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, crop-weed interactions, 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). 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, Jannie Olsen, Lars Pødenphant Kiær, Wibke Wille and Hans-Werner Griepentrog (University of Hohenheim). 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):

   

   

   

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, but emprical support for this hypothesis is dubious. We created a realistic scenario in which competition below ground could be size-asymmetric, by providing a “preemptable” high nutrient patch relatively deep in a lower nutrient soil. There was little evidence of size-asymmetric below ground competition, and the evidence was strongest under homogeneous, low soil nutrient conditions. Below-ground competition may be slightly size-asymmetric in some situations, but strong size asymmetry arises from competition for light. In collaboration with Kristian Thorup-Kristensen, Anne Nygaard Weisbach (Ph.D. student) and Camilla Ruø Rasmussen (M.Sc. student).

"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 the model has also been implemented in NetLogo (Chu et al. 2008; Jia et al. 2011, Oikos; code available from Xin Jia xinjia830424@gmail.com). Simulations addressing a specific question can serve as the basis for a Master Thesis (Speciale) for students interested in ecological modelling or plant population ecology. Interested students should contact me (jw@life.ku.dk).

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2. 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, see Publications.)
   

Does variation in the degree of specialization help maintain local diversity?

According to current theory, tradeoffs are central to the maintenance of local diversity, but there are many potential tradeoff axes, so it may be useful to look for general axes, which can summarize many dimensions. The continuum between generalist and specialist adaptations may be such a summary axis, and we explore this idea with a conceptually simple simulation model in which there are patch types to which species arrive as propagules and compete. A specialist species is top competitor for a specific patch type, but has a low average competitive ability across patch types, whereas a generalist species has a high average competitive ability, but is not a top competitor in any patch type.  When fecundity of all species (and therefore density-dependent mortality) is high, specialists outcompete generalists. When fecundity is low, generalists outcompete specialists. There is a range of fecundity levels in which specialists, generalists and intermediates coexist, and the number of species is much greater than the number of patch types. The continuum from specialist to generalists is a "major axis" in multi-dimensional tradeoff space, and may play an important role in maintaining local diversity. In collaboration with Xiao Sa (Lanzhou University; see Weiner & Xiao under Publications)

                          

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).