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 ecology and evolutionary biology to agriculture. Areas of special interest include

Evolutionary Agroecology 

Evolutionary Agroecology (also called Darwinian Agriculture or Crop Evolutionary Ecology) is an attempt to apply ecological and evolutionary theories to improve 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 is unutilized potential in selecting for attributes that increase crop yield but reduce plants' individual fitness: “group selection”. Together with colleagues from 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 yield (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. Crop plants still have many “selfish” behaviors that reduce population yield, such as proliferation of roots in response to the presence of neighboring roots (Zhu et al. 2019 under Publications). In collaboration with Feng-min Li, Yanlei Du, and Cong Zhang (Lanzhou University), and Yong-He Zhu (Nanjing Agricultural University).

Increasing the suppression of weeds by cereal crops

The disproportionate 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 in wheat (Olsen et al. 2005a,b; 2006; 2012; Kristensen et al. 2006, 2008; Wu et al. 2021; Xi et al. 2022) and maize (Marín & Weiner 2014).

The short-term goal is to reduce or eliminate herbicide application in conventional farming and provide 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 Jannie Olsen (VKST Field Trials), Hans-Werner Griepentrog (University of Hohenheim) and Nianxun Xi (Hainan University). Funding has come 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):

Low crop density (200 seeds/m2)

Crop sown in rows

High crop density (600 seeds/m2)

Crop sown in rows

High crop density (600 seeds/m2)

Crop sown in a uniform pattern

Allocation, yield and yield stability

Crop plants grow and then they allocate resources to different structures, including seeds and fruits, which represent yield in most crops. The relationship between growth and yield is allometric. Ecological theory and simple allometric models based on observed patterns predict a tradeoff between (a) yield stability - the ability of a genotype to produce yield over a wide variety of conditions and (b) potential yield - the ability to produce very high yields under optimal or near-optimal conditions. Yield stability in crops corresponds to bet-hedging in evolutionary ecology. It is the most appropriate strategy for smallholder farmers in developing countries, a group that comprises most of the farmers in the world (Weiner et al. 2021 under Publications). In collaboration with Feng-Min Li, Yan-Lei Du and Yi-Min Zhao (Lanzhou University).

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. Total standing biomass initially increases in proportion to density, levels off, and then remains constant as density increases further. We have reviewed the 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 are currently reviewing the relevant data to test the pattern’s generality and we now have experimental evidence that Constant Final Yield applies to multispecies communities as well as single-species populations (Cavalieri et al. 2022).  In collaboration with Andrea Cavalieri (University of Copenhagen), Wei-Ping Zhang (China Agricultural University), Wibke Wille and Jiangping Cai (Chinese Academy of Sciences, Shenyang).

The ecological basis of agricultural sustainability

Making agriculture more sustainable is one of the world's most important and urgent 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 replacement of mineral nutrients removed through harvesting. I argue that increased plant biomass density in the field is the key to increased sustainability and reduced use of fertilizers and pesticides, while maintaining high yields. Further research in this direction can lead us to new, productive yet sustainable plant production systems. Sustainability does not arise spontaneously in a market economy, so it must be a policy objective.