Growth and photosynthesis characteristics of invading larch saplings in an occasionally flooded dry stream bed in cool-temperate Japan

Semi-naturally occurring Japanese larch (Larix kaempferi) saplings originating from planted trees are often observed in riparian habitats around Nikko National Park, Japan, and they may be affecting the structure and functioning of endemic riparian vegetation. To evaluate possible larch establishment in riparian habitats, we examined the aboveground structure and leaf (needle)-level ecophysiological characteristics of larch saplings (~3 m in height and ~8 years old) grown in a dry-wet stream bed (site DW, well-drained but floods after heavy rains) and in neighboring non-riparian vegetation (site M, stably mesic) for comparison. DW saplings were generally shorter than M saplings for their ages, and the total length of current-year stems in DW saplings was about a half that of M saplings. DW saplings produced a larger number of short shoots (shoots without apparent stem elongation), resulting in a similar amount of leaves to those of the M saplings. Leaves of DW saplings had lower nitrogen contents (N) and photosynthetic capacity (light-saturated net CO₂ assimilation rate and electron transport rate measured on excised branches) than those of M saplings. These results suggest that larch saplings were subject to some environmental stressor, but were able to grow slowly yet steadily in riparian environments for a relatively long period, implying considerable potential for colonization of riparian ecosystems. Further studies on the structure, functioning and management of cool-temperate Japanese ecosystems should consider the unintended expansion of introduced larch into surrounding habitats.