Long-term growth of three sympatric Dryopteris fern species shows the accumulation of climatic effects over 2 years because of organ preformation

We hypothesized that yearly response of plant growth to seasonal weather conditions would provide insight into the biogeographic status of three closely related and sympatric Dryopteris fern species and elucidate their potential future performance in conditions of climate change. In a 9-year experiment, we surveyed the yearly performance of 108 ex situ pre-grown sporophytes of D. carthusiana, D. dilatata, and D. expansa in six forests in Estonia. We used multi-factorial models to assess species-specific responses of frond traits and population parameters to seasonal temperature, precipitation, and snow cover, and also considered the effects of continentality and overstory density. Frond length was highly limited by plant age, and even the 11-year-old plants did not reach their mature size. The number of fronds stabilized already in the first few years. Climate effects on leaf traits accumulated over 2 years, as their growth benefited from the abundant precipitation during two growing seasons and the presence of dry cold winters. Milder climate near the coast and semi-open overstory provided some extra support for the growth. Species-specific effects were rare. The species’ responses to habitat and climate effects only partly mirrored their present biogeographic distribution. Evidently, possible effects of climate change might go unnoticed for decades in these ferns since it would be mitigated by (1) their slow ontogenesis, (2) organ preformation during preceding year(s), and (3) region-specific and habitat-specific effects of environmental conditions.