Provenance-by-environment interaction of reproductive traits in the invasion of Spartina alterniflora in China.

Ecological invasions are facilitated by pre-adaptation and phenotypic plasticity, upon which evolution can act. The rapid invasion of the intertidal grass Spartina alterniflora in China during the last 36 yr is a test case for the roles of these mechanisms. A previous study of S. alterniflora in China found strong latitudinal clines in vegetative and sexual traits and concluded that most of this variation was due to phenotypic plasticity. Recent observations suggested provenance-by-environment interactions, and we employed common gardens at multiple latitudes as a test of this idea. Phenotypically, field plant height, which correlates strongly with biomass and other indices of vegetative performance in this species, showed a hump-shaped relationship across 10 sites, covering 19° of latitude; field seed set increased linearly with latitude. To assess the role of plasticity vs. genetic differentiation in these patterns, we grew plants from the ten field sites in three common gardens at low (20.9° N), mid (28.3° N), and high (38.0° N) latitudes to maturity, at 18 months. Plant height varied among common gardens, with the tallest plants at mid latitude, mirroring the field pattern, consistent with the previous study. Within the gardens, latitude of origin also affected plant height. Seed set varied among the gardens, with the greatest values at high latitudes, again mirroring the field pattern and indicating substantial plasticity. Evidence of evolution was found as increasing seed set with latitude among provenances within common gardens. However, the effect differed among common gardens, with the greatest slope in the high-latitude garden, lower slope in the mid-latitude garden, and no relationship in the low-latitude garden, indicating a provenance-by-environment interaction. The number of surviving plants also suggested a provenance-by-environment interaction; no relationship with latitude among provenances in the two southern gardens and increasing survival with latitude in the northern garden. Field seedling density was ~200-fold greater at high than at low latitude sites. The profuse seed germination and recruitment in the north could have created high selection intensity resulting in evolution of reproductive traits at high latitudes with the result that the mechanisms of the invasion differ with latitude.