I pursue research in several areas of ecology, including plant-plant interactions, plant growth and resource allocation, individual variation within plant populations, crop-weed competition, 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 (Ph.D. student) 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 empirical 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 evidence of some degree of size-asymmetric below ground competition, and the evidence was strongest under homogeneous, low soil nutrient conditions. Below-ground competition may be size-asymmetric to some degree, but the data to date support the hypothesis that strong size asymmetry arises from competition for light. In collaboration with Kristian Thorup-Kristensen, Anne Nygaard Weisbach and Camilla Ruø Rasmussen (M.Sc. student).
How general is Constant Final Yield?
Constant Final Yield is a general pattern concerning total biomass production of plant stands growing at different densities after a period of time. Total standing biomass initially increases in proportion to density, levels off and then remains constant as density increases further. We recently reviewed the empirical bases for this phenomenon, mathematical models of it, mechanisms, and we argued for its central importance for understanding plant populations and communities (Weiner & Freckleton 2010, under Publications). We did not, however, test the pattern’s generality by reviewing as much of the relevant data as possible. If Constant Final Yield is close to universal, then exceptions are of special interest. We have therefore undertaken an extensive review of the published literature. In collaboration with Wibke Wille and Rob Freckleton (University of Sheffield).
"Zone of Influence" model of stand development
We have developed a spatially explicit, individual-based plant competition model based on overlapping zones of influence, which has been used in studies by many researchers. 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).
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.)
Developing novel cropping systems utilizing “subsidiary crops”
A European research project aims to develop novel cropping systems based on subsidiary crops (“cover crops” and “living mulches”) - crops sown with or after the main, harvested crop for their ecological services, to increase sustainability and reduce the need for inputs. The 4 year project, called OSCAR (Optimising Subsidiary Crop Application in Rotations) brings together 20 partners from 11 countries and is led by University of Kassel, Germany. OSCAR aims to increase the use of subsidiary crops, to increase the duration of soil coverage by plants, to introduce diversity to crop rotations, and to reduce the need for weed control and the intensity of soil tillage. Subsidiary crops offer multiple functions in agriculture, such as protecting soil against erosion, increasing soil fertility and structure and reducing weed losses. Our focus within the project is on managing competition between the main crop and the subsidiary crop to increase the long-term benefits of subsidiary crops while reducing short term yield loss in the main crop due to competition from the subsidiary crop. In the above-mentioned “high density cropping systems”, competition among plants in the field is seen as something to manipulate, not something to avoid. Subsidiary crops are an essential element in the development of truly sustainable agroecosystems.
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).