Introduction
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The effect of pollinator exclusion on the fruit set of Solidago rigida and Salvia azurea. N. Boersma, T. Simmelink, and L. Furlong Biology Department, Northwestern College, Orange City Iowa Abstract - Long-term persistence of native plant species in Iowa’s fragmented tallgrass prairies will be dependent upon several interacting factors. For outcrossing species, the lack of pollinators may limit seed set and will limit genetic diversity. This field study addressed the effect of experimental pollinator exclusion on the fruit set of two native prairie plants, Solidago rigida and Salvia azurea. Exclusion was accomplished by enclosing inflorescences in mesh bags before the flowers had fully opened. We manipulated the degree of exclusion by varying mesh size. Individual plants were subjected to both treatments, and unbagged inflorescences on the same plants were used as controls. Following fruit set, inflorescences were removed from the plants and dry weights of the fruit were compared. Pollinator exclusion resulted in a significantly lower fruit weight in Salvia azurea but not in Solidago rigida. Our study indicates that Solidago may be more tolerant of self pollination than Salvia, suggesting that Salvia populations are less likely to persist in small prairie fragments or fragments with low pollinator abundance. Introduction Outcrossing plant species rely upon external factors, such as wind, water and animals, for pollen transport (Howe and Westley 1997). Species relying upon animal pollinators will be adversely affected by declining pollinator abundance and richness. Even those species capable of self-pollination may face reduced fitness in the long term if pollinators are in short supply. For both obligate and facultative outcrossing species, the negative affects of limited pollinators is often observed as lower reproductive success (Matsui et al. 2001, Kalinganire et al. 2001). Relative reproductive success may be measured in a number of ways including fruit set, seed production (number and/or mass), germination success, and seedling survival (Herrera 2000). Given the insular nature of remnant and restored tallgrass prairie sites in a landscape dominated by row-crop monoculture, pollinator limitation may endanger the long term genetic diversity of many populations of prairie plants. This study explores the effects of pollinator limitation through an exclusion experiment conducted at our field site, the Northwestern College Prairie (NCP). We chose two of the NCP’s most prolific fall blooming plants, Solidago rigida (rigid goldenrod) and Salvia azurea (pitcher sage), as the foci of our pollinator exclusion experiment. With this manipulation, we hoped Figure 1. Location of the Northwestern College Prairie. North Figure 2. Example of minimum exclusion treatment on Solidago. Materials and Methods The experiment was conducted at the Northwestern College Prairie (NCP), a 13 acre restoration site located approximately 3 miles east of Hawarden Iowa (figure 1). At the time of this study the prairie was in its fourth year of restoration. Prior to the restoration, the site supported an alfalfa field. At present, the study site supports 6 native grass species and approximately 30 species of native forbs. The NPC is bordered by corn/soy cultivation on the east and an alfalfa field on the west. On September 7, 2004, 10 representative plants of each species ( Solidago rigida and Salvia azurea) were randomly selected for study. On each plant, immature inflorescences were subjected to the following treatments: 1) an inflorescence was enclosed with maximum exclusion netting (1 mm openings); 2) an inflorescence was enclosed with minimum exclusion netting (3 mm openings); and 3) an inflorescence was marked with cotton string (control). The netting material consisted of nylon mesh and the net exclusion “bags” were large enough that they did not interfere with floral development. The bags were secured to the plants with cotton string (figure 2). We harvested the inflorescences on October 12 (Salvia) and October 19 (Solidago). The inflorescences were dried and the fruits and attached calyxes were weighed as a unit. In the following discussion, “fruit” refers to the nutlets or achens and their attached calyxes. For each species, we analyzed mean fruit weights per plant and mean fruit weights per treatment using one-way ANOVAs. Data were log transformed as necessary to satisfy the assumptions of the ANOVA. Non-significant subsets were identified with Tukey’s multiple comparison test.
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The effect of pollinator exclusion on the fruit set of Solidago rigida and Salvia azurea. N. Boersma, T. Simmelink, and L. Furlong Biology Department, Northwestern College, Orange City Iowa. Materials and Methods - PowerPoint PPT Presentation
Transcript of Introduction
The effect of pollinator exclusion on the fruit set of Solidago rigida and Salvia azurea.N. Boersma, T. Simmelink, and L. Furlong
Biology Department, Northwestern College, Orange City Iowa
Abstract - Long-term persistence of native plant species in Iowa’s fragmented tallgrass prairies will be dependent upon several interacting factors. For outcrossing species, the lack of pollinators may limit seed set and will limit genetic diversity. This field study addressed the effect of experimental pollinator exclusion on the fruit set of two native prairie plants, Solidago rigida and Salvia azurea. Exclusion was accomplished by enclosing inflorescences in mesh bags before the flowers had fully opened. We manipulated the degree of exclusion by varying mesh size. Individual plants were subjected to both treatments, and unbagged inflorescences on the same plants were used as controls. Following fruit set, inflorescences were removed from the plants and dry weights of the fruit were compared. Pollinator exclusion resulted in a significantly lower fruit weight in Salvia azurea but not in Solidago rigida. Our study indicates that Solidago may be more tolerant of self pollination than Salvia, suggesting that Salvia populations are less likely to persist in small prairie fragments or fragments with low pollinator abundance.
Introduction
Outcrossing plant species rely upon external factors, such as wind, water and animals, for pollen transport (Howe and Westley 1997). Species relying upon animal pollinators will be adversely affected by declining pollinator abundance and richness. Even those species capable of self-pollination may face reduced fitness in the long term if pollinators are in short supply. For both obligate and facultative outcrossing species, the negative affects of limited pollinators is often observed as lower reproductive success (Matsui et al. 2001, Kalinganire et al. 2001). Relative reproductive success may be measured in a number of ways including fruit set, seed production (number and/or mass), germination success, and seedling survival (Herrera 2000).
Given the insular nature of remnant and restored tallgrass prairie sites in a landscape dominated by row-crop monoculture, pollinator limitation may endanger the long term genetic diversity of many populations of prairie plants. This study explores the effects of pollinator limitation through an exclusion experiment conducted at our field site, the Northwestern College Prairie (NCP). We chose two of the NCP’s most prolific fall blooming plants, Solidago rigida (rigid goldenrod) and Salvia azurea (pitcher sage), as the foci of our pollinator exclusion experiment. With this manipulation, we hoped to determine the potential impacts of pollinator limitation on the reproductive success of these species. We hypothesized that decreasing pollinator access would result in decreased fruit set.
Figure 1. Location of the Northwestern College Prairie.
North
Figure 2. Example of minimum exclusion treatment on Solidago.
Materials and Methods
The experiment was conducted at the Northwestern College Prairie (NCP), a 13 acre restoration site located approximately 3 miles east of Hawarden Iowa (figure 1). At the time of this study the prairie was in its fourth year of restoration. Prior to the restoration, the site supported an alfalfa field. At present, the study site supports 6 native grass species and approximately 30 species of native forbs. The NPC is bordered by corn/soy cultivation on the east and an alfalfa field on the west.
On September 7, 2004, 10 representative plants of each species (Solidago rigida and Salvia azurea) were randomly selected for study. On each plant, immature inflorescences were subjected to the following treatments:
1) an inflorescence was enclosed with maximum exclusion netting (1 mm openings);
2) an inflorescence was enclosed with minimum exclusion netting (3 mm openings); and
3) an inflorescence was marked with cotton string (control).
The netting material consisted of nylon mesh and the net exclusion “bags” were large enough that they did not interfere with floral development. The bags were secured to the plants with cotton string (figure 2).
We harvested the inflorescences on October 12 (Salvia) and October 19 (Solidago). The inflorescences were dried and the fruits and attached calyxes were weighed as a unit. In the following discussion, “fruit” refers to the nutlets or achens and their attached calyxes.
For each species, we analyzed mean fruit weights per plant and mean fruit weights per treatment using one-way ANOVAs. Data were log transformed as necessary to satisfy the assumptions of the ANOVA. Non-significant subsets were identified with Tukey’s multiple comparison test.
ResultsSalvia azurea
Mean fruit weight per flower ranged from 2.70 mg to 5.20 mg, with seed weight increasing with increased pollinator access (figure 3). An ANOVA of the log transformed data indicated that the treatments differed significantly (F 2,26 = 19.01, P < 0.001).
The fruit weights of the maximum and minimum exclusion treatments were significantly lower than those of the control (Tukey’s q 3,26 < 0.05).
Differences in the mean fruit weights per flower of individual plants were not significant (ANOVA, F 9,19 = 0.27, P = 0.97).
Solidago rigida
Mean fruit weight per flower head ranged from 17.2 mg to 14.5 mg, with seed weight increasing with decreased pollinator access (figure 4). An ANOVA of these indicated that the treatments did not differ significantly (F 2,24 = 1.53, P =0.23).
Differences in mean fruit weights per head of individual plants were significantly different (ANOVA, F 9,17 = 4.12, P < 0.005).
Figure 2. Mean Salvia fruit weight per flower under varying degrees of pollinator exclusion. Exclusion treatments differed significantly
from the control (ANOVA F 2,26 = 19.01, P < 0.001). Error bars represent standard deviations.
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Figure 4. Mean Solidago fruit weight per flower under varying degrees of pollinator exclusion. Exclusion treatments differed
significantly from the control (ANOVA, F 2,24 = 1.53, P =0.23). Error bars represent standard
deviations.
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