Seedling emergence is one of the early life cycle constraints on population demography and dynamics. To better quantify this vital rate it is important to understand how variability across the landscape can fluctuate temporally and in response to exogenous factors such as disturbance. We monitored seedling emergence of an agricultural weed, the summer annual Amaranthus palmeri, across a large geographic area in response to the timing of disturbance. Seedlings of A. palmeri were monitored weekly throughout the growing seasons of 2013 and 2014 in 1-m2plots established in agricultural fields in five US States (Arkansas, Illinois, Indiana, Missouri, and Tennessee) subject to three disturbance treatments: no-tillage, early tillage, and late tillage to simulate different grower practices. Soil temperature and soil moisture were recorded hourly using data loggers. Statistical analyses of these data included use of Kolomogorov-Smirnov tests and Mixed-models to compare the effects of location, disturbance, and year on various emergence parameters, and the fitting of splines (piecewise polynomials) to model the effect of thermal time accumulation and disturbance timing on proportional cumulative seedling emergence.
Substantial differences in seedling emergence occurred between years, among locations, and among disturbance treatments. Cumulative seedling emergence differed significantly among all three disturbance treatments in both years in three locations (AR, IN, MO), with more limited differences in TN and IL. Mixed-model analyses showed that the total number of emerging seedlings, the last date of emergence, and the duration of emergence varied in response to the interaction of location and year, but not disturbance. However, disturbance affected the date of initial emergence, peak emergence, and maximum flush dates, reducing time to reach these emergence properties with later timing of tillage. Proportional cumulative seedling emergence was modelled fitting splines to thermal time accumulation and indicated significant effects of disturbance on emergence rate. Seedling emergence varies across a large geography and between years, but can be overridden by disturbance. The timing and duration of seedling emergence dictates the ability of a seedling to compete successfully with neighbors, including for this plant with crops. The findings presented here not only provide insight into the timing of seedling emergence in response to disturbance, but can inform growers on management strategies to combat the discontinuous patterns of emergence exhibited by this annual plant.