Nicholas R. Jordan

SOIL CARBON ADDITION CONTROLS WEEDS AND FACILITATES PRAIRIE RESTORATION

Soil nitrogen enrichment and consequent vigorous weed growth are thought to hinder the restoration of tallgrass prairie. Adding carbon to the soil may facilitate prairie restoration by inducing immobilization of plant-available nitrogen. Early attempts to use this method, however, have had mixed results. Success of C addition depends on three conditions: weeds must suppress prairie species in the absence of C addition, weeds must be nitrophilic relative to prairie species, and C addition must result in a large enough decrease in N to alter the balance of competition among weeds and prairie species. We examined these conditions by comparing productivity of 10 weeds and 11 tallgrass prairie species under 14 levels of C addition, ranging from 84 to 3346 g C/m 2 . Carbon was tilled into the soil prior to planting. To control for non-N effects of C addition, N was added to a subset of plots. Relative to untreated plots, the highest level of C addition resulted in an 86% decrease in available NO3 -N, a1 43 increase in early season light availability, a 54% decrease in weed biomass, and a sevenfold increase in prairie biomass. Nitrogen addition significantly reduced or reversed all of these effects. Significant species-specific responses to C addition included decreased biomass for six annual weeds and increased biomass for six prairie species, one annual weed, and three perennial weeds. These results suggest that C addition may be a useful tool for restoring N-limited plant communities.

Prospects for Weed Control Through Crop Interference

Crop interference with weed growth is a fundamental method of nonchemical weed control. Crop interference, entailing weed suppression, should be distinguished from crop tolerance of weeds, i.e., relatively small yield loss due to the presence of weeds. Tolerance is less desirable than interference in that it may lead to increases in weed seed populations that can cause future yield losses. Ideally, interference should occur as early as possible in growth to prevent resource consumption by weeds. A path analysis of crop-weed interference is presented. The model can be used to estimate the relative magnitude of early vs. later crop interference with weed growth, and to identify crop traits that contribute to the interference with weed growth. Such analysis can identify crop varieties that show strong early interference with weed growth, and traits associated with strong early interference. For illustrative purposes, the model is applied to characterize differences among soybean varieties in interference with common cocklebur. Weed control by crop interference could be enhanced by crop breeding and management. However, several benefits and costs of interference should be evaluated before undertaking such approaches. First, the degree to which crop interference can substitute for other forms of weed control must be determined. Second, yield costs associated with interference should be measured; these may occur in crop varieties that interfere strongly with weeds, particularly when water, temperature, and mineral nutrients are limiting. A more mechanistic understanding of crop interference is needed so that interference can be enhanced through crop breeding and management while minimizing yield costs.

Soil modification by invasive plants: effects on native and invasive species of mixed-grass prairies

Invasive plants are capable of modifying attributes of soil to facilitate further invasion by conspecifics and other invasive species. We assessed this capability in three important plant invaders of grasslands in the Great Plains region of North America: leafy spurge (Euphorbia esula), smooth brome (Bromus inermis) and crested wheatgrass (Agropyron cristatum). In a glasshouse, these three invasives or a group of native species were grown separately through three cycles of growth and soil conditioning in both steam-pasteurized and non-pasteurized soils, after which we assessed seedling growth in these soils. Two of the three invasive species, Bromus and Agropyron, exhibited significant self-facilitation via soil modification. Bromus and Agropyron also had significant facilitative effects on other invasives via soil modification, while Euphorbia had significant antagonistic effects on the other invasives. Both Agropyron and Euphorbia consistently suppressed growth of two of three native forbs, while three native grasses were generally less affected. Almost all intra- and interspecific effects of invasive soil conditioning were dependent upon presence of soil biota from field sites where these species were successful invaders. Overall, these results suggest that that invasive modification of soil microbiota can facilitate plant invasion directly or via ‘cross-facilitation’ of other invasive species, and moreover has potential to impede restoration of native communities after removal of an invasive species. However, certain native species that are relatively insensitive to altered soil biota (as we observed in the case of the forb Linum lewisii and the native grasses), may be valuable as ‘nurse’species in restoration efforts.

Take a Closer Look: Biofuels Can Support Environmental, Economic and Social Goals

The US Congress passed the Renewable Fuels Standard (RFS) seven years ago. Since then, biofuels have gone from darling to scapegoat for many environmentalists, policy makers, and the general public. The reasons for this shift are complex and include concerns about environmental degradation, uncertainties about impact on food security, new access to fossil fuels, and overly optimistic timetables. As a result, many people have written off biofuels. However, numerous studies indicate that biofuels, if managed sustainably, can help solve pressing environmental, social and economic problems (Figure 1). The scientific and policy communities should take a closer look by reviewing the key assumptions underlying opposition to biofuels and carefully consider the probable alternatives. Liquid fuels based on fossil raw materials are likely to come at increasing environmental cost. Sustainable futures require energy conservation, increased efficiency, and alternatives to fossil fuels, including biofuels.

Innovative Education in Agroecology: Experiential Learning for a Sustainable Agriculture

The transdisciplinary field of agroecology provides a platform for experiential learning based on an expanded vision of research on sustainable farming and food systems and the application of results in creating effective learning landscapes for students. With increased recognition of limitations of fossil fuels, fresh water, and available farmland, educators are changing focus from strategies to reach maximum yields to those that feature resource use efficiency and resilience of production systems in a less benign climate. To help students deal with complexity and uncertainty and a wide range of biological and social dimensions of the food challenge, a whole-systems approach that involves life-cycle analysis and consideration of long-term impacts of systems is essential. Seven educational case studies in the Nordic Region and the U.S. Midwest demonstrate how educators can incorporate theory of the ecology of food systems with the action learning component needed to develop student potentials to create responsible change in society. New roles of agroecology instructors and students are described as they pursue a co-learning strategy to develop and apply technology to assure the productivity and security of future food systems.

FROM FLUORESCENCE TO FITNESS: VARIATION IN PHOTOSYNTHETIC RATE AFFECTS FECUNDITY AND SURVIVORSHIP

Genetic variation in photosynthetic traits within populations provides the potential for evolution, but few studies relate phenotypic variation in these traits to variation in fitness. We tested the prediction that a lower photosynthetic rate reduces fecundity and survivorship by comparing wild-type (WT) Amaranthus hybridus family lines to those having a single-gene mutation that confers resistance to atrazine (R) and lowers the rate of photosynthetic carbon assimilation. Wild-type and R family lines with nearly uniform nuclear genomes were used to minimize the confounding effects of other loci. We established experimental populations in agricultural and one-year-old field plots and measured chlorophyll fluorescence, gas exchange, and the fecundity and survivorship of WT and R genotypes for two generations. The R genotype had a lower efficiency of electron transport through photosystem II, which translated into a 20–30% decrease in photosynthetic rate at light levels above 400 μmol·m−2·s−1. Compared to the WT, the R genotype also had lower water-use efficiency, higher specific leaf area, and greater leaf nitrogen concentration on a mass, but not area, basis. In five of six replicate populations, the R genotype had lower fecundity than the WT in the first generation. Survivorship of seed over winter was similar for the two genotypes, but survivorship of R seedlings during early establishment was lower than the WT in the agricultural field. The consistent pattern of selection against the R genotype during vegetative growth stages suggests that a lower photosynthetic rate reduces fitness. This selection, paired with heritable variation for photosynthetic traits within populations, provides a more complete scenario for the evolution of photosynthetic traits.

Weed Control as a Rationale for Restoration: The Example of Tallgrass Prairie

The potential weed control benefits of ecological restoration are rarely cited and largely unstudied. Nevertheless, the nature of many restoration target communities, i.e., diverse, late-successional communities, suggests that restoration may control weeds and that the invasibility of plant communities may decrease with both diversity and successional age. Given the high cost of weed control in nonagricultural land, weed control benefits could be a strong incentive for restoration efforts. We examined the cumulative effects of restoration on weed populations 7 yr after tallgrass prairie restoration on a Minnesota sand plain. The numbers and biomass of volunteer weeds were compared among randomized plots with (1) no restoration, (2) prairie seed addition, and (3) site preparation plus prairie seed addition. After 7 yr, comparison with unrestored sites showed that site preparation plus prairie seed addition had reduced weed biomass by 94%, total weed stem number by 76%, and the stem numbers of four individual weed species. Prairie seed addition alone had no significant effect on weed biomass but reduced weed stem number by 45%. Restoration also reduced available light, which is consistent with the hypothesis that restoration may limit weed invasion by decreasing resource availability.

Contribution of photosynthetic rate to growth and reproduction in Amaranthus hybridus

Abstract While it is known that genetic variation for photosynthetic and growth traits exists in natural populations, the functional significance of this variation remains unclear, particularly for photosynthetic traits. To test the hypothesis that photosynthetic rate has direct effects on reproduction as well as contributing indirectly to reproduction through effects on growth, we compared wild-type Amaranthus hybridus families to those with a single gene mutation that confers a lower photosynthetic rate. Wild-type and photosynthetic-mutant families were grown in competitive and non-competitive environments and we compared size, biomass allocation, architecture, and reproduction at three developmental stages. To assess the contributions of individual growth traits to reproduction, we calculated covariances between standardized traits and relative fitness (selection differentials), and compared selection between the two biotypes. Finally, we used path analysis to calculate the indirect effects of photosynthetic rate on fitness through growth. The size, allocation, and architecture of photosynthetic mutants did not differ from those of the wild type in either the competitive or non-competitive environment, with the exception that they were taller by the last developmental stage. However, the reproductive biomass of the photosynthetic mutants was significantly reduced compared to the wild type. In the competitive environment, the wild type achieved greater fitness because, while similar in size to the mutants, at any given size it produced more reproductive biomass. This suggests that photosynthetic rate affected the linkage between plant size and reproduction and is evidence of an indirect contribution to fitness. In the non-competitive environment, there were fewer differences in selection differentials between the two plant genotypes, suggesting fewer indirect effects. Path analysis showed that variation in photosynthetic biotype had indirect effects on reproductive biomass, via growth traits, and that there were no direct effects. Photosynthetic rate appears to have fitness consequences primarily through multiple contributions to growth throughout development.

Responsiveness of certain agronomic weed species to arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) are plant root symbionts that provide many benefits to crop production and agro-ecosystem function; therefore, management of AMF is increasingly seen as important to ecological farming. Agronomic weeds that form a symbiotic relationship with AMF can increase diversity and abundance of agronomically beneficial AMF taxa. Also, AMF can strongly affect plant community composition, and may thus provide some degree of biological control for weeds. Therefore, relationships between weeds and AMF have a dual significance in ecological farming, but are relatively unexamined. In glasshouse experiments, seedlings of 14 agronomic weed species were grown in the presence or absence of AMF inocula sampled from each of three types of cropping systems: organic, transitional-organic or high-input/conventional. For each weed species, AMF root colonization rates and growth responses to AMF were assessed. On the basis of observed colonization levels, the species were classified as strong hosts (five species), weak hosts (three) and non-host species (six). Among species, biomass responses to AMF were highly variable. Strong hosts showed more positive responses to AMF than weak hosts, although the range of responses was great. Non-hosts did not suffer consistent negative biomass responses to AMF, although strong biomass reductions were noted for certain species–inoculum combinations. Biomass responses to inocula from different cropping systems varied significantly among weed species in one of two experiments. Results suggest that weed–AMF interactions can affect weed community dynamics. We recommend investigation of these interactions in agro-ecosystems that use management methods likely to intensify weed–AMF interactions, such as conservation tillage and cover cropping.

Effects of the triazine-resistance mutation on fitness in Amaranthus hybridus (smooth pigweed)

1. Fitness costs of maternally inherited triazine resistance were estimated in populations of Amaranthus hybridus (smooth pigweed) from Maryland and Virginia (USA), through fitness comparisons of lines that bear comparable nuclear genomes but had either resistant or susceptible cytoplasm. Under glasshouse conditions only modest costs were evident. However, larger fitness costs were often evident when seedlings were transplanted, over 3 years of field studies, into a diverse community of annual weeds in the field ; the relative fitness of resistant lines ranged from 0.24 to 1.12 (mean 0.62). 2. Fitness costs of resistance appeared to be larger in the population from Virginia. In the glasshouse, resistant plants of this population suffered a definite cost in terms of early seedling growth and biomass production (mean relative fitness of resistant lines = 0.82), while very little cost was evident in the Maryland population. In the field, the relative fitness of resistant lines from the Virginia population averaged 0.44 over 3 years, while that of resistant lines frm Maryland averaged 0.82. However, at high levels of neighbour interference in the field, the fitness costs were similar in the two populations and very large (relative fitness of resistant lines = 0.26). 3. A path analysis was used to estimate fitness differences between resistant and susceptible lines at different stages of growth in the field. The analysis apportioned total differences in seed production between resistant and susceptible lines into a series of independent components associated with three successive periods of growth. Large (e.g. 33%) fitness differences were evident during some growth periods in some years. However, costs were absent or statistically insignificant in other instances. These results do not support the hypothesis that a fitness cost of triazine resistance was consistently operative over the whole growing season. Rather, the mechanism(s) causing the fitness costs appeared to function sporadically.