Claudio M. Ghersa

ENVIRONMENTAL CONTROL OF DORMANCY IN WEED SEED BANKS IN SOIL

Dormancy is a common attribute of many weed seed populations and this usually hampers the task of predicting timing and extent of emergence of weeds. Both the number of established plants and the timing of emergence of a weed are strongly related to the dynamics of dormancy release of the seed population. In this paper, we discuss the different factors that affect dormancy in weed seed banks in soil, aiming to set a conceptual basis that will facilitate the construction of predictive models. From the long list of factors that are known to control dormancy under field conditions, we distinguish those that modify the dormancy level of the population (i.e. soil temperature and soil hydric conditions) from those that terminate dormancy or in other words, remove the ultimate constraints for seed germination once the degree of dormancy is sufficiently low (i.e. light, fluctuating temperatures, nitrate concentration). We also discuss the effect of agricultural practices on dormancy of weed seed populations, making reference to studies that have evinced clearly the factor(s) involved in determining a particular pattern of response. Overall, we stress the importance of clarifying, both qualitatively and quantitatively, the interaction between soil thermal and hydric conditions in the modification of the degree of dormancy of seed populations. Similarly, it is essential that we understand the extent to which such changes in dormancy comprise changes in sensitivity to factors that terminate dormancy. # 2000 Elsevier Science B.V. All rights reserved.

Modeling seedling emergence

Most common approaches to predicting or documenting seedling emergence are imprecise. Mechanistic models that simulate seed dormancy and germination and seedling elongation as functions of measured or estimated environmental variables seem to be the most promising approach to the problem, but they also are the most difficult models to develop. These models will need to integrate soil water potential and soil temperature (hydrothermal time), diurnal soil temperature fluctuations, oxygen deficiency, light quality, and seed burial depth to better describe the direct and interactive effects on and among seed dormancy alleviation and induction, seed germination, and seedling elongation. In the meantime, creation and use of simpler empirical models, which also employ microclimate and soil factors for predictions, may provide sufficiently accurate predictions of seedling emergence until better mechanistic models are developed. Published by Elsevier Science B.V.

Symbiotic fungal endophytes control insect host–parasite interaction webs

Symbiotic microorganisms that live intimately associated with terrestrial plants affect both the quantity and quality of resources, and thus the energy supply to consumer populations at higher levels in the food chain. Empirical evidence on resource limitation of food webs points to primary productivity as a major determinant of consumer abundance and trophic structure. Prey quality plays a critical role in community regulation. Plants infected by endophytic fungi are known to be chemically protected against herbivore consumption. However, the influence of this microbe–plant association on multi-trophic interactions remains largely unexplored. Here we present the effects of fungal endophytes on insect food webs that reflect limited energy transfer to consumers as a result of low plant quality, rather than low productivity. Herbivore–parasite webs on endophyte-free grasses show enhanced insect abundance at alternate trophic levels, higher rates of parasitism, and increased dominance by a few trophic links. These results mirror predicted effects of increased productivity on food-web dynamics. Thus ‘hidden’ microbial symbionts can have community-wide impacts on the pattern and strength of resource–consumer interactions.

Limits to tree species invasion in pampean grassland and forest plant communities

Factors limiting tree invasion in the Inland Pampas of Argentina were studied by monitoring the establishment of four alien tree species in remnant grassland and cultivated forest stands. We tested whether disturbances facilitated tree seedling recruitment and survival once seeds of invaders were made available by hand sowing. Seed addition to grassland failed to produce seedlings of two study species, Ligustrum lucidum and Ulmus pumila, but did result in abundant recruitment of Gleditsia triacanthos and Prosopis caldenia. While emergence was sparse in intact grassland, seedling densities were significantly increased by canopy and soil disturbances. Longer-term surveys showed that only Gleditsia became successfully established in disturbed grassland. These results support the hypothesis that interference from herbaceous vegetation may play a significant role in slowing down tree invasion, whereas disturbances create microsites that can be exploited by invasive woody plants. Seed sowing in a Ligustrum forest promoted the emergence of all four study species in understorey and treefall gap conditions. Litter removal had species-specific effects on emergence and early seedling growth, but had little impact on survivorship. Seedlings emerging under the closed forest canopy died within a few months. In the treefall gap, recruits of Gleditsia and Prosopis survived the first year, but did not survive in the longer term after natural gap closure. The forest community thus appeared less susceptible to colonization by alien trees than the grassland. We conclude that tree invasion in this system is strongly limited by the availability of recruitment microsites and biotic interactions, as well as by dispersal from existing propagule sources.

The Role of Fluctuating Temperatures in Germination and Establishment of Sorghum halepense. Regulation of Germination at Increasing Depths

Field and laboratory tests explored the response of Sorghum halepense seeds to fluctuations in soil temperature, and whether that response serves as a mechanism for sensing depth and regulating seed germination. Seedling production in the field was severely curtailed when seeds were planted at increasing soil depths. Soil-temperature fluctuations were strongly dependent on depth. When soil columns containing seeds were incubated at alternating temperatures of 20 and 30°C, however, seed germination did not diminish with increasing depth, and an evaluation of how soil-surface shading affects the perception of depth by the seeds indicated that soil cover reduced germination of only those seeds in the upper strata of the profile (...)

Neotyphodium endophyte infection frequency in annual grass populations: relative importance of mutualism and transmission efficiency

Persistence and ubiquity of vertically transmitted Neotyphodium endophytes in grass populations is puzzling because infected plants do not consistently exhibit increased fitness. Using an annual grass population model, we show that the problems for matching endophyte infection and mutualism are likely to arise from difficulties in detecting small mutualistic effects, variability in endophyte transmission efficiency and an apparent prevalence of non-equilibrium in the dynamics of infection. Although endophytes would ultimately persist only if the infection confers some fitness increase to the host plants, such an increase can be very small, as long as the transmission efficiency is sufficiently high. In addition, imperfect transmission limits effectively the equilibrium infection level if the infected plants exhibit small or large reproductive advantage. Under frequent natural conditions, the equilibrium infection level is very sensitive to small changes in transmission efficiency and host reproductive advantage, while convergence to such an equilibrium is slow. As a consequence, seed immigration and environmental fluctuation are likely to keep local infection levels away from equilibrium. Transient dynamics analysis suggests that, when driven by environmental fluctuation, infection frequency increases would often be larger than decreases. By contrast, when due to immigration, overrepresentation of infected individuals tends to vanish faster than equivalent overrepresentation of non-infected individuals.

Ecological correlates of weed seed size and persistence in the soil under different tilling systems: implications for weed management

Abstract In the pursuit of more sustainable approaches to agricultural management, there is an obvious need to integrate knowledge of the agricultural sciences and ecology. We used recent findings on ecological correlates of seed characteristics as a framework to discuss the impact of shifting from soil ploughing to farming with reduced or zero tillage on weed communities. In the future, knowledge about seed bank ecology may become increasingly applicable towards improving the way we manage weeds. It is also possible that the effects of cultural activities on moulding weed characteristics will become more apparent.

Morphological responses of Datura ferox L. seedlings to the presence of neighbours Their relationships with canopy microclimate

SummaryWe studied the effects of density on the dynamics of seedling growth and canopy microclimate within experimental stands composed of Datura ferox L. seedlings grown in individual pots. Interception of photosynthetically active radiation (PAR) by seedlings was evaluated either indirectly, by measuring leaf area, proportion of leaf area shaded by neighbouring individuals and laminar orientation with respect to sunlight, or directly, by measuring PAR at individual leaves at their natural angle of display. An integrating cylinder, with a geometry approximating that of a stem, was used within the canopies to measure the red:far-red (R:FR) ratio of the light flux from all compass points parallel to the soil surface. Seedlings responded rapidly (i.e. 1–2 weeks) to increased density by producing longer internodes and partitioning more dry matter to stems relative to leaves. These responses were observed before either PAR interception of growth rate were reduced by the presence of neighbours. Conversely, morphogenetic adjustment was preceded by a consistent effect of plant density on the R:FR ratio of the light received by the integrating cylinder. Air and soil temperature were not affected by density in these experiments. Differences in wind velocity within the canopy associated with plant density were avoided by the experimental procedure. The results support the idea that the drop in R:FR ratio of the light flux parallel to the ground — e.g. reflected sunlight — is an early signal that allows rapid adjustment of plant form to changes in canopy structure.

The role of fluctuating temperatures in the germination and establishment of Sorghum halepense (L.) Pers. Regulation of germination under leaf canopies

1 The regulation of germination of Sorghum halepense (L.) Pers. under plant canopies, and its relation with temperature fluctuation requirements of these seeds, were studied. 2 Seed germination was depressed under closed leaf canopies and both seedling emergence and soil thermal amplitude were strongly related to canopy height. 3 A similar effect was observed when seed banks were subjected to increasing artificial shading. Heating the soil under dense canopies resulted in the same germination flux as observed in bare soil. 4 Germination responses to different alternating temperature regimes under controlled conditions were in agreement with the results of field experiments where soil temperature was modified either by shading or heating. 5 These results demonstrate that the alternating temperature requirements for germination of S. halepense seeds have a role in regulating seed germination under leaf canopies. Some ecological implications of this mechanism are discussed. Key-words: Sorghum halepense, fluctuating temperatures, germination

Woody species invasion in the Rolling Pampa grasslands, Argentina.

Information on the geographical distribution of plant invasion has been recorded in detail in some areas of the world; however, in large regions such as South America there are a few, if any, records of the spread of alien plants and invasive species and even less information about their effects on ecosystems at different levels of organization. This study examines the extent to which woody species introduced during the last centuries are invading the Rolling Pampa (which is typical of the entire region of the Argentina pampas) and discusses whether this invasion is related to the species’ genetics or to environmental factors. All woody species were surveyed along landscape corridors (highways and intersecting secondary dirt roads and streams), as well as in farmed fields under three different tillage systems: zero tillage in the entire field for all crops in the rotation (where tillage was replaced by a presowing herbicide application), zero tillage for selected crops, and conventional tillage. Landscape corridors along the roads had been invaded by 40 woody species (mostly trees). On the farmed land, fields under the zero tillage farming system were invaded by seven woody species (three tree species and four shrubs). With zero tillage for select crops only, woody species richness was reduced to three (one tree and two shrub species). In the conventional tillage, there were only three invading species, all shrubs. In both the roadside and riparian corridors, the species with the highest constancy values were Gleditsia triacanthos L., Morus alba L., and Melia azedarach L. In both types of zero tillage farmed fields, M. alba was absent, but G. triacanthos and M. azedarach remained the species with the highest constancy values. Both genetic and ecological factors were important determinants for the invasion of the pampas by woody species. The woody invasion process has reached a point at which the pampean grasslands on the better-drained soils will no longer be restored to a grassland biome without human intervention.