F. Xavier Sans

A comparative growth analysis between alien invader and native Senecio species with distinct distribution ranges

ABSTRACT A good way to check hypotheses that explain the invasion of ecosystems by exotic plants is to compare congeneric alien and native species. To test the hypothesis that alien invaders grow faster than natives, the maximum relative growth rate and its components were compared in controlled growth conditions between four Senecio species, two aliens introduced from southern Africa (S. inaequidens and S. pterophorus) and two European natives (S. malacitanus and S. jacobaea). The four species colonize similar habitats, but the frequency and abundance of their populations and their distribution ranges differ. The two aliens showed a higher relative growth rate than the natives, and although there were differences between species for leaf area ratio, leaf dry matter content, and dry matter partition between stems, leaves, and roots, no clear pattern was detected to explain the differences in growth rates: several combinations of the components of the relative growth rate can give similar results. The higher relative growth rate of the alien species, combined with other ecological and life-history traits, may enhance their invasive capacity.

How organic farmers practice conservation agriculture in Europe

The interest of organic farmers in adopting conservation agriculture principles, including minimal soil disturbance, permanent soil cover and crop rotation has been growing since the early 2000s. However, currently there is no network for organic farmers practicing conservation agriculture, and a lack of knowledge on how organic farmers implement conservation agriculture in practice. Consequently, few technical references are available for organic farmers when they start applying conservation agriculture practices, in particular on controlling weeds without the use of herbicides. The main objectives of this study were: (1) to explore the diversity of conservation agriculture techniques (i.e., reduced tillage, no-tillage and green manures) practiced among European farmers, and (2) to identify farmers’ main strategies for implementing conservation agriculture and the agronomic and environmental factors that determine these strategies. Strategies were identified by analyzing survey results on: (1) the type and degree of use of conservation agriculture practices by farmers, and (2) the effects it produces in terms of soil disturbance and soil cover (low, medium and high). We carried out a survey of 159 European organic farmers and collected 125 data sets on management of winter-sown crops. Among the conservation agriculture practices, reduced tillage was used by 89%, no-tillage by 27% and green manure by 74% of the 159 interviewed farmers. Green manures were more frequently used in northern Europe than in the south (below 45°N). Most of the farmers used crop rotations, with a mean duration of 6 years. A wide diversity of conservation agriculture practices were used, with farmers rarely using all three techniques (no-till, reduced till and green manures) within one system. The range of practices was grouped into five strategies ranging from intensive non-inversion tillage without soil cover to very innovative techniques with no-tillage and intercrops. The five strategies for conservation agriculture could be grouped into two larger categories based on weed control approach: (1) intensification of the mechanical work without soil inversion or (2) biological regulation of weeds with cover crops. The diversity of strategies identified in this study shows that organic farmers use innovative approaches to implement conservation agriculture without herbicides. This studys findings will help organic farmers to experiment with innovative practices based on conservation agriculture principles and also benefit conventional farmers who use conservation agriculture practices and would like to reduce or eliminate the use of herbicides.

Weeds, aphids, and specialist parasitoids and predators benefit differently from organic and conventional cropping of winter cereals

The aphid–natural enemy interaction in winter wheat fields constitutes a complex system that has been frequently studied because of its implication for biological control. However, not all of the aphids living in cereal fields are crop pests, as there are also aphids living on weeds that may serve as alternative hosts or prey for aphid parasitoids or predators. In this context, a concomitant survey of the plant and insect communities was conducted to understand how different plant communities affect the abundance and richness of aphids and the interactions with their natural enemies. The plant community was split into functional groups (grasses, legumes and forbs), and the aphid community was divided into feeding groups according to their host preferences (specialists in grasses or forbs). The grass aphids, which dominated the total aphid catches, responded positively to grass cover, which was particularly enhanced in the conventional fields. Conversely, the forb aphids, which mainly conditioned the total species richness of the aphids, were closely correlated with the local abundance of legumes. The system of cereal aphid-parasitoids was enhanced in the conventional fields, where the abundance of grasses was higher, whereas the legumes of the organic fields indirectly played a key role in enhancing the richness of the parasitoids and the abundance of predators. Our findings indicate that a bottom-up effect exists throughout the plant community, aphids, and aphidophagous insects and that plant community characteristics should be considered to better understand cereal aphid control.

A novel index of land use intensity for organic and conventional farming of Mediterranean cereal fields

The conservation of biodiversity in agro-ecosystems is closely related to land use. Intensive land use is considered to be a major cause of biodiversity loss. Most studies addressing the effect of land use intensity on biodiversity have compared organic and conventional systems. However, little is known about the heterogeneity of the management intensity within each farming system. We hypothesise that there is a gradient of land use between and within the farming systems and that an index of management intensity is more useful than the farming system for evaluating the effects of the management practices on weed flora. In this study, 18 pairs of organic and conventional cereal fields were selected in northeastern Spain. The farmers were interviewed to gather information on the management practices performed. We selected the five following variables from these interviews: nitrogen inputs, crop diversity, weed control, seed origin and cereal ratio. We used principal components analysis to create a new management index. Weed species richness was recorded in 10 field pairs before crop harvest. Our results showed that the index values displayed huge variation within each farming system. Index values of conventional fields varied between −0.01 and 1.00, whereas within organic ones the values ranged from −1.19 to 0.18. The index better explained weed species richness than did the farming system. The index values demonstrate the existence of a land use intensity gradient, which indicates that it is an over-simplification to always equate organic farming with low intensity management. Here, we also prove that this new index is more appropriate for evaluating the effects of management practices on weed species richness than the classical organic-conventional dichotomy.

Positive vs. negative interactions in Picris hieracioides L., a mid-successional species of Mediterranean secondary succession

A field experiment was designed to evaluate the importance offacilitative and competitive interactions in Picrishieracioides, a facultative biennial that colonises the early andthemid-stages of secondary succession in the Mediterranean region. Seedlings ofPicris hieracioides from populations of the early- (1year)and the mid-stages of field abandonment (15–40 years) were transplantedintwo adjacent old fields, abandoned for 4 (F4) and 20 years (F20) and thatdiffered markedly in floristic composition and vegetation structure. For twoyears, we experimentally manipulated competition (no-neighbours vs. naturalvegetation) and resource availability (addition of water and fertiliser vs.controls) in an attempt to evaluate their influence on survival, reproductivetiming, growth and reproductive output throughout the life cycle. Earlymortality was higher in non vegetated plots in both fields. Mortality ofseedlings was mainly due to herbivory by larvae of genusAgriotes. Flowering throughout the whole experiment wasalso facilitated by vegetation in the F4 field as a result of the positiveeffect of annual vegetation and remained unaffected in the F20 field because ofthe high competitive effect of established perennial vegetation. The additionofresources altered the effect of facilitation and competition on late seedlingsurvival. Survival was enhanced in the vegetated plots of the F4 field, becauseresource addition increased the shade provided by the canopy of vegetation,protected seedlings from temperature extremes and reduced water loss. Seedlingmortality also decreased in the F20 field but in a similar manner to vegetatedand non-vegetated subplots, and consequently the outcome of positive andnegative interactions remained neutral. The net effect of facilitation andcompetition resulted in interference later in the life cycle and appearedthrough final lower growth and reproduction in both fields. However, thegreatercompetition in 1994 than in 1995 in both fields, probably because the size ofthe rosettes, makes them less susceptible to competition, illustrates thedifficulty in predicting the outcome of competition solely of one season forfacultative biennial plants. The relative competition intensity calculatedusingonly survivors (RCI1) was unaffected by habitat fertility in bothfields. In striking contrast, the relative competition intensity calculatedusing seedling mortality (RCI2) was significantly higher in subplotswithout resource addition in both fields because of high seedling mortality invegetated subplots. Finally, there were no differences in the net effect offacilitation and interference processes among populations from early and midsuccessional stages showing that phenotypic plasticity buffers theenvironmentalselective pressures linked to successional processes.

Herbicide impact on the growth and reproduction of characteristic and rare arable weeds of winter cereal fields

The decline of arable species characteristic of winter cereal fields has often been attributed to different factors related to agricultural intensification but most importantly to herbicide use. Herbicide phytotoxicity is most frequently assessed using short-term endpoints, primarily aboveground biomass. However, short-term sensitivity is usually not sufficient to detect actual effects because plants may or may not recover over time following sublethal herbicide exposures. Therefore, it is important to assess the long-term effects of herbicide applications. Annual species rely on renewable seed production to ensure their persistence; hence, assessment of herbicide sensitivity is more accurately estimated through effects on reproduction. Here we aim to assess the phytotoxicity of two commonly used herbicides: tribenuron and 2,4-D on eight plant species belonging to four families, each with one rare and one more common species. Specifically we examined the pattern of sensitivity using short-term and long-term endpoints (total aboveground biomass, total seed biomass and number of seeds) of these species; we determined the levels of and time to recovery in terms of stem length and fruit number, and assessed whether their rarity relates to their sensitivity to herbicide application. Our results suggest that although differences in herbicide sensitivity are not a direct cause of rarity for all species, it may be an important driver of declining arable plants.

Organic farmers’ motivations and challenges for adopting conservation agriculture in Europe

Conservation agriculture and organic farming are considered as promising sustainable agricultural system for producing food, while minimizing environmental impacts. Despite an increasing number of experimental data on organic conservation practices and various studies dealing with the adoption of conservation agriculture by farmers, none of those studies have specifically addressed conservation agriculture adoption under organic conditions in Europe. We carried out a survey with 159 farmers located in 10 European countries. These farmers had applied at least two of the following conservation practices: (i) no-tillage, (ii) reduced tillage and (iii) green manures. Each farmer assessed socio-economic, agronomic and environmental motivations and problems for each conservation practice, using a Likert scale. For each conservation practice, we ranked motivations and problems and carried out a principal component analysis, followed by clustering to identify groups of farmers. Independent of the conservation practices, the most important motivations were related to soil fertility preservation and challenges were mainly linked to crop management, machinery and yield performances. For all conservation practices, we identify three groups of farmers that shared the same type of motivations and challenges across Europe: “soil conservationists,” “agro-technically challenged farmers,” and “indifferent farmers.” Soil conservationist farmers were strongly motivated by soil preservation and minimizing environmental impacts. Agro-technically challenged farmers mainly expressed agronomic problems and challenges. There were no clear effects of location or farm characteristics explaining these attitudes, but they depended on farmers’ environmental concerns and beliefs. The study demonstrated that research priorities should address agronomic problems caused by the adoption of conservation practices in organic farming, weed control in particular.

Factors affecting plant species richness in field boundaries in the Mediterranean region

Field boundaries are expected to support the maintenance of biodiversity in agroecosystems, since they provide the habitat for a range of plant species. However, plant diversity in field boundaries has decreased substantially in recent decades. This pattern is generally linked with the intensification of agricultural land use at field and landscape level. Therefore, we aimed to test the effect of farming management (field and boundary management), boundary structure (width and habitat assemblage considering the Mediterranean grassland element), and landscape heterogeneity on plant species richness of field boundaries. Plants were recorded along 30 field boundaries next to organic fields and 30 next to conventional fields located in 15 agrarian localities of NE Iberian Peninsula along a gradient of landscape complexity. A total of 517 plant species were identified in the 60 field boundaries. We recorded 162 species (31%) catalogued as rare, very rare or extremely rare in the flora of the Catalan Countries. Our results showed the importance of landscape heterogeneity, field management and habitat assemblage, since they were found to be the most influential variables for plant species richness; whereas boundary width and boundary management were seen to contribute less to explaining plant diversity. Accordingly, agri-environmental schemes should be designed to promote organic farming and maintain the structure of the landscape mosaic in order to benefit plant diversity in field boundaries in the Mediterranean region.

Assessing the effect of inbreeding and long-distance gene flow on the invasive potential of Senecio pterophorus (Asteraceae)

Alien invasive species undergo genetic bottlenecks during the colonisation of new areas that can lead to a loss of genetic variation within populations and to subsequent reproductive constraints. We analysed the self-compatibility and the effects of inbreeding and inter-population gene flow in the fitness of one native and one introduced population of the invasive Senecio pterophorus D.C. Plants were self-pollinated and outcrossed within families, within populations, between local populations and between populations located in the native and introduced range. Self-pollinated individuals from both populations produced almost no seeds, thus revealing self-incompatibility. High family-level variation was observed in the effect of pollination treatment on seed set and total fitness. Overall, in the Spanish population, related crosses produced fewer seeds and lower germination rates than unrelated crosses. In the South African population, inbreeding depression affected the probability of flowering. Heterosis was found to affect seed set in both populations and growth and mean pre-reproductive time in the Spanish population. We discuss the effects of the incompatibility system, inbreeding depression and long-distance gene flow within the introduced population with respect to the invasive potential of S. pterophorus in north-eastern Spain.

Comparing the effect of habitat on the magnitude of inbreeding depression in the Mediterranean native Senecio malacitanus and the alien S. inaequidens: consequences for invasive ability

We studied the effects of inbreeding depression and the level of self-compatibility on overall fitness parameters in the invasive species Senecio inaequidens DC. and the native Senecio malacitanus Huter, in plots with and without inter- specific competition by natural vegetation. Competition had a stronger effect on fitness parmeters for both species, but it mostly affected the survival of S. malacitanus during the first year, and particularly the survival of individuals issued from inbred crosses. Inbreeding depression decreased the seed production in both species. Summer drought in the second year reduced the fitness of all inbreeding levels, masking the effect of inbreeding.The alien species had a shorter pre-reproductive time, a greater head production, and greater resistance to competition from established vegetation. In addition, a nega- tive relationship was found between inbreeding coefficient and herbivory in the native species only. All these factors may help to explain the invasive ability of S. inaequidens. The magnitude of inbreeding depression and the environ- mental conditions can thus determine the success or failure of an invasion.