Frond size, shape and fertility of Thelypteris confluens (Thunb.) C. V. Morton in wetlands disturbed by human activities in Hokkaido, northern Japan

Abstract

Thelypteris confluens (Thunb.) C. V. Morton exhibits dimorphism between sterile and fertile fronds when fully matured. To assess the frond fertility patterns in this species, the frond size and the shape and degree of fertile fronds were measured in individuals growing in disturbed wetlands in Hokkaido, northern Japan, between late summer and early autumn in 2017 and 2018. This species often establishes on human-disturbed wetlands in boreal regions. The measured parameters were frond fertility, number of veins (NV), dry weight (DW), frond length (FL), blade length (BL), blade width (BW), frond area and blade perimeter. The Dissection Index, aspect ratio and specific frond area were calculated. The data were analyzed by generalized linear models (GLMs) and standardized major axis tests to investigate the determinants of frond fertility and the differences between the sterile and fertile fronds. All the GLMs for predicting frond fertility contained NV, FL and BL, suggesting that NV had a specific role in fertility. Compared with the use of destructive measurements, the use of nondestructive measurements in the GLMs led to slightly lower prediction accuracies. The frond sizes and shapes in relation to fertility were not strongly related to the disturbance rank scores, while the populations in highly disturbed sites showed low numbers of fertile fronds. These findings strongly suggested that the shape- and size-related variables and NV affected frond fertility differently. NV provides more precise measurements of frond maturity and population dynamics for nondestructive long-term monitoring.