Neighbourhood crowding severely limits seed offspring recruitment in a temperate mesic old-field meadow

Abstract

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.