Tradeoffs in life history investment of eelgrass Zostera marina across estuarine intertidal conditions

Plant ecological strategies are associated with tradeoffs between investment in each module (e.g. leaf, stem) and the longevity or rate of production of these modules, while clonal species can exhibit additional tradeoffs between asexual and sexual reproduction. To evaluate intraspecific life history patterns, we applied multivariate analyses to a suite of 7 distinct traits plus density in a clonal marine angiosperm (eelgrass Zostera marina) across sites and seasons. We expected life history tradeoffs for ecotypes at different sites, but positively correlated investment seasonally due to variation in photoperiod at 46 degrees N. Relative growth rate (RGR), an indicator of suitable growing conditions, varied seasonally by 3-fold and strongly tracked photoperiod, with small variation among sites. Consistent with tradeoffs across ecotypes, shoot size (length, mass), module production (leaf and shoot emergence rates), and flowering tended to load in opposite ways on the first 2 principal components (62% of total trait variation). Annual life histories were separated from perennial life histories in both space (higher tidal elevations) and time (2 mo lag in flowering). In perennial populations, flowering peaked seasonally with daylength, but size continued to increase after the summer solstice, and asexual branching occurred in early spring and late summer, so investment in most life history traits did not align with RGR. Seasonal variation in shoot density was pronounced for small ecotypes, but for larger ecotypes, variation occurred through change in shoot size. Because the functional traits of foundation species help shape their role, documented covariation in these traits addresses a gap between environmental conditions and community response that is mediated by biogenic structure.

Automated summary

Plant ecological strategies involve tradeoffs between investment in different plant modules and the longevity or production rate of these modules. Clonal species can also have tradeoffs between asexual and sexual reproduction. In this study, we analyzed the traits and density of a clonal marine angiosperm, eelgrass, to evaluate intraspecific life history patterns. We found tradeoffs between different ecotypes at different sites and a positive correlation between investment and photoperiod. The relative growth rate varied seasonally and showed a strong relationship with photoperiod. Shoot size, module production, and flowering had opposite effects on principal components, indicating tradeoffs between these traits. Annual and perennial life histories were distinguished by spatial and temporal differences in flowering. Our findings highlight the importance of functional traits in shaping the role of foundation species and address the gap between environmental conditions and community response mediated by biogenic structure.