Testing the full impact of neighbourhood crowding within natural vegetation requires more than just effects incurred by established plants. It must also include measurements that take into account suppressive effects on the earliest plant life stages of resident individuals — seeds, their germination (emergence of radicles and cotyledons), and very young rooted seedlings. In this study, we explored the potential for these effects in a field experiment spanning three years, using a novel design for controlling granivory and small mammal herbivory. This allowed us to assess the limitations of natural crowding on seed recruitment success for non-resident species introduced into both natural and denuded neighbourhood plots within a temperate mesic old field meadow in eastern Ontario, Canada. Our results show that crowding by standing vegetation of resident species caused an overall reduction of seed recruitment success by more than 90%. These data provide strong inference that suppression resulting directly from near neighbour effects are likely to impose routinely intense natural selection within temperate mesic old field habitats like our study site. The consequences of this selection, in terms of traits promoting plant fitness under competition, are traditionally interpreted in terms of superior resource depletion/uptake, typically associated with greater growth accumulation and larger potential body size. We suggest, however, that these consequences are rare. Individuals of any species approach maximum potential body size only when near neighbour effects are relatively weak — not within crowded neighbourhoods. Recent studies suggest that severe neighbourhood crowding (where virtually all resident plants are forced to remain relatively small) selects instead for ‘reproductive economy’ — i.e., capacity to produce at least a few (or even at least one) offspring despite severe body size suppression, involving a relatively small minimum reproductive threshold size. Potential for additional component traits of reproductive economy are also suggested for investigation in future research.
The study was conducted between 2013 and 2016 at the Queen’s University Biological Station (QUBS) near Chaffey’s Locks, Ontario, Canada (44°33’ N, 76°21’ W), using an old field meadow on a section of QUBS land known locally as the Bracken tract. The climate is temperate mesic with mean annual precipitation (1981-2010) of 950 mm (based on data from the weather station at nearby Kingston; ECCC 2017), and the soil is clay loam. The field is rectangular, roughly 150 m × 200 m, and surrounded by a mix of shrubland and mature woodland. The field was last tilled and sown (with an unknown forage seed mix) sometime in the early half of the last century, and was used periodically for light cattle grazing and occasional hay harvest until 2010. A survey of the field in 2010 recorded 52 resident species (Appendix A1).
Selection of Introduced Species
In the summer field season of 2013, seeds were collected from wild herbaceous vegetation near QUBS for as many species (n = 46; Table 1) as were encountered during haphazard field excursions and as time permitted. Species were chosen that are at least occasionally found in old field habitats, but also (and in some cases more commonly) found in more recently disturbed (e.g., roadside) habitats — but only species not already resident within the Bracken old field. At least 40,000 seeds (from one or several populations combined) were collected for each species and kept in paper bags stored at ambient (room) temperature until December (2013) when they were placed within experimental field plots (protected from granivory and from physical disturbance by wind and water)—described below—in order to allow breaking of dormancy under natural overwintering conditions.