´tat de Washington. On a prote´ge´ des plantes te´moins contre herbivorie a` l’aide d’enclos barbele´s; des plantes ont e´te´ coupe´es a` la main pour simuler herbivorie; les plantes paˆture´es ont e´te´ laisse´es expose´es a` l’herbivorie par le wapiti. Quatre-vingt-dix pourcent des plantes qui ont perdu leur inflorescence ont rejete´ de multiples tiges florales; ces plantes ont produit moins de fruits et de graines que les plantes prote´ge´es, ce qui indique que l’I. aggregata ne compense pas totalement pour l’herbivorie, en de´pit d’une plus forte biomasse e´pige´e. Les plantes qui produisent de multiples tiges florales sont e´galement plus susceptibles de former des rosettes auxiliaires, ce qui pourrait augmenter la compensation au cours des anne´es. Bien que l’herbivorie re´duise la fe´condite´ initiale, le paˆturage ge´ne`re des changements morphologiques qui pourraient permettre a` la plante de re´aliser une compensation supe´rieure, avec le temps. Mots cle´s : compensation, fe´conditie´, interactions plante–herbivore, rejet, rosettes auxiliaires. [Traduit par la Re´daction]
Introduction Herbivory can significantly impact a plant’s overall fitness through reduced biomass and reproductive output. To minimize the impact of grazing, a plant can deter herbivores Received 13 March 2006. Published on the NRC Research Press Web site at http://canjbot.nrc.ca on 15 September 2006. K.M. Becklin1,2 and H.E. Kirkpatrick. Biology Department, University of Puget Sound, 1500 North Warner Street, Tacoma, WA 98416, USA. 1Corresponding
author (e-mail:
[email protected]). address: Division of Biological Sciences, 105 Tucker Hall, University of Missouri–Columbia, Columbia, MO 652117400, USA.
2Present
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by using defensive adaptations, or cope with the damage through tolerance traits (Juenger and Bergelson 1997). Tolerance is the ability to maintain fitness by growing and reproducing after herbivore damage (Mothershead and Marquis 2000). Compensatory growth occurs through the release of dormant lateral buds (Juenger and Bergelson 2000b). By keeping some of its buds inactive, a plant retains the potential to initiate the growth of new flowering stalks when herbivores remove the active meristem (Nilsson et al. 1996). Owing to the cost of bud dormancy (reduced number of active meristems and resources available to the ungrazed plant), this bet-hedging strategy is beneficial only if the risk of herbivory is high (Tuomi et al. 1994). The timing of defoliation is also important since meristems differentiate into floral rather than vegetative tissues as a plant begins to
doi:10.1139/B06-099
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flower (Lennartsson et al. 1998). These constraints on bud dormancy suggest that this tolerance trait is correlated with the timing, frequency, and severity of local grazing (see Paige 1994). Since grazing is a highly variable event, plant tolerance and compensation also vary among species and populations. Scarlet gilia (Ipomopsis aggregata (Pursh) V. Grant) is one plant that utilizes tolerance traits to compensate for herbivory. Compensation by scarlet gilia has been studied extensively in the western United States, particularly in Arizona and Colorado. The degree of compensation achieved by scarlet gilia populations varies from undercompensation to overcompensation depending on the intensity and timing of the grazing events, the fitness component measured (male or female), and the availability of nutrients and pollen to the populations (Paige and Whitham 1987b; Maschinski and Whitham 1989; Bergelson and Crawley 1992a; Bergelson et al. 1996; Brody and Mitchell 1997; Juenger and Bergelson 1997; Paige 1999; Juenger and Bergelson 2000a). The removal of apical dominance and subsequent release of dormant buds that contribute to inflorescence production was cited in these studies as the primary compensatory mechanism. Typically, scarlet gilia is monocarpic (Campbell 1991, 1997); however, some individuals have been shown to produce ancillary rosettes that enable the plant to bloom the following year (Paige and Whitham 1987a; Maschinski and Whitham 1989). If herbivory is associated with ancillary rosette formation this structural response could act as a second mechanism of compensation. The release of dormant buds, which enhances inflorescence production, contributes to compensation in the current year, whereas the initiation of new rosettes contributes to compensation in subsequent years. Moreover, energy allocated to these rosettes may come at an energy cost to seed production in the current year. By quantifying seed production in the context of inflorescence production and rosette formation, we may gain insight into how resource allocation and resource limitations affect plant tolerance for herbivory. We examined trade-offs between vegetative regrowth and seed production following defoliation of scarlet gilia. Preliminary observations suggest that ancillary rosette formation may be a significant factor in our study population as a mechanism to promote reblooming (B. Kirkpatrick, personal observation). In this study we asked two questions regarding scarlet gilia’s response to herbivory: (1) What is the structural response of this population to apical meristem removal in terms of the number of inflorescences and rosettes produced? and (2) Does herbivory affect an individual’s potential to bloom for an additional season?
Methods and Materials Study system Ipomopsis aggregata (Polemoniaceae) is a self-incompatible hermaphrodite (Grant and Wilken 1986). Following germination the plant typically spends 2–8 years as a hardy rootstalk bearing a small, leafy rosette (Bergelson and Crawley 1992b). Once it reaches maturity, the plant extends a single 30–60 cm flowering stalk. Since the majority of scarlet gilia plants bloom and die over the course of one season, an individual’s lifetime fitness can be quantified by its seed produc-
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tion for that season (Campbell 1991). Ancillary rosette formation is a relatively rare, but potentially important occurrence in some scarlet gilia populations (Paige and Whitham 1987a; Maschinski and Whitham 1989). Seed set varies extensively among individuals, populations, and years, with a reported range of 3–20 seeds per fruit. Most seeds germinate within one year, with approximately 8% of those surviving to flower (Juenger and Bergelson 2000c). The low seedling survivorship increases the importance of maximum seed production to individual fitness. Scarlet gilia is found throughout the western mountain ranges in montane habitats. This study was conducted in the eastern Cascade Mountains of Washington State during the 2003 reproductive season (June to October). In this region, scarlet gilia is found between 760 and 1980 m elevation in open, dry sites containing coniferous trees and sagebrush (Grant and Wilken 1986). Elk and mule deer, scarlet gilia’s primary herbivores, are common throughout the Cascade Mountains. According to Paige and Whitham (1987b), ungulate herbivory occurs after stem elongation and prior to flowering. Ungulates typically consume a plant’s reproductive tissue (Collins and Urness 1983), leaving the rosette intact for regrowth (Paige 1994), although occasionally the entire plant is uprooted and eaten (B. Kirkpatrick, personal observation). Elk and mule deer are prominent herbivores during their peak active season from June until October (Merrill 1994), which corresponds to the periods of stem elongation and flowering in scarlet gilia. At our field site in the eastern Cascade Mountains, elk are a common herbivore on scarlet gilia. Experimental design We tagged 80 mature individuals distributed over a 35 m by 45 m area on a south-facing slope in the Wenatchee National Forest (1851 m elevation; 46854’10@N, 121809’56@W) in early June 2003. Individuals were randomly assigned to three treatment groups, stratified by plant size to prevent treatment biases resulting from differences in initial plant size. An analysis of variance indicated that the mean basal diameters of plants assigned to each treatment were similar (F = 1.365, df = 2, p = 0.4032). One-meter high conical wire cages protected 40 plants from herbivory. Twenty of the protected plants were hand-clipped to simulate herbivory. The remaining 40 plants were left vulnerable to natural herbivory. The unprotected plants were monitored for trends in the timing and intensity of herbivory at the site. We handpollinated the mature flowers on all 80 plants once a week to minimize any negative effects of the cages on pollinator behavior and the percent pollination achieved by the control and clipped groups. Handpollination also maximized seed set by an individual, which increased the likelihood of detecting overcompensation. Twenty percent of the unprotected plants were consumed by elk prior to flowering in the first 2 weeks of July. A Student t test indicated that the elk did not preferentially graze larger plants (t = 1.536, df = 39, p = 0.1319). The stalks remaining after elk consumption ranged from 1 to 10 cm in height (average 5 cm). Based on the timing and extent of local herbivory, we clipped the primary stalks of the assigned individuals to a height of 5.0 cm on July 9. We continued #
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Fig. 1. The average number of fruits (A), seeds (B), and seeds per fruit (C) (±1 SE) produced by Ipomopsis aggregata plants with one or more inflorescences (N = 39 and 24, respectively). Reproductive output was significantly reduced in plants that had multiple inflorescences (fruit: p = 0.0263, seed: p = 0.0036, seeds/fruit: p = 0.0223). The multiple inflorescence category includes both cut and grazed plants since their responses were similar; the single inflorescence category includes both protected and ungrazed plants.
Table 1. Mean values for plant (Ipomopsis aggregata) growth and reproductive output. Herbivory treatment No. of individuals No. of fruits No. of seeds No. of seeds/fruit Height (cm) Dry mass (g)
Control 36 23 428 19 33.3 2.083
Cut 18 14 241 15 18.6 2.848
Grazed 11 16 259 17 27.4 3.482
weekly hand-pollinations until the plants were harvested in October. Final plant height (measured as the height of the tallest inflorescence), number of stalks, and number of ancillary rosettes were compared with similar measurements recorded in June to determine the morphological effects of herbivory on structural growth. We also determined plant biomass in grams after drying the plants at 60 8C for 4.5 d. Reproductive output was defined as the number of seeds produced by an individual. We quantified seed production per fruit by counting scars left on the central column of each fruit. These scars indicate where individual seeds were attached to the central column. For each missing column, the plant’s average number of seeds per fruit was added to its total seed count. On average control, clipped, and grazed plants were missing 6, 4, and 3 central columns, respectively. A comparison of actual seed numbers with scar counts from greenhouse grown plants resulted in a significant correlation between the two measures (r2 = 0.859, n = 55). Scar counts may have overestimated fecundity by approximately 17%, but it would have done so equally for all treatments. Statistical analysis Since artificial and natural herbivory resulted in the same
Fig. 2. Seed production in relation to dry plant biomass for Ipomopsis aggregata plants with one (~) or more (*) inflorescences. Seed production increased with plant size. The rate of increase was significantly reduced in multi-stalked plants (p = 0.0191; multiple: r2 = 0.111; single: r2 = 0.171).
regrowth responses, we focused on the number of flowering stalks rather than treatment in the following analyses. To determine the fitness cost of regrowing multiple inflorescences, we conducted an analysis of variance with the number of stalks (single or multiple) as the independent variable, and the mean number of fruits, seeds, and seeds per fruit as the dependent variables. To account for the size difference between plants with one stalk verses those with multiple stalks, we conducted an analysis of covariance with the number of stalks as the independent variable, seed production as the dependent variable, and biomass as the covariate. To determine the fitness cost of producing ancillary rosettes, we conducted a second analysis of variance with rosette formation as the independent variable and the various fitness measurements as the dependent variables. The probability of a plant producing an ancillary rosette based on its number of stalks was characterized by a w2 analysis. One control plant showed significant signs of secondary defoliation by insects. Several control and clipped plants were uprooted or #
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Fig. 3. The average number of fruits (A), seeds (B), and seeds per fruit (C) (±1 SE) produced by Ipomopsis aggregata plants with and without ancillary rosettes. Rosette formation was correlated with a significant decrease in fruit (p = 0.0204) and seed (p = 0.0373) production. These differences were greatest in single-stalked plants.
died partway through the study. These plants were omitted from the analyses.
Results Ninety percent of grazed and clipped individuals demonstrated extensive compensatory growth through the release of lateral inflorescence buds. On average, clipped plants regrew six flowering stalks and grazed plants regrew five flowering stalks. Clipped plants were significantly shorter than both control (F = 15.501, df = 2, p < 0.0001) and grazed (F = 15.501, df = 2, p = 0.0110) plants (Table 1). Despite hand-pollination, plants with multiple flowering stalks produced significantly fewer fruits (F = 4.298, df = 1, p = 0.0263) and seeds (F = 7.949, df = 1, p = 0.0036) than plants with a single inflorescence (Fig. 1). Plants with only one inflorescence set, on average, three more seeds per fruit than plants with multiple inflorescences (F = 7.033, df = 1, p = 0.0223, Fig. 1). When using plant size as a covariate, we found that for two plants of the same mass, a multi-stalked plant produced significantly fewer seeds than a singlestalked plant (F = 4.411, df = 2, p = 0.0191, Fig. 2). Ancillary rosettes were produced by 13% of the experimental population. Multi-stalked plants were more likely to produce ancillary rosettes than single-stalked plants ( 2 = 34.333, p = 0.0372). Only 6% of single-stalked plants produced ancillary rosettes, while 28% of multi-stalked plants developed new rosettes by the end of the season. Rosette formation was correlated with a significant decrease in average fruit (F = 4.484, df = 1, p = 0.0204) and seed (F = 3.567, df = 1, p = 0.0373) production, but did not affect the number of seeds set per fruit (Fig. 3). Decreased reproductive output was greatest in single-stalked plants that produced ancillary rosettes.
Discussion Grazed and clipped plants invested in two types of vegetative growth: multiple inflorescences and ancillary rosettes.
Multiple inflorescences enabled immediate compensation while ancillary rosettes presumably enabled future compensation. We found a negative relationship between rosette formation and initial reproductive output, and herbivory apparently induced the formation of ancillary rosettes. Previous studies characterized 96% of scarlet gilia plants as monocarpic (Campbell 1991, 1997). In those studies, ancillary rosette formation occurred only under specific conditions that included pollinator limitation, but not herbivory (Paige and Whitham 1987a; Maschinski and Whitham 1989). Paige and Whitham (1987a) found that rosette formation occurred when seed set fell to 30%–40% of normal productivity. Of the plants that did produce ancillary rosettes in that study, 10% flowered in the following year. The disparity between our results and those of these authors may stem from population and environmental variation. Grazing intensity did not seem to affect the level of compensation achieved by our population since regrowth was sufficiently stimulated in all grazed individuals, but the timing of defoliation, which is linked to plant development and resource availability, may have had an important effect (Frank and McNaughton 1992; Maschinski and Whitham 1989; Lennartsson et al. 1998; Juenger and Bergelson 2000a). Generally, plants grazed early in the year are able to compensate more because they have access to more resources (Lennartsson et al. 1998). For instance, in a study by Paige (1994), a twofold difference in individual fecundity was attributed to a 2-week difference in the timing of herbivore damage. At sites used in previous studies, defoliation occurred at the end of May and beginning of June. Since most grazing at our site occurred in early July, there was less time for our plants to compensate compared with other studies, and the plants were doing so later in the season when resources were scarce. The availability of soil resources has been shown to limit plant fitness and compensation in montane environments (Paige 1994; Caruso 1999). In our case, water limitation may explain the trends we saw in vegetative growth and #
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seed production by clipped and grazed individuals. A moderate El Nino event during 2002 led to reduced snow pack throughout most of Washington (Moore et al. 2003), which contributed to relatively dry spring conditions in 2003. Above average temperatures and low precipitation in June, July, and August further reduced soil moisture and generated concern over the possibility of forest fires in the eastern Cascade Mountains (National Agricultural Statistics Service 2004). Similar drought conditions have been shown to decrease overall seed set (Lennartsson et al. 1998) and limit compensation in other scarlet gilia populations (Levine and Paige 2004). Consequently, grazed individuals may not have been able to accumulate the resources necessary to produce costly fruits and seeds. Further research on the direct effect of water availability on regrowth and rosette formation is needed to determine what factors influence the specific compensatory mechanism employed by individuals in a population. The suboptimal growing conditions and shorter period for regrowth may have limited the level of compensation our population could achieve, thereby making our plants more likely to fall below the 30%–40% threshold that promotes rosette formation. Consequently, the timing of meristem removal and the combined stress of herbivory and semidrought conditions could have prompted a greater percentage of our plants to allocate their resources to ancillary rosettes instead of reproductive output. This form of bethedging allows the plant to flower a second season when the conditions might be more optimal. Thus, even though rosette formation uses limited soil resources and is associated with reduced fruit and seed set, the combined seed set of two or more reproductive seasons may counteract the initial negative relationship between rosette formation and individual fitness.
Conclusion This study demonstrated the potential for I. aggregata to compensate for herbivory through rosette formation as well as regrowth. These results suggest that herbivores generate a significant selection pressure on scarlet gilia populations via their indirect effect on plant structure. Furthermore, compensation is mediated by environmental factors and plant adaptations to local conditions. To fully understand the particular adaptations of I. aggregata in the Pacific Northwest, future research should further explore the abiotic and biotic conditions that promote rosette formation, and how this response influences long-term compensation.
Acknowledgements This work was supported by the University of Puget Sound and the Summer Science Research Program. We thank K. Lubetkin for her assistance with data collection. We also thank A. DeMarais, C. Galen, and two anonymous reviewers for their thoughtful comments and suggestions regarding this manuscript.
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