Morphological traits for rapid and simple separation of native and introduced common reed (Phragmites australis)

Abstract Effective management of the introduced invasive grass common reed [Phragmites australis (Cav.) Trin. ex Steud.] requires the ability to differentiate between the introduced and native subspecies found in North America. While genetic tools are useful for discriminating between the subspecies, morphological identification is a useful complementary approach that is low to zero cost and does not require specialized equipment or technical expertise. The objective of our study was to identify the best morphological traits for rapid and simple identification of native and introduced P. australis. A suite of 22 morphological traits were measured in 21 introduced and 27 native P. australis populations identified by genetic barcoding across southern Ontario, Canada. Traits were compared between the subspecies to identify measurements that offered reliable, diagnostic separation. Overall, 21 of the 22 traits differed between the subspecies, with four offering complete separation: the retention of leaf sheaths on dead stems; a categorical assessment of stem color; the base height of the ligule, excluding the hairy fringe; and a combined measurement of leaf length and lower glume length. Additionally, round fungal spots on the stem occurred only on the native subspecies and never on the sampled introduced populations. The high degree of variation observed in traits within and between the subspecies cautions against a “common wisdom” approach to identification or automatic interpretation of intermediate traits as indicative of aberrant populations or hybridization. As an alternative, we have compiled the five best traits into a checklist of simple and reliable measurements to identify native and introduced P. australis. This guide will be most applicable for samples collected in the late summer and fall in the Great Lakes region but can also inform best practices for morphological identification in other regions as well.