Sprout recruitment and self-thinning of Erica multiflora after clipping

Regrowth after clipping and the effect of local competition were studied in a natural population of Erica multiflora in a Mediterranean shrubland, by removing neighbours at 1 and 2 m around the target plants during four growing seasons. Removal of surrounding natural vegetation increased the number, the density (number of sprouts per stump area) and the biomass of the sprouts growing from clipped plants. Target plants ònly interacted with their near neighbours. Target plants had a negative relative increment in the number of sprouts per stump during the 18 months immediately following treatment, but a positive increment thereafter, which suggests that there was a constant or episodic recruitment of sprouts within the stump after clipping. Competition treatment had a non-significant effect on the negative increment of sprouts per stump. The self-thinning trajectory was different for the different competition treatments: there was an allometric negative relationship between density of sprouts and mean biomass of survivors during all sampling periods in genets without neighbours in a 1-m radius; the self-thinning trajectory of sprouts in genets without neighbours in a 2-m radius was short, a net increase in sprouts per stump area was accompanied by an increase in mean sprout biomass 30 months after clipping. During the same period, however, plants with neighbours showed a decline in both the sprout biomass and density.     

EFFECTS OF COMPETITION AND DISTURBANCE ON THE RESPROUTING PERFORMANCE OF THE MEDITERRANEAN SHRUB ERICA MULTIFLORA L. (ERICACEAE)

Two field experiments were designed to evaluate the importance of competition, fire, repeated disturbance, and their interactions on the vegetative and reproductive performance of the Mediterranean shrub Erica multiflora over a 2.5-yr period. In a burn experiment, fire was applied to the ground-level stumps of previously clipped 13-yr-old plants with a propane torch and competition was diminished by removal of neighboring plants. Fire resulted in a reduction of sprout vigor and biomass of flowers; mature neighbors also reduced E. multiflora sprout vigor and flowering. The interaction between fire and competition was nonsignificant. In a stand burned by a wildfire we studied the effects of regenerating neighbors on target plants by removing all neighbors or only Quercus coccifera, the most dominant species in the burned stand. In this stand we also simulated herbivory by repeatedly clipping the sprouts of E. multiflora. Regenerating neighbors did not affect target plant sprout vigor after the wildfire, but did cause a decrease in the biomass of flowers per plant. Survival decreased after repeated clipping but was not affected by neighborhood treatment. The results suggest that the importance of competition on resprouting vigor was temporally variable. Variables related to plant size rather than species determined competitive superiority: resprouting neighbors did not affect resprouting performance of target plants, but mature neighbors did. In nature, fire may directly reduce vegetative and reproductive biomass by the heating effect. But it may have an indirect positive effect on biomass, by reducing competition among plants. Frequent disturbances that removed aboveground biomass of E. multiflora had a detrimental effect on target plant survival independent of neighborhood effect.     

Facilitation in an unproductive boreal forest understorey community

Questions: We tested the hypothesis that if competition had a significant influence in structuring this boreal plant community, removal of neighbours, addition of fertilizer and addition of water would all benefit the transplanted seedlings. Alternatively, if facilitation had a greater influence, then removal of neighbours would be detrimental to the transplants but fertilization and watering would still be beneficial. Location: Understorey of the boreal forest in southwestern Yukon Territory, Canada (138°22′W; 61°02′N). Methods: Ten of the most common species were transplanted as seedlings into transects from which all neighbours had been removed, and also into transects with intact vegetation. We used a factorial design with two levels of watering and two levels of fertilization; this allowed us to test effects at both species and community level. Results: The summed survival and total biomass of all transplants was significantly higher in the presence of neighbours than without neighbours, indicating a facilitative effect of neighbouring plants, but there were significant increases in only six of the ten species. The combined survival and biomass of all species increased with watering, survival decreased and biomass increased with fertilization, but only two species had significant responses to fertilization: Anenome parviflora decreased and Mertensia paniculata increased in biomass. Watering increased the biomass of Achillea millefolium, Festuca altaica and Solidago multiradiata; there were also some interaction effects. Conclusions: (1) The presence of neighbours was generally facilitative. (2) Fertilization had negligible effects, and watering had minor beneficial effects. (3) This study demonstrates the importance of facilitation in structuring this boreal understorey community.     

川滇高山栎灌丛萌生过程中的营养元素供应动态

萌生更新是森林更新的重要方式, 是硬叶栎林受到干扰后植被恢复的主要机制。以位于青藏高原东南缘的川西折多山东坡川滇高山栎(Quercus aquifoliodes)灌丛为研究对象, 调查分析了砍伐后灌丛萌生过程中基株根系和萌株生物量动态、营养元素含量, 以及基株根系和土壤对萌株生长过程中的营养元素供应动态。结果表明, 川滇高山栎灌丛平均地上和地下生物量分别为(11.25 ± 0.92) t·hm^-2和(34.85 ± 2.02) t·hm^-2, 具有较大的根冠比(3.10:1); 萌生过程中, 萌株生物量呈线性增加趋势, 以灌丛活细根生物量变化为最大, 其次是活中根和活粗根, 树桩和根蔸生物量变化最小; 萌生过程中, 灌丛细根和中根N、P含量表现为先增加、后降低的变化趋势, 萌生初期树桩、粗根和根蔸中N和K的含量明显下降, 根蔸中Ca含量略有下降, 而P没有明显下降, 根系Mg含量变化幅度较大, 灌丛地下根系储存了较多的营养元素; 土壤、树桩、粗根和根蔸是川滇高山栎灌丛砍伐后0-120天萌生生长的主要营养来源, 砍伐后60天, 萌株生长所需的营养除K元素主要来源于根系外, 其余营养元素主要来源于土壤; 在砍伐后60-120天, 基株根系对萌株生长所需的N、K和Ca贡献较大, 而对P和Mg的贡献较小; 在砍伐后120-180天, 根系除K元素对萌生生长还保持较大的贡献外, 对其余营养元素的贡献均较小。高山栎林管理要注重加强地下根系的保护。     

The influence of below ground herbivory and plant competition on growth and biomass allocation of purple loosestrife

Experiments investigating plant-herbivore interactions have primarily focused on above-ground herbivory, with occasional studies evaluating the effect of below-ground herbivores on plant performance. This study investigated the growth of the wetland perennial Lythrum salicaria (purple loosestrife) under three levels of root herbivory by the weevil Hylobiustransversovittatus and three levels of plant competition by the grass Phleumpratense in a common garden. Plant growth, flowering phenology, and biomass allocation patterns of purple loosestrife were recorded for two growing seasons. During the first year, root herbivory reduced plant height; plant competition delayed flowering; and the interaction of root herbivory and plant competition resulted in reductions in plant height, shoot weight and total dry biomass. Plant competition or larval feeding did not affect the biomass allocation pattern in the first year. These results indicate the importance of interactions of plant competition and herbivory in reducing plant performance – at least during the establishment period of purple loosestrife. In the second growing season, root herbivory reduced plant height, biomass of all plant parts, delayed and shortened the flowering period, and changed the biomass allocation patterns. Plant competition delayed flowering and reduced the dry weight of fine roots. The interaction of root herbivory and plant competition delayed flowering. Root herbivory was more important than plant competition in reducing the performance of established purple loosestrife plants. This was due, in part, to intense intraspecific competition among the grass individuals effectively preventing shoot elongation of P. pratense and resulting in a carpet like growth. Received: 3 April 1997 / Accepted: 27 July 1997     

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.     

Demographic consequences of drought in the herbaceous perennial Cryptantha flava: effects of density,associations with shrubs,and plant size

The demographic consequences of a severe drought year were examined for two experimental plantings of the herbaceous desert perennial Cryptantha flava(Boraginaceae) in northeastern Utah, United States. A total of 6680 nutlets were planted individually or in clusters of four both under shrubs and in open microhabitats within two natural populations. Survival, growth, and flowering as a function of density and microhabitat were followed for 7 years, including 1 year when precipitation just before and during the growing season was 74.5% below normal. The design permitted assessment of how intraspecific density and shrub cover affect demographic response to drought. Mortality increased and flowering decreased dramatically during drought but neither varied with density or between shrub and open microhabitats. For plants growing under shrubs, survival (at Site 1) and growth (at Site 2) varied with shrub species. Average aboveground plant size also decreased during drought. Population size hierarchies were rearranged because larger plants lost leaf rosettes while many smaller plants grew. Density and microhabitat affected plant performance in non-drought years but more often at Site 1 than at Site 2. Individuals growing alone often were more likely to flower and/or produced more inflorescences when they did flower than did individuals growing with at least one other plant. However, for 2 years, survival rates at Site 1 were higher for plants growing in clumps than for single individuals. Shrubs also had mixed effects on plant performance. In some years, survival was higher under shrubs, but at Site 1 plants in the open often were more likely to flower and/or produced more inflorescences. Thus despite severe demographic consequences of drought, the study provided no evidence that intraspecific competition, interference by shrubs, or facilitation by shrubs increases under limited soil water.     

Drought and soil fertility modify fertilization effects on aphid performance in wheat

Agricultural intensification and climate change are expected to affect pest performance through excessive inputs of chemical fertilizers and increased probability of extreme drought events. Potential interactive effects of fertilization and water availability on aboveground pest performance may depend on soil fertility because of its effect on nutrient availability. In a greenhouse experiment, we examined the effects of inorganic fertilization on the performance of the grain aphid (Sitobion avenae, F.), an important pest of wheat, under different conditions of soil fertility and water availability. We found soil fertility and water availability to influence the positive effects of inorganic fertilizers on aphid growth, i.e. fertilization promoted faster aphid development time and higher fecundity and biomass under low fertility and under well-watered conditions. Moreover, although increased soil fertility favored aphid growth under well-watered conditions, it simultaneously sustained plant development. The current practices promoting soil fertility do not have direct negative consequence on crop protection under conventional cropping systems.     

Nutrient and carbon relations in subalpine dwarf shrubs after neighbour removal or fertilization in northern Italy

Two subalpine dwarf-shrub heath communities with differing levels of soil nutrient availability were subjected to a 3-year experimental manipulation, including nutrient addition or removal of one of the two co-dominant species from each community. The main objective of our study was to assess the relative importance of interspecific competition versus nutrient limitation in relation to soil fertility. We also aimed to investigate if and to what extent current-year shoot size, leaf-based rates of net photosynthesis and foliar nutrient status accounted for the observed changes in the aboveground biomass of the shrubs. At the end of the experiment, neighbour removal increased the aboveground biomass of all shrubs, especially in the more fertile community, while fertilization did not. We concluded that: (1) competition is more effective than nutrient limitation in structuring the vegetation of subalpine heathlands; and (2) competition intensity is stronger in the more fertile community. The observed patterns of variations in aboveground biomass were not consistently related to net photosynthetic rates, size of individual shoots and foliar nutrient status. Hence, we also concluded that the growth response of dwarf shrubs to altered environmental conditions is primarily determined by developmental plasticity.     

Multiple effects of shrubs on annual plant communities in arid lands of South Australia

Abstract The presence of shrubs in arid lands creates spatial heterogeneity that affects the distribution and performance of annual plants; several possible mechanisms have been implicated. A preliminary survey in a chenopod shrubland in South Australia showed differences in the distribution of annual plants under canopies of Atriplex vesicaria and Maireana sedifolia (the two dominant shrub species) and open spaces. A series of experiments were conducted to test the potential contribution to these patterns of nutrient enrichment under shrubs, differential seed accumulation, stress reduction by the canopy, competition by shrub roots, and protection against grazing. The germinable soil seed‐bank under A. vesicaria and M. sedifolia was different from that in open spaces, but these differences can only explain a fraction of the differences observed in the growing annual plant community in different microsites. The soil under A. vesicaria had higher total nitrogen content than soil in open spaces, whereas soil under M. sedifolia had lower available phosphorus than open spaces. Although annual plant densities under A. vesicaria were higher than in open spaces, experimental removal of shrubs increased their density, suggesting that shrub canopies inhibit annual plants in this system. Surprisingly, trenching of open areas close to shrubs (severing lateral shrub roots) decreased annual plant density. We suggest that water moves laterally through shrub roots, in a process akin to a hydraulic lift, increasing water availability for the annual plants. Exclusion of vertebrate grazers had a stronger effect on annual plant biomass in open spaces than under M. sedifolia, suggesting that this shrub provides shelter against herbivory. Overall our results show that shrubs can have simultaneously facilitative and inhibitory effects on the annual plant community through different mechanisms, but more importantly that different shrub species have different effects. This is a potential mechanism allowing for species coexistence of annual plants.     

Effect of arbuscular mycorrhiza on inter- and intraspecific competition of two grassland species

We were interested in the role of arbuscular mycorrhiza (AM) in the competition between plants of different sizes. A pot experiment of factorial design was established, in which AM root colonization and competition were used as treatments. Five-week-old Prunella vulgaris seedlings were chosen as target plants (i.e. plants whose response to competition was studied) and the following (13 replicates of each) were used as neighbours: (1) a large, 10-week-old P. vulgaris, (2) two P. vulgaris seedlings, and (3) a large, 10-week-old Fragaria vesca. In the experiment where small neighbours were grown together with small target plants, competition did not reduce target plant weight significantly, compared to the other two treatments. The competitive effects of large neighbours were significant, regardless of species (both older neighbours reduced the weights of target plants similarly), but there was a clear difference between intra- and interspecific competition when plants were mycorrhizal. In intraspecific competition with a large neighbour, the target plant shoot weight was reduced 24% when inoculated with AM. Thus, AM amplified rather than balanced intraspecific competition. In interspecific competition with old F. vesca, the shoot weights of target plants were 22% greater when inoculated with AM than when non-mycorrhizal. The results showed that, for given soil condition, AM might increase species diversity by increasing competitive intraspecific suppression and decreasing the interspecific suppression of small plants by larger neighbours.     

The effects of nutrient addition on floral characters and pollination in two subalpine plants, <Emphasis Type="Italic">Ipomopsis aggregata</Emphasis> and <Emphasis Type="Italic">Linum lewisii</Emphasis>

The availability of soil and pollination resources are main determinants of fitness in many flowering plants, but the degree to which each is limiting and how they interact to affect plant fitness is unknown for many species. We performed resource (water and nutrients) and pollination (open and supplemental) treatments on two species of flowering plants, Ipomopsis aggregata and Linum lewisii, that differed in life-history, and we measured how resource addition affected floral characters, pollination, and reproduction (both male and female function). We separated the direct effects of resources versus indirect effects on female function via changes in pollination using a factorial experiment and path analysis. Resource addition affected I. aggregata and L. lewisii differently. Ipomopsis aggregata, a monocarp, responded to fertilization in the year of treatment application, increasing flower production, bloom duration, corolla width, nectar production, aboveground biomass, and pollen receipt relative to control plants. Fertilization also increased total seed production per plant, and hand-pollination increased seeds per fruit in I. aggregata, indicating some degree of pollen limitation of seed production. In contrast, fertilization had no effect on growth or reproductive output in the year of treatment on L. lewisii, a perennial, except that fertilization lengthened bloom duration. However, delayed effects of fertilization were seen in the year following treatment, with fertilized plants having greater aboveground biomass, seeds per fruit, and seeds per plant than control plants. In both species, there were no effects of resource addition on male function, and the direct effects of fertilization on female function were relatively stronger than the indirect effects via changes in pollination. Although we studied only two plant species, our results suggest that life-history traits may play an important role in determining the reproductive responses of plants to soil nutrient and pollen additions.     

Diversity of diploid androgenic Brussels sprout plants of R0 and R1 generations

Androgenic Brussels sprout plants were produced by the use of anther culture from the donor cultivar 'Philemon F1'. A total of 96 plants obtained from 20 androgenic R0 genotypes assigned as diploids were evaluated both in the generative and vegetative stage, in respect of their morphological characters: mean plant height; leaf size, colour and waxiness; leaf blade shape, blistering and attitude; number of sprouts; as well as their self-incompatibility and fertility. Androgenic R0 plants derived from each of the 20 embryos were highly diversified and differed from the donor in one or more morphological traits in the vegetative stage. Evaluated populations also varied in fertility and self-incompatibility. Six androgenic genotypes that set a sufficient amount of seeds of the R1 generation and 'Philemon F1' were evaluated in the field in respect of plant height, total and marketable yield per plot, shape of stem with sprouts, shape and density of sprouts, and spacing between sprouts. Only four diploid R0 and R1 populations may have some value for further breeding, as they are characterised by good vigour, high or medium ability for sprout generation, and sufficient fertility.     

Factors determining plant–neighbour interactions on different spatial scales in young species-rich grassland communities

In naturally colonised species-rich grassland communities, we examined the properties of a plant’s aboveground neighbourhood that affect its performance (aboveground biomass). To this end a range of neighbourhood parameters were measured: number, biomass and species richness of the neighbours, number and biomass of the conspecific neighbours, and light availability at the base of the target plant. We also determined at which neighbourhood size the strongest target plant–neighbour interactions occurred, and whether conspecific neighbours affected competitively stronger or weaker target species differently. Target plant performance varied with target identity, and was significantly affected by light availability and the number of neighbouring plants (neighbourhood density). Depending on the target species, there was also an effect of total neighbour biomass on plant performance. The target plants were most strongly affected by their neighbours within a 3-cm distance, which could account for 78% of the variance in target biomass. Number or biomass of the conspecific neighbours did not contribute to the explanation of target performance in any of the target species. Whereas in an 8-cm neighbourhood the amount of light penetration was the strongest predictor of target performance, the number of neighbours was more important in a 3-cm neighbourhood. These experimental results might be useful to extend existing neighbourhood competition models for one or two species to multi-species competition models.     

Coexistence of plant species in a biodiversity hotspot is stabilized by competition but not by seed predation

Understanding the mechanisms of species coexistence is a key task for ecology. Recent theory predicts that both competition and predation (which causes apparent competition among prey) can either promote or limit species coexistence. Both mechanisms cause negative interactions between individuals, and each mechanism promotes stable coexistence if it causes negative interactions to be stronger between conspecifics than between heterospecifics. However, the relative importance of competition and predation for coexistence in natural communities is poorly known. Here, we study how competition and apparent competition via pre‐dispersal seed predators affect the long‐term fecundity of Protea shrubs in the fire‐prone Fynbos biome (South Africa). These shrubs store all viable seeds produced since the last fire in fire‐proof cones. Competitive effects on cone number and pre‐dispersal seed predation reduce their fecundity and can thus limit recruitment after the next fire. In 27 communities comprising 49 990 shrubs of 22 Protea species, we measured cone number and per‐cone seed predation rate of 2154 and 1755 focal individuals, respectively. Neighbourhood analyses related these measures to individual‐based community maps. We found that conspecific neighbours had stronger competitive effects on cone number than heterospecific neighbours. In contrast, apparent competition via seed predators was comparable between conspecifics and heterospecifics. This indicates that competition stabilizes coexistence of Protea species, whereas pre‐dispersal seed predation does not. Larger neighbours had stronger competitive effects and neighbours with large seed crops exerted stronger apparent competition. For 97% of the focal plants, competition reduced fecundity more than apparent competition. Our results show that even in communities of closely related and ecologically similar species, intraspecific competition can be stronger than interspecific competition. On the other hand, apparent competition through seed predators need not have such a stabilizing effect. These findings illustrate the potential of ‘community demography’, the demographic study of multiple interacting species, for understanding plant coexistence.     

Effect of local competition on resprouting of Arbutus unedo after clipping

Abstract. To evaluate the effects of local competition on the growth and size variability of sprouts following disturbance in a natural population of Arbutus unedo in Catalonia, plants were cut at the base and a neighbor removal experiment was performed. Removal of neighbors resulted in an increase in the number and biomass of sprouts at 2 and 7.5 months after clipping. Number and biomass of sprouts was also correlated with initial plant size (stump area). Inter-genet competition appeared to be symmetric, and acted to delay the onset of interference among sprouts within a genet. Size variability of sprouts on an individual was positively correlated with their density on the stump, supporting the hypothesis that competition among ramets (sprouts) within a genet is asymmetric. Reduced inter-genet competition from neighbor removal resulted in an increase in the number and biomass of sprouts growing from a stump. This resulted in an increase in the asymmetry of competition among sprouts, and therefore an increase in the size variability of these sprouts.     

Inhibition of growth,and effects on nutrient uptake of arctic graminoids by leaf extracts — allelopathy or resource competition between plants and microbes?

Previous research has shown that plant extracts, e.g. from boreal dwarf shrubs and trees, can cause reduced growth of neighbouring plants: an effect known as allelopathy. To examine whether arctic and subarctic plants could also be affected by leaching of phytochemicals, we added extracts from the commonly occurring arctic dwarf shrubs Cassiope tetragona and Empetrum hermaphroditum, and from mountain birch, Betula pubescens ssp. tortuosa to three graminoid species, Carex bigelowii, Festuca vivipara and Luzula arcuata, grown in previously sterilized or non-sterilized arctic soils. The graminoids in non-sterilized soil grew more slowly than those in sterilized soil. Excised roots of the plants in non-sterilized soil had higher uptake rate of labelled P than those in sterilized soil, demonstrating larger nutrient deficiency. The difference in growth rate was probably caused by higher nutrient availability for plants in soils in which the microbial biomass was killed after soil sterilization. The dwarf shrub extracts contained low amounts of inorganic N and P and medium high amounts of carbohydrates. Betula extracts contained somewhat higher levels of N and much higher levels of P and carbohydrates. Addition of leaf extracts to the strongly nutrient limited graminoids in non-sterilized soil tended to reduce growth, whereas in the less nutrient limited sterilized soil it caused strong growth decline. Furthermore, the N and P uptake by excised roots of plants grown in both types of soil was high if extracts from the dwarf shrubs (with low P and N concentrations) had been added, whereas the P uptake declined but the N uptake increased after addition of the P-rich Betula extract. In contrast to the adverse extract effects on plants, soil microbial respiration and soil fungal biomass (ergosterol) was generally stimulated, most strongly after addition of the Betula extract. Although we cannot exclude the possibility that the reduced plant growth and the concomitant stimulation of microbial activity were caused by phytochemicals, we believe that this was more likely due to labile carbon in the extracts which stimulated microbial biomass and activity. As a result microbial uptake increased, thereby depleting the plant available pool of N and P, or, for the P-rich Betula extract, depleting soil inorganic N alone, to the extent of reducing plant growth. This chain of events is supported by the negative correlation between plant growth and sugar content in the three added extracts, and the positive correlation between microbial activity, fungal biomass production and sugar content, and are known reactions when labile carbon is added to nutrient deficient soils.     

Interactive effects of soil fertility and herbivory on Brassica nigra

Soil nutrient availability may affect both the amount of damage that plants receive from herbivores and the ability of plants to recover from herbivory, but these two factors are rarely considered together. In the experiment reported here, I examined how soil fertility influenced both the degree of defoliation and compensation for herbivory for Brassica nigra plants damaged by Pieris rapae caterpillars. Realistic levels of defoliation were obtained by placing caterpillars on potted host plants early in the life cycle and allowing them to feed until just before pupation on the designated plant. Percent defoliation was more than twice as great at low soil fertility compared to high (48.2% and 21.0%, respectively), even though plants grown at high soil fertility lost a greater absolute amount of leaf area (38.2 cm² and 22.1 cm², respectively). At both low and high soil fertility, total seed number and mean mass per seed of damaged plants were equivalent to those of undamaged plants. Thus soil fertility did not influence plant compensation in terms of maternal fitness. However, the pathways used to achieve compensation in seed production were different at low and high soil fertility. At low soil fertility, relative leaf growth rates (area added per inital area per day) of damaged plants were drastically reduced over the second week of caterpillar feeding. Damaged plants recovered the leaf area lost to herbivory in the two weeks following insect removal by increasing leaf relative growth rates above the levels seen for undamaged plants, but the replacement of leaf tissue lost to herbivory came at the expense of stem biomass. At high soil fertility, relative leaf growth rates of damaged plants were similar to those of undamaged plants both over the second week of caterpillar feeding and following caterpillar removal, and stem biomass was not affected by herbivory. These results suggest that higher levels of soil nutrients increased the ability of plants to stay ahead of their herbivores as they were being eaten. Because damaged plants at high soil fertility were able to maintain leaf growth rates to a greater extent than damaged plants at low soil fertility, they did not fall as far behind undamaged plants over the period of insect feeding and did not have as much catching up to do after feeding ended to compensate for herbivory.     

The potential of soil amendment with insect exuviae and frass to control the cabbage root fly

Reliable options to control the cabbage root fly, Delia radicum L., are lacking in many countries as restrictions on insecticide use have tightened due to environmental concerns. Although microbial control agents are often considered as a sustainable alternative, their application in agriculture is constrained by inconsistent efficacy owing to low field persistence. To stimulate naturally occurring beneficial microbes, soil amendment with the residual streams of insect production has been suggested as an alternative to synthetic fertilization and a new approach to microbial crop protection. In a set of greenhouse experiments, exuviae and frass of black soldier fly larvae, Hermetia illucens L., house crickets, Acheta domesticus L. and exuviae of mealworms, Tenebrio molitor L., were added to soil from an organically managed field. Exuviae and frass treatments were compared to treatments with synthetic fertilizer. Brussels sprouts, Brassica oleracea L., plants were grown in amended soil for 5 weeks before being infested with cabbage root fly larvae. Insect and plant performance were assessed by recording cabbage root fly survival, biomass and eclosion time and seed germination and plant biomass, respectively. Whereas soil amendment with black soldier fly frass or exuviae reduced cabbage root fly survival and biomass, respectively, amendment with house cricket or mealworm residual streams did not negatively affect root fly performance. Furthermore, seed germination was reduced in soil amended with house cricket exuviae, while amendment with either residual stream derived from black soldier fly larvae or house crickets resulted in lower plant shoot biomass compared with the synthetic fertilizer treatment. Amending soil with black soldier fly residual streams could become a novel and low-cost tool to be integrated in cabbage root fly management programmes, especially where methods currently available are insufficient. Therefore, the mechanisms underlying the effects of insect-derived soil amendments described here should be the focus of future research.     

A negative heterogeneity–diversity relationship found in experimental grassland communities

Recent meta-analyses and simulation studies have suggested that the relationship between soil resource heterogeneity and plant diversity (heterogeneity–diversity relationship; HDR) may be negative when heterogeneity occurs at small spatial scales. To explore different mechanisms that can explain a negative HDR, we conducted a mesocosm experiment combining a gradient of soil nutrient availability (low, medium, high) and scale of heterogeneity (homogeneous, large-scale heterogeneous, small-scale heterogeneous). The two heterogeneous treatments were created using chessboard combinations of low and high fertility patches, and had the same overall fertility as the homogeneous medium treatment. Soil patches were designed to be relatively larger (156 cm²) and smaller (39 cm²) than plant root extent. We found plant diversity was significantly lower in the small-scale heterogeneous treatment compared to the homogeneous treatment of the same fertility. Additionally, low fertility patches in the small-scale heterogeneous treatment had lower diversity than patches of the same size in the low fertility treatment. Shoot and root biomass were larger in the small-scale heterogeneous treatment than in the homogeneous treatment of the same fertility. Further, we found that soil resource heterogeneity may reduce diversity indirectly by increasing shoot biomass, thereby enhancing asymmetric competition for light resources. When soil resource heterogeneity occurs at small spatial scales it can lower plant diversity by increasing asymmetric competition belowground, since plants with large root systems can forage among patches and exploit soil resources. Additionally, small-scale soil heterogeneity may lower diversity indirectly, through increasing light competition, when nutrient uptake by competitive species increases shoot biomass production.