Two-dimensional seed spread function in simulations, sans mathematical integration.
Two-dimensional seed spread function accounts successfully for wind, and shelter.
Unawned Bromus weed seeds were spread less within wheat, when sheltered by a hedge.
Seed movement of both Apera spica-venti and Bromus sterilis slower nearer to hedgerow.
Accurate predictions of weed seed dispersal are important for spatial modelling of weed population dynamics. However there are few multi-year data sets available, and they are seldom analysed well. Here we gathered together and re-examined population growth and seed spread data from three grass weed species over 3 years. Using a new seed spread function in the simulation model SOMER it was possible to accurately parametrise population growth and seed spread to replicate the field data of Apera spica-venti (APESV) and Bromus sterilis (BROST). In contrast, the greatest increases in Alopecurus myosuroides (ALOMY) numbers occurred in the direction of machinery movement, which could not be predicted using this function. A probability-sum, exponential-type function gave an excellent fit to the field data for APESV and BROST, with changes in seed spread distances due to species differences, wind, and ‘shading’ from the hedge, accommodated by alterations in the scale, shape, and wind dispersal parameters. Here we describe a method of parametrising the probability function from within a stochastic simulation. Working within the simulation allowed empirical population size data sets to be successfully parameterised, without mathematical integration. This function is fully explained with stochastic simulated examples, and fitted to various two-dimensional longitudinal data sets. Enough detail to enable its parametrisation (including simplification) within a majority of spatial stochastic population models is included.
Many weeds have seeds or seedpods that cling to the fur of animals and the clothing of humans as a result of sticky hairs, hooked spines, or barbed awns. Examples of weeds using this method of dispersion include Lappa minor (Lesser Burdock), Torilis arvensis (Common Hedge Parsley), and Bidens frondosa (Common Beggar’s Ticks). Other weeds have seeds that become sticky while wet, which enables them to cling to the bottom of hooves, webbed feet, or shoes. Species using this method of seed dispersal include Plantago lanceolata (English Plantain) and several Chamaesyce spp. (Prostrate Spurges).
Machine Agency. People spread many weeds around through their machines. For example, automobiles and trucks raise gusts of wind as they travel up and down highways. As a result, tiny seeds or seeds with tufts of hair are easily carried aloft in the wind and blown along the highway with each passing motor vehicle. In this manner, many weeds are distributed along roadsides. Similarly, locomotives and their freight cars generate even stronger gusts of winds as they travel up and down railroad tracks. As a result, weeds having tiny seeds or seeds with tufts of hair are distributed up and down railroads. For example, Chaenorrhynum minus (Dwarf Snapdragon) is common along railroad tracks because it is well-adapted to dry gravelly areas and its tiny seeds are blown back and forth by passing trains.
Dispersion of Weeds
Lawn maintenance companies help to spread weeds from one lawn to another in urban areas. This is because weed seeds can become trapped within the undercarriage of power mowers amid the grass clippings. Eventually, some of the old grass clippings and the weed seeds of other lawns are blown out from the power mower onto a new lawn. Farmers help to spread weeds to other agricultural areas through shipments of contaminated grains and bales of hay. The transportation of livestock by trucks and freight train spreads weeds along roadsides and railroads through the droppings of such animals while they are being transported from one area to another. Wind and Water. Gusts of wind resulting from local weather conditions helps to spread those weeds with seeds that are tiny or have tufts of hair. Even large seeds can be blown across the ground or carried aloft by the strong winds of thunderstorms and hurricanes. Examples of wind-dispersed weeds include Asclepias syriaca (Common Milkweed), Taraxacum officinale (Dandelion), and Conyza canadensis (Horseweed). Each of these species have seeds with tufts of hair. Weeds with tiny seeds that are easily blown about by the wind include Artemisia vulgaris (Mugwort) and Consolida ajacis (Rocket Larkspur). Some plants produce seeds or seedpods that are flat, light-weight, and surrounded by a thin papery membrane (i.e., they are winged). This enables even larger seeds to be blown about by the wind. Weeds adopting this method of wind-dispersion include Linaria vulgaris (Butter-and-Eggs) and Thalaspi arvense (Field Pennycress). Another group of plants break off at the base of the stem (or the base of the inflorescence) and act like a “tumbleweed” in an open terrain, scattering their seeds along the way. Weeds adopting this method of wind-dispersion include Sisymbrium altissimum (Tumble Mustard) and Kochia scoparia (Kochia). Such weeds often assume a more or less spheroid shape, which enables them to roll across the ground in response to strong winds.
Mechanical Ejection. Some plants fling their seeds several inches or several feet by mechanical means. This typically involves the release mechanism of their seedpods as they mature. Weedy plants that rely on the mechanical ejection of their seeds, in whole or in part, include Erodium cicutarium (Stork’s Bill), Oxalis stricta (Yellow Wood Sorrel), and some Euphorbia spp. (Spurges).