Interactions between seedlings of the invasive tree Ailanthus altissima and the native tree Robinia pseudoacacia under low nutrient conditions

The allelopathic effect of A. altissima and the nitrogen fixing ability of R. pseudoacacia make the interaction between these two species important to community dynamics. A replacement series greenhouse experiment was used to investigate the type of interaction between seedlings of A. altissima and R. pseudoacacia at high and low soil nutrition states. Also, seeds of A. altissima from its native (China) and invasive ranges (USA) were used to compare the effect of the different seed sources on the interaction with R. pseudoacacia. Robinia pseudoacacia was the better competitor. The presence of A. altissima significantly inhibited nodulation of R. pseudoacacia roots. In summary, in low nutrient early succession sites, seedlings of R. pseudoacacia and A. altissima will coexist and R. pseudoacacia will be the better competitor. However, A. altissima can increase its competitive ability by inhibiting nitrogen fixation by R. pseudoacacia. Differences between the competitive ability of the Chinese and US seed lots of A. altissima did not support the rapid evolution theory for invasive species success.     

Analysis of interactions between the invasive tree-of-heaven (<Emphasis Type="Italic">Ailanthus altissima</Emphasis>) and the native black locust (<Emphasis Type="Italic">Robinia pseudoacacia</Emphasis>)

Invasive exotic plants can persist and successfully spread within ecosystems and negatively affect the recruitment of native species. The exotic invasive Ailanthus altissima and the native Robinia pseudoacacia are frequently found in disturbed sites and exhibit similar growth and reproductive characteristics, yet each has distinct functional roles such as allelopathy and nitrogen fixation, respectively. A four-month full additive series in the greenhouse was used to analyze the intraspecific and interspecific interference between these two species. In the greenhouse experiment, the inverse of the mean total biomass (g) response per plant for each species was regressed on the density of each species and revealed that the performance of the plants was significantly reduced by interspecific interference and not by intraspecific interference (p < 0.05). Other biomass traits such as root dry weight, shoot dry weight, stem dry weight, and leaf dry weight were also negatively affected by interspecific interference. Competition indices such as Relative Yield Total and Relative Crowding Coefficient suggested that A. altissima was the better competitor in mixed plantings. Ailanthus altissima consistently produced a larger above ground and below ground relative yield while R. pseudoacacia generated a larger aboveground relative yield in high density mixed species pots.     

Regeneration of <Emphasis Type="Italic">Robinia</Emphasis><Emphasis Type="Italic">pseudoacacia</Emphasis> riparian forests after clear-cutting along the Chikumagawa River in Japan

Regeneration processes of riparian Robinia pseudoacacia forests after clear-cutting were investigated through dendroecological and microsatellite polymorphism analyses. Age determination of regenerated R. pseudoacacia trees based on the dendroecological analysis revealed that forests regenerating after clear-cutting were composed of trees that mostly established within a few years after clear-cutting. This suggests that the stimulus to form new shoots was evoked by clear-cutting but faded within a few years. Genet identification via the microsatellite polymorphism analysis showed that ramet trees belonging to the same genet were distributed in a cluster. Almost all trees regenerated asexually through new ramet formation on the cut stumps and residual horizontal roots after clear-cutting. AMOVA with microsatellite markers showed that among- and within-population variations contributed 6 and 94% to the total variation, respectively, suggesting that R. pseudoacacia trees in the forests were initially established from seeds dispersed randomly from mother trees in a wide area.     

Photosynthetic activity and water relations of sprouts ofPasania edulis

In order to investigate the factors causing fast growth of sprouts ofPasania edulis, photosynthetic activity and water relation characteristics of lower (mature) leaves and upper (expanding) leaves of the sprouts were compared with those of seedlings and adult trees ofP. edulis. Apparent quantum yield was generally low. Maximum photosynthetic rate was highest in the lower leaves of sprouts. Stomatal frequency was higher in sprout leaves than in seedling leaves. Osmotic potential at the water saturation point and water potential at the turgor loss point, in leaves, were higher in sprouts than in seedlings and adult trees. Symplasmic water content per unit leaf area was higher in sprouts than in seedlings. These water relation parameters in leaves indicated that sprout leaves are superior in maintaining cell turgor against water loss, but are not tolerant to water stress. In field measurements, sprout leaves showed higher stomatal conductance and transpiration rates. These results indicated that sprout leaves fully realized their high potential productivity even under field conditions. The leaf specific conductance, from the soil to the leaf, was higher in sprouts than in seedlings. Large and deep root systems of the original stumps of the sprouts may be attributed to the high leaf specific conductance.     

Sprout initiation and growth for three years after cutting in an abandoned secondary forest in Kyoto,Japan

Secondary forests in Japan have been abandoned and the ecosystem has degraded since the high economic growth period. We carried out cutting in January in three small areas of a long-abandoned secondary forest and investigated the sprout initiation and growth of woody plants for three years in order to reveal the early stage of sprout regeneration and to understand the sprouting ability and characteristics of each species for effective management. The percentage of sprouted stumps and the number of sprouting shoots was substantially maximized in autumn in the first year. These results suggest that autumn monitoring in the first year after cutting shows the maximum percentage of sprouted stumps and the maximum sprout number when cutting was conducted in the dormant season. With regards to species characteristics, Eurya japonica showed a low percentage of sprouted stumps in the lower plot, where the mean diameter at breast height for this species was smaller than in the other plots. The sprouting ability of E. japonica was deemed to be influenced by parent tree size. Ilex pedunculosa and Lyonia ovalifolia var. elliptica had high percentages of sprouted stumps and many sprouts. These species are useful for obtaining sprouting shoots (e.g., for firewood), but it is difficult to control their sprouting.     

Comparison of leaf decomposition and macroinvertebrate colonization between exotic and native trees in a freshwater ecosystem

One of the most important sources of energy in aquatic ecosystems is the allochthonous input of detritus. Replacement of native tree species by exotic ones affects the quality of detritus entering freshwater ecosystems. This replacement can alter nutrient cycles and community structure in aquatic ecosystems. The aims of our study were (1) to compare leaf litter decomposition of two widely distributed exotic species (Ailanthus altissima and Robinia pseudoacacia) with the native species they coexist with (Ulmus minor and Fraxinus angustifolia), and (2) to compare macroinvertebrate colonization among litters of the invasive and native species. Litter bags of the four tree species were placed in the water and collected every 2, 25, 39, 71, and 95 days in a lentic ecosystem. Additionally, the macroinvertebrate community on litter bags was monitored after 25, 39, and 95 days. Several leaf chemistry traits were measured at the beginning (% lignin; lignin:N, C:N, LMA) and during the study (leaf total nitrogen). We detected variable rates of decomposition among species (k values of 0.009, 0.008, 0.008, and 0.005 for F. angustifolia, U. minor, A. altissima and R. pseudoacacia, respectively), but we did not detect an effect of litter source (from native/exotic). In spite of its low decay, the highest leaf nitrogen was found in R. pseudoacacia litter. Macroinvertebrate communities colonizing litter bags were similar across species. Most of them were collectors (i.e., they feed on fine particulate organic matter), suggesting that leaf material of either invasive or native trees was used as substrate both for the animals and for the organic matter they feed on. Our results suggest that the replacement of the native Fraxinus by Robinia would imply a reduction in the rate of leaf processing and also a slower release of leaf nitrogen to water.     

Influence of crop background on aphids and other phytophagous insects on Brussels sprouts

More Brevicoryne brassicae and other alate aphids were caught in yellow water-traps in a weed-free crop of Brussels sprouts than in a crop with a weedy background. More B. brassicae colonized Brussels sprout plants in bare soil than in weeds; very few alatae were attracted to cruciferous weeds in the crop. Results in 1 yr suggest that initially larger populations on the weed-free sprouts became smaller than populations on the weedy sprouts because the larger aphid population attracted more natural enemies. Aleyrodes brassicae and certain Lepidoptera were also more abundant on sprout plants in bare soil than on sprouts surrounded by weeds; more adult A. brassicae were caught in water traps over the bare soil. More A. brassicae were present on sprout plants surrounded by a green than by a brown cloth background but the differences were not significant (P < 0–05). Numbers of B. brassicae on sprout plants with green and brown backgrounds varied greatly and did not differ significantly. In field cages, B. brassicae alatae were more attracted to potted sprout plants surrounded by bare soil than to ones surrounded by rings of living or cut grass or by artificial green rings. This effect was greater with small than with large sprout plants surrounded by grass rings. The maintenance of a limited weed cover is considered potentially useful in integrated control of some brassica pests.     

Seedling and Sprout Response to Slash-and-Burn Agriculture in a Tropical Deciduous Forest1

Survival and growth of seedlings and sprouts were assessed in three plots for 16 mo following the slashing and burning of a tropical deciduous forest in Jalisco, Mexico. We encountered a total of 47 species: 21 seedling species and 35 sprout species. Calliandra formosa and Piptadenia flava were the most common seedling species; Bursera arborea, Cordia alliodora, and Piptadenia constricta were the most common sprouts. Colubrina triflora, Diphysa occidentalis, and Cnidoscolus spinosus had limited sprouting ability. Twenty-six species were represented by one seedling or one sprout. Thirty-eight percent of the seedlings were tree species, 59 percent were shrub species, and 2 percent were vines species. In contrast, 86 percent of the sprouts were from trees and 13 percent from shrubs. One year after the initial measurements, 29 percent of the seedlings and 13 percent of the sprouts were dead. Each of the seedling means (number of stems/individual, height and diameter of the tallest stem, and elliptical crown area) was significantly smaller (P < 0.05) than that of sprouts at all three measurement periods, but relative growth rates were similar. Total canopy area of seedlings had a larger relative increase than did the canopy area of sprouts. The presence of seedlings increased species diversity compared to calculated diversity excluding seedlings. Timing of fruit dispersal in relation to the date of burning and the high number of Leguminosae species in the forest appeared to favor seedling establishment for some species.     

SPATIAL DISTRIBUTION AND DEVELOPMENT OF ROOT SPROUTS IN FAGUS GRANDIFOLIA (FAGACEAE)

Root sprouts around 31 Fagus grandifolia parent trees, some declining due to beech bark disease, were studied to describe the pattern of sprout distribution, the ecological importance of this pattern, and the relationship between sprout patterns and parent vigor. Microtome sections of roots were studied to determine the histological origin of sprouts. Sprout distribution was circular and usually centered on the parent tree. Most sprouts were within 8 m of the parent and remained attached to the parent root system even after the sprouts reached 10 yrs of age. Root sprouting in F. grandifolia may be effective in replacing dead parent trees, but the potential for clones to spread is limited. Spatial distribution of sprouts was mostly unaffected by tree vigor. Root sprouts originated from callus tissues associated with wounds on superficial woody roots.     

Sprouting in temperate trees: A morphological and ecological review

Sprouting in trees, which results in the production of secondary trunks, is an induced response to injury or to a dramatic change in surrounding environmental conditions. This article reviews the forestry and ecology literature to produce an integrated view of the role of sprouting in both disturbed habitats and closed-canopy forests. Sprouting is a universal attribute of temperate angiosperm trees through the sapling stage of development but is much less common among gymnosperms. Four basic types of sprout morphologies are described: collar sprouts from the base of the trunk, sprouts from specialized underground stems (lignotubers and rhizomes), sprouts from roots, and opportunistic sprouts from layered branches. In a survey of 68 species of trees native to northeastern North America, 41% were found to retain the ability to sprout from the collar into adulthood; 26% sprout from branch layers under natural conditions; and 25% have the capacity to form root suckers. Sprouting in seedlings promotes their survival under a variety of stressful conditions, including suppression by canopy trees, herbivory, site exposure, and desiccation. In contrast, sprouting in mature trees extends the life span of the individual following damage and, in the case of root-suckering species, promotes the colonization of new ground. Although the sprouting of mature trees is more conspicuous than the sprouting of seedlings, its ecological significance is not as great. As a broad generalization, species that grow in stressful sites or sites with frequent disturbances are likely to sprout more vigorously and to retain the sprouting ability longer than are species that grow in less stressful sites or sites with less frequent disturbance. Near the limits of a species’ altitudinal or latitudinal range, the production of basal sprouts, root suckers, rhizomes, and/or branch layers allows trees to spread into adjacent areas, thereby circumventing the difficulties associated with seedling establishment.     

Size-dependent Allometry of Tree Height, Diameter and Trunk-taper

The allometry of tree height with respect to trunk diameterand the allometry of trunk diameter with respect to distancefrom the top of the tree (i.e. trunk taper) were determinedfor 27 Robinia pseudoacacia trees differing in age and sizegrowing in an open field. The allometric (scaling) exponentfor height was > 1 for small and young trees and decreasedto 2/3 and then 1/2 as tree size and age increased. Similarly,the exponent for taper was > 1 near the tips of young andold trunks and converged onto values of 2/3 and 1/2 toward thebase of mature tree trunks. These observations indicate thata single 'optimal mechanical design principle' (i.e. elastic,stress or geometric self-similarity) neither holds true throughoutthe lifetime of R. pseudoacacia trees, nor does a single designprinciple govern the taper of a trunk throughout its entirelength. Rather, over the course of growth and development, theallometry of R. pseudoacacia tree height and trunk taper progressivelychanges, complying with geometric self-similarity for youngplants (and young portions of old plants) and subsequently givingthe appearance of elastic or stress self-similarity as plants(or portions of plants) get older and therefore larger. Analysesof published (and new) data suggest that the conclusions drawnfor R. pseudoacacia trees are likely to hold true for othertree species because stem growth in diameter is 'indeterminate'whereas growth in overall tree height is asymptotic and thereforeessentially 'determinate'.Copyright 1995, 1999 Academic Press Scaling, woody plants, Robinia pseudoacacia     

Occurrence and development of Eucryptorrhynchus brandti and E. chinensis (Coleoptera: Curculionidae) on Ailanthus altissima trees subjected to different levels of mechanical damage

Eucryptorrhynchus brandti (Harold) and E. chinensis (Oliver) are herbivores of Ailanthus altissima (Mill.) Swingle (tree of heaven) in China. Eucryptorrhynchus brandti has been recommended as a potential biological control agent of A. altissima in North America. In China, the majority of adults of both species were found on tree trunks 0–5 m above the ground, from May to September. In October and November, most adults were found at the base of A. altissima trees. Trees were subjected to different levels of mechanical injury: very low mechanical injury, one to two 0.015 m² bark sections were removed; low, two to four bark sections were removed; moderate, 8 to 22 bark sections removed; high, tree was girdled; and extreme, tree was felled and the remaining stump was sampled. Very low and low mechanical injury trees had very few adult emergence holes with few larvae developing beyond the first or second instar after 29 months. The health of these trees remained good. An increase in adult emergence holes occurred in moderately injured trees that appeared as healthy as very low to low mechanically injured trees. This may indicate that only a small increase in tree stress can increase the adult emergence rate and thus hasten a decline in tree health. Moderate, high and extreme mechanically injured trees had significantly more emergence holes/m than very low and low injured trees and late larval stages were present. The development of Eucryptorrhynchus species was more successful the greater the degree of mechanical injury. Based on this study both Eucryptorrhynchus species appear to be secondary pests of A. altissima in China.     

Distribution and characteristics of the soil seed bank of the black locust (<Emphasis Type="Italic">Robinia pseudoacacia</Emphasis>) in a headwater basin in northern Japan

Invasive expansion of Robinia pseudoacacia is a worldwide problem. A method to control existing R. pseudoacacia populations is urgently needed. In addition to plants above the ground, seed banks should be targets for the management of R. pseudoacacia. We targeted an old plantation of R. pseudoacacia and its surroundings in a headwater basin to determine the spatial characteristics of the distribution. We established 49 square plots in which the number of buried seeds and seeds dispersed by wind, and several other related environmental factors were surveyed. The relationship between the spatial extent of seeds dispersed by wind and the distribution of the seed bank was analyzed. Environmental factors that potentially influenced the density of buried seeds were also analyzed. We found that barochory and anemochory were the main factors that contributed to R. pseudoacacia forming seed banks. The greatest factor controlling the density of buried seeds was slope angle; thus, transfer of seeds by rolling is important for the formation of seed banks in headwater basins. From these results, some guidelines for the management of seed banks in headwater basin are presented. First, unnecessary soil disturbance within about a 100-m buffer area around seeding trees should be avoided. Second, plantation soil from gentle slopes should not be applied for revegetation of other locations. In situations where R. pseudoacacia populations are planted on steep slopes near streams, there is a chance that seeds will be carried away and form seed banks at lower or downstream sites.     

A Test for the Resource Remobilization Hypothesis: Tree Sprouting using Carbohydrates from Above-ground Parts

To test the resource remobilization hypothesis, i.e. the hypothesisthat some trees sprout from root-collars or from the lower partof trunks using resources obtained from above-ground parts ratherthan from resources reserved in their roots, we conducted cuttingexperiments forEuptelea polyandra, a frequently sprouting treespecies with little carbohydrate reserves in its roots,Quercusserrata,a frequently sprouting tree species with large reservesin the roots, andMallotus japonicus, a rarely sprouting treespecies. Trees of each species were cut down in winter leavingtwo kinds of stumps, those approx. 1.5 m in height and thosecut off near the ground. The number and total dry weight ofnewly sprouted shoots per stump were compared between the twotreatments and among the three species at the end of the followinggrowing season. InE. polyandra,both the number and total dryweight of sprouts per stump were very small for both treatmentsand were similar to, or less than, those ofM. japonicus. Onthe other hand,Q. serratasprouted abundantly in both treatments.These results indicate thatE. polyandracannot sprout sufficientlywithout a considerably large volume of above-ground parts orthat additional structures such as foliage and branches maybe necessary for sprouting. We conclude that the resource remobilizationhypothesis is supported for this species.Copyright 1998 Annalsof Botany Company Euptelea polyandraSieb. et Zacc,Quercus serrataThunb,Mallotus japonicus(Thunb.) Muell. Arg., tree sprouting, cutting experiment, resprouter, resource movement, carbohydrate allocation, ground-surface disturbance, root stock, resource remobilization hypothesis.     

Differences in nitrogen use strategies between native and exotic tree species: predicting impacts on invaded ecosystems

Background and aim

Exotic plant species can alter the nitrogen cycle in invaded ecosystems. We assess the differences in nitrogen use strategies and litter production and dynamics among three native riparian trees (Fraxinus angustifolia, Populus alba and Ulmus minor) and three co-occurring exotics (Ailanthus altissima, Robinia pseudoacacia and Ulmus pumila), currently spreading throughout river banks in inner Spain. We aim to predict the ecological consequences of a replacement of the natives by the exotics.

Methods

We compared the leaf lifespan, nitrogen resorption efficiency in leaves, nitrogen mean residence time, amount and timing of litter production and amount of nitrogen returned to soils between these native and exotic species.

Results

We found differences among species in all the variables measured, but not between native or exotic origins. Species were ranked from the most to the least conservative nitrogen use strategy as follows: U. pumila was the most conservative species, followed by the three natives (with an intermediate strategy), A. altissima and finally by the nitrogen-fixer R. pseudoacacia. The studied exotic species would produce contrasting impacts on the nitrogen cycle upon invasion.

Conclusions

On the basis of our results, we predict little impacts on the nitrogen cycle if U. pumila dominates the landscape. Despite being nitrogen-fixer R. pseudocacia would not increase soil nitrogen availability in the study area due to its low litter production and litter decomposition rates. In contrast, we predict an increase in nitrogen availability of soils upon A. altissima invasion, as this species produces a high amount of nitrogen rich and labile litter. This study offers a striking example of the contingencies involved in predicting the ecosystem impacts of exotic plant invasion.     

Phytophagous insect-woody sprout interactions in tropical eucalypt forest. I. Insect herbivory

Abstract Foliar insect damage levels on woody sprouts in the ground layer of two tropical eucalypt forest communities on Melville Island were between 7.8 and 43.2%. Of eight common tree species, Eucalyptus confertiflora was damaged most by insects and Buchanania obovata and Terminalia ferdinandiana the least. Seasonal trends in insect damage were not consistent between plant species and were not always consistent between vegetation types for a particular plant species. The results of this study are not consistent with hypotheses suggesting that insect grazing is a critical determinant of tree species dominance or woody sprout dormancy.     

Allelopathic potentials of exotic invasive and native trees over coexisting understory species: the soil as modulator

Ailanthus altissima and Robinia pseudoacacia are two aggressive exotic tree species invading riparian ecosystems in Central Spain. We explored their allelopathic potentials as a possible mechanism explaining their success in these ecosystems. Specifically, we aimed (1) to compare the phytotoxic effects of the exotic and native (Fraxinus angustifolia and Populus alba) trees on the fitness of several understory plants coexisting in riparian ecosystems, and (2) to assess the capacity of the riparian soil to modulate the phytotoxic effects. In laboratory bioassays, aqueous leaf litter extracts from the donor tree species at field-realistic concentrations were tested on different fitness indicators of 13 understory target species, using germination paper and soil as substrates in petri dishes. Using germination paper, we found species-specific effects between donor and target species, but the phytotoxicity of the exotic trees as a group was not greater than that of the natives. Nevertheless, the exotic R. pseudoacacia was the most effective donor species reducing the radicle growth of the target species. Over riparian soil substrate, the aqueous leaf litter extracts did not produce any phytotoxic effect on the target species, except in one case. Altogether, our results highlight the importance of using both a native control when assessing the phytotoxicity of nonnative plants and also the natural soil in the modulation of phytotoxic effects. Ignoring both factors in laboratory bioassays would have led to the overestimation of the phytotoxicity of the exotic species as a mechanism contributing to their invasion success.     

黄土高原羊圈沟小流域人工物种和自然物种径向生长对气候变化的响应差异

为揭示黄土高原人工和自然物种径向生长对气候变化的响应差异,在延安羊圈沟小流域分别获取人工和自然物种的树木年轮材料并构建标准年表,其中人工物种为刺槐(Robinia pseudoacacia)和柠条(Caragana korshinskii),自然物种为山杏(Armeniaca sibirica)和荆条(Vitex negundo var.heterophylla),并对年表中的气候信号进行了统计分析。结果表明:1)人工物种年表中的气候信号较强,主要表现在5—8月份,与温度呈负相关关系(刺槐:r=-0.427—-0.511,P0.05;柠条:r=-0.227—-0.738,P0.05),与降雨则呈正相关关系,但相关系数未达到显著性水平;自然物种年表中的气候信号较弱,与温度和降雨的相关关系均较低;2)不同于自然物种,人工物种树轮年表还与去年夏季(7—9月份)温度(负相关)和降水(正相关)存在相关关系,表明人工物种树木生长对气候因子存在一定滞后性;3)人工物种树轮年表与PDSI干旱指数在各月份均维持正相关关系,在生长季(刺槐4—9月、柠条4—8月)达到显著水平(刺槐:r=0.481—0.704,P0.05;柠条:r=0.314—0.610,P0.05);而自然物种年表与PDSI干旱指数的相关关系较弱,均未达到相关性水平。从各年表与气候要素(温度、降雨、PDSI)响应强度来看,黄土高原人工物种树木生长受水分胁迫显著,且以刺槐最为明显,其次是柠条;自然物种树木生长则没有明显干旱胁迫的影响,仅山杏生长受一定水分胁迫影响,荆条生长则与各气候要素关系较弱,水分胁迫对其生长的影响已很小。本研究的结果表明黄土高原人工物种生长明显受到水分条件限制,而自然恢复物种生长则受水分条件影响较小,能适应黄土高原干旱半干旱气候条件。     

Influence of crop background on natural enemies of aphids on Brussels sprouts

The most abundant natural enemies of aphids on Brussels sprout crops were Syrphidae, different species being attracted differentially to weedy or weed-free plots according to whether they were more host-plant-orientated (e.g. Melanostoma spp., Platycheirus spp.) and thus affected directly by the background, or more aphid-orientated (e.g. Syrphus balteatus) and so less affected by background than by aphid numbers. Oviposition by Melanostoma spp. was usually much greater in weedy sprout crops than on sprouts in bare soil, and their eggs were also very abundant on weeds. Eggs of other syrphid species were scarcer on weeds. In contrast to Melanostoma, Platycheirus spp. usually oviposted preferentially on sprouts growing in bare soil. Oviposition by S. balteatus was in response to aphid abundance and thus tended to become greater on sprouts in bare soil. Notably more adults of non-aphido-phagous Syrphidae were caught over weedy than over non-weedy Brussels sprout plants. Anthocoris nemorum nymphs and adults were very common on sprout plants and weeds in the weedy crop but were scarce on sprouts in bare soil; A. nemorum oviposited on white and yellow charlock (Raphanus raphanistrum and Sinapis arvensis) occasionally. Parasitism of Brevicoryne brassicae by Diaeretiella rapae appeared to be related to aphid numbers and was only indirectly influenced by the crop background. Field experiments with green and brown cloth backgrounds showed that some syrphids were attracted to green; A. nemorum was relatively scarce over both artificial backgrounds. It is concluded that decreased natural enemy action is partly responsible for the initially greater abundance of B. brassicae in a weed-free crop of Brussels sprouts than in a weedy crop.     

Leaf litter age regulates the effect of native and exotic tree species on understory herbaceous vegetation of riparian forests

Decomposing litter is regarded as the most important source of allelochemicals released into soil. In this study, a greenhouse experiment was designed to assess the net effect of differently aged leaf litter from exotic (Ailanthus altissima, Robinia pseudoacacia and Ulmus pumila) and native riverine trees (Populus alba, Populus nigra and Ulmus minor) on the germination and growth of three herb species (Trifolium repens, Dactylis glomerata and Chenopodium album). We also characterized the chemical composition of litter samples at different litter ages (0, 1, 2 and 3 months) based on phenolic compounds, fibers and ergosterol (as a measure of fungal biomass) contents. Overall, litter from both native and exotic species had a negative effect on shoot and root growth of target species, indicating that phytotoxic effects of litter predominate over positive effects. The inhibition effect of the exotic species was similar or even lower than that of the natives, which does not support the Novel Weapons Hypothesis. Among exotic trees, U. pumila showed the highest inhibition effect on the growth of the target species. T. repens was the most sensitive target species. The importance of litter age varied with both target and donor species. In general, D. glomerata was more inhibited by fresh litter, C. album by half-decomposed litter of U. pumila and R. pseudoacacia and by fresh litter of A. altissima and T. repens was more inhibited by fresh litter of A. altissima and P. alba and by highly decomposed litter of U. minor. The concentration of total phenolics and flavonoids decreased while acid detergent fiber, lignin and ergosterol increased with increasing litter age. Hydroxybenzoic and protocatechuic acids and the flavonoid quercetin were detected in all litter species and at most of the litter ages, while gallic, chlorogenic, vanillic, coumaric and rosmarinic acids were species-specific and they were only detected in fresh litter. Ergosterol concentration appeared as the strongest constrictor of inhibitory effects of litter on understory species. The results of this study contribute to the understanding of the net effect of fresh and decomposed litter from exotic and native trees on the growth of understory species.