Post-dispersal factors influence recruitment patterns but do not override the importance of seed limitation in populations of a native thistle

Abstract

Whether plant populations are limited by seed or microsite availability is a long-standing debate. However, since both can be important, increasing emphasis is placed on disentangling their relative importance and how they vary through space and time. Although uncommon, seed addition studies that include multiple levels of seed augmentation, and follow plants through to the adult stage, are critical to achieving this goal. Such data are also vital to understanding when biotic pressures, such as herbivory, influence plant abundance. In this study, we experimentally added seeds of a native thistle, Cirsium canescens, at four augmentation densities to plots at two long-term study sites and quantified densities of seedlings and reproductive adults over 9 years. Recruitment to both seedling and adult stages was strongly seed-limited at both sites; however, the relative strength of seed limitation decreased with plant age. Fitting alternative recruitment functions to our data indicated that post-dispersal mortality factors were important as well. Strong density-dependent mortality limited recruitment at one site, while density-independent limitation predominated at the other. Overall, our experimental seed addition demonstrates that the environment at these sites remains suitable for C. canescens survival to reproduction and that seed availability limits adult densities. The results thus provide support for the hypothesis that seed losses due to the invasive weevil, Rhinocyllus conicus, rather than shifting microsite conditions, are driving C. canescens population declines. Shifts in the importance of density-dependent recruitment limitation between sites highlights that alternate strategies may be necessary to recover plant populations at different locations.

Regulation of Jacobaea vulgaris by varied cutting and restoration measures

“…This phenomenon, i.e. spatial variation in the factors limiting plant populations can modulate treatment effects on plant population dynamics, was also shown by Rand et al [ 53 ]. Thus, strategies for population management may have to pay attention to specific site conditions.…”

Persistent effects of landscape connectivity on recruitment dynamics in secondary forests

“…Life history tradeoffs link the two feedback loops, as both acquisitive traits associated with fast growth under favorable conditions (e.g., photosynthetic rates, SLA) and traits associated with fecundity and dispersal (e.g., seed size) have been found to trade off with the ability to survive in the shade in tropical forests (Lai et al, 2021;Turner, 2008) (but see Clark et al, 2004). That implies that, because we analyzed sapling recruitment rather than seed dispersal, the pattern of increasing CWM seed mass may reflect that larger-seeded species became less dispersal limited, but also that decreasing light levels increasingly limit seedling establishment and survival of smallerseeded species (Gratzer et al, 2022;Kraft et al, 2015;Rand et al, 2020). An earlier trait-based study in our field site provides support for the latter, showing that maximum photosynthesis was a better predictor of successional changes in species' recruitment and mortality rates than seed mass (Lai et al, 2021).…”

Agricultural Research Service Weed Science Research: Past, Present, and Future

“…For instance, seed transport associated with flooding patterns might augment propagule pressure and shorten timelines for Russian olive ( Elaeagnus angustifolia L.) stand increase (West et al 2020). Rand et al (2020) demonstrated seed predation by a biological control agent reduces already limited seed availability, further reducing populations of a non-target thistle ( Cirsium canescens Nutt. ), even when recruitment pressures differ across sites.…”