A mosaic of phenotypic variation in giant ragweed (Ambrosia trifida): Local‐ and continental‐scale patterns in a range‐expanding agricultural weed

Abstract

Spatial patterns of trait variation across a species' range have implications for population success and evolutionary change potential, particularly in range‐expanding and weedy species that encounter distinct selective pressures at large and small spatial scales simultaneously. We investigated intraspecific trait variation in a common garden experiment with giant ragweed (Ambrosia trifida), a highly variable agricultural weed with an expanding geographic range and broad ecological amplitude. Our study included paired populations from agricultural and natural riparian habitats in each of seven regions ranging east to west from the core of the species' distribution in central Ohio to southeastern Minnesota, which is nearer the current invasion front. We observed trait variation across both large‐ and small‐scale putative selective gradients. At large scales, giant ragweed populations from the westernmost locations were nearly four times more fecund and had a nearly 50% increase in reproductive allocation compared to populations from the core. The degree of surface texture on fruits also declined from east to west. Greater fecundity in the west represents a putative trade‐off between fruit size and fruit number across the study region, although no such trade‐off was found across individual plants. This pattern may effectively result in greater propagule pressure closer to the invasion front. At smaller spatial scales, plants from agricultural populations emerged later and were smaller than plants from riparian populations. However, because plants from agricultural populations allocated more biomass to reproduction, total fecundity did not differ across habitats. Our emergence data are consistent with previous observations showing delayed emergence in agricultural compared to natural populations; thus evolutionary change may be predictable as giant ragweed continues spreading into agricultural fields throughout North America. These shifts in life‐history strategy apparently bear no fecundity cost, suggesting that giant ragweed's success can be attributed at least in part to its substantial adaptive potential.

Phenotypic Trait Variation in Populations of a Global Invader Mayweed Chamomile (Anthemis cotula): Implications for Weed Management

“…Second, A. cotula in the PNW mainly occurs in agricultural fields, hence localized differences in agricultural management practices such as the competitiveness of the planted crop (Ogg et al, 1994), crop rotations, herbicide applications (Lyon et al, 2017, Hovick et al, 2018, tillage (Ghersa and Martínez-Ghersa, 2000), and fertilizer application (Zeng et al, 2017) could have selected for local adaptation in A. cotula traits across populations. For example, farming practices including tillage, fertilizer, herbicide regime, crop type, sowing date, and fallow can affect the flowering phenology of associated weeds (Ekeleme et al, 2000;Fried et al, 2012;Gaba et al, 2017).…”

“…If these factors are operating simultaneously, they may also interact in their effects on the weed. Variable farm management practices such as the timing of planting or other mechanical weed management inputs in crop fields (Hovick et al, 2018) will be influenced by climatic and edaphic factors such that both climate and farming practice together exert unique selection on specific populations. Variations among populations could reflect long and differing histories of selection, drift, or pre-adaptation in the native range before introduction (Keller and Taylor, 2008;Schlaepfer et al, 2010;Cadotte et al, 2018).…”

Ambrosia trifida L

“…It measures from 0.5 to 1.2 cm long and from 0.3 to 0.5 cm wide. A. trifida is characterized by enormous variability in the size and shape of its seeds, which may correspond to an ability to germinate in a variety of conditions (Harrison et al , 2007; Hovick et al , 2018; Fig. 1).…”

“…In North America, there is variation in A. trifida plant traits at both large and small geographic scales. Populations in the western USA corn belt had nearly four times greater fecundity and a 50% greater allocation to reproduction than populations in the eastern USA corn belt (Hovick et al , 2018). In addition, seedling emergence patterns differ among populations in agricultural fields (Sprague et al , 2004; Schutte et al , 2006, 2008).…”

“…Seedling emergence patterns also differ between agricultural and non‐agricultural environments. Populations from agricultural habitats exhibited a more prolonged emergence pattern than those from riparian, early successional, railroad siding or forest border habitats (Schutte et al , 2012; Hovick et al , 2018).…”