| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
3Biology Department, Virginia Tech, Blacksburg, Virginia 24061, 4CSIRO Cotton Research Unit, Locked Bag 59, Narrabri, New South Wales, Australia; and 5USDA Forest Service Southern Research Station, Coweeta Hydrologic Laboratory, Otto, North Carolina 28763
Received for publication May 11, 1998. Accepted for publication March 2, 1999.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
|---|
Key Words: allelopathy • deciduous forest • ectomycorrhizae • Ericaceae • Rhododendron • seedling inhibition • southern Appalachian mountains • subcanopy
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
|---|
40% cover. There is clear evidence that the coverage of R.m. thickets has been increasing over the past 30 yr. For example, the area occupied by R.m. thickets in the Coweeta basin has increased from 14.8% in 1976 to 31.7% in 1993 (Dobbs, 1995
). The increase in abundance of this species is a concern for forest managers because recruitment of canopy tree seedlings under R.m. thickets is substantially inhibited. Several studies over the past 50 yr document the extent of seedling inhibition and the potential for inhibition of canopy tree productivity by R.m. (Minkler, 1941
; Wahlenberg, 1950
; Barden, 1979
; Day, Phillips, and Monk, 1988
). These studies show that recruitment of most dominant canopy tree species are inhibited by R.m. However, the mechanisms by which this inhibition occurs are unknown.
The presence of R. maximum thickets in the subcanopy of Appalachian forest is important for the basic ecology of the forest and for designing forest management protocols. Rhododendon maximum is considered a "keystone species" because of its dominance in stream-side communities. Nutrient and water fluxes between the terrestrial and aquatic ecosystems are filtered and possibly impacted by this shrub (Swank and Crossley, 1988
). Furthermore, R.m. is most abundant in sites with the highest potential for forest productivity (north-facing slopes and cove positions), which increases the importance of the influence that R.m. has on these forests. Consequently, critical adjustments in forest management protocols and forestry practices may be required based on the mechanisms by which R.m. inhibits canopy tree seedling recruitment.
Inhibition of canopy tree seedlings by R.m. in the subcanopy represents a syndrome in many other forests around the world. In fact, there are subcanopy evergreen layers in many deciduous, tropical, and evergreen forests (such as dwarf bamboo species, tropical palms, and other species in the Ericaceae), and inhibition of canopy tree seedlings is a common trait of these situations (Denslow, Newell, and Ellison, 1991
; Nakashizuka, 1991
; Messier, 1993
; Fischer et al., 1994
; Mallik, 1995
; Widmer, 1998
). Therefore, knowing the mechanism by which R.m. inhibits canopy tree recruitment may have implications for similar processes in many other forest locations.
There are several potential allelopathic processes that may limit seedling recruitment under R.m. thickets, compared to that in forest without a subcanopy of R.m. Among these possibilities are inhibition of seed germination, retardation of root growth, and suppression of ectomycorrhizae by leachates from the R.m. canopy or from decomposing R.m. litter. The potential significance of allelopathy to seedling inhibition is suggested by several lines of evidence. Leaves of R.m. contain secondary compounds that have metabolic effects on humans and insects (Davidian, 1989
), and many of the molecules that confer medicinal or antiherbivory properties to plants are the same as those that cause allelopathy (Rice, 1979
). The relatively high quantity and the quality of secondary chemicals in R.m. leaves may be a reason why there is a paucity of chewing insects that attack Rhododendron (Nielson, 1980
). Also, other species of Rhododendron have been documented to have medicinal properties in China (R. dauricum, R. dabanshanense), Korea (R. chrysanthum), and Russia (R. luteum). Field research indicates that light limitation in the understory is not the only factor reducing survivorship of Acer rubrum in R.m. thickets (Clinton and Vose, 1996
). In addition, allelopathy is cited as one of the factors that maintains heath bald communities (dominated by R. catawbiense and R.m.) of the southern Appalachian mountains (Thomas and Pittillo, 1987
). Moreover, species in other genera of the Ericaceae and Empetraceae (Kalmia, Gaultheria, Empetrum, Ceratiola) have been shown to have some allelopathic potential (Messier, 1993
; Nilsson, 1994
; Fischer et al., 1994
; Mallik, 1995
). Based on these results, the potential exists for allelopathic inhibition of seedling recruitment under R.m.
The overall objective of this research was to determine whether R.m. has the potential for allelopathic inhibition of seed germination, root growth, and ectomycorrhizal growth. We evaluate mycorrhizal growth because in previous research we have shown that seedlings growing under R.m. thickets have reduced incidence of mycorrhizal colonization compared to those in forest without R.m. (Walker et al., 1999
). The experiments were designed to answer five questions. (1) Does rain that passes through the canopy (throughfall) of forest sites with a subcanopy of R.m. (+R.m. sites) inhibit seed germination more than that from similar forest sites without a subcanopy of R.m. (-R.m. sites)? (2) Do leachates of forest floor substrate from +R.m. sites inhibit seed germination more than that from -R.m. sites? (3) Will root growth be retarded for seeds germinating in the presence of leachates of forest floor substrates from +R.m. sites compared with that from -R.m. sites? (4) Does litter from R.m. inhibit the growth of fungi (known to form ectomycorrhizae with canopy tree species) more than litter from other canopy trees? (5) Does the +R.m. forest floor substrate inhibit native tree seed germination and seedling survival in situ more than that in -R.m. sites?
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
|---|
20 – 25 m high. Soils were deep and well-drained coarse-loam of the Edneyville series (Thomas, 1996
). The mineral soil is covered with a 5 cm thick layer of decomposing litter (organic layer) and a similar thickness of intact leaf litter (litter layer). The regional climate is classified as marine, humid with cool summers, mild winters, and with adequate rainfall (1800 mm annually) during all seasons (Swank and Crossley, 1988
).
Leachate sampling design
Throughfall was collected with standard wet-dry collectors throughout the year. Following storm events during each month of the year, the throughfall was collected and immediately frozen (-25°C) until used in bioassays. Forest substrate was collected at five randomly selected locations in both +R.m. and -R.m. sites. Collections were made in the late spring (the time of most seed germination and seedling establishment). Forest floor substrate was separated into litter and partially decomposed organic layer and pooled by forest type (+R.m. or -R.m.). Leachates used in bioassays were made by mixing specified masses of fresh litter or organic layer with distilled water in zip-loc bags. The standard ratio of litter or organic layer to water was 1:5, and controls were distilled water alone in the zip-locs. The mixtures were incubated at 25°C for 24 h in a growth chamber and periodically shaken, before filtering through Whatman number 3 filter paper. Leachates were kept in the refrigerator until used within 5 d. The pH and osmolality of leachate solutions were measured before all experiments with a pH meter (Accumet pH meter Model 610, Fisher, Pittsburg, Pennsylvania) and a vapor pressure osmometer (Vapor pressure osmometer Model 5520, Wescor, Logan, Utah), respectively.
Bioassay of solution toxicity
Two bioassay species, lettuce (Lactuca sativa L. var. "Black Seeded Simpson" lot number 371, Wyatt-Quarles Seed Co., Garner, North Carolina) and cress (Lapidum sativum L. var. "Upland" lot no l278, Wyatt-Quarles Seed Co., Garner, North Carolina) were used to assay potential inhibition of seed germination and radical elongation by leachates and throughfall. These species were selected because they germinate rapidly and have been used in many bioassay experiments in the past (Rice, 1979
). In each bioassay experiment, two layers of Whatman number 1 filter paper were inserted in a sterile petri dish. The filter paper was moistened with 3 mL of leachate or throughfall solution at room temperature. One lot number of lettuce and cress seeds were used for all experiments. In seed germination tests, 30 lettuce or cress seed were randomly placed in the petri dishes, sealed with Parafilm, and placed in a growth chamber at 25°C, 60% relative humidity (RH), and a 12/12 h day/night (photosynthetic photon flux density = 90 (µmol photons·m-2·s-1). Germination percentage was counted in all petri dishes daily at noon for 7 d (no more germination occurred after this time). The experiment was replicated five times.
Root elongation was studied by placing 20 seeds in two rows down the middle of the petri dish at five replicates per treatment. Each seed was oriented so that the roots grew out toward the edge of the plate. To permit repeated census at the same time each day an image was made of each bioassay plate with the use of an image analysis program on a MAC platform (NIH IMAGE). Length of root for each seedling was measured using a calibrated scale placed in each petri dish. Mean root length was determined for each plate on each sampling date.
Native seed germination experiments
Individual lots of seed for seven species native to the research site were obtained from seed suppliers: eastern hemlock – Tsuga canadensis (L.) Carr, northern red oak –Quercus rubra L., and red maple –Acer rubrum L. seeds from Sheffield's Seed Co. Inc., Locke, New York; yellow poplar –Liriodendron tulipifera L. from F. W. Schumacher Co. Inc., Sandwich, Massachusetts; white pine –Pinus strobus L., black tupelo –Nyssa sylvatica Marshall, black birch –Betula lenta L. from Herbst Tree Seed, Inc., Fairview, North Carolina. Initial germination trials indicated that germination rates of three species (Liriodendron tulipifera, Acer rubrum, and Betula lenta) were too low (5-10%) to be useful in these experiments. Therefore, only four species, Quercus rubra, Pinus strobus, Nyssa sylvatica, and Tsuga canadensis, were analyzed at the end of the experiment. In each zip-loc bag, 50 seeds of each species were placed in 250 g of substrate brought to field capacity with distilled water. Three substrate types were used: forest floor substrate (litter and organic layer) from +R.m. sites, forest floor substrate from -R.m. sites, and vermiculite. The experiment was replicated three times for each species. Seed were imbibed for 3 mo at 3°C and then transferred into a growth chamber at 25°C, 60% RH, and a 12/12 h day/night. Germinating seed were counted each week for the next 5 mo. Moisture lost from bags was replaced as needed on a weekly basis.
Field substrate transplant experiments
A total of six 0.25-ha main plots were randomly located within (N = 3) and outside (N = 3) of the R.m. thicket. In each main plot, 15 2 x 2 m subplots were established resulting in a total of 90 plots, 45 within the R.m. thickets (+R.m.) and 45 in forest free of R.m. (-R.m.). The 15 subplots were randomly assigned to five substrate manipulation treatments: four combinations of -R.m. and +R.m. litter, and -R.m. and +R.m. organic layer, plus a control with no disturbance to the substrate (Fig. 1). In the fall of 1995, litter and the organic layer were scraped separately from the subplots, bulked, and equally redistributed back to appropriate assigned subplots. Each of the five treatment types was represented by three replicate subplots per main plot. The aim of the substrate manipulation was to evaluate the possible inhibitory effects of the R. maximum litter and organic layers. The experimental plots were allowed to settle over the winter before seeds were planted in the following spring.
|
Bioassay of litter toxicity to ectomycorrhizae
Decoction experiments were conducted using solid Hagem's media modified by Cripps and Miller (1995)
with and without the addition of 16 g/L of chopped leaf materials. All leaf litter was collected during July and stored frozen until used. The leaves were cut into fine pieces and incorporated into the media, which was then autoclaved for 18 min at 250°C. The media was poured on petri plates while warm with constant agitation to evenly distribute the leaf materials among the plates. After cooling, the decoction media plates and controls were inoculated with the mycelium of ectomycorrhizal fungi prepared as follows.
Two ectomycorrhizal bioassay experiments were performed. In the first experiment, one litter type (R.m.) and three ectomycorrhizae species, Cenococcum geophilum Fr. (VTCC 1409), Pisolithus tinctorius Pers. (VTCC 3303), and Suillus pictus (Pk.) Smith and Theirs (VTCC 3326), were used. Voucher cultures for all three strains of the fungal species are being maintained in the Virginia Tech Culture Collection. In the second experiment, five litter types (Rhododendron maximum, Tsuga canadensis, Quercus rubra, Betula lenta, and Prunus serotina) and two mycorrhizae species (Cenococcum geophilum and Pisolithus tinctorius) were used. For both experiments, the fungi were initially grown on solid Hagem's media as modified by Cripps and Miller (1995)
, for 1 mo at 20°C. Uniform squares of mycelium and agar were cut from these plates, near the actively growing margin of the fungal mycelium, and used to center inoculate the bioassay plates. Immediately after inoculation, two perpendicular lines were drawn on the lid of the plate. Measurements of radial growth of the fungi were taken along the predetermined lines from the edge of the inoculum block. In the first experiment, measurements were taken over a period from 28 to 37 d at 1-wk intervals. The second experiment was only measured once, 6 wk after inoculation.
Differences between mean radial growth on control plates vs. decoction media plates were analyzed using Students t tests for each fungal strain on each measurement date for the first experiment. Analysis of variance was used for the second experiment to examine the main effects of inoculum species and the leaf material added (SAS, 1998). Ten replicates were used per treatment in the first experiment and seven per treatment for the second experiment.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
|---|
|
85% germination when exposed to leachate made with the organic layer from either forest type (Fig. 3C). However, cress seed imbibed in leachate made with -R.m. litter reached only 68% and that imbibed with +R.m. leachate attained 52% germination (Fig. 3B).
|
|
|
Root lengths for lettuce seedlings were longer when tested with leachates of litter from -R.m. sites in comparison to that from +R.m. sites (Fig. 4B). It is important to note that when leachates of litter from +R.m. sites were used as a bioassay solution, the resultant root elongation rate was not significantly different from that of distilled water. These initial root length experiments showed that root elongation was stimulated compared with the distilled water control by most leachates and that stimulation increased from throughfall, to litter, to organic layer.
Measurement of solution pH indicated that the distilled water control had a pH close to neutrality (Table 2). The pH of the +R.m. site organic layer was below 5, while that of the -R.m. site organic layer was slightly higher. The pH of the R.m. litter was the highest of all leachates (6). Osmotic potential of all solutions was <10 mmol/kg (Table 2) with no discernible differences among leachates. Neither pH nor osmotic potential was a likely factor associated with reduced seed germination and root elongation in the +R.m. litter leachates because there was very little variation in these factors among all treatments.
|
|
|
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
|---|
; Putnam and Tang, 1986
; Fischer et al., 1994
; Nilsson, 1994
; Mallik, 1995
) including Rhododendron (Yang and Wang, 1978
; Nielson, 1980
; Pittillo, 1980
; Wang and Yang, 1981
). Even though several Rhododendron species can have negative influences on other species, there have been no studies of the allelopathic potential of these taxa native to North America and Europe. This is surprising because several members of the genus can become invasive or suppressive in these regions (Fuller and Boorman, 1977
; Monk, McGinty and Day, 1985
). To infer that Rhododendron maximum exerts dominance over competing species through allelopathy, we must first demonstrate its potential toxicity. Did throughfall from +R.m. sites inhibit seed germination more than that from -R.m. sites? No, throughfall from neither forest type inhibited germination of any species at any time of the year. Compared to the distilled water control, throughfall from both forest types actually stimulated seed germination. Stimulation of germination could have been the result of nutrients in the throughfall or the slightly acidic pH compared to the control.
Did the forest floor substrate (litter or organic layer) from +R.m. sites inhibit seed germination more than that from -R.m. sites? We found contrasting results. Seed germination of cress was not inhibited by leachates of the organic layer from either forest type when compared to the distilled water control. In contrast, seed germination of cress was stimulated to a lesser degree by leachates from fresh litter compared to that from organic layer or throughfall. Furthermore, the only difference in germination percentage of cress between leachates of the two forest types was attributable to litter. Leachates of litter from +R.m. sites may have been more toxic than that from -R.m. sites because other potential effects on germination, such as pH or osmolality, were not different between litter leachates. Alternatively, the nutritional quality of leachate from litter in +R.m. sites could have been deficient enough to limit germination (R.m. leaves have relatively low nutrient content; Monk, McGinty, and Day, 1985
; personal observation).
To determine whether results from cress seed (which germinates in 7 d) were relevant to natural conditions, similar experiments were performed with seed of native tree species. The lack of significant difference between +R.m. and -R.m. sites in this experiment indicates that the forest floor substrate from locations with a R.m. thicket does not inhibit germination of native seeds any more than that from locations without a R.m. thicket. These results were supported by germination trials with northern red oak in field plots. However, the two deciduous species (northern red oak, black gum) had higher germination in either forest substrate treatment than in the vermiculite control. In comparison, both evergreen species (white pine, eastern hemlock) had lower germination in either substrate treatment than in the vermiculite. This difference was unrelated to forest substrate type and thus must be related to seed germination requirements and the differential structure or chemistry of substrate vs. vermiculite.
Was root length shorter in the presence of forest floor leachates from +R.m. sites compared with that from -R.m. sites? The answer to this question was different depending on which substrate was used to make the leachate. Root growth of lettuce seedlings was enhanced compared to the distilled water control in all treatments except litter leachate from +R.m. sites. No differences in root length of lettuce seeds were found in -R.m. sites for throughfall or organic layer leachates. However, litter leachates from +R.m. sites resulted in a significantly reduced root length compared to that from -R.m. sites.
Does leaf material from R.m. inhibit ectomycorrhizal growth? Yes, in one of the ectomycorrhizal species studied, Pisolithus tinctorius, growth was inhibited by R.m. leaf material. However, inhibition was less than that from leaves of other canopy tree species. Therefore, it is not likely that leaf litter on the forest floor under a thicket of R.m. will be any more inhibitory to ectomycorrhizal growth than leaf litter from forest without R.m..
Our experiments with native seedling survivorship in situ also showed that there was no effect of substrate type in either forest type. If substrate from +R.m. sites was toxic by itself, then there should have been a clear treatment effect on seedling survival in both forest types. Furthermore, if toxicity were due to the combination of substrate and long-term throughfall effects, there should have been a treatment effect in +R.m. sites, but we did not detect any significant interaction of treatment with forest type.
It is important to note that seedling growth (not germination) in the field was significantly lower for +R.m. plots compared with -Rm. plots (data not presented). If this difference was due to allelopathy it could not be the result of litter, organic matter, or short term throughfall components of the system. The reduced growth rate of seedlings growing in +R.m. sites is most likely due to reduced resource (light) availability. However, a long-term continual influx of toxin from root exudates and throughfall may be an additional factor reducing seedling growth rate.
Although our study did not discern a significant allelopathic influence in the field, there are examples in other forest situations where allelopathy may be an important process for shrub inhibition of canopy tree seedlings. For example, there is strong evidence that allelopathy is an important mechanism by which black spruce seedlings are inhibited by Kalmia angustifolia in Newfoundland (Mallik, 1995
). Moreover, toxicity of Kalmia angustifolia may be manifest through an inhibition of ectomycorrhizae (Yamanashi et al., 1998
). More studies on the allelopathic potential of subcanopy evergreen shrubs are needed before a generalized model can be developed.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
|---|
Data collected in this study clearly indicate that for those species tested, direct allelopathic influences of R.m. on seed germination, initial root growth, or ectomycorrhizal growth cannot be considered an important factor associated with the inhibition of seedling survival under R.m. Our results with ectomycorrhizae do not apply to species that are colonized by vesicular-arbuscular (VA) mycorrhizae such as Prunus serotina or Liriodendron tulipifera. Further studies on toxicity to VA mycorrhizae is important to completely rule out allelopathy as a significant factor determining low seedling recruitment under R.m. thickets. Our data also do not rule out indirect allelopathic effects. For example, litter leachates and throughfall may not be inhibitory to the growth of mycorrhizal mycelia, but may be inhibitory to mycorrhizal colonization of tree seedlings under R.m. thickets. Since a majority of native species require mycorrhizal synthesis for survival in the subcanopy, allelopathic inhibition of this colonization could reduce seedling survival. Also, the release of allelochemicals in R.m. litter may inhibit the normal action of bacteria and invertebrates resulting in reduced soil nutrient availability. Lower soil nutrient availability is a characteristic of forests where R.m. is present and may be an important process for seedling survival. In fact, the combination of resource limitation (nutrients and light) and inhibition of mycorrhizal synthesis may be the predominant process regulating canopy tree seedling survival in the southern Appalachian mountains.
|
FOOTNOTES |
|---|
2
Author for correspondence: (phone: 540 231-5674; FAX: 540231-9307; enilsen{at}vt.edu
).
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
|---|
Clinton, B. D., and J. M. Vose. 1996 Effects of Rhododendron maximum L. on Acer rubrum L. seedling establishment. Castanea 61: 38–45.
Cripps, C. L., and O. K. Miller. 1995 Ectomycorrhizae formed in vitro by quaking aspen: including Inosybe lacera and Aminita pantherina. Mycorrhiza 5: 357–370.
Day, F. P., D. L. Phillips, and C. D. Monk. 1988 Forest communities and patterns. In W. T. Swank and D. A. Crossley, Jr. [eds.], Forest hydrology and ecology at Coweeta, 141–149. Springer-Verlag, New York, NY.
Davidian, H. H. 1989 The Rhododendron species, vol. II, Elepidotes. Timber Press, Portland, OR.
Denslow, J. S., E. Newell, and A. M. Ellison. 1991 The effect of understory palms and cyclanths on the growth and survival on Inga seedlings. Biotropica 23: 225–234.[CrossRef][ISI]
Dobbs, M. M. 1995 Spatial and temporal distribution of the evergreen understory in the southern Appalachians. Master's thesis, University of Georgia, Athens, GA.
Fischer, N. H., G. B. Williamson, J. D. Weidenhamer, and D. R. Richardson. 1994 In search of allelopathy in the florida scrub: the role of terpenoids. Journal of Chemical Ecology 20: 1355–1380.[CrossRef][ISI]
Fuller, R. M., and L. A. Boorman. 1977 The spread and development of Rhododendron ponticum L. on the dunes at Winterton, Norfolk, in comparison with invasion by Hippophae rhamnoider at Salt Fleetby, Lincolnshire. Biological Conservation 12: 82–94.
Mallik, A. U. 1995 Conversion of temperate forests into heaths–role of ecosystem disturbance and ericaceous plants. Environmental Management 19: 675–684.[CrossRef]
Messier, C. 1993 Factors limiting early growth of western redcedar, western hemlock and sitka spruce seedlings on ericaceous-dominated clearcut sites in coastal British Columbia. Forest Ecology and Management 60: 181–206.[CrossRef][ISI]
Minkler, L. 1941 Planting white pine in laurel and Rhododendron slicks. Journal of Forestry 39: 1036.
Monk C. D., D. T. McGinty, and F. P. Day, Jr. 1985 The ecological importance of Kalmia latifolia and Rhododendron maximum in the deciduous forest of the southern Appalachians. Bulletin of the Torrey Botanical Club 112: 187–193.[CrossRef][ISI]
Nakashizuka, T. 1991 Population dynamics of coniferous and broad-leaved trees in a Japanese temperate mixed forest. Journal of Vegetation Science 2: 413–418.[CrossRef][ISI]
Nielson, D. G. 1980 Strategy for minimizing insect damage to Rhododendron. In J. L. Lutegen and M. E. O'Brien [eds.], Contributions toward a classification of Rhododendron, 305–318. Allen Press, Lawrence, KS.
Nilsson, M.-C. 1994 Separation of allelopathy and resource competition by the boreal dwarf shrub Empetrum hermaphroditum Hagerup. Oecologia 98: 1–7.[CrossRef][ISI]
Pittillo, J. D. 1980 Status and dynamics of balds in the southern Appalachian mountains. In P. R. Sanders [ed.], Status and management of Southern Appalachian Mountain balds, 39–51. SARRMC, Western Carolina University, Cullowhee, NC.
Putnam, A. R., and C.-S. Tang. 1986 The science of allelopathy. John Wiley&Sons. New York, NY.
Rice, E. L. 1979 Allelopathy, an update. Botanical Review 45: 15–109.
SAS. 1998 SAS/STAT user's guide, release 6.03. SAS Institute Inc., Cary, NC.
Swank W. T., and D. A. Crossley, Jr. [eds.]. 1988 Forest hydrology and ecology at Coweeta. Springer-Verlag, New York, NY.
Thomas, D. J. 1996 Soil survey of Macon county, North Carolina. U.S. Department of Agriculture, Washington, DC.
Thomas R. B., and J. D. Pittillo. 1987 Invasion of Fagus grandifolia Ehrh. into a Rhododendron catawbiense Michx. heath bald at Craggy Gardens, North Carolina. Castanea 52: 157–165.[ISI]
Wang, S.-X., and H.-R., Yang. 1981 Chemical investigation of Rhododendron habanshanense II. The isolation and identification of (+)-catechin, hyperin, and toxicant components. Acta Botanica Sinica 23: 47–51.
Wahlenberg, W. G. 1950 From bush to pine. Southern Lumberman 180: 40–41.
Walker, J. R., T. Lei, S. Semones, E. Nilsen, B. D. Clinton, and O. K. Miller. 1999 Suppression of ectomycorrhizae on canopy tree seedlings in Rhododendron maximum L. (Ericaceae) thickets in the southern Appalachians. Mycorrhiza 9: 49–56.
Widmer, Y. 1998 Pattern and performance of understory Bamboos (Chusquea spp.) under different canopy closures in old-growth oak forests in Costa Rica. Biotropica 30: 400–415.[CrossRef][ISI]
Yamanashi, S. H., J. W. Fyles, K. N. Egger, and B. D. Titus. 1998 The effect of Kalmia angustifolia on the growth, nutrition, and ectomycorrhizal symbiont community of black spruce. Forest Ecology and Management 105:197–207.
Yang, H.-R., and S.-X. Wang. 1978 Chemical studies of Rhododendron habanshanense I. The isolation and identification of four phenolic components. Acta Botanica Sinica 20: 355–359.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
