Predicting spatio-temporal changes in suitable climate conditions for tree regeneration using niche differences between adult and juvenile trees
To assess climate change impacts on tree species’ regeneration niche separately with growth niche, we constructed a size-based species distribution model (SDM), which enabled us to predict both adult and juvenile habitats simultaneously. We then predicted the two objects from the SDM. One is spatiotemporal changes in distribution overlaps through classifying four potential habitats (PHs) as (1) overlapped areas between adults and juveniles, (2) only-adults, (3) only-juveniles, and (4) non-habitats, for the baseline year and the years 2080-2099. The other is the year of habitat loss for juveniles by predicting juvenile PHs for every single year between 2011 and 2099. Siebold’s beech (Fagus crenata) was selected as target species since the species has size dependent niche differences along snow gradient. Distribution datasets for adults (canopy and sub-canopy layers) and juveniles (herb layer) were extracted from the phytosociological relevé database (PRDB). Five climatic datasets (warmth index, mean of the daily minimum temperature of the coldest month, summer precipitation, winter rainfall, and maximum snow water equivalent) were calculated from 24 general circulation models (GCMs). For the juvenile PHs, we used four representative GCMs.
By the end of this century, overlapped areas between adults and juveniles were predicted to decrease with future warming. Only-adult PHs were also predicted to decrease widely in less snowy regions. On the other hand, only-juvenile PHs were predicted to increase in snowy regions. These spatio-temporal patterns were consistent among the 24 GCMs, suggesting high certainty under predicted future climate. The year of habitat loss for juvenile PHs was earlier in warm and less snowy regions than cold and snowy regions. Furthermore, juvenile PHs were predicted to be lost widely not only at the end of this century but also in the 2060s, considerably due to rapid warming around the decade. This temporal pattern in the lost year of juvenile PHs was consistent among the representative four GCMs. We conclude that Fagus crenata’s regeneration will start declining from the 2060s but snow rich condition will postpones the timing of regeneration losses, causing increases of only-juvenile PHs in snow rich region. Our size-based SDM approach showed potential to assess spatio-temporal changes in tree species’ regeneration niche especially when a target species had the size dependent niche difference along specific climatic conditions.