Abstract: Genetically based trait covariances are critical for invasive plant adaptation. Clonal growth can facilitate local expansion; however, the evolutionary relationship between clonal timing and performance remains underexplored. Particularly, it is unclear how these traits covary to affect adaptation across latitudes. Here, we investigated variation and covariance in two clonal traits (the timing of the first ramet emergence and peak number of ramets within a growing season) of Spartina alterniflora, an aggressive coastal invader in China. We sampled eight populations spanning the entire latitudinal range of S. alterniflora along the Chinese coast (21° N–38° N). These seeds were cultivated in three common gardens at low (21° N), mid (28° N), and high (38° N) latitudes. Plants from low-latitude garden exhibited significantly earlier ramet emergence and greater ramet production than those in mid- and high-latitude gardens across the growing season, highlighting substantial phenotypic plasticity. The timing of the first ramet emergence showed a provenance-by-environment interaction, and was driven by abiotic factors at original sites. Furthermore, we found a generally negative covariance between ramet emergence time and ramet production, populations originating from low-latitudes consistently exhibited earlier ramet emergence with more ramets. Our findings suggest that clonal traits covariances were likely to benefit the local adaptation of S. alterniflora at low latitudes, and such covariances appeared to be driven by genetic admixture. Our study advances the understanding of how variable life-history strategies enable invasion success across environmental gradients in the introduced range, and highlights the need for region-specific management strategies tailored to local phenology.

Key words: biological invasion, clonal growth, common garden, latitudinal gradient, life-history strategies, trait covariances