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

Abstract

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 (eelgrassZostera 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°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.