Ontogenetic niche dynamics, that is the change in environmental requirements throughout an individual's development, remains an underexplored aspect in ecological research with potentially major implications for predicting species responses to climate change. Despite a growing body of research in tree regeneration dynamics, multispecies assessments to establish the expectations for niche ontogeny are still lacking. Here, we used country-wide repeated inventory data from the Swiss National Forest Inventory to: (i) compare juvenile and adult climate niches across 43 tree species, (ii) investigate the direction and magnitude of ontogenetic niche shifts across species, and (iii) investigate the temporal consistency of these niche estimates over inventories. Additionally, for the five best replicated species, we investigate niche overlaps and shifts in further detail over seven size classes, spanning from juveniles under 10 cm in height to large adult trees. Ontogenetic niche conservatism, that is large niche overlap between size classes, was the prevailing pattern across species. Non-overlapping fractions of the niche were small and tended to be suitable to juveniles only, suggesting slightly larger juvenile than adult niches. In-detail analyses of size class transitions also suggested high niche overlap between consecutive sizes classes (>93% in average). Niche estimations were temporally consistent over inventories for both adults and juveniles, though lower for juveniles. Finally, juvenile niche shifts did not align with climate change expectations, being highly variable across species. Consecutive size class transitions similarly indicated niche shifts that were not consistent within- nor across species. Synthesis. Our results strongly suggest niche conservatism as the expected patterns across ontogeny over Swiss tree species. This imply that approaches relying exclusively on adult presence are still likely to be representative of all life stages, at least for temperate trees at mesic conditions, which are those best represented across Swiss forests. At the same time, the lack of clear climate change responses in juveniles (or across size classes) for most species emphasizes the importance of species-specific and non-climatic factors in predicting climate change range shifts. How these shifts in potential niches translate to in situ responses or tolerance to extremes of seedlings living under the canopy remain to be tested.
Manzanedo et al. (Fri,) studied this question.