Distinct barrens and forest vegetation types grow in close proximity on the Pocono Plateau of northeastern Pennsylvania.  This study investigated whether organic chemicals produced by ericaceous shrubs in the barrens inhibit the growth of forest tree species.  Germination of Betula populifolia (Gray birch, Marshall, Betulaceae) and Tsuga canadensis, (Eastern hemlock, L., Pinaceae) were not inhibited by aqueous extracts of barrens leaf litter or organic soil.  Activated carbon applied to tree seedlings planted in the barrens and forest reduced mortality of Acer rubrum (Red maple, L., Aceraceae), Prunus serotina (Black cherry, Ehrh., Rosaceae), and Betula lenta (Sweet birch, L., Betulaceae) in the barrens but not in the forest.  It increased the growth of Acer and Prunus seedlings in the barrens, but did not increase their growth in the forest.  Organic chemicals that were absorbed by activated carbon in this experiment may contribute to the persistence of distinct forest and barrens vegetation types.  These chemicals may reduce nutrient availability in the barrens, contributing to the continued dominance of ericaceous shrubs that are adapted to low nutrient conditions.

        One of the central questions of ecology is why particular combinations of species exist in particular places.  Usually the plant species present in a place are attributed to geography and successional stage.  However, in some cases, neither succession nor abiotic factors can explain why particular plant species are prevalent in a place.  Some conditions may allow for "alternative stable states": multiple community types persisting on sites with similar abiotic factors (Petraitis and Latham, 1999).  This may be the case on the Pocono Plateau, where distinct forest and heath vegetation on seemingly equivalent areas has puzzled ecologists.  This paper is an attempt to understand the biotic processes that maintain the existence of distinct vegetation types in the Poconos.

The Pocono Plateau

        The Pocono Plateau of northeastern Pennsylvania is host to beech-northern hardwoods forest, hemlock forest, and mixed deciduous forest.  It also contains three types of barrens areas, each characterized by predominant shrub species: shrub oak barrens, rhodora barrens, and heath barrens (Latham et al., 1996).  Both barrens and forest areas are primarily underlain by glacial till and have rich, fine-loamy soil.  Barrens occur primarily on areas of Illinoian glacial till.  However, the barrens vegetation type is present on only 23% of the area covered by Illinoian till on the Pocono Plateau, and barrens also exist on soil underlain by bedrock and Wisconsonan glacial till.  The location of barrens is not a result of underlying soil type (Latham et al., 1996).  In a period covering a range of weather conditions including severe drought, there were no significant differences in soil water content between the barrens and forest areas.  Water table depth does not differ significantly between barrens and forest areas, although in some cases there are significant differences between more specific barrens and forest vegetation types (Eberhardt and Latham, 2000). Thus, water availability also does not seem to be responsible for the presence of barrens.  The coexistence of forest and barrens is also intriguing because of the very sharp divisions that often exist between the two vegetation types.  Latham et al. (1996) speculated that the dominant plant species of forests and barrens may sustain their communities by influencing soil properties.  Certain plants may alter their environments in ways that inhibit the growth of species from the other community type.  This could be an explanation of the coexistence of barrens and forest on similar underlying soils and of the sharp divisions between these communities.

Allelopathy and the Ericaceae

        Plants alter their environment in several ways that can promote the persistence of their own species.  Plants that are fire-tolerant may generate highly flammable compounds that promote more frequent fires, harming less fire-tolerant species.  Plants may invest in superior regeneration strategies, which gives them a superior ability to colonize after disturbance.  If one species gains dominance over an area, it may maintain its dominance by reducing the availability of a resource it can effectively monopolize or sequester, such as nutrients, water, or light.  Plants can also produce chemicals that modify soils in ways that directly or indirectly inhibit the growth of other plant species.  This process is known as allelopathy (Inderjit and Weiner, 2001).
        This research focuses on the Pocono heath barrens, which are dominated by two plant species from the family Ericaceae: Kalmia angustifolia (Sheep-laurel, L.) (mean 31% cover), and Vaccinium angustifolium (Lowbush-blueberry, Aiton)(mean 32% cover) (Latham et al., 1996).  Ericaceous plants (including the subfamily Empetraceae) (Zomlefer, 1994) have several characteristics that could allow them to modify their environments in ways that inhibit other species.  They have highly flammable, sclerotized leaves and vegetative regeneration strategies (Mallik, 1993).   Ericaceous shrubs have been shown to inhibit the growth of tree species in the forests of Sweden (Nilsson and Zackrisson, 1992), France (Gallet, 1994; Jäderlund et al., 1996), and Newfoundland (Zhu and Mallik, 1994).  The conversion of forests to ericaceous-dominated heaths in western Europe and  coastal British Columbia has also been blamed on allelopathy (Mallik, 1995).  Ericaceaous plants are generally dominant in very particular soil conditions, which include low pH, high level of organic matter, and low level of available nutrients (Read, 1983).  They produce large quantities of phenolic compounds in their leaves (Read, 1983).  Phenolics are often defense compounds against herbivores or other plant species that act by binding to lipids and amino acids or creating oxygen radicals (Appel, 1993).  The production of phenolic compounds that inhibit the growth of tree species may be a shared characteristic of ericaceous shrubs that contributes to their widespread ability to thrive in nutrient-poor and acidic soils.
        The allelopathic compounds produced by ericaceous shrubs inhibit trees in different ways.  Allelochemicals have been found to directly inhibit seed germination or growth in several cases.  Water extracts of senescent leaves of Empetrum hermaphroditum (Bilberry, Hagerup), an ericaceous shrub in Swedish forests, inhibit the growth of Pinus sylvestris (Scots pine, L., Pinaceae) seedlings.  Oden et al. (1992) isolated the phenolic compound batatasin-III (3, 3í dihydroxy-5-methoxydihydrostilbene) as the active allelopathic compound.  Phenolic compounds from the leaves of Vaccinium myrtillus (Bilberry, L., Ericaceae), and from humus surrounding this plant, inhibit the germination and seed growth of several tree species in Swedish and French forests (Gallet, 1994; Pellissier et al., 1994; Jaderlund et al., 1996).
        Allelopathic chemicals produced by ericaceous shrubs have also been found to inhibit tree fungal symbionts, or mycorrhizae.  Phenolic compounds in concentrations found in humus from Vaccinium- dominated areas inhibit the Picea abies (Norway spruce, L., Pinaceae) fungal symbionts Cenococcum graniforme and Laccaria laccata (Boufalis and Pellissier, 1994).  Souto et al. (2000b) found that such humus extracts inhibit the growth of the Picea symbiont Hebeloma crustuliniforme but not of the Vaccinium symbiont Hymenoscyphus ericae.
 Many ecologists have noted that the mere existence of inhibitory compounds does not prove that these compounds have significant effects on communities in the field.  There is evidence that E. hermaphroditum phenolic compounds have an ecologically significant effect from the work of Nilsson (1994).  Nilssonís study attempted to isolate the influences of competition and allelopathy in the field and concluded that allelopathy had significant effects in both the presence and absence of competition.

Allelopathy and Kalmia angustifolia

        Although allelopathy has not yet been studied in the Pocono barrens, a dominant shrub in heath barrens, Kalmia angustifolia, has been studied as a potential allelopathic plant in the forests of Newfoundland, Canada.  Aqueous extracts of Kalmia leaves inhibit the growth of Picea mariana (Black spruce, Miller, Pinaceae) germinants (Zhu and Mallik, 1994).  Growth of P. mariana seedlings was also inhibited by adding Kalmia litter to humus (Inderjit and Mallik, 1996).  P. mariana growing in areas without Kalmia are taller and thicker than those growing in Kalmia cover (Mallik, 2001).
        Allelopathic compounds in Kalmia leaves may affect Picea mariana seedlings by reducing the nutritive quality of the soil.  Forest soil where Kalmia grows has less organic matter, organic nitrogen, Fe, Mn, and phosphate than soil from areas without Kalmia.  The areas where Kalmia is present also has a higher pH and carbon/nitrogen ratio (Inderjit and Mallik, 1999).  Addition of Kalmia litter to humus caused a linear decrease in its mineral nitrogen content (Inderjit and Mallik, 1996).  There is evidence that Kalmia litter causes nutrient deficiencies in P. mariana.  Yamasaki et al. (1998) found that Picea  seedlings within 1m of Kalmia have lower levels of nitrogen and phosphorus in their leaves than seedlings further from Kalmia plants.
        Decreased nitrogen availability in the presence of Kalmia may be due to polyphenolic compounds that complex organic nitrogen, making it unavailable to many plants and soil microbes.  Many plant-produced polyphenolics can bind to proteins and amino acids (Appel, 1993), and polyphenolics in plant litter can decrease the release of mineral nitrogen during decomposition (Northup et al., 1995).  Polyphenolic tannins from Kalmia leaves reduce both nitrate and ammonium leaching from soil, while Abies balsamea (Balsam fir, Pinaceae) needles reduce ammonium leaching only.  This indicates that Kalmia tannins reduce conversion of organic nitrogen to mineral nitrogen, probably by forming complexes with organic nitrogen in the soil (Bradley et al., 2000).
        Kalmia is able to reduce mineral nitrogen availability without inhibiting its own growth because it can utilize varied nitrogen sources more effectively than other species.  While both Kalmia and Betula papyrifera (Paper birch, Marsh., Betulaceae) can acquire nitrogen from Betula-dominated soils, only Kalmia can acquire nitrogen from Kalmia-dominated soils (Bradley et al., 1997b).  Betula- dominated soil planted with Kalmia had higher levels of mineral N than such soil planted with Betula or Picea mariana.  This probably indicates that Kalmia utilizes primarily organic nitrogen.  Ericoid mycorrhizae can utilize organic nitrogen much more than other types of fungal symbionts (Read, 1983), so it is not surprising that Kalmia may require less mineral nitrogen than most plants.
         Allelopathy seems to be a widespread phenomenon in the Ericaceae in general, and a cause of inhibition of Picea mariana by Kalmia in particular.  However, the interactions between ericaceous shrubs, forest trees, and soil biota can be very complex.  The inhibitory effects of allelochemicals may vary on different tree species and may depend on soil type, pH, or nutrient status.  The fact that Kalmia inhibits  Picea mariana in Newfoundland does not necessarily mean that it inhibits forest trees in the Poconos.

Hypothesis and Experimental Design

        I tested the hypothesis that organic compounds produced by Kalmia leaves or roots inhibit the growth or germination of forest tree species in the Pocono heath barrens.  One component of the study tested whether growth of three common forest species was inhibited by Kalmia allelochemicals in the field.  Activated carbon was used to alleviate the effects of allelochemicals on tree seedlings planted in the forest and in the barrens.  Activated carbon has been shown to remove from soil the inhibitory compounds produced by E. hermaphroditum (Zackrisson and Nilsson, 1992; Nilsson and Zackrisson, 1992).  It is thought to mimic the effects of naturally produced charcoal, which has been shown to absorb phenolics from soil solutions as determined by germination bioassays (Zackrisson et al., 1996).  Because of its nonpolar surface, activated carbon easily adsorbs organic compounds from solution, even at low concentrations (Cheremisinoff and Cheremisinoff, 1993) but does not as readily adsorb inorganic ions, which comprise most plant nutrients (Cheremisinoff and Morresi, 1978; Radovic et al., 2001).
        The research design of the field experiment allowed a comparison of the effects of organic compounds on growth in the two areas.  I did not attempt to determine the mechanism of allelopathic inhibition in the field, but focused on the existence of inhibition in an ecologically relevant situation.  A second component of the study tested the effects on tree seed germination of solutions made from litter and organic soil from the forest and barrens.  This experiment investigated the existence of allelopathic effects on germination, which was not tested in the field experiment.

Field Experiment

        A total of sixteen 1m x 2m plots were randomly located in both barrens and forest areas at sites in Long Pond, PA and Albrightsville, PA (Figure 2).  The two sites were about 5 km. apart.  The plots within the forest or barrens of each site were not more than 15 meters from one another.  Each of the plots was cleared of aboveground biomass.  On 15 and 16 June 2001, each plot was planted with eighteen bare-root seedlings of a mixture of three species: Betula lenta, Acer rubrum, and Prunus serotina.  All of these species are present in Pocono Plateau forests, and Acer rubrum has invaded some areas of heath barrens (Latham, pers. comm.)  The seedlings were 1-2 years old and were obtained from Pikes Peak Nursery (Indiana, PA).  The trees on one-half of each plot were treated with 2L dechlorinated water (designated as the control treatment), which was poured around the bases of the nine plants.  The trees on the other half of each plot were similarly treated with 2L of 125g/L powdered activated carbon (100-400 mesh; Sigma) (designated as the carbon treatment).  The experiment thus consisted of four treatment conditions: forest control, barrens control, forest carbon, and barrens carbon.  The carbon soaked several inches into the soil.  Carbon applied in this way can persist in the soil for many years (Nilsson, pers. comm.).
        After two weeks, the plots were covered with plastic netting to protect them from herbivory.  On 27 and 28 July 2001, plant mortality was surveyed, and it was found that mortality was particularly high in barrens areas.  The soil near the surface in these areas was observed to be very dry in comparison with the forest areas.  Average daily solar irradiation on the barrens and forest plots was measured using a Solar Pathfinder (Pleasantville, TN), and it was determined that the barrens plots received approximately five times the energy from direct sunlight as the forest plots.  In order to prevent further mortality in the barrens due to excessive heat and dryness, burlap screening was put over the top and two most south-facing sides of the plots in the barrens on 5 August 2001.  On 8 September 2001, the trees were pulled out of the ground and frozen.  Their stems were dissected into leaves, stem growth from this season, live older stems, and dead tissue.  Growth from this season was determined to be any stems without secondary growth, and live tissue was determined to be any stem section that appeared to have functioning xylem.  The shoot sections were dried at 600 C for several days and weighed.
         The percent mortality of seedlings under the four experimental conditions were analyzed with a c2 test, using Yateís Correction Factor where applicable (Zar, 1999).  The ratios of new stem growth to total stem live biomass of the seedlings were analyzed using analysis of variance (ANOVA) of the effects of site, community, treatment, and plot (nested in site and community).  The community x site and community x treatment interaction effects were also determined.  ANOVA was used despite the fact that the low sample size within a plot (at most four for any particular species) did not allow a test for normality, as nonparametric tests that calculate interaction terms are not available.  ANOVA was performed with JMP software (SAS Institute, Inc.).

Germination Experiment

         The laboratory experiment was designed to determine how water-soluble chemicals from the soil and litter of forest and barrens areas affect germination rates of forest tree seeds.  Leaf litter and soil from the organic layer was collected from two places in forest area and two places from the barrens at the Long Pond site.  Samples of the soil were weighed, dried at 600 C, and weighed again to determine the ratio of dry weight to wet weight for each of the four soil types (forest litter, forest organic soil, barrens litter, and barrens organic soil.)  The equivalents of 20g/L of the dry litter and 60g/L of the dry organic soils were soaked in deionized water for twenty-four hours at room temperature and then filtered through Whatman #1 filter paper (modified from Nilsen et al., 1999) The osmotic potential of the extracts, measured using a vapor-pressure osmometer, was found to be between 20 and 23 mmol/kg for all extracts.  This is much too low to have any inhibitory effect on germination from solute concentration alone (Jäderlund et al., 1996).  The extracts were frozen until thawed for use in germination assays.
         Germination assays were performed on seeds of Betula populifolia and Tsuga canadensis.  Tsuga is common in Pocono Plateau forests, while Betula is common in disturbed forest areas and is sometimes sparsely present in the barrens (Latham, pers. comm.).  Seeds were obtained from Sheffieldís Seed Co., Lock, NY.  Tsuga seeds were soaked for 24 hours and then placed on disks of Whatman #3 filter paper inside petri dishes (50 seeds/dish).  The seeds were treated with either 4.5 mL of one of the four soil extracts or with 4.5 mL of deionized water.  They were then covered with a piece of Whatman #3 filter paper, and 5.0 mL of extract or water was pipetted over the paper (modified from Gallet, 1994).  The paper was misted to saturation with additional deionized water.  The petri dishes were covered and stratified in a refrigerator for one month at approximately 3 0 C before being placed in the greenhouse.  Deno (1993) found that Tsuga canadensis germination rates are increased by a stratification period.
         After one month, the petri dishes were placed in trays lined with wet paper towels, and the trays were placed in clear frosted plastic bags open at one end.  The trays were placed in a greenhouse.  The tray linings were kept moist, and the seeds were misted periodically with deionized water.
         This procedure was repeated with Betula populifolia seeds, with the exceptions that Whatman #1 filter paper was used as a covering, and only 4.0 mL of treatment solution was applied over the paper.  These modifications were due to the smaller seed size of Betula.  These seeds were also not stratified before being placed in the greenhouse.
         Germination was periodically assayed for all species until most of the seeds had germinated or additional germination ceased (53 days for Tsuga; 17 days for Betula).
         The germination data from the laboratory experiments were analyzed with the Kruskal-Wallis H test using Statview statistical software.

Soil Analysis

         Samples of organic soil were collected from two places in the forest and two places in the barrens at the Long Pond site.  After collection, the samples from within a community type were mixed, and all samples were frozen.  Two samples were taken from each community type and thawed, dried, and sent to the Agricultural Analytical Services Laboratory of The Pennsylvania State University (University Park, PA).  Samples were tested for pH, P, K, Mg, Ca, nitrate-N, and ammonium-N.

        Mortality over the first month of the experiment averaged 20% for the three species (Table 1; Figure 3).  For all species, mortality was greater in barrens communities than in forest communities.  Activated carbon treatment reduced mortality in the barrens for all species and did not reduce mortality in the forest for any species.  c2 analysis showed a significant community x treatment interaction effect for Prunus serotina mortality (df=1; X2=10.2; p<.01).  A significant community effect was calculated for Betula lenta mortality (df=1; X2=13.8; p<.01).  The pattern of mortality for Acer rubrum among the different treatments did not differ significantly from a random distribution.
          The ratio of new growth to total stem biomass after the twelve week growing period was greater in the forest control plots than in the barrens control plots for Betula and Prunus (Figure 4).  The opposite trend was true for Acer.  The carbon treatment in the barrens plots increased the growth ratio for Prunus and Acer.  The carbon treatment in the forest plots resulted in a lower growth ratio for Prunus and Betula and did not affect the growth ratio of Acer.  ANOVA showed a significant effect of community x treatment for Prunus (F=4.55; p=0.038) and Acer (F=4.121; p=0.054) growth ratios (Table 2).

Germination Experiment
 
        There were no significant differences between the germination rates of Betula populifolia or Tsuga canadensis seeds in distilled water or in aqueous extracts of forest and barrens litter and organic soil (Figure 5).

Soil Analysis
         Barrens and forest organic soils had fairly similar pH and levels of K, Ca, Mg, and nitrate-N (Table 3).  Ammonium-N levels were about four times as high in the forest as in the barrens for both sets of soil samples.  Phosphorus levels were also higher in the forest than in the barrens.

Germination Experiment

         Germination of Tsuga canadensis and Betula populifolia seeds was not inhibited by extracts of barrens or forest litter or organic soil.  This indicates that any allelopathic chemicals present in the soil influence seedling growth and not seed germination.  However, it is possible that germination inhibition in these experiments was masked by overall high variability in germination among petri dishes within a treatment.  Individual petri dishes likely experienced different levels of moisture and different microbial populations.  Germination experiments that better control for these factors would reduce variation within treatments and might allow treatment effects to be seen.  Germination inhibition could also exist for other species that were not tested.

Allelopathy: Potential Processes

         Allelopathic compounds can inhibit plant growth through a wide range of effects on soil ecology (Inderjit and Weiner, 2001).  Although allelopathy was originally conceived of as direct plant-plant inhibition, recent researchers have expanded this conception to include inhibition of mycorrhizal symbionts, disruption of microbial communities, and influences on nutrient cycling.  This field experiment did not distinguish between any of several potential mechanisms of allelopathy in the barrens.
         Barrens allelochemicals may limit nutrient availability.  This hypothesis is supported by the finding that ammonium and phosphorus levels were lower in the barrens than in the forest.  More extensive analyses of the Long Pond and Schoch Heath sites have also found that barrens areas have lower mineral nitrogen levels (A. Wibiralske, unpublished data; Yorgey, 1999).   Polyphenolic allelochemicals often work by complexing organic nitrogen, reducing its availability to most plants and ectomycorrhizal fungi (Hättenschwiler and Vitousek, 2000).  This would be a particularly advantageous strategy for ericaceous plants, which can utilize organic nitrogen sources through their mycorrhizal symbionts (Read, 1983).  These fungi have also been found to utilize proteins bound to polyphenolics (Bending and Read, 1996a; Bending and Read, 1996b).  Thus, Kalmia could potentially produce polyphenolic compounds that reduce nitrogen availability to other plants without reducing limiting its own access to nitrogen.
        There are also other ways that phenolics can reduce nitrogen availability besides directly binding organic nitrogen.  They may increase numbers of carbon-limited soil bacteria, which then utilize available mineral and organic nitrogen sources.  Humus phenolics have complex effects on the populations of many species of microbes.  Low (<.25micromol/g soil) concentrations of phenolics have been shown to increase the growth of phenolic-utilizing bacteria (Blum et al., 2000).  Phenolics can also inhibit the growth of microbes that contribute to nitrogen mineralization (Souto et al., 2000a).  Although soil microbes are generally thought to be nitrogen limited, Kalmia-associated microbes have an unusually high energy/nutrient demand, and so are more likely to be carbon limited than other microbe systems (Bradley et al., 1997a; Bradley et al., 1997b). However, Kalmia- derived tannins have not been shown to decrease mineral nitrogen availability when added to humus, and may inhibit the survival or growth of microbes with high nitrogen requirements (Bradley at al., 2000).  Thus it seems more likely that Kalmia allelochemicals reduce nitrogen availability through binding organic nitrogen than by stimulating microbes.
        It is likely that Kalmia, like other ericaceous plants, has low levels of phosphorus and nitrogen in its leaf litter (Latham et al., 1996; Read, 1983).  This would contribute to nitrogen deficiencies in the soil that could be exacerbated by allelochemicals.  This study and others have found that Kalmia- dominated soils are low in phosphorus (Inderjit and Mallik, 1999).  However, addition of Kalmia litter to soil increases phosphate content (Inderjit and Mallik, 1996).  Phenolics in Kalmia litter may bind to soil particles and soluble Al and Fe, which would otherwise bind phosphate ions (Inderjit and Mallik, 1996).  This might increase available phosphate in the short run but eventually reduce total phosphorus levels through leaching.  Kalmia- associated microbes may also contribute to phosphorus deficiencies in soils (Inderjit and Mallik, 1999).
        While the available literature on Kalmia and soil phenolics makes it seem likely that Kalmia allelochemicals work by reducing nutrient availability, the direct inhibition of growth via toxicity to plant physiological functioning was not ruled out by this study.  Kalmia allelochemicals may also be toxic to mycorrhizal symbionts of other plant species.  This would have similar effects on forest trees as a reduction in soil nutrients.  Phenolic compounds mimicking humus solutions have been found to inhibit the growth of Picea abies mycorrhizal fungi (Boufalis and Pellissier, 1994).  Bending and Read (1996a) found that the soluble polyphenol tannic acid inhibits the growth of several ectomycorrhizal species, but does not inhibit ericoid mycorrhizal species.  However, inhibition of ectomycorrhizae in these studies could also be caused by reduction of available nitrogen.  Direct inhibition of forest trees could be tested with laboratory experiments on mycorrhizal and nonmycorrhizal seedlings grown in a greenhouse.  Such a study found that mycorrhizal infection causes Pinus sylvestris seedlings to be more inhibited by E. hermaphroditum alleolochemicals, providing an example of allelochemical inhibition of mycorrhizae in the Ericaceae (Nilsson et al., 1993).

Alternative Community States: Theories of Stabilization

        If Kalmia is able to so effectively inhibit the growth of tree species, one might wonder why it does not gradually invade forest areas in the Poconos, as it has done in disturbed areas of Newfoundland.  In many areas I observed Kalmia growing in the forest understory, yet it does not seem to be taking over these areas.  In fact, the opposite trend seems to be taking place, as the size of the heath barrens has shrunk in relation to the forest in the last 40-60 years (Latham et al., 1996).  However, barrens can persist for decades without invasion by forest trees (Latham et al., 1996).  The barrens and forest are thought to be the persistent results of diverging processes of succession (Latham et al., 1996).  If this is the case, they must have stabilizing factors to maintain the division between them.
        The hypothesis that Kalmia inhibits tree growth by reducing nutrient availability in the soil provides a possible explanation for how it is able to coexist with trees in forest areas.  The shrub may be unable to take over these areas because of the higher nutrient levels in their soils.  Although Mallik (1996) found that NPK fertilization increased Kalmia growth in a greenhouse, Prescott et al. (1995) found that nitrogen fertilization of a jack pine forest reduced Kalmiaís competitive ability.  Kalmia may not make sufficient allelochemicals to significantly reduce levels of available nitrogen in forests.  This would prevent it from gaining a competitive advantage there, since forest trees have better access to light.  Thus the cycle of nutrient depletion that comes with increased Kalmia dominance does not get a chance to get started, and Kalmia remains a sparse understory plant.
        While Kalmia exerts an influence on the nutrient quality of surrounding soils, nutrient availability may also be affecting its production of phenolic compounds.  According to the carbon-nutrient (C/N) balance hypothesis, the quantity of carbonaceous defense chemicals a plant produces is related to the ratio of available carbon to available nutrients.  Plants that have excess photosynthate products available because they are nutrient-limited will create carbon-based secondary compounds, while plants with an adequate nutrient supply will invest their energy in growth (Harborne, 1993).  Thus shaded Kalmia plants in more nitrogen-rich forest soil may not be inhibiting tree growth because they are producing fewer allelochemicals than their cousins in the barrens.  The production of more allelochemicals in nutrient-limited, highly irradiated barrens Kalmia may help promote its cycle of dominance.
In addition to nutrient levels, pH may impact how Kalmia allelochemicals work to exclude tree species.  Acidity has been found to increase the extent of growth inhibition by Kalmia phenolics (Zhu and Mallik, 1994), but to decrease the ability of soil polyphenols to complex proteins (Bending and Read, 1996b).  The oxidation state of phenolic compounds, which determines their mode of action, is highly dependent on pH (Appel, 1993).  If acidity and nitrogen availability work synergistically to inhibit tree growth, then Kalmia may not be able to inhibit tree growth in forests because in low numbers it is unable to sufficiently affect both pH and nitrogen levels.  Soil acidity also affects the solubility and availability of many nutrients.  In acidic conditions, Fe is thought to leach out of upper soil layers and precipitate into a semi-impermeable "hard pan" in the lower, less acidic layers (Sugden-Newbury, 1999).  However, this study and others have not found that soils of forests and heath barrens have substantially different pH (Wibiralske, unpublished data).  In the forests of Newfoundland, Kalmia has been found to decrease soil acidity (Inderjit and Mallik, 1999).
        The production of high levels of organic compounds in leaves resistant to decay make ericaceous plants particularly flammable.  Latham et al. (1996) theorized that fire may have historically played an important role in the maintenance of barrens vegetation.  Fire could favor Kalmia dominance by giving it an advantage over other species that are less fire-tolerant.  While fire in forests produces charcoal that mitigates the effects of allelopathy (Wardle et al., 1998; Zackrisson et al., 1996), barrens areas might not have sufficient biomass to create substantial amounts of charcoal.  Fire might also remove nitrogen and other nutrients from barrens areas (Whelan, 1995).  Pocono barrens areas in general are more likely to transition to forests if they have not been burned (Sugden-Newbery, 1999).  However, heath barrens can continue to exist in the absence of fire (Latham et al., 1996).
The persistence of barrens in the absence of fire indicates that other processes, such as the action of allelochemicals, are acting to maintain Kalmia dominance.  The existence of multiple inhibitory elements may complicate vegetation transitions, necessitating that many changes in soils and vegetation come about before transition can occur.  Nutrient availability, fire, regeneration strategy, light availability, and direct inhibition may all interact so that a temporary change in any one element does not often result in a vegetation transition.  Fire may be the original stimulus for ericaceous proliferation, which in turn increases flammability.  Meanwhile, superior regeneration strategies allow the shrubs to persist during the periods between fires.  The long-term result of both fire and ericaceous dominance would be reduction of nitrogen availability, which further isolates the barrens from the forest.  Production of greater quantities of allelochemicals as a result of high light availability in the barrens would strengthen these positive feedback processes through increased nitrogen sequestration and/or tree inhibition.  It may take multiple processes, including allelopathy, to maintain the divisions between forest and barrens in the Poconos.

I would like to thank Roger Latham, Philip Johns, Amy Vollmer, Marcus McFerren, Jose-Luis Machado, and Tom Valente for their generous advice and support.  Tom Dee and Phil Everson helped with statistical analysis.  Jack Borrebach, Jason Burton, Julie Cohen, Sorelle Friedler, Irene Garcia, Abby Lowther, Ned Schneider, Trudy Schneider, Harold Schneider, and Laura Valentine helped with field and laboratory work.  Bill Pinder, John Kelly, and Matt Powell provided crucial technical assistance.  Sara Hiebert provided the use of her vapor pressure osmometer.  This research was conducted on land owned or managed by The Nature Conservancy and Doug Fogel  The Swarthmore College Department of Biology provided funding for this project.

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