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Allelopathic effects of goldenrod species on turnover in successional communities.

INTRODUCTION

Goldenrods (largely Solidago spp.) are herbaceous perennials commonly found in meadows, prairies, roadsides and abandoned agricultural land in eastern North America. They are self-incompatible, produce numerous wind-dispersed seeds and germinate easily on a wide range of soils (Weber, 2001). Once a population is established through seed establishment, energy allocation shifts to vegetative growth (Hartnett & Bazzaz, 1985; Bazzaz, 1996; Meyer and Schmid, 1999; Weber, 2001) leading to genetically uniform clonal patches of goldenrods. Vegetative propagation is important as a competitive mechanism for gaining territory for expansion (Salisbury, 1942; Werner, 1976; Grime, 2001) and is an important process in successional systems (Grime, 2001). Together, these two modes of reproduction are important in determining the competitive success and impacts of goldenrods in successional systems (Hartnett and Bazzaz, 1985; Myster and Pickett, 1992; Long et al., 2003). This competitive ability has contributed to their invasiveness in Europe where they are not attacked by herbivores (Weber, 2001).

Beyond direct competitive interactions, goldenrods have the potential to interact indirectly with their neighbors through allelopathy. Allelopathy has generally come to mean the deleterious effect that one plant has on another through the production of chemical retardants (Martin and Rademacher, 1960; Muller, 1965; Jackson and Willemsen, 1976). However, this process is more complex because allelopathic plants are also capable of stimulatory effects (Jackson and Willemsen, 1976). The chemical producing plant may also inhibit itself with the same chemicals that inhibit its neighbors (Kumari and Kohli, 1987).

Previous studies have tested Solidago species for allelopathic inhibition of germination and have found varying results. For example, Fisher et al. (1977) found that goldenrods reduced the germination and growth of maples even in the absence of competing vegetation. Bingyao et al. (2006) observed both inhibitory and stimulatory effects of S. canadensis against several target species. However, in higher chemical concentrations the interactions were predominantly inhibitory. In another study, Euthamia graminifolia and S. canadensis reduced germination of seeds in target species, but inhibition was tissue-specific, only occurring with leachates of leaves (Butcko and Jensen, 2002).

The effect of allelopathic chemicals is often tested through bioassays, typically by testing the effects of plant tissue extracts on the germination of seeds. However, there are issues in relating laboratory bioassays to allelopathic interactions in the field. The few studies that examine both find a conflict between lab and field data where allelopathy in the laboratory is not always demonstrated in the field (Keever, 1950; Muller and Muller, 1956; Jameson, 1970; del Moral and Cates, 1971; Neill and Rice, 1971; Stowe, 1979). This difficulty may relate to low concentrations of the allelopathic agent or fluctuations in toxicity in time and space, or be a sign that chemical interactions are offset by other processes. In spite of these limitations, laboratory bioassays are useful for identifying potential allelopathic interactions that must be then verified in the field.

The primary interest of this study was the potential for allelopathic interactions of plants and their importance in shaping the surrounding plant community. Goldenrods were chosen as the focus for this study because they are documented as being allelopathic, they are important in successional systems, and they are becoming invasive worldwide. We addressed the primary question, the importance of allelopathy in determining the success of and impact of goldenrods in old field succession, in two ways. First, we experimentally assessed the allelopathic potential of each goldenrod species in the lab. Second, we used long-term vegetation data to determine the influence of goldenrods on the surrounding community. Finally, we related these two data sets to determine whether allelopathic interactions could explain vegetation patterns seen in the field.

MATERIALS AND METHODS

Six species of goldenrods were included in this study: Euthamia graminifolia Nutt. (formerly Solidago graminifolia), S. canadensis L. (synonymous with S. altissima), S. gigantea Alton., S. juncea Alton, S. nemoralis Alton. and S. rugosa Miller. Although closely related species, they vary considerably in morphology, habitat selectivity and invasiveness. Two of the species, S. juncea and S. nemoralis, produce basal rosettes of leaves whereas the other species produce upright stems. All species reproduce clonally; however, S. canadensis and S. rugosa are the most aggressive vegetative spreaders (Gleason and Cronquist, 1991). Solidago gigantea allocates most of its energy into sexual reproduction and produces large flower heads and achenes, contributing to its invasiveness (Abrahamson et al., 2005). Species also vary in their niche preferences. Euthamia graminifolia and S. gigantea are often associated with more mesic conditions, contrasting with S. juncea and S. nemoralis which occur in drier areas (Abrahamson and Gadgil, 1973; Werner and Platt, 1976). Solidago canadensis and S. rugosa prefer intermediate conditions (Abrahamson and Gadgil, 1973; Werner and Platt, 1976). Additionally, E. graminifolia and S. rugosa have higher abundances on acidic soils (Abrahamson et al., 2005). At least three goldenrod species, E. graminifolia, S. canadensis and S. gigantea have been observed to be invasive in Europe (Abrahamson et al., 2005).

ALLELOPATHIC TESTING

To assess allelopathic potential, all six goldenrods were tested with laboratory bioassays following Butcko and Jensen (2002). Leaf samples were collected from the Hutcheson Memorial Forest in Jul. 2007. For each species, leaves from at least 20 plants were collected and air dried for 2 d. Extracts were made from 12.5 g of dried leaf tissue placed in 500 ml of distilled water. The mixture was placed on a magnetic stirrer for 24 h at room temperature and was strained through cheesecloth to remove particulate plant material. Dilutions of each extract, ranging from 0% to 100% in 10% increments were made. Filter paper was placed in 90 mm petri plates with 20 seeds of the target species. Five trials were run for each dilution for each goldenrod species tested. We used two target species, lettuce (Lactuca sativa L., 'Black Seed Simpson') and radish (Raphanus sativus L., 'Early Scarlet Globe': Bay Farm Services, Inc., Bay City, MI). Lettuce and radish seeds were selected because they are commonly used in allelopathic testing. Four mL of extract was added to each plate and incubated at 25 C for a 12/12 h light/dark cycle. The plates were removed after 4 d and germinated seeds were counted.

The proportion of seeds that germinated in each dish was analyzed in two ways. First, an ANCOVA of goldenrod species and extract concentration was run to determine overall effects in both target species. This analysis was followed by a separate regression analysis for each goldenrod to quantify the slope of the inhibition response. Pair-wise differences between species were determined using t-tests with Bonferroni correction for multiple comparisons. All statistical analyses were conducted using SPSS 15.0 (SPSS Inc., Chicago Illinois).

FIELD DATA

Field data were obtained from the Buell-Small Successional Study (BSS), a long-term study of old field successional dynamics. The study is located within the Hutcheson Memorial Forest on former agricultural land in the Piedmont region of New Jersey, (40[degrees]30' N, 74[degrees]34' W). The BSS contains 10 replicated fields each having 48 permanently marked plots (0.5 x 2.0 m). Fields were abandoned in pairs from 1958-1966. The identity and percent cover of all species present in each plot were recorded for each year (alternate years since 1979).

To determine the population dynamics of each goldenrod species during succession, percent plot cover and frequency were calculated for each species in all 10 fields (C3, C4, C5, C6, C7, D1, D2, D3, E1 and E2 named for their position in the field). From the original six species, four that attained a minimum of 3.5% cover per plot were selected for further analysis: Euthamia graminifolia, Solidago canadensis, S. juncea and S. rugosa. The remaining species were too infrequent for analysis. Cover of goldenrods varied among fields and was examined using an ANOVA for each of the four selected species using plot data. Correlation analyses were also run to test for associations among goldenrod species.

COMMUNITY EFFECTS OF GOLDENROD

To document the influence of goldenrod species on plant community structure, the effect of goldenrods on species richness during peak cover was analyzed. Years 23 and 24 post abandonment were chosen for this analysis because all goldenrods were abundant during this time period. Because fields were sampled in alternate years, both years were used to include data from all 10 fields. Since fields varied dramatically in cover for goldenrod species, a subset of fields was selected for each species based on average plot cover. Fields which did not achieve an average percent cover of at least 3.5% in years 23/24 for a species were excluded from analysis in order to prevent bias from poorly colonized fields. The influence of plot goldenrod cover (continuous) and field identity (categorical) on species richness was analyzed with an ANCOVA. Pearson correlations between species richness and goldenrod cover were calculated for each field to assess variation among fields and to interpret ANCOVA interactions. To assess goldenrod impacts on individual species, the percent cover of the four most abundant resident species, Centaurea dubia Suter., Fragaria virginiana Duchesne., Hieracium caespitosum Dumort. and Poa compressa L., was correlated with percent cover of each goldenrod species.

To determine the underlying dynamics which generated goldenrod effects on associated species richness, colonization and extinction rates were also calculated. These rates were calculated for the periods of 23-25 and 24-26 y for all fields which met abundance criteria. Data collection in alternate years set a minimum interval of 2 y for this analysis. Rates were calculated as the number of species which appeared or disappeared from each plot during the interval. Colonization and extinction rates were analyzed via ANCOVA as above. Pearson correlations were conducted for species gain/loss in each field to assess variation among fields and to interpret interactions between goldenrod cover and turnover.

RESULTS

ALLELOPATHY DATA

Allelopathic effects were seen in all goldenrod species through reduced seed germination in lettuce and radish. There was significant variation among goldenrod species in toxicity and separation between target species in their susceptibility to allelochemicals (Table 1; Fig. 1). Radish seeds showed more separation among goldenrod species compared to lettuce seeds. In lettuce, the most toxic species were Euthamia graminifolia and Solidago juncea, whereas the weakest inhibitors were S. canadensis and S. gigantea. Radish yielded different species in terms of strongest (E. graminifolia and S. canadensis) and weakest (S. gigantea and S. nemoralis) germination inhibitors.

FIELD DATA

Goldenrod cover was generally low in early succession and rapidly increased at mid-succession (years 10-20; Fig. 2). In late succession (years 30-40), the cover and frequency of goldenrods decreased for all species. However, two species, Solidago gigantea and S. nemoralis, remained at a relatively consistent, but low cover throughout succession. These two species were dropped and only the four most abundant species were analyzed further.

[FIGURE 1 OMITTED]

There was dramatic variation among fields in goldenrod cover (Fig. 3). Euthamia graminifolia ([F.sub.9,470] = 4.39, P < 0.001), S. canadensis ([F.sub.9,470] = 2.79, P = 0.003), S. juncea ([F.sub.9,470] = 20.2, P < 0.001) and S. rugosa ([F.sub.9,470] = 9.26, P < 0.001) all had significant variation among fields in plot cover. Interestingly, these species also showed some spatial segregation within the fields. Solidago juncea was negatively correlated with both S. canadensis and S. rugosa (R = -0.143, P = 0.002 and R = -0.178; P < 0.001 respectively). Although most fields had a single dominant goldenrod, all species were present in most fields (Fig 3.)

[FIGURE 2 OMITTED]

COMMUNITY EFFECTS OF GOLDENROD

Species richness was influenced by either cover or the cover X field interaction for three of the four goldenrod species tested (Table 2). Species richness was overall positively associated with Solidago juncea cover, but this association varied among fields. Solidago juncea was positively correlated with richness only in fields C5 (R = 0.450, P = 0.001), D2 (R = 0.428, P = 0.002) and E2 (R = 0.480, P = 0.001), whereas cover in the other fields was not correlated with species richness. The only significant model term for Euthamia graminifolia was a field effect on species richness. For S. rugosa, the only significant effect on richness was the cover X field interaction. The only field where cover of S. rugosa was associated with richness was field C7 (R = 0.348; P = 0.015); all other fields were non-significant. Solidago canadensis was similar to S. rugosa in that there was a significant cover X field interaction, although the field term was also significant. The relationship between S. canadensis and species richness varied across fields with field C5 (R = -0.313, P = 0.030) negatively correlated and field D2 (R = 0.323; P = 0.025) positively correlated.

[FIGURE 3 OMITTED]

To understand these influences on species richness, cover of the four most abundant resident species was correlated with goldenrod cover (Table 3). These results were largely similar to the relationships with species richness in that positive associations were found. Each goldenrod had at least two positive associations with resident species, except for S. rugosa which had only one positive association (Table 3).

Colonization and extinction rates of associated species varied greatly among fields for all four goldenrod species. However, only Solidago juncea cover affected colonization rates (Table 4). As S. juncea showed a nearly significant cover X field interaction for colonization, Pearson correlations were used to identify which fields were associated with species turnover. Fields C6 (R = 0.335, P = 0.020), D2 (R = 0.311, P = 0.032), and E1 (R = 0.292, P = 0.044) showed positive associations of the species with colonization. Laboratory bioassay data did not predict cover of goldenrods in the field as correlations between the allelopathic toxicity and field cover were not significant for either target species (Fig. 4; both P [greater than or equal to] 0.125).

DISCUSSION

Although laboratory bioassays confirmed the presence of at least some allelopathic capability for all of the goldenrod species tested, there was dramatic variation among species. Moreover, toxicity of goldenrod species varied between target species. This toxicity, however, was not consistent with the observed dynamics in the field. While some goldenrod species reached high cover in the field, we found no inhibitory effects of goldenrod cover on species richness or turnover. In contrast, the field data revealed a significant, but minor facilitative effect of goldenrods on other species. Finally, there was no correlation between allelopathic potential and the cover achieved by each species in the long-term data, suggesting that allelopathy is not an integral part of a goldenrod's success in its native range.

[FIGURE 4 OMITTED]

Our results are consistent with experimental work done at the BSS site. In a field experiment, Facelli and Pickett (1991) tested litter of three species: Quercus alba L., Setaria faberii Herrm. and Solidago spp. (mostly S. canadensis) for persistence and the ability to reduce total plant cover. They found that goldenrod litter was the most persistent of the three species tested, but had little impact on plant cover (Facelli and Picket, 1991). While Facelli and Pickett (1991) would have integrated both the physical and chemical influences of goldenrod litter, the lack of impacts suggests little direct influence of allelochemicals on the community.

Although it is evident that goldenrod species have allelopathic potential, it does not appear to be sufficiently strong to affect surrounding plants in this system. If allelochemicals primarily function as germination inhibitors, the total availability of seeds should determine the net effect of allelopathy on the community. In this study, plant extracts reduced germination on average by 54.8% for lettuce seeds and 37.8% in radish seeds at full concentration. If the plant community was not seed limited (i.e., if seed availability was high), then there may have been too many seeds present to reduce colonization rates. Therefore, the importance of allelopathy may vary along gradients of propagule pressure, with the greatest effects occurring in areas of low seed availability. As target species appear to vary greatly in their response to plant extracts, the diversity of seeds available in a habitat may include a mix of susceptible and resistant species, further reducing the observed impacts.

The rate at which allelochemicals are released and their concentration in the soil could be affected by several factors such as rainfall, concentration of chemicals produced in goldenrod tissues and decomposition rates. Once incorporated into the environment, allelochemicals may have been metabolized by soil organisms so rapidly that their concentration in the soil decreased and, therefore, their effect was minimized. As the allelochemicals that we investigated would typically be released into the environment upon leaf abscission, their effects would be mixed with other allelopathic old-field plants that may mask the community level influences of goldenrods (Keever, 1950; Jackson and Willemsen, 1976; Gomez-Aparicio and Canham, 2008).

Allelopathy is often thought to be an important mechanism in determining the success of non-native species in their introduced ranges (Inderjit et al., 2008). In contrast to our results, allelopathy has been shown to be important in invasive Solidago canadensis populations in Europe (Abhilasha et al., 2008). Nearly all of the native European species tested against S. canadensis showed reduced performance in the presence of allelochemicals. This result may be due to the lack of evolutionary exposure of plants in the introduced range to the novel chemicals produced by the invading species (Callaway and Aschehoug, 2000; Abhilasha et al., 2008). Within their native range, herbivore pressure may ameliorate goldenrod effects on the community (Carson and Root, 2000). Insect herbivory on goldenrods can promote plant species richness and coexistence, primarily by augmenting light availability to suppressed understory species but also by increasing soil moisture and nitrogen levels in the soil (Carson and Root, 2000; Long et al., 2003). However, herbivory may stimulate the production of allelochemicals (Thelen et al., 2005; Abhilasha et al., 2008), potentially mitigating these effects.

The contradiction between lab and field data suggests that allelopathy, although present, plays a minimal role in the success of goldenrods within their native range. Their dominance within successional systems appears primarily due to more direct competitive strategies. However, the contrast between allelopathy in native and introduced ranges indicates the potential for conditionality in allelopathy. The context-dependent nature of allelopathy in goldenrods makes them a useful model system to explore the range of allelopathic impacts seen in plant communities.

Acknowledgments.--We thank L.M. Ladwig, K.A. Lang, J.M. Lyerla-Kirkton, N.C. Morris and G.N. Presley for comments on previous drafts of this manuscript; funding was provided by NSF Award DEB0424605 and an REU Supplement.

SUBMITTED 10 NOVEMBER 2008 ACCEPTED 29 APRIL 2009

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NIKKI L. PISULA (1) AND SCOTT J. MEINERS

Department of Biological Sciences, Eastern Illinois University, Charleston 61920

(1) Corresponding author: Department of Biological Sciences, Eastern Illinois University, Charleston, Illinois 61920; Telephone: (217) 621-7901; e-mail: nlpisula@eiu.edu
Table 1.--Influence of goldenrod species identity and extract
concentration on the germination of radish and lettuce seeds.
Significant values are indicated by bolding

Source                       df         MS         F         P
  Radish
  Species                   5           43.27     0.331     0.896
  Concentration             1       80,561.52   609.69     <0.001#
  Species X Concentration   5         1907.52    14.44     <0.001#
  [R.sup.2]                 0.72
Lettuce
  Radish                    5          428.97     2.01      0.076
  Species                   1       33,509.33   157.32     <0.001#
  Concentration             5         4827.06    22.66     <0.001#
  Species X Concentration   0.57
  [R.sup.2]

Note: Significant values are indicated by bolding with #.

Table 2.--Influence of goldenrod cover and field identity on local
(plot scale) species richness using analysis of covariance of the four
most abundant goldenrods within the BSS data. Significant values are
indicated by bolding

Source            df     MS       F     [R.sup.2]     P

E. graminifolia
  Field            4   397.81   21.37     0.33      <0.001#
  Cover            1     1.13    0.06                0.806
  Field X Cover    4    21.37    1.15                0.335
S. canadensis
  Field            6   315.29   14.85     0.09      <0.001#
  Cover            1     0.04    0.00                0.966
  Field X Cover    6    47.94    2.26                0.038
S. juncea
  Field            6   187.40   10.32     0.35      <0.001#
  Cover            1   466.64   25.70               <0.001#
  Field X Cover    6    38.51    2.12                0.051
S. rugosa
  Field            4    47.55    2.27     0.27       0.062
  Cover            1     3.27    0.16                0.693
  Field X Cover    4    57.01    2.73                0.030#

Note: Significant values are indicated by bolding with #.

Table 3.--Correlations of the four most common associated species in
relation to goldenrod cover within the BSS data. Values reported are
Pearson correlation coefficients (p-value). Significant values are
indicated by bolding

                  Centaurea dubia    Fragaria virginana

E. graminifolia    -0.03 (0.619)        0.17 (0.009)#
S. canadensis      -0.01 (0.911)        0.06 (0.305)
S. juncea          -0.05 (0.345)        0.29 (<0.001)#
S. rugosa          -0.09 (0.151)       -0.05 (0.476)

                  Hieracium caespitosum    Poa compressa

E. graminifolia       0.00 (0.947)          0.21 (<0.001)#
S. canadensis         0.17 (0.002)#         0.14 (0.010)#
S. juncea             0.29 (<0.001)#       -0.05 (0.340)
S. rugosa            -0.02 (0.788)          0.16 (0.014)#

Note: Significant values are indicated by bolding with #.

Table 4.--Influence of goldenrod cover and field identity on local
(plot scale) colonization and extinction rates of the four most
abundant goldenrods within the BSS data. Significant values are
indicated by bolding

                  E.graminifolia    S. canadensis      S. juncea
                   F       P        F        P        F       P

Colonization
  Field           2.58    0.038#   5.23    <0.001#   2.86    0.010#
  Cover           0.03    0.854    0.93     0.336    8.78    0.003
  Field X Cover   1.99    0.097    0.51     0.801    2.11    0.052
  [R.sup.2]       0.07             0.11              0.13
Extinction
  Field           4.33    0.002#   6.73    <0.001#   3.26    0.004#
  Cover           1.19    0.276    0.32     0.570    0.22    0.638
  Field X Cover   0.76    0.554    0.71     0.642    0.95    0.461
  [R.sup.2]       0.10             0.16              0.09

                    S. rugosa
                   F       P

Colonization
  Field           3.92    0.002#
  Cover           0.03    0.861
  Field X Cover   0.41    0.844
  [R.sup.2]       0.09
Extinction
  Field           3.43    0.005#
  Cover           1.24    0.267
  Field X Cover   1.25    0.286
  [R.sup.2]       0.11

Note: Significant values are indicated by bolding with #.
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Author:Pisula, Nikki L.; Meiners, Scott J.
Publication:The American Midland Naturalist
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Date:Jan 1, 2010
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