Population differentiation in the tiller architecture of Microstegium vimineum (Poaceae) in relation to habitat

Grass tillers grow by addition of modular units known as phytomers. Differences in phytomer organ size produce subindividual variation with potential adaptive value. Here, patterns in organ mass along tillers in the invasive annual Microstegium vimineum are related to habitat and tiller architecture. In an earlier study, seed families were collected from two populations: one from a sunny, woodland edge and the other from a shady understory in New Jersey, USA. Plants from these seeds were grown in a greenhouse. Phytomers along primary tillers were divided into culms, leaves, and cleistogamous or chasmogamous spikelets and seeds, dried and weighed. These data were used to examine the quantitative genetics of subindividual variation among families and populations, and the relationship of tiller fitness (based on total seed mass) to the mean or subindividual variance of phytomer traits. Phytomer position along a tiller was the major determinant of organ mass. Leaf mass increased from basal to upper nodes; cleistogamous reproductive mass decreased from upper to lower nodes. Phytomer organs were heaviest in the population from the sunny habitat. Family explained < 18% of variation in organ mass. Tiller fitness was positively correlated with mean culm, and leaf mass, but negatively correlated with coefficients of variation. Field‐collected tillers showed evidence of selection for increased leaf mass. Subindividual variation in M. vimineum is mostly due to phytomer position along a tiller and the prevailing light environment. Differentiation between sunny and shady populations suggests selection favors heavier phytomer leaves and culms, especially in the shady understory where this species is most invasive.