Leaf and needle litter of the deciduous trees beech, birch, oak, ash, common maple, hornbeam, hazelnut, willow, poplar, hawthorn and the coniferous trees pine, spruce-fir, douglas-fir and larch were collected immediately after “leaf-fall” in October and analysed for their water-soluble phenolic compounds. By collecting leaf and needle litters three times in the period from October to January leaching rates of these organic compounds could be determined. There were considerable differences in the amount and leaching rate of water-soluble phenolics between litter-types. The concentration found in freshly deposited litter of the leaf-forming species was high and most of the water-soluble phenolics in these litter types were released in the period from October to January. In needle litters concentrations of water-soluble phenolics were low and they leached at a slow rate. By using gas chromatography, 14–18 phenolic compounds could be identified in the leachates. Benzoic acids dominated the phenolic spectra in October. In the course of time cinnamic acids became more important in the water-leachates. The results are discussed in view of their importance for the relationship between woodland-types and understory vegetation.

Leaching of phenolic compounds from leaf and needle litter of several deciduous and coniferous trees

Phenolic compounds have been identified previously as potentially responsible for allelopathic interferences in spruce forest at high altitude. They have now been analyzed in canopy leachates, snow, and soil solutions collected from the three layers of the podsolic soil: OA, E. and B. Leachates were characterized by high tanning capacity and by p-hydroxyacetophenone (found at 10−6 M) also detected as the major (10−7 M) monomeric compound in snow. At least 10 phenolic monomers, including vanillic, p-hydroxybenzoic, and protocatechuic acids were identified in capillary waters extracted from the OA layer with their sum reaching 2 × 10−6 M. These monomers were only a small part (1%) of the total phenolics. In soil solutions, significant decreases in phenolic concentrations with depth were observed between the E and B layers, with qualitative modifications of the phenolic pattern. Spruce leachates and soil solutions exhibited high temporal variability, resulting in transitory allelopathic potential towards both aerial and subterranean parts of spruce seedlings. The occurrence of various and soluble phenolic coumpounds in the whole forest system, including p-hydroxyacetophenone, which persist from green needles to soil solutions, suggests that they could be involved in complex and interactive processes occurring during organic matter accumulation on the coniferous forest floor.

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Phenolic Compounds in Natural Solutions of a Coniferous Forest

Agroforestry is a modern tool to develop sustainable land use and to increase food production by growing woody species (trees, shrubs, palms, bamboos, etc.) with agricultural crops and/or animals in some form of spatial arrangement or temporal sequence. Because these species co-exist with the agricultural crops, their allelopathic compatibility may be crucial to determine the success of an agroforestry system. A survey of the available information reveals that most of the agroforestry species (AF species) have negative allelopathic effects on food and fodder crops. Therefore, it is desirable to do further research in this direction so that AF species with no or positive allelopathic effects on the companion crops may be promoted for agroforestry programs. As AF species remain a part of the agroecosystem for a longer period, and most of them produce a large amount of leaves and litter, their allelochemicals may play an important role in developing an eco-friendly pest management strategy. Besides these generally studied aspects of allelopathy, some comparatively newer aspects of research have been identified, such as evaluation of qualitative yield of agroforestry systems, selective behavior of the allelochemicals, effect on soil quality, and the role of tree allelochemicals in animal and human nutrition. If given due consideration, allelopathy could play a pivotal role in conservation of the highly threatened environment, biodiversity, natural resource base, and making agriculture more sustainable through broadening the scope of agroforestry.

Allelopathic Interactions in Agroforestry Systems

1. The allelopathic effect of the exudate secreted by Cistus ladanifer leaves was tested on different plant species. Cynodon dactylon and Rumex crispus, species absent from C. ladanifer scrub (jarales), were clearly inhibited by the exudate.



2. Species present in the jarales, Medicago polymorpha and Lolium rigidum, showed no direct inhibition of germination, but the process this was delayed, and cotyledon and root size was diminished. This implies inhibition of seedling growth, and may explain the low abundance of these species inside the jarales.



3. The active constituents in the inhibition of germination are low molecular weight phenolic compounds. The flavonoids appear to exert no direct effect on germination of the species tested, but do provoke a reduction in cotyledon and root size, and could thereby inhibit subsequent seedling development.

Allelopathic effect of Cistus ladanifer on seed germination

Eleven allelochemicals (ferulic acid, cinnamic acid, 4-hydroxybenzoic acid, hydroxycinnamic acid, methyl propionate, oxalic acid, methylmalonic acid, p-anisic acid, butyric acid, 3-hydroxybutyric acid, and azulene) were identified in the exudate of Cistus ladanifer L. We studied the effect of each on germination, cotyledon emergence, root length, and cotyledon length of Rumex crispus. Three groups were distinguished with respect to phytotoxic activity: compounds with low activity (ferulic acid, 4-hydroxybenzoic acid, oxalic acid, methylmalonic acid, p-anisic acid, hydroxybutyric acid, and azulene), with intermediate activity (cinnamic acid and hydroxycinnamic acid), and with high activity (methyl propionate and butyric acid). The effect of the interaction of the compounds was studied. When acting conjointly, all combinations tested produced a more negative effect on both germination and seedling growth than when acting alone. The interaction affected cotyledon emergence and root length more negatively than germination and cotyledon length. When hydroxycinnamic acid and cinnamic acid were added to these mixtures there was an enhancement in the phytotoxic activity, accentuating the effect of the other allelochemicals.

Identification and Effects of Interaction Phytotoxic Compounds from Exudate of Cistus ladanifer Leaves

Correlation between the distributional frequency of undergrowth species of red pine forest and their germination and growth effected by pine extracts and leachates was found. It was made clear by germination and growth tests that pine toxic substances inhibit the germination and growth of low frequency species more than high frequency species in a red pine forest and that these substances are contained in descending concentration in fresh and fallen leaves, roots, pine forest soil, and pine rain. The concentration of pine toxic substances in extracts or leachates was affected by extracting or leaching within a given period of time, requiring a few hours for extracts or a few days for leachates. The amount of dry weight inhibition of the undergrowth treated by pine leachates was expressed as a growth inhibition index (GII) for the comparison of tolerance in various species. GII is a relative value (%) of the test groups against the control and it is an exponential function of the amount of pine toxic substances affecting the dry weight of the undergrowth. The substances were analyzed by paper and gas chromatography. Benzoic acid and 11 phenolic acids were identifed by gas chromatography. Benzoic acid was considered to be a key factor of allelopathy in the red pine forest.

Yang Jai Yim

Papers

Allelopathic effects ofPinus densiflora on undergrowth of red pine forest.