I pursue research in several areas of ecology, including plant competition at the individual and population levels, plant growth and resource allocation, individual variation within plant populations, the application of ecological and evolutionary knowledge to plant production systems, and the relationship between ecology and environmental sciences. Projects currently underway include
Increasing the suppression of weeds by cereal crops
Studies of the size advantage in competition among individual plants suggest that the potential for many crops to suppress weeds is much greater than generally appreciated, and that this potential can be realized if (i) the crop density is increased substantially, and (ii) the crop is uniformly distributed in two-dimensional space rather than sown in traditional rows (see Weiner, Griepentrog & Kristensen 2001 under Publications). Recent experiments investigating the effects of different crop sowing patterns, density, fertility level and weed growth form on weed suppression (Olsen et al. 2005a, b; Olsen et al. 2006; Kristensen et al. 2006, 2008 under Publications) have provided strong support for this approach. We are currently applying evolutionary theory to further improve weed suppression (an approach we call “Evolutionary Agroecology”; see Weiner et al. 2010 under Publications), investigating genetic variation in weed suppression at high density, and testing the hypothesis that reduced morphological plasticity can be advantageous. The short-term goal is to reduce environmental impacts of agriculture by reducing herbicide application in conventional farming and providing an alternative to mechanical weed control in organic farming. The long-term goal is to develop "high density" cropping systems, in which crops themselves can suppress weeds much more effectively than under current practices, while offering other major improvements in sustainability. In collaboration with Sven Bode Andersen, Hans-Werner Griepentrog, Jannie Olsen, Lars Pødenphant Kiær and Wibke Wille (Ph.D. student). Previous funding from the National Research Council and the Department of Environmental Protection; current funding from the University of Copenhagen Program of Excellence.
Experiment with spring wheat (Triticum aestivum). The “weed” is Brassica napus (yellow flowers):
"Zone of Influence" model of stand development
We have developed a spatially explicit, individual-based plant competition model based on overlapping zones of influence. Plants are modelled as circles growing in two dimensions. The area of the circle represents resources available to the plant, and is allometrically related to its biomass. Plants compete for resources in areas in which they overlap, and the mode of competition is reflected in the rules for dividing up the overlapping areas. We have used the model to investigate and generate testable hypotheses on the effects of density, size-asymmetry and spatial pattern on size variability (Weiner et al. 2001 under Publications), the effect of the size-asymmetric competition on density-dependent mortality (“self-thinning”; Stoll et al. 2002), the relationship between asymmetric resource competition, density and growth (Weiner & Damgaard 2006), and the effects of facilitation on size-density relationships (Chu et al. 2008), size variation (Chu et al. 2009), and self-thinning (Chu et al. 2010). The computer code for the model is freely available to researchers (zoicode.zip), and it has also been implemented in the freely-available, multi-agent programming language and modeling environment NetLogo (Chu et al. 2008). Simulations addressing a specific question can serve as the basis for a Master Thesis (Speciale) for students interested in computer modelling and plant population ecology. Interested students should contact me (jw@life.ku.dk).
Subplots at t = 10, 20, at low (100) and high (992) density, growing under asymmetric and symmetric competition, in random and uniform patterns. Plants with growth rate of 0 are shown with a gray outline. (From Weiner, J., Stoll, P., Muller-Landau, H. and Jasentuliyana, A. 2001. The effects of density, spatial pattern and competitive symmetry on size variation in simulated plant populations. American Naturalist 158, 438-450 under Publications.)
Can below-ground competition be “size asymmetric”?
The evidence to date suggests that the mechanism of size asymmetry in competition among individual plants is competition for light, which is a ”one sided” interaction, since higher leaves shade lower leaves, while lower leaves do not shade higher leaves. Competition below ground appears to be size symmetric, i.e. a larger plant with larger roots may have an advantage over a smaller plant with smaller roots, but this advantage is not “over-proportional”, which is the definition of size-asymmetric competition. It has been hypothesized that competition below ground can be size asymmetric if soil resources occur in patches that larger plants can reach and preempt before the roots of smaller plants can get their share. Attempts to create size-asymmetric competition below ground under controlled conditions have not been successful to date, but nor have these attempts been very rigorous. We are attempting to create a realistic scenario in which competition below ground may be size-asymmetric, by providing a high nutrient patch relatively deep in a lower nutrient soil. Such a patch may be “preemptable” by larger individuals, resulting in a size asymmetric interaction. In collaboration with Kristian Thorup-Kristensen, Anne Nygaard Weisbach (Ph.D. student) and Camilla Ruø Rasmussen (M.Sc. student)
Constant Final Yield
When plant populations are grown at different densities for a given period of time, total standing biomass initially increases linearly with density and then levels off, remaining constant at even higher densities. This phenomenon is known as “Constant Final Yield”, and it is one of the most well-established patterns in plant ecology. In a paper in press in Annual Review of Ecology, Evolution and Systematics (see Weiner & Freckleton under Publications), we review the empirical basis for this general pattern, mathematical formulations of it, its relationship to self-thinning, possible mechanisms, and exceptions. Finally, we describe its fundamental importance for plant ecology. In collaboration with Robert Freckleton (University of Sheffield).
Biomass-density relationships of a crowded stand over time.
Positive and negative correlations between neighbor sizes
When large individual plants have large neighbors, it is often considered evidence for spatial heterogeneity. When large plants have small neighbors, it is thought to reflect competition among individuals. We use simple models to demonstrate that competition can result in either a positive (Wyszomirski & Weiner 2009 under Publications) or a negative correlation between plant size and neighbor size. In collaboration with Tomasz Wyszomirski (University of Warsaw).