Dead litter of resident species first facilitates and then inhibits sequential life stages of range‐expanding species

Abstract

Resident species can facilitate invading species (biotic assistance) or inhibit their expansion (biotic resistance). Species interactions are often context‐dependent and the relative importance of biotic assistance versus resistance could vary with abiotic conditions or the life stage of the invading species, as invader stress tolerances and resource requirements change with ontogeny. In northeast Florida salt marshes, the abundant dead litter (wrack) of the native marsh cordgrass, Spartina alterniflora, could influence the expansion success of the black mangrove, Avicennia germinans, a tropical species that is expanding its range northward. We used two field experiments to examine how S. alterniflora wrack affects A. germinans success during (a) propagule establishment and (b) subsequent seedling survival. We also conducted laboratory feeding assays to identify propagule consumers and assess how wrack presence influences herbivory on mangrove propagules. Spartina alterniflora wrack facilitated A. germinans establishment by promoting propagule recruitment, retention and rooting; the tidal regime influenced the magnitude of these effects. However, over time S. alterniflora wrack inhibited A. germinans seedling success by smothering seedlings and attracting herbivore consumers. Feeding assays identified rodents—which seek refuge in wrack—as consumers of A. germinans propagules. Synthesis. Our results suggest that the deleterious effects of S. alterniflora wrack on A. germinans seedling survival counterbalance the initial beneficial effects of wrack on A. germinans seed establishment. Such seed‐seedling conflicts can arise when species stress tolerances and resource requirements change throughout development and vary with abiotic conditions. In concert with the tidal conditions, the relative importance of positive and negative interactions with wrack at each life stage can influence the rate of local and regional mangrove expansion. Because interaction strengths can change in direction and magnitude with ontogeny, it is essential to examine resident–invader interactions at multiple life stages and across environmental gradients to uncover the mechanisms of biotic assistance and resistance during invasion.

Litter accumulation, not light limitation, drives early plant recruitment

“…Litter can also inhibit germination through leachates (Ruprecht et al, 2008) or by blocking incoming light and precipitation (Facelli & Pickett, 1991). In addition, rodents may be more active under the protective cover of litter, since dead plant material can provide shelter and cover, increasing seed and seedling predation (Edwards & Crawley, 1999;Fraser & Madson, 2008;Hulme, 1996;Smith et al, 2021). In our study, recruitment success was not associated with light intensity and quality, soil moisture, soil temperature or soil C:N ratio, suggesting that litter likely acted as a mechanical barrier, rather than affected seedling recruitment via any F I G U R E 5 Relationships between total seedling richness and (a) litter depth, (b) soil moisture, (c) soil temperature, (d) light intensity at the ground (under litter and live vegetation when present), (e) red to far red ratio and (f) soil C:N ratio.…”

Litter facilitates plant development but restricts seedling establishment during vegetation regeneration

“…In addition, the factors driving the effect of litter on seedling density are different from emergence and establishment, such as single litter constraining emergence and mixed litter constraining density, and more significant for greenhouse seedling emergence and establishment and for field seedling density, respectively. We speculated two plausible explanations to address this distinction; one is differences in litter structure (Mohler et al, 2021) and the other is differences in the feedback of seedling growth and expansion to environmental stress (Smith et al, 2021). In general, single litter is more likely to form dense mats to form a mechanical barrier that hinders emergence (Koorem et al, 2011), and mixed litter decomposes faster and releases more nutrients and allelochemicals (Richards et al, 2010).…”

Using ecosystem engineers to enhance multiple ecosystem processes

“…The wrack can smother vegetation underneath it, leach nutrients and provide habitat for nest building species. Thus, the legacy engineering effect of Spartina wrack often has strikingly different effects from its live counterpart due to differences in orientation, location and traits (Smith et al, 2018(Smith et al, , 2021.…”

“…In addition to the central roles of EEs, it is increasingly recognized that most ecosystems sustain multiple EEs that interact; yet only recently have their interactive dynamics been fully investigated (Altieri et al, 2007; Bishop et al, 2012; Gribben et al, 2019; Smith et al, 2018; Smith et al, 2021). These interactions may be key to the EEs' success, as well as producing important synergies that benefit ecosystem processes from the interactions of their engineering (Gribben et al, 2019), or antagonisms that diminish net processes (Berkenbusch & Rowden, 2003; Gamfeldt et al, 2013; Woodin, 1976; Zavaleta et al, 2010).…”