Self‐thinning and size‐dependent flowering of the grass Spartina alterniflora across space and time

Abstract

Plants adjust their size and reproductive effort in response to numerous selection pressures and constraints. The self‐thinning law describes a well‐known trade‐off between size and density. Plants also trade‐off investment into growth vs. sexual reproduction, as described by life‐history theory. We build on past work on plant allometry and life history by examining both self‐thinning and size‐dependent reproduction in a single plant species, the saltmarsh grass Spartina alterniflora, across a wide range of settings: three landscape positions, two habitats and eight sites, across sixteen years. Plants in different landscape positions and years varied tremendously in size and shoot density. However, all this variation could be explained by a single allometric relationship consistent with the self‐thinning law, but with a lower slope. Flowering was size‐dependent, and the size at which plants had a 50% probability of flowering varied among habitat, sites and years. Plants that were stressed reproduced at a smaller size than plants that were growing under good conditions, and this pattern was consistent among habitat, sites and years. Finally, reproductive biomass and the proportion of shoots flowering increased with increasing vegetative size (plant height or shoot biomass). Combining these two patterns, S. alterniflora plants growing high density are small and reproduce at a smaller size than large plants growing at low density. Although there is tremendous spatial and temporal variation in S. alterniflora growth and reproductive patterns, all this variation can be understood as resulting from two simple allometric trade‐offs. Because saltmarsh plants often occur in monospecific stands, they may serve as simple, model systems for studies of plant life history. A free Plain Language Summary can be found within the Supporting Information of this article.

Contrasting plant adaptation strategies to latitude in the native and invasive range of Spartina alterniflora

“…However, at lower latitudes in the native range, high temperatures reduced S. alterniflora performance (Więski & Pennings, ), consistent with previous findings in China (Liu et al , , ; Qiu et al , ). Because shoot density is negatively related to plant height (Liu & Pennings, ), shoot density was lowest when plants were tallest. Plant height increased with tide range, this result is consistent with the findings of Turner () for S. alterniflora in its native range, and may be related to a decrease in sediment redox potential (Castillo et al , ).…”

“…One caveat to our results is that we only sampled the tall form S. alterniflora . In the native range, S. alterniflora varies greatly in height, shoot density and flowering across elevation as a function of abiotic stress (Richards et al , ; Liu & Pennings, ). S. alterniflora in China also retains similar plastic responses to elevation (Peng et al , ; Zhu et al , ).…”

“…Because the two measures showed the same pattern (Liu et al , ), we only choose the seed set as the indicator of sexual reproduction. Year‐to‐year variation affects plant growth and sexual reproduction (Liu & Pennings, ), but this variation is weaker than latitudinal patterns (Liu et al , ).…”

“…Recent research has found that fecundity plays a critical role in the spread of introduced species, especially under variable environmental conditions (Hayes & Barry, ; Pyšek & Richardson, ; Simberloff, ; van Kleunen et al , ). Fecundity is driven in part by plant size, which determines the availability of resources for sexual reproduction (Klinkhamer et al , ; Liu & Pennings, ). Plant size will also affect competitive interactions with native species and conspecifics, because taller plants are better competitors for light (Keddy, ).…”

Variation in synchrony of production among species, sites, and intertidal zones in coastal marshes

“…How coastal wetlands function should be strongly affected by whether or not primary production in these systems is synchronous. Aboveground primary production in coastal wetlands varies two‐ to threefold from one year to another (Morris et al 1990, Więski and Pennings 2014, Liu and Pennings 2019), and because the primary producers are foundation species (Pennings and Bertness 2001), these fluctuations are likely to affect most other species in the community and all aspects of ecosystem function (Lamy et al 2020). Because coastal wetlands provide numerous ecosystem services to mankind (Barbier et al 2011, Mitsch et al 2013), whether primary production and the abundance of the marsh species that contribute to these services (Morris et al 1990, Rochlin and Morris 2017) is synchronous or asynchronous at the landscape scale has important management implications.…”