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. Ongoing projects 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 misunderstanding that natural selection inevitably works to increase the survival or performance of the population or species: the evolutionary interest of the individual is often in conflict with the interests of the population or species. When this occurs, natural selection will increase individual fitness at the expense of population performance. 

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 have tested 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, see Weiner et al. 2017 under Publications).  Similarly, suppression of weeds by a crop (described below) is a group activity (Weiner et al. 2010 under Publications).  It will be most successful if the individual crop plants do not use resources competing with each other, but cooperate in suppressing weeds. Such a strategy would not evolve in nature, because it reduces individual fitness. In collaboration with Lars Pødenphant Kiær (University of Copenhagen), Wibke Wille (Phillip University of Marburg), Feng-min Li, Yanlei Du, Cong Zhang, Yong-He Zhu (Lanzhou University) and Xiao-Liang Qin (Northwest Agriculture and Forestry University).


Increasing the suppression of weeds by cereal crops

The size advantage in competition among individual plants suggests 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 good 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. 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 (University of Copenhagen) and Jiangping Cai (Chinese Academy of Sciences, Beijing).

The ecological basis of agricultural sustainability

Making agriculture more sustainable is one of the world's most important challenges. "Sustainability" has become a "buzz-word”, so it is misused to promote specific interests. Most of the agricultural methods and practices that are called "sustainable" would more correctly be referred to as "slightly less unsustainable". Sustainability is an ecological phenomenon, almost by definition. We have the basic ecological knowledge needed to develop and practice truly sustainable agricultural systems (see Weiner 2017 under Publications). At the local level, agricultural sustainability is about the maintenance and improvement of soil fertility and the recycling of mineral nutrients removed through harvesting. Further research can lead us to new possibilities for productive yet sustainable plant production systems. Sustainability does not arise spontaneously in the current economic or political context, so it must be a policy objective.