Geographical distribution and phenotypic differentiation in populations of Dactylis glomerata L. in Japan

Although the perennial grass Dactylis glomerata L. has established dominant populations in Japan since its introduction in the 1870s, there have been marked reductions in its abundance in southern and northeastern regions. In order to examine the effects of climatic factors on distribution and differentiation of the naturalized populations of D. glomerata, abundance of 26 populations over a distance of 1500 km along a latitudinal gradient was recorded at each site, and life-history traits of each population were measured in a common garden. It was found that the reduction in abundance was related to the mean summer temperature in southern regions and to the lowest temperature before snow cover in northeastern regions. Recent climatic records in Japan show an increase in the mean summer temperature but no apparent changes in the lowest temperature before snow cover. These data suggest that, assuming the recent trend in climatic changes continues, the population abundance will decrease in southern regions and will change little in the northeastern regions. Germination speed, leaf width and reproductive allocations showed clinal variation over a geographical range, and the southern populations had more rapid germination, narrower leaves and lower reproductive allocation than did the northern populations. On the other hand, seed size and germination date showed margin-center differentiation. Marginal populations in both distributional borders had smaller seeds and lower germination rates than did the central populations.