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, and the application of ecological and evolutionary knowledge to plant production systems. Projects currently underway include

Evolutionary Agroecology ("Darwinian Agriculture")

Evolutionary Agroecology is an attempt to apply ecological and evolutionary theories to address problems in agriculture. Darwinian evolution by natural selection is driven primarily by differential survival and reproduction among individuals within a population. It is a common popular scientific misunderstanding that natural selection inevitably works to increase the survival or performance of the population or species: over the past 30 years evolutionary biologists, using data from molecular biology to social behavior, have shown clearly that evolutionary interest of the individual is often in conflict with the interests and even the survival of the population or species. When this occurs, genes that increase individual fitness at the cost of population performance will increase in frequency.

According to this line of reasoning, plant breeding for agriculture is unlikely to improve attributes already favored by millions of years of natural selection, whereas there may be unutilized potential in selecting for attributes that increase crop yield but reduce plants' individual fitness, i.e. group selection. In collaboration with colleagues at the Institute of Arid Agroecology, Lanzhou University, we are testing the core hypothesis of Evolutionary Agroecology: genotypes that have the high individual fitness in a mixture of genotypes do not produce the highest population fitness (yield).  Similarly, suppression of weeds by a crop (described below) is a group activity.  It will be most successful if the individual crop plants do not use resources competing with each other, but cooperate in suppressing weeds.  According to evolutionary theory, such a strategy would not evolve in nature, because it reduces individual fitness. But it may be useful in agriculture, where ecosystems are directed to meet human needs.

In collaboration with Lars Pødenphant Kiær (University of Copenhagen), Wibke Wille (Phillip University of Marburg), Feng-min Li, Yanlei Du, Cong Zhang (Lanzhou University) and Xiao-Liang Qin (Northwest Agriculture and Forestry University).

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 in wheat. A study on maize in Colombia (Marín & Weiner 2014) also showed very promising results.

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. We argue that increased plant density in the field is the key to increased sustainability and reduced use of pesticides, while maintaining high yields. In collaboration with Jannie Olsen (Agronova) and Hans-Werner Griepentrog (University of Hohenheim). Previous funding from the Danish National Research Council, the Department of Environmental Protection and the University of Copenhagen Program of Excellence.

Experiment with spring wheat (Triticum aestivum). The “weed” is Brassica napus (yellow flowers):




How general is Constant Final Yield? Does it apply to plant communities?

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 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, so we have now undertaken such a review. If Constant Final Yield is close to universal, then exceptions are of special interest. In a new project, funded by the Danish Natural Science Research Council, we will ask if Constant Final Yield applies to multispecies communities as well as single-species populations.  If it does, it can play an important role in plant community ecology.  In collaboration with Wibke Wille (Phillip University of Marburg), Andrea Cavalieri (Univeristy of Copenhagen) and Jiangping Cai (Chinese Academy of Sciences, Shenyang).

Can below-ground competition be “size asymmetric”?

The evidence to date suggests that the primary 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, but not vice versa. 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.  In collaboration with Kristian Thorup-Kristensen, Anne Nygaard Weisbach and Camilla Ruø Rasmussen, University of Copenhagen.

Developing novel cropping systems utilizing “subsidiary crops”

A recently concluded European research project focussed on the development of 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. Subsidiary crops offer multiple functions in agriculture, such as protecting soil against erosion, increasing soil fertility and structure and reducing weed losses. The 4 year project, called OSCAR (Optimising Subsidiary Crop Application in Rotations) brought together 20 partners from 11 countries. OSCAR 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. Our focus within the project was 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. Subsidiary crops are an essential element in the development of truly sustainable agroecosystems. In collaboration with Jörg Peter Baresel (Technical University Munich) and all participants in the OSCAR project, lead by Maria Finckh (University of Kassel, Witzenhausen).



High crop density (600 seeds/m2)

Crop sown in rows

Low crop density (200 seeds/m2)

Crop sown in rows

High crop density (600 seeds/m2)

Crop sown in a uniform pattern