Weed populations and crop rotations: Exploring dynamics of a structured periodic system

The periodic growing of a certain set of crops in a prescribed order, called a crop rotation, is considered to be an important tool for managing weed populations. Nevertheless. the effects of crop rotations on weed population dynamics are [lot well understood. Explanations for rotation effects on weed populations usually invoke the diversity of environments caused by different crops that a weed population encounters. Using a periodic matrix model, we show that the number of different crops is not the sole factor, and that the sequence of a given set of crops can play an important role, In the model the weed population is structured by seed depth in the soil. and plowing moves seeds between layers, For illustration of concepts, we use parameter values thought to be characteristic for Polygonum persicaria growing in carrots (crop A) and spring wheat (crop B) in the Netherlands. We systematically examine the population growth rates for P. persicaria and their sensitivity to changes for all rotations of 2-6 years based on crops A and B. We include eight scenarios that differ in the effects of plowing and seed survival over winter. Differences between rotations can be striking. For example the weed population growth rate in the baseline rotation AABB assuming 100% winter survival) is nearly 25% lower than in rotation ABAB. The elasticity (a measure to quantify the effect of proportional changes in model parameters on population growth) to seedling survival is nearly 75% higher in the B years of rotation ABAB than in the B years of rotation AABB. Changing parameter values changes the relation between population dynamics and rotation organization, but not the conclusion that there are consequences for population dynamics and management due to choice of a rotation. While our example is an agronomic one, the question "Does sequence matter'?" and the methods applied should be of interest to researchers and managers concerned with the periodic management of other ecosystems.