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Ecology |
2Department of Forest Science, Colorado State University, Fort Collins, Colorado, 80523 USA; 3Archbold Biological Station, P.O. Box 2057, Lake Placid, Florida 33862 USA
Received for publication August 16, 2001. Accepted for publication February 14, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: allelopathy • Ceratiola • Empetraceae • fire • Florida, USA • rosemary scrub
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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), is thought to be an important factor in many natural and agricultural communities (Rice, 1984
; Einhellig, 1995
). Chemicals extracted from plant roots or shoots have been shown to directly inhibit or stimulate germination, growth, and development of other plants (Rice, 1984
; Putman and Tang, 1986
). Allelochemicals may also indirectly affect plants through inhibition of microorganisms, including nitrogen-fixing and nitrifying bacteria (Rice, 1964
) and ectomycorrhizae (Walker et al., 1999
). Given these effects, allelopathic plants have the potential to alter individual plant fitness and thus plant population and community dynamics.
Yet it remains difficult to develop generalized models on allelopathy as studies often have found confounding evidence of allelopathic effects. In an attempt to differentiate ecological patterns that are a result of allelopathy, competition, or herbivory, controlled greenhouse studies are typically conducted. In much of these studies, mimicking of natural field conditions has been attempted using methods that are not realistic (Qasem and Hill, 1989
), which has led to a general skepticism in the ecological significance of allelopathy. To combat such skepticism, investigations into allelopathy have considered results from field studies, controlled greenhouse experiments, and chemical analysis of allelochemicals (Foy, 1999
). In addition, many attempts have been made to make controlled greenhouse studies more comparable to field conditions (Foy, 1999
). In this study we draw on findings from previous studies investigating the chemical analysis of allelochemicals from our target species and field observations of allelopathy in Florida scrub and further examine allelopathic interactions in this environment.
In systems in which bare soil or areas of low plant density occur, limitation of herbs to these areas has been attributed to allelopathy by the dominant woody plant (Muller, 1966
). Often, plant distributions in these habitats are actually caused by multiple factors, including allelopathy, competition, herbivory, seed dispersal, and seed predation (Bartholomew, 1970
; Halligan, 1973
). In Florida's rosemary scrub, several distribution patterns have been attributed to allelopathy by the dominant shrub Ceratiola ericoides (Florida rosemary) (Richardson and Williamson, 1988
). Rosemary scrub tends to occupy knolls in sand pine scrub and is often surrounded by oak scrub or scrubby flatwoods (Abrahamson et al., 1984
). Unlike surrounding oak scrub, gaps of bare sand up to several meters in diameter are commonly found between Florida rosemary shrubs or groups of shrubs. Allelopathy, fire, competition, or a combination of these factors may be responsible for creating and maintaining these gaps.
Gaps in rosemary scrub are the primary habitat for many rare herbaceous species endemic to Florida's Lake Wales Ridge (Menges and Hawkes, 1998
). Many of these species favor microhabitats in gaps, at a distance from Florida rosemary shrubs (Menges and Kimmich, 1996
; Quintana-Ascencio and Morales-Hernandez, 1997
). As rosemary scrub patches age and Florida rosemary canopy increases, gaps will persist (Hawkes and Menges, 1996
), but populations of some herbaceous species within these gaps will often decline (Menges and Kimmich, 1996
; Quintana-Ascencio and Morales-Hernandez, 1997
). We suggest that increased allelopathic and competitive effects from Florida rosemary might be responsible for this pattern.
We examine leaf and litter allelochemicals and spatial distribution of roots from the dominant shrub Florida rosemary to determine how this species might affect spatial distribution of subordinate species. The focus of the first part of the study is the effects of allelochemicals on germination of other species found in rosemary scrub. Germination is very important for many rosemary scrub species, most of which are killed by infrequent fires. Such species rely on seedling recruitment to recolonize rosemary scrub patches after fire or to colonize unoccupied gaps in rosemary scrub (Johnson and Abrahamson, 1984
; Hawkes and Menges, 1995
; Menges and Kohfeldt, 1995
; Menges and Kimmich, 1996
; Quintana-Ascencio and Morales-Hernandez, 1997
). Specifically, we ask the following questions. Do allelochemicals from Florida rosemary inhibit germination of species found in rosemary scrub? Do rosemary scrub and gap specialists differ from other community and microhabitat specialists in their response to Florida rosemary allelochemicals? Are litter or leaves more effective agents of Florida rosemary allelochemicals? What is the distribution of Florida rosemary litter and roots in gaps of older and younger rosemary scrub stands?
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Of the seven target species exhibiting varying degrees of community specialization are two found almost exclusively in rosemary scrub (Eryngium cuneifolium and Hypericum cumulicola). The remaining species occur in rosemary scrub as well as other upland habitats (Lechea deckertii, Paronychia chartacea, Polygonella basiramia, Liatris ohlingerae, and Palofoxia feayi) (Table 1). Four of the seven species recover from fire via seedling recruitment (three from a seed bank) (Eryngium cuneifolium, Hypericum cumulicola, Paronychia chartacea, and Polygonella basiramia), while two (L. ohlingerae and P. feayi) can also resprout after fire (Menges and Kohfeldt, 1995
). The seven species also display different microhabitat specializations. Five of the seven are found almost exclusively in gaps between shrubs (Eryngium cuneifolium, Hypericum cumulicola, Lechea deckertii, Paronychia chartacea, and Polygonella basiramia) (Table 1). The remaining species (Liatris ohlingerea and Palofoxia feayi) do not exhibit a high degree of microhabitat specialization and are considered generalists for the purpose of this study (Table 1).
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). Hydrocinnamic acid has been shown to suppress germination and growth of several species and has been proposed as the primary allelopathic compound released from Florida rosemary (Fischer et al., 1994
). All pots were placed on benches in an unheated greenhouse and given 5 h of supplementary lighting daily. Leachate application was designed to mimic natural rainfall under a rosemary canopy, by spraying water through foliage and litter onto the pots. The timing of this study coincided with the season of germination to address the concern of seasonal variation of allelochemical production. This was done to assure that the concentration of the experimental leachate closely mimicked that that would actually be found under a rosemary canopy. We placed a standard amount of rosemary leaves or litter (300 g leaves, 150 g litter, or 150 g leaves + 75 g litter) in an even layer on nylon mesh screens, placed screens over groups of treatment pots, and sprayed water through the screens. Using this standard amount of leaves and litter allowed an equal number of branches or litter to be placed over each pot in a treatment. During the winter months (December 1998, January and February 1999) plant material was collected on a weekly basis and stored in a refrigerator until used (1–2 d). Once transferred to the screens in the greenhouse, the material was used for three consecutive waterings conducted every other day. During March and April 1999, as the weather became warmer and drier, we collected plant material twice a week and used it for two consecutive waterings. Screens were stored on separate benches between waterings.
There were two treatment replicates, thus there were a total of 20 pots per species (200 seeds) per treatment or 560 pots with 5600 seeds in all. The total number of seedlings in each pot was recorded every other week. Data were analyzed using univariate analysis of variance with a Bonferroni adjustment to control effects of multiple comparisons. Species were grouped according to community specialization and analyzed. The two community specialization categories were rosemary scrub specialists (Eryngium and Hypericum) vs. rosemary scrub and scrubby flatwood specialists (all other species). Species were also grouped according to microhabitat specialization and analyzed. The three microhabitat specialization categories were locally rare gap specialists (Eryngium, Hypericum, and Lechea), locally abundant gap specialists (Polygonella and Paronychia), and generalists (Liatris and Palofoxia).
Field distribution of Florida rosemary roots and litter
We examined the distribution of Florida rosemary roots and litter as a function of distance from Florida rosemary individuals. Sampling was conducted at five separate rosemary scrub patches at ABS, two of which were considered young in 1998 (12 or 13 yr since last fire) and two of which were long unburned (>31 yr since last fire). An additional rosemary scrub patch was sampled twice, in a young portion and in a long unburned portion. A transect was placed at each site along the longest portion of the site, and ten points were randomly chosen along this transect. For each point, we sampled the nearest rosemary shrub that was adjacent to a gap. Measurements of canopy width and shrub height were taken for each sampled shrub.
To determine distribution of Florida rosemary roots in the field, we took soil cores at two depths and three distances from the randomly selected Florida rosemary shrubs. A 785 cm3 (10 cm wide and 15 cm tall) volume soil auger was used to take soil cores directly under the canopy of Florida rosemary (0 m) and at 1 m and 2 m from the edge of a rosemary canopy. At every distance, we removed cores from two depths, 0–15 cm and 15–30 cm. Soil from the cores was immediately sieved through a number 18 wire mesh sieve to remove sand. Florida rosemary roots, which are distinctively reddish and flaky, were visually separated from other roots and placed in a bag. We returned roots to the laboratory and recorded fresh mass. Root mass data were analyzed using univariate ANOVA and independent t tests.
At four of these sites (two young and two old) rosemary litter was sampled at the same shrubs. This was done by pressing a 1.5 cm wide test tube 5–6 cm into the ground and removing rosemary litter. Samples were taken at 10-cm intervals from near the center of the shrub towards the edge of the canopy until no litter was present. Samples were sieved through a number 18 wire mesh sieve to remove sand and weighed. Distances of the extension of rosemary litter from the center of the shrub were correlated with canopy radius.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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For Florida rosemary root abundance, there was a significant effect of distance (F = 13.60, df = 1, 348, P < 0.001), site age (F = 17.46, df = 2, 348, P < 0.001), and depth (F = 6.52, df = 1, 348, P = 0.011), but none of their interactions were significant (F < 2.9, P > 0.05) indicating that the increase in root abundance spans all distances and depths in similar proportions. The interaction of distance and depth was only marginally significant (F = 2.97, df = 2, 348, P = 0.0525). Rosemary roots were less abundant at the 1-m distance (P = 0.0002) and the 2-m distance (P < 0.0001) compared to the 0-m distance (Fig. 5). There was no significant difference in root abundance between the 1- and 2-m distance (P = 0.2112) (Fig. 5). Overall roots were significantly more abundant in the 0–15 cm depth than the 15–30 cm depth (P = 0.0111) (Fig. 5). Root abundance also significantly increased with time since fire (95% confidence interval for difference between means 0.336–0.664).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Hawkes, 2000
). While we only address germination in this study, field studies suggest that allelochemicals may impact plant growth as well. For example, demography studies on Eryngium suggest that its survival is highly correlated with distance from Florida rosemary individuals (Menges and Kimmich, 1996
). Also, leachates from Florida rosemary have been shown to suppress growth of other species in laboratory settings (Fischer et al., 1994
).
Overall, litter induced stronger allelopathic patterns than the leaves. Florida rosemary litter may release stronger or more concentrated allelochemicals. These results are consistent with previous studies on rosemary leachates and grasses found in the sandhill community (Richardson and Wiilliamson, 1988
). Chemical analysis showed that a novel compound called ceratiolin is released from fresh Florida rosemary leaves by rain (Tanrisever et al., 1987
). While ceratiolin was found to be only mildly allelopathic, it degrades to hydrocinnamic acid, which was found to have much stronger allelopathic properties (Fischer et al., 1994
).
Germination of rosemary scrub and gap specialists (Eryngium, Hypericum, Lechea) was particularly reduced by Florida rosemary allelochemicals. Other allelopathy studies have found that different plant species often respond differently to allelochemicals (Richardson and Williamson, 1988
), presumably due to variation among evolutionary histories. In Florida scrub, gap specialization may have evolved as a response to competition with Florida rosemary with consequent delay or removal of the opportunity to develop resistance to its allelochemicals. Alternatively, the species limited to gaps may have a shorter history of interaction with Florida rosemary or may not have the genetic background to develop tolerance to its allelochemicals. An alternate explanation is that Florida rosemary selected for allelochemicals that deter plants that are strong competitors in early seral environments. In this community, such species would be other rosemary scrub and gap specialists. Previous studies in Florida scrub have shown that allelopathic effects are stronger in young, open sites compared to more mature sites with closed canopies (Richardson and Williamson, 1988
).
Both Eryngium and Hypericum have persistent seed banks and may additionally use Ceratiola allelochemicals as a cue to induce seed dormancy in order to avoid competition with Ceratiola. Recent studies have shown that seeds use environmental cues such as smoke and removal of a chemical to break dormancy when conditions for survival are favorable (Baldwin and Morse, 1994
; Preston and Baldwin, 1999
). This could explain why recruitment of many of these herbaceous species increases after a fire, which generally kills Florida rosemary shrubs. Unfortunately, it is not known how long rosemary allelochemicals persist in the soil after fire or how they are altered by heat. The chemicals themselves are quickly leached through the sandy soil with rain (Williamson, Richardson, and Fischer, 1992
), but buried rosemary skeletons might provide a continual prolonged source of allelochemicals. Although the results of this study do not show that Florida rosemary leachates are inducing dormancy, the possibility of this scenario still exists. A more extensive study on dormancy patterns in natural settings would need to be conducted in order to determine how Florida rosemary is affecting seed dormancy.
Florida rosemary root exudates may also impact growth and germination of other plants. Florida rosemary root abundance increases with age at varying distances from Florida rosemary while Florida rosemary litter does not in the absence of disturbance. This indicates that root competition and potentially root allelopathy would be increasing as rosemary patches age. However, the significance of allelochemicals released from leaves or litter cannot be ruled out, as, although the distance of litter from the shrub edge did not change with site age, the shrubs themselves increase in circumference, increasing in the overall amount of litter at a site and a consequent decrease in the overall size of aboveground, allelochemical-free gaps.
Because Florida rosemary roots are found in relatively high abundance in the upper soil depth (0–15 cm), they are likely to be interacting with roots of many subordinate species found in that depth, including the seven focal species of this study. While increased root competition would seem to be a likely reason for decline in herbaceous species, results from previous studies indicate that allelopathy may be important. In a field study, transplants of Hypericum, Eryngium, and Polygonella had lower reproductive output and growth rates near Florida rosemary as opposed to transplants near oaks (Quintana-Ascencio and Menges, 2000)
. Our study clearly shows that many Florida scrub species are affected by Florida rosemary allelochemicals. The fact that leaf and litter leachates suppressed germination of many species explains, at least in part, why these plants are consistently found at a distance from Florida rosemary. Rosemary scrub is the only habitat in this community where one finds large gaps of bare sand, which is the primary habitat for these species. The evolution of gap specialization explains why these plants endure the threat of Florida rosemary allelochemicals instead of moving to other gap-free habitats. While allelopathy from Florida rosemary litter and leaves is likely to be an important factor in influencing vegetation patterns seen in rosemary scrub, the roles of belowground competition and root allelopathy are still not well understood.
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FOOTNOTES |
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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0.05). N = 140 for the control, leaf, and leaf + litter treatments. N = 139 for litter treatment
